solar detoxification - for cleaning toxic sites and materials

8/8/2019 Solar Detoxification - for Cleaning Toxic Sites and Materials

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SolarDetoxificationby Julian Blanco Galvez, Head of Solar Chemistry and

Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area,

Plataforma Solar de Almeria, Spain

United Nations Educational, Scientific and Cultural Organization

2003

Electronic copy only


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TABLE OF CONTENTS

PART A. SOLAR DETOXIFICATION THEORY

1. Introduction

Aims

Objectives

Notation and units

1.1 Solar Chemistry

1.2 Water contaminants

1.3 Photodegradation principles

1.3.1 Definitions1.3.2 Heterogeneous photocatalysis

1.3.3 Homogeneous photodegradation

1.4 Application to water treatment

1.5 Gas-phase detoxification

Summary of the chapter

Bibliography and references

Self-assessment questions

Answers

2. Solar irradiation

Aims

Objectives

Notation and units

2.1 The power of light

2.1.1 Ultraviolet light

2.1.2 Visible light

2.1.3 Infrared light

2.2 The solar spectrum

2.3 Solar ultraviolet irradiation

2.4 Atmospheric attenuation of solar radiation

2.4.1 Annual available ultraviolet radiation

2.5 Solar radiation measurement

2.5.1 Detectors

2.5.2 Filters

2.5.3 Input Optics




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Answers

3. Experimental systems

Aims

Objectives

Notation and units

3.1 Laboratory systems3.2 Solar detoxification pilot plants

3.3 Operation of pilot plant

3.3.1 Once-through operation

3.3.2 Batch operation

3.3.3 Modelling once-through and batch operation

3.4 Evaluation of solar UV radiation inside

photoreactors

3.4.1 Radiometers calibration

3.4.2 Correlation between radiometric andspectroradiometric data

3.4.3 Collector efficiency

3.4.4 Actinometric experiments

3.5 Simplified method for the evaluation of solar UV

radiation inside photoreactors


Bibliography and referencesSelf-assessment questions

Answers

4. Fundamental parameters in photocatalysis

Aims

Objectives

Notation and units

4.1 Direct photolysis

4.2 Oxygen influence4.3 pH influence

4.4 Catalyst concentration influence

4.5 Initial contaminant concentration influence

4.6 Radiant flux influence

4.7 Temperature influence

4.8 Quantum yield



Answers


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5. Water decontamination by Solar Detoxification

Aims

Objectives

Notation and units

5.1 Detoxification of pollutants

5.1.1 Total mineralization

5.1.2 Degradation pathways

5.1.3 Toxicity reduction

5.1.4 Detoxification of inorganic pollutants

5.2 Quantum yield improvement by additional

oxidants

5.2.1 Hydrogen peroxide

5.2.2 Persulphate

5.2.3 Other oxidants

5.3 Catalyst modification

5.3.1 Metal semiconductor modification

5.3.2 Composite semiconductors

5.3.3 Surface sensitisation

5.4 Recommended analytical methods

5.4.1 Original contaminants

5.4.2 Mineralization measurements (TOC)

5.4.3 Intermediate analysis (GC-MS/HPLC-MS)

5.4.4 Extraction methods

5.4.5 Toxicity analysis




Answers

PART B. SOLAR DETOXIFICATION ENGINEERING

6. Solar Detoxification Technology

Aims

Objectives

Notation and units

6.1 Solar collector technology generalities

6.2 Collectors for solar water detoxification.

Peculiarities

6.2.1 Peculiarities of solar UV light utilization

6.2.2 Parabolic trough collectors

6.2.3 Non-concentrating collectors

6.2.4 Compound parabolic concentrator (CPC)


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6.2.5 Holographic collectors

6.3 Concentrated versus non-concentrated sunlight

6.4 Technology issues

6.4.1 Reflective surfaces

6.4.2 Photocatalytic reactor

6.5 Catalyst issues

6.5.1 Slurry versus supported catalyst

6.5.2 Catalyst recuperation and re-use




Answers

7. Solar Detoxification Applications

Aims

Objectives

Notation and units

7.1 Introduction

7.2 Industrial waste water treatment

7.2.1 Phenols

7.2.2 Agrochemical compounds

7.2.3 Halogenated hydrocarbons7.2.4 Antibiotics, antineoplastics and other

pharmaceutical biocide compounds

7.2.5 Wood preserving waste

7.2.6 Removal of hazardous metals ions from water

7.2.7 Other applications

7.3 Maritime tank terminals

7.4 Groundwater decontamination

7.5 Contaminated landfill cleaning

7.6 Water disinfection7.7 Gas-phase treatments




Answers

8. Economic Assessment

Aims


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Objectives

Notation and units

8.1 Photochemical and biological reactors coupling

8.2 Cost calculations

8.2.1 Example A: TiO2-based detoxification plant

8.2.2 Example B: Photo-Fenton based detoxification

plant

8.3 Solar or electric photons?8.4 Solar resources assessment

8.5 Comparison with other technologies

8.5.1 Thermal oxidation

8.5.2 Catalytic oxidation

8.5.3 Air stripping

8.5.4 Adsorption

8.5.5 Membrane technology

8.5.6 Wet oxidation

8.5.7 Ozone oxidation8.5.8 Advanced oxidation processes




Answers

9. Project engineering

Aims

Objectives

Notation and units

9.1 Feasibility study

9.1.1 Identification of target recalcitrant hazardous

compounds

9.1.2 Identification of possible pre-treatments9.1.3 Identification of most adequate photocatalytic

process

9.1.4 Determination of optimum process parameters

9.1.5 Post-treatment process identification

9.1.6 Determination of treatment factors

9.2 Feasibility study example

9.2.1 Background

9.2.2 Experimentation. TiO2-Persulphate tests

9.2.3 Photo-Fenton tests

9.2.4 Conclusions and Treatment Factors

9.3 Preliminary design


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9.4 Preliminary design example

9.5 Final design and project implementation

9.6 Example of final design and project

implementation



Answers

10. International collaboration

Aims

Objectives

Notation and units

10.1 International Energy Agency: The SolarPACES

Program

10.2 The European Union

10.3 The CYTED Program

10.4 Main research activities

10.4.1 United Stated

10.4.2 Spain

10.5 Guidelines to successful water treatment projects

in developing countries




Answers


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SOLAR DETOXIFICATION

1. INTRODUCTION

AIMS

This unit describes an alternative source of energy that combines sunlight and chemistry toproduce chemical reactions. It outlines the basic chemical and physical phenomena that are

related with solar chemistry. This chapter will review approaches that have been taken,progress that has been made and give some projections for the near and longer term prospectsfor commercialisation of solar photochemistry. It also introduces the focus of this book: SolarDetoxification.

OBJECTIVES

By the end of this unit, you will understand the main factors causing the photochemicalreactions and you will be able to do five things:1. Distinguish perfectly between thermochemical and photochemical processes.2. Understand the impact of pollutants on the environment.3. Calculate the energy flux of a light source and its relationship with semiconductor

excitation.4. Understand the basic principles sustaining advanced oxidation processes.5. Describe the most important features of heterogeneous photocatalysis making it applicable

to the treatment of contaminated aqueous effluents.

NOTATION AND UNITS

Symbol UnitsA Absorbance at wavelength AOPs Advanced Oxidation Processesc Light speed nm/s, m/s

ci Concentration of component i molesEG Semiconductor band-gap energy eV, JE Spectral irradiance W m

-2 nm-1

Eo Spectral irradiances incident into the medium W m-2 nm-1

El Spectral irradiances at a distance l W m-2 nm-1

EC50 Concentration that produce an effect in 50% of a population mg/L, mg/kgGAC Granulated activated carbonh Plancks constant J sLC50 Concentration that produce death in 50% of a population mg/L, mg/kg

NOEL No observed effect level mg/kg/daypi partial pressure of component i atm

U energy of a photon eV, J Absorption coefficient cm

-1 atm-1

extinction coefficient mol-1 cm-1

quantum yield. Wavelength nm, m

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1.1 SOLAR CHEMISTRY

The dramatic increases in the cost of oil beginning in 1974 focussed attention on the need todevelop alternative sources of energy. It has long been recognised that the sunlight falling onthe earths surface is more than adequate to supply all the energy that human activity requires.The challenge is to collect and convert this dilute and intermittent energy to forms that are

convenient and economical or to use solar photons in place of those from lamps. It must bekept in mind that today there is a clear world-wide consensus regarding the need for long-term replacement of fossil fuels, which were produced million of years ago and today aremerely consumed, by other inexhaustible or renewable energies. Under these circumstances,the growth and development of Solar Chemical Applications can be of special relevance.These technologies can be divided in two main groups:

1. Thermochemical processes: the solar radiation is converted into thermal energy thatcauses a chemical reaction. Such a chemical reaction is produced by thermal energyobtained from the sun for the general purpose of substituting fossil fuels.

2. Photochemical processes: solar photons are directly absorbed by reactants and/or acatalyst causing a reaction. This path leads to a chemical reaction produced by the

energy of the suns photons, for the general purpose of carrying out new processes.It should be emphasized, as a general principle, that the first case is associated with processesthat are feasible with conventional sources of energy. The second is related only tocompletely new processes or reactions that are presently carried out with electric arc lamps,fluorescent lamps or lasers.

Heat Photons

Thermochemical ProcessSteam reforming of methane

CH4 + H2O CO + 3H2 - 206 kJ/mol600 - 850C

Photochemical ProcessExcitation of a semiconductor

h + SC e- + p+h EG of SC

Increase ofTemperature

Modification ofchemical bonds

Figure 1.1 Schematic view of Solar Chemical Applications

From the outset, it was recognized that direct conversion of light to chemical energy held promise for the production of fuels, chemical feedstock, and the storage of solar energy.Production of chemicals by reactions that are thermodynamically uphill can transform solarenergy and store it in forms that can be used in a variety of ways. Wide ranges of suchchemical transformations have been proposed. A few representative examples are given in

Table 1.1 to illustrate the concept.

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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

Table 1.1 Representative chemical reactions that can store solar energy (Thermochemicalprocesses)

These processes generally start with substances in low-energy, highly-oxidized forms. Theessential feature is that these reactions increase the energy content of the chemicals using

solar energy. For such processes to be viable, they must fulfil the following requirements, asoutlined by NREL (1995) and slightly modified by the authors: The thermochemical reaction must be endothermic. The process must be cyclic and with 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 the energy content. The products of the photochemical reaction should be easy to store and transport.

The other pathway for the use of sunlight in photochemistry is to use solar photons asreplacements for those from artificial sources. The goal in this case is to provide a cost-effective and energy-saving source of light to drive photochemical reactions with useful

products. Photochemical reactions can be used to carry out a wide range of chemicalsyntheses ranging from the simple to the complex. Processes of this type may start with morecomplex compounds than fuel-producing or energy-storage reactions and convert them tosubstances to which the photochemical step provides additional value or destroy harmful

products. The principles of photochemistry are well understood and examples of a wide rangeof types of synthetic transformations are known (Figure 1.2). Therefore, the problem becomesone of identifying applications in which the use of solar photons is possible and economicallyfeasible. The processes of interest here are photochemical, hence, some component of the

reacting system must be capable of absorbing photons in the solar spectrum. Because photonscan be treated like any other chemical reagent in the process, their number is a critical elementin solar photochemistry (see Chapter 2).

OCHO

OCHO

O O

h


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Because they are very technologically and environmentally attractive, solar chemicalprocesses have seen spectacular development in recent years. In the beginning, research insolar chemistry was centered only on converting the solar energy into chemical energy, whichcould then be stored and transferred over long distances. Together with this importantapplication, other environmental uses have been developed, so that today the entire range of

solar chemical applications has a promising future. In principle, any reaction or processrequiring an energy source can be supplied by solar energy.

1.2 WATER CONTAMINANTS

Environmental pollution is a pervasive problem with widespread ecological consequences.Recent decades have witnessed increased contamination of the Earths drinking waterreserves. The inventory ofpriority pollutants compiled by the U.S. Environmental ProtectionAgency provides a convenient frame of reference (in Table 1.2 only a partial list is shown) forunderstanding the importance of removing such contamination from the Earth.

1,1,2,2-Tetrachloroethane

1,l -Dichloroethane1,2,4-Trimethylbenzene1,2-Dibromoethane1,2-Dichlorobenzene1,2-Dichloropropane1,2-Dinitrotoluene1,2-Diphenylhydrazine1,4-Dioxane2,2,4-Trimethylpentane2,4,6-Trichlorophenol2,4,6-Trinitrotoluene2,4 Diaminoanisole2,4-Dichlorophenol

2,4-Dinitrophenol2,4-Dinitrotoluene2,4-Toluene diamine2-Chloroethyl vinyl ether2-Chlorophenol2-Nitropropane4,4-Diaminodiphenyl ether4,4-Methylenedianiline4-Aminoazobenzene4-Methylphenol5-Nitro-o-anisidineAcetaldehydeAcetamide

AcetoneAcetonitrileAcetophenoneAcroleinAcrylamideAcrylic acidAcrylonitrileAldrinAnilineAnthraceneAtrazineBenzamideBenzene

Benzidine

Benzo(a)pyreneBenzyl chlorideBenzenehexachloride)BiphenylBis(2-Chloroethoxy)methaneBromoethaneCaptanCarbarylCarbon disulfideCarbon tetrachlorideCatecholChlordane

Chloroacetic acidChlorobenzeneChlorodibenzodioxins,variousChlorodibenzofuranso-,m-,p-CresolsCumeneCyclohexaneDiazomethaneDibenzofuranDichlorvosDicofolDiepoxybutane

DiethanolamineDimethyl phthalateDisulfotonEndosulfanEpichlorohydrinEthylbenzeneEthylene glycolEthylene thioureaFluometuronFormaldehydeHexachlorobenzeneHexachloroethaneHexane

Hydroquinone

IsophoroneIsopropyl alcoholLindaneMalathionManebMechlorethamineMelamineMethanolMethoxychlorMethyl acrylateMethyl isocyanateMethyl tert-butyl etherMethylene bromide

MethylhydrazineMirexMustard gas

Nitrilotriacetic acidNitrobenzeneNitrofenNitrogen mustardNitroglycerinNitrophenoln-Butyl alcoholn-Dioctyl phthalate

N-NitrosodiethylamineN-Nitrosopiperidine

N-Nitroso-N-ethylureaOctachloronaphthaleneOctaneOxiraneo-Anisidinehydrochlorideo-Nitroanilineo-ToluidinehydrochlorideParathion (DNTP)PCBsPentachlorobenzenePentachlorophenol

Phenanthrene

PhosgenePhthalic anhydridePolybrominatedbiphenylsBeta-PropoxurPyrene

p-Chloro-m-cresolQuinoneQuintozeneSafroleSet-Butyl alcoholSevin (carbaryl)StyreneTerephthalic acid

Tert-Butyl alcoholTetrachlorvinphosTetrahydrofuranThioacetamideThioureaTolueneToluene diisocyanateTotal xylenesToxapheneTriaziquoneTrichlorfonTrifluralinUrethane (ethyl carbamate)

Vinyl bromideVinyl chlorideVinylidene chlorideXylene (mixed isomers)Zineb

Table 1.2. Organic compounds that are included in various lists of hazardous substances identified bythe U.S. EPA

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In any case, a consensus exists that the environmental impact of a given contaminant dependson the degree of exposure (its dispersion and the resulting concentration in the environment)and on its toxicological properties. The assessment of exposure involves comprehension ofthe dispersion of a chemical in the environment and estimation of the predicted concentration

to which organisms will be exposed. For example, the pesticide fenaminphos oxidizes veryquickly (half-life 10 days) into sulphoxide and sulphone, while its pesticidal properties remainunaffected. A half-life of 70 days has been found for degradation of fenaminphos and its twometabolites. Furthermore, the two metabolites are more mobile (soluble) than fenaminphos(Hayo and Werf, 1996). Assessment of the contaminants effect involves summarizing dataon the effects of the chemical on selected representative organisms and using these data to

predict a no-effect concentration on a specific niche. Organisms may consume chemicalsthrough ingestion of food and water, respiration and through contact with skin. When achemical crosses the various barriers of the body, it reaches the metabolic tissue or a storagedepot. Toxicity of a chemical is usually expressed as the effective concentration or dose of thematerial that would produce a specific effect in 50% of a large population of test species

(EC50 or ED50). If the effect recorded is lethal, the term LC50 (or LD50) is used. The noobserved effect level (NOEL or NOEC) is the dose immediately below the lowest leveleliciting any type of toxicological response in the study. For example, the pesticidemethamidophos, which has been classified as a Restricted-Use Pesticide (RUP) by the U.S.EPA, is highly toxic for mammals (acute oral LD50 = 16 mg/kg in rats and 30-50 mg/kg inguinea pigs), birds (bobwhite quail 8-11 mg/kg) and bees. The 96-hour LC50 is 25-51 mg/L inrainbow trout, but concentrations as low as 0.22 ng/L are lethal to larval crustaceans in 96-hour toxicity tests. A 56-day rat feeding study resulted in a NOEL of 0.03mg/kg/day (Tomin,1994).

Decontamination of drinking water is mainly by procedures that combine flocculation,filtration, sterilization and conservation, to which a limited number of chemicals are added.

Normal human sewage water can be efficiently treated in conventional biological processingplants. But very often, these methods are unable to reduce the power of the contaminant. Inthese cases, some form of advanced biological processing is usually preferred in the treatmentof effluents containing organic substances. Biological treatment techniques are wellestablished and relatively cheap. However, these methods are susceptible to toxic compoundsthat inactivate the waste degrading microorganisms. To solve this problem, apart fromreducing emissions, two main water treatment strategies are followed: (i) chemical treatmentof drinking water, contaminated surface and groundwater and (ii) chemical treatment of wastewaters containing biocides or non-biodegradable compounds.

Chemical treatment of polluted surface and groundwater or wastewater, is part of a long-termstrategy to improve the quality of water by eliminating toxic compounds of human origin

before returning the water to its natural cycles. This type of treatment is suitable when abiological processing plant cannot be adapted to certain types of pollutants that did not existwhen it was designed. In such cases, a potentially useful approach is to partially pre-treat thetoxic waste by oxidation technologies to produce intermediates that are more readily

biodegradable. Light can be used, under certain conditions, to encourage chemicals to breakdown the pollutants to harmless by-products. Light can have a dramatic effect on a moleculeor solid, because, when it absorbs light, its ability to lose or gain electrons is often altered.This electronically excited state is both a better oxidizing and a better reducing agent than its

counterpart in the ground. Electron transfer processes involving excited-state electrons andthe contact medium (for example water) can therefore generate highly reactive species like

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hydroxide (OH) and superoxide (O2-) radicals (see Table 1.3). These can then be used to

chemically decompose a pollutant into harmless end-products. Alternatively, light can be useddirectly to break up pollutant molecule bonds photolytically. These processes are calledAdvanced Oxidation Processes (abbreviated as AOPs). Many oxidation processes, such asTiO2/UV, H2O2/UV, Photo-Fenton and ozone processes (O3, O3/UV, O3/H2O2) are currently

employed for this purpose.

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

Table 1.3. Oxidation potentials of common substances and agents for pollution abatement.The more positive the potential, the better the species is an oxidizing agent

1.3 PHODEGRADATION PRINCIPLES

1.3.1 Definitions

For the benefit of those who may have a limited background in photochemistry, a brief outline

of some basic concepts of photochemistry is presented here. In order for photochemistry totake place, photons of light must be absorbed. The energy of a photon is given by

hcU= (1.1)

where h is Plancks constant (6.626 10-34 J s), c is the speed of light and is the wavelength.For a molecules bond to be broken, Umust be greater than the energy of that bond.

When a given wavelength of light enters a medium, its spectral irradiance E (W m-2 nm-1)is attenuated according to the Lambert-Beer law, which is expressed in two ways, one for gas

phase and the other for liquid phase:

lpEEln ilo

=)/( gas phase (1.2)

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lcEElog ilo

=)/( liquid phase (1.3)E

o and El are the incident spectral irradiances and at a distance linto the medium, is the

absorption coefficient (cm-1 atm-1), pi is the partial pressure (atm) of component i, is theextinction coefficient (M-1 cm-1), and ci is the concentration (M) of component i. Theabsorbence A

at wavelength is the product

cil. The photochemical quantum yield () is

defined as the number of molecules of target compound that reacts divided by the number of photons of light absorbed by the compound, as determined in a fixed period of time.Normally, the unit is the maximum quantum yield attainable.

The term photocatalysis implies the combination of photochemistry with catalysis. Both lightand catalyst are necessary to achieve or to accelerate a chemical reaction. Photocatalysis may

be defined as the acceleration of a photoreaction 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 aphotoreaction begins with the absorption of a photon, being sunlight the source of photons in

solar photocatalysis. In the case of homogeneous photocatalytic processes, the interaction of aphoton-absorbing species (transition metal complexes, organic dyes or metalloporphyrines), asubstrate (e.g. the contaminant) and light can lead to a chemical modification of the substrate.The photon-absorbing species (C) is activated and accelerates the process by interactingthrough a state of excitation (C*). In the case of heterogeneous photocatalysis, the interactionof a photon produces the appearance of electron/hole (e- and h+) pairs, the catalyst being asemiconductor (e.g. TiO2, ZnO, etc). In this case, the excited electrons are transferred to thereducible specimen (Ox1) at the same time that the catalyst accepts electrons from theoxidizable specimen (Red2) which occupies the holes. In both directions, the net flow ofelectrons is null and the catalyst remains unaltered.

C C

C R R C

R P

C C e h

h Red Ox

e Ox Red

h

h

+ +

+

+

+

+

+

*

* * *

*

( )

2 2

1 1

(1.4)(1.5)

(1.6)

(1.7)

(1.8)

(1.9)

1.3.2. Heterogeneous photocatalysis

The concept of heterogeneous photocatalytic degradation is simple: the use under irradiationof a stable solid semiconductor for stimulating a reaction at the solid/solution interface. Bydefinition, the solid can be recovered unchanged after many turnovers of the redox system.When a semiconductor is in contact with a liquid electrolyte solution containing a redoxcouple, charge transfer occurs across the interface to balance the potentials of the two phases.An electric field is formed at the surface of the semiconductor and the bands bend as the fieldforms from the bulk of the semiconductor towards the interface. During photoexcitation (a

photon with appropriate energy is absorbed), band bending provides the conditions for carrierseparation. In the case of semiconductor particles, there is no ohmic contact to extract themajority carriers and to transfer them by an external conductor to a second electrode. This

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means that the two charge carriers should react at the semiconductor/electrolyte interface withthe species in solution. Under steady state conditions the amount of charge transferred to theelectrolyte must be equal and opposite for the two types of carriers. The semiconductor-mediated redox processes involve electron transfer across the interface. When electron/hole

pairs are generated in a semiconductor particle, the electron moves away from the surface to

the bulk of the semiconductor as the hole migrates towards the surface (see Figure 1.3). Ifthese charge carriers are separated fast enough they can be used for chemical reactions at thesurface of the photocatalyst, i.e., for the oxidation or reduction of pollutants.

Oxid1

Red1

Red2

Oxid2

h

recombination

recombination

Figure 1.3. Fate of electrons and holes within a particle of illuminated semiconductor incontact with an electrolyte.

Metal oxides and sulphides represent a large class of semiconductor materials suitable forphotocatalytic purposes. Table 1.4 lists some selected semiconductor materials, which havebeen used for photocatalytic reactions, together with band gap energy required to activate thecatalyst. The final column in the table indicates the wavelength of radiation required toactivate the catalysts. According to Planks equation, the radiation able to produce this gapmust be of a wavelength () equal or lower than that calculated by Eq. 1.10.

GE

hc= (1.10)

where EG is the semiconductor band-gap energy, h is Plancks constant and c is the speed oflight.

Material Band gap (eV) Wavelength corresponding to band gap (nm)

BaTiO3 3.3 375

CdO 2.1 590

CdS 2.5 497

CdSe 1.7 730

Fe2O3 2.2 565

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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

Table 1.4. Selected properties of several semiconductors

Summarizing, a semiconductor particle is an ideal photocatalyst 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 (generated by the absorption of photons

with energy greater than that necessary to move an electron from the valence band to theconduction band)

Photon energy is not stored in the final products, being an exothermic reaction and onlykinetically retarded.

1.3.3. Homogeneous photodegradation

The use of homogeneous photodegradation (single-phase system) to treat contaminated waters

dates back to the early 1970s. The first applications concerned the use of UV/ozone andUV/H2O2. The use of UV light for photodegradation of pollutants can be classified into twoprincipal areas: Photooxidation. Light-driven oxidative processes principally initiated by hydroxyl

radicals. Direct photodegradation. Light-driven processes where degradation proceeds following

direct excitation of the pollutant by UV light.

Photooxidation involves the use of UV light plus an oxidant to generate radicals. Thehydroxyl radicals then attack the organic pollutants to initiate oxidation. Three major oxidantsare used: hydrogen peroxide (H2O2), ozone and Photo-Fenton reaction. H2O2 absorbs fairly

weakly in the UV region with increasing absorption as the wavelength decreases. At 254 nm, is 18 M-1 cm-1, whereas at 200 nm is 190 M-1 cm-1. The primary process for absorption oflight below 365 nm is dissociation to yield two hydroxyl radicals:

OHOH h 222 (1.11)

The use of hydrogen peroxide is now very common for the treatment of contaminated waterdue to several practical advantages: (i) the H2O2 is available as an easily handled solution thatcan be diluted in water to give a wide range of concentrations; (ii) there are no air emissions;(iii) a high-quantum yield of hydroxyl radicals is generated (0.5). The major drawback is thelow molar extinction coefficient, which means that in water with high UV absorption thefraction of light absorbed by H2O2 may be low unless very large concentrations are used.

Furthermore, especially as concerns the focus of this text, H2O2 absorption is very low in theSolar UV range (up 300 nm).

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Ozone is generated as a gas in air or oxygen in concentrations generally ranging from 1 to 8%(v/v). It has a strong absorption band centered at 260 nm with = 3000 M-1 cm-1. Absorptionof light at this wavelength leads to formation of H2O2:

21

3 )( ODOOh + (1.12)

2221 )( OHOHDO + (1.13)

Hydroxyl radicals are then formed by reaction of ozone with the conjugate base of hydrogenperoxide:

+ ++ OHHOOHOH 32222 (1.14) ++ 2332 HOOOHO (1.15)

++ OHHOOHO 323 (1.16)

23 OOHHO + (1.17)

Since the net result of ozone photolysis is the conversion of ozone into hydrogen peroxide,UV-ozone would appear to be only a rather expensive method of making hydrogen peroxide.However, there are other oxidation-related processes occurring in solution, such as the directreaction of ozone with a pollutant (see Table 1.3). Ozone may have advantages in water withhigh inherent UV absorbence, but it involves the same problem as hydrogen peroxide for usein solar energy processes.

The essential step of the Fenton reaction is the same as for all AOPs. Highly reactive radicals(like HO and HO2

) oxidize nearly all organic substances to yield CO2, water and inorganicsalts. In the case of Photo-Fenton, Fe2+ ions are oxidized by H2O2 while one

OH is produced(1.18), and the Fe3+ or complexes obtained then act as the light absorbing species that produceanother radical while the initial Fe2+ is recovered (1.19 and 1.20).

++

+++ OHOHFeOHFe3

222

(1.18)+++ ++++ OHHFehOHFe 22

3 (1.19)

++ +++ RCOFehROOCFe 222)]([ (1.20)

Note that in equation (1.20) the ligand R-COO can be replaced by other organic groups(ROH, RNH2 etc.). Compared to other homogeneous photooxidation processes, theadvantages of Photo-Fenton are the improved light sensitivity (up to a wavelength of 600 nm,corresponding to 35% of the solar radiation). On the other hand, disadvantages, such as thelow pH values required (usually below pH 4) and the necessity of removing iron after thereaction, remain.

Some pollutants are able to dissociate only in the presence of UV light. For this to happen, thepollutant must absorb light emitted by a lamp (or the sun) and have a reasonable quantumyield of photodissociation. Organic pollutants absorb light over a wide range of wavelengths,

but generally absorb more strongly at lower wavelengths, especially below 250 nm (Figure1.4). In addition, the quantum yield of photodissociation tends to increase at lowerwavelengths, since the photon energy is increasing (eq. 1.1). The net chemical result of

photodissociation is usually oxidation, since the free radicals generated can react withdissolved oxygen in the water. In practice, the range of waste waters that can be successfullytreated by UV alone is very limited. This defect is more relevant when solar energy is used

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

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Figure 1.4. UV spectra between 200 and 400 nm of Acrinathrin and sunlight.

1.4 APPLICATION TO WATER TREATMENT

As mentioned above, UV light can be used in several ways. But direct photolysis can occur

only when the contaminant to be destroyed absorbs incident light efficiently. In the case ofUV/ozone and UV/hydrogen peroxide this does not happen. But here too, absorption by somesensitizer must initiate the reaction, and limited absorption by the solute or the additiverestricts efficiency. Furthermore, these mixtures often still require large quantities of addedoxidant. By contrast, in heterogeneous photocatalysis, dispersed solid particles absorb largerfractions of the UV spectrum efficiently and generate chemical oxidants in situ fromdissolved oxygen or water (see Figure 1.5). These advantages make heterogeneous

photocatalysis a particularly attractive method for environmental detoxification. The mostimportant features of this process making 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 can be reused. The catalyst can be attached to different types of inert matrices.

O2

OH + H+

e-

O2-

H2O

TiO2 Particle

WATER

h 3.0eV

h+

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

For all of the above reasons, from now on only this method is dealt with in this text.

Whenever different semiconductor materials have been tested under comparable conditionsfor the degradation of the same compounds, TiO2 has generally been demonstrated to be the

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most active. Only ZnO is as active as TiO2. TiO2s strong resistance to chemical andphotocorrosion, its safety and low cost, limits the choice of convenient alternatives (Pelizzetti,1995). Furthermore, TiO2 is of special interest since it can use natural (solar) UV. This is

because it has an appropriate energetic separation between its valence and conduction bandswhich can be surpassed by the energy content of a solar photon (see Table 1.4). Other

semiconductor particles, e.g., CdS or GaP absorb larger fractions of the solar spectrum andcan form chemically activated surface-bond intermediates, but unfortunately, thesephotocatalysts are degraded during the repeated catalytic cycles involved in heterogeneousphotocatalysis. Therefore, degradation of the organic pollutants present in waste water usingirradiated TiO2 suspensions is the most promising process and R&D in this field has grownvery quickly during the last years.

++

+

+

+

++

+++

++

)(2)(2

22

2

22

22

)()(

'/

adsBCads

IVIVBV

IVIV

h

OeO

HOHTiOTiOhOHTiOTiO

horandheatTiOTiOhe

TiOheTiO

(1.21)

(1.22)

(1.23)

(1.24)

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


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Water-miscible solvents (ethanol, alkoxyethanol, etc.). These compounds are verydifficult to detoxify since they are resistant to treatment and are poorly adsorbed on GAC.

Pesticides. Contaminate waters where agricultural runoff is important. Among the recentlyinvestigated compounds are triazines, organophosphorous, carbamates, phenoxyacids,organochlorines, chloronicotinics, etc.

Surfactants. Surface active agents enter domestic and industrial waste waters in increasingamounts. Because their biodegradability may be one of the more important constraints intheir use, photocatalytic degradation has received increasing attention.

Dyes. Strongly colored compounds can be removed by adsorption but it is always better todestroy them by oxidation.

Three exhaustive reviews by Blake (1994, 1995, 1997) describe almost 1800 studies carriedout before 1996.

Despite encouraging laboratory-scale data and some industrial-scale tests, chemical oxidationdetoxification is still restricted to a few experimental plants. The broader application of thosetechnologies requires: i) reactor optimization and modeling and ii) assessment of theefficiency of oxidation technology to reduce the toxicity of effluents. The following chaptersof this book will attempt to highlight these matters.

1.5 GAS-PHASE DETOXIFICATION

Airborne pollutants (such as volatile compounds) can be treated during the gas phase with theUV/TiO2 process. Gas-phase treatment offers several advantages. In general, substrate mass-transport is an order of magnitude faster in the gas phase than in the liquid phase. This in turnleads to much faster reaction rates. Oxidant starvation (such as O2 supply) may be less of a

problem in the gas-phase than in water. There is also no interference on the photocatalytic

surface from other species that are invariably present in aqueous treatment media (forexample anions). In addition, photocatalysis separation after use is not a problem unlike withaqueous slurry suspensions. By using solar energy to drive the process, no fuel is required,gaseous affluent volume is reduced, no NOx is generated, no products of incompletecombustion are produced, CO2 emanating from fuel burning is avoided and substantial fuelsaving may be achieved. Since no burning takes place, oxygen is only necessary atstoichiometric ratio. Solar concentrators provide the opportunity for small-size solar furnacesand even mobile solar parabolic dishes for on-site destruction of low productions of highlytoxic compounds

On the other hand, there are indications that mineralization may not be complete with some

organic substrates in the gas-phase. The TiO2 photocatalyst loses its activity after prolongeduse and must be reactivated with moist air that presumably restores the original degree ofhydroxylation on the oxide surface. There are also indications that product (or intermediate)adsorption on the TiO2 surface may be problematic during the course of the reaction.

Pollutant substrates like trichloroethylene, acetone, formaldehyde, m-xylene and Nox havebeen treated with TiO2/UV in the gas-phase in bench-scale tests. Field tests have also beenconducted to treat effluent air emissions using this technique at different manufacturing plantsin the USA (Rajeshwar, 1996).

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Class of Compound Chemicals Tested

Aromatics

Nitrogen-containing ring

Aldehydes

Ketones

Alcohols

Alkanes

Terpenes

Sulfur-containing Organics

Chlorinated E!hylenes

Acetyl Chlorides

Benzene, Toluene

Pyridine, Picoline, Nicotine

Acetaldehyde. Formaldehyde

Acetone

Methanol. Ethanol, Propanol

Ethylene. Propene, Tetramethyl Ethylene

-Pinene

Methyl Thiophene

Dichloroethylene.Trichloroethylene.Tetrachloroetylene

Dichloroacetyl Chloride, Tetrachloroacetyl Chloride

Table 1.5. VOCs amenable to treatment via Photocatalytic Oxidation (Jakobi et al., 1996).

SUMMARY OF THE CHAPTER

A description is given of how solar chemistry could become a significant segment of thechemical industry and how it can be used, under certain conditions, to provoke chemical

breakdown of pollutants into harmless by-products. The behaviour of contaminants inenvironmental water is summarised. The basic concepts of photochemistry relating to

photolysis of chemical bonds, homogeneous photodegradation and heterogeneous photocatalysis are reviewed. The use of semiconductors for wastewater treatment, withparticular reference to TiO2, has been discussed. Examples of the waste materials that havebeen treated successfully using TiO2, have been presented. Gas-phase photocatalysis has alsobeen introduced.

BIBLIOGRAPHY AND REFERENCES

Blake, D.M.;Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air. National Technical Information Service, US Depart. ofCommerce, Springfield, VA22161, USA, May 1994. Update Number 1 To June 1995,October 1995. Update Number 2 To October 1996, January 1997.

Hayo, M.G. and van der Werf. Assessing the impact of pesticides on the environment. Agric.Ecosys. Environ., 60, 81-96, 1996.

Jacoby, W.A., Blake, D.M., Fennell, J.A., Boulter, J.E., Vargo, L.M., George, M.C. andDolberg, S. K. Heterogeneous Photocatalysis for Control of Volatile OrganicCompounds in Indoor Air. J. Air Waste Manage. Assoc. 46, no. 9, 891-8, 1996.

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National Renewable Energy Laboratory, Solar Photochemistry-Twenty Years of Progress,Whats Been Accomplished, and Where Does It Lead? Report NREL/TP-433-7209,Golden, Colorado, USA, 1995.

Pelizzetti, E. Concluding Remarks on Heterogeneous Solar Photocatalysis. Solar En. Mat.Sol. Cells, 38, 453-457, 1995.

Rajeshwar, K. Photochemical Strategies for Abating Environmental Pollution. Chemistry &Industry, 17, 454-458, 1996.Tomin, C. The Pesticide Manual, a World Compendium. 10th Edition. British Crop Protection

Council. Croydon, UK, 1994.

SELF-ASSESSMENT QUESTIONS

PART A. True or False?

1. The solar energy is useful only to substitute fossil fuels converting it into thermal energythus provoking chemical reactions.

2. Toxicity of a chemical is the same for all the species.3. Biological treatment techniques are the cheapest wastewater treatment methods.4. The energy of a photon depends of the ambient temperature.5. Heterogeneous photocatalysis employs liquid catalysts.6. Light driven oxidative processes are initiated by excited electrons of the catalyst surface.7. Ozone can be produced from air.8. The most important characteristics of a photocatalysts are: stability to chemical and

photocorrosion, safety, cost and band-gap.9. The electron/hole recombination can be avoided increasing reaction temperature.10. Heterogeneous photocatalysis can be applied only to monoaromatics.

PART B.

1. Which is the most important difference between thermochemical and photochemical solarprocesses?

2. Which are the usual ways to express the toxicity of a chemical in the environment?3. Why biodegradation, which is a major mechanism in wastewater treatment, is quite

inefficient to treat certain types of wastewater?4. What is the percentage of absorbed photons in a solution with the following

characteristics: extinction coefficient = 1327 cm-1 M-1, concentration of substrate 0.01 M,illuminated pathlength = 5.6 cm? And if the extinction coefficient is 0.3?

5. What is the wavelength able to excite a semiconductor which band-gap is 4.0 eV?6. Name three important characteristics of heterogeneous photocatalysis to be used as watertreatment process.

7. Why TiO2 is the most suitable photocatalyst for wastewater treatment?8. Which is the more important electron acceptor in water?9. Which is the most important product of photocatalytic degradation with organic

contaminants?10. Why hydroxyl radicals react with organic substances?

Answers

Part A

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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 is converted into thermal energy, in

photochemical processes the solar photons are absorbed directly by the reactants givingrise to the reaction.2. Toxicity of a chemical is usually expressed as the effective concentration or dose of the

material that would produce a specific effect in 50% of a large population of test species(EC50 or ED50).

3. Because when compounds are very toxic, the micro-organisms need an extended period ofadaptation, when they are not invaible.

4. 100% and 3.8 %.5. 310 nm6. The process takes place at ambient temperature, the oxygen necessary for the reaction is

obtained from the atmosphere, the catalyst is cheap, innocuous and can be reused.

7. It has exhibited the highest activity. It is high stable to chemical and photocorrosion. Itcan use natural UV.

8. Dissolved oxygen.9. Carbon dioxide.10. Because of its very high oxidation potential.

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2. SOLAR IRRADIATION

AIMSThis unit describes the power of light as a source of energy. It outlines the basic principlesthat are related to the light spectrum and specifically to the solar spectrum. This chapterdiscusses solar UV radiation and its photon flux in more detail, because this part of the solarspectrum is the most important for driving chemical processes. Moreover, the majoratmospheric variables determining the amount of UV solar radiation on the earths surface arediscussed. A method for calculating UV attenuation at a given site is presented. Finally, solarradiation measurement systems are described.

OBJECTIVES

At the end of this unit, you will understand the main factors affecting solar radiationbehaviour and you will be able to do six things:1. Discriminate between the different components of solar radiation and their principal

characteristics.2. Recognize typical solar spectra and understand the effect of sun position on the solar

power reaching the earths surface.3. Find the photon flux of a polychromatic source of energy with simple calculations.4. Describe the most important components of the earths atmosphere and the consequences

for power and spectral distribution of the solar radiation.5. Understand the procedures that permit solar power to be calculated from available

radiation at any given site.

Comprehend the basic principles on which solar radiation measurement is based.

NOTATION AND UNITS

Symbol UnitsAM Air mass ratiofn Clouds factorf Fraction of power associated with a wavelength nm

-1

H Radiance exposure monthly average kJ m-2

TBDH TBDUV radiance exposure kJ m-2

I Photon flux density Einstein s-1 m-2

Na Quantity of photons absorbed by the system Photons s-1

N0 Avogadros number, 6.023 x 1023 Photons mol-1

N Number of photons supplied by a source of light of wavelength Photons s-1

Q Energy of a monochromatic source of light of wavelength W m-2 m-1

T TransmittanceT Transmittance of direct-bean solar radiation under cloudless skies

at a specific wavelengthTa, Transmittance related to absorption and dispersion by aerosolsTg,, Transmittance resulting from absorption of atmospheric gasesTo, Transmittance related to the effect of the ozone layerTR, Transmittance related to the molecules of air

Tv,, Transmittance resulting from absorption by steam.

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TBDUV Typical best day. Completely clear sky during all the hours ofsunlight

U Energy of one photon eV, JUVD Direct ultraviolet light W m

-2

UVG Global ultraviolet light W m-2

UV Ultraviolet irradiance associated with a wavelength W m-2

nm-1

Quantum yield No units. Wavelength nm, m

2.1 THE POWER OF LIGHT

Light is just one of various electromagnetic waves present in space. The electromagneticspectrum covers an extremely broad range, from radio wavelengths of a meter or more, downto x-rays with wavelengths of less than one billionth of a meter. Optical radiation lies betweenradio waves and x-rays on that spectrum and has a unique combination of ray, wave, andquantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to bequite particle-like in its behaviour, whereas toward the long wavelength 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).

UV

100-400 nm

Infrared

770-10 6nm

Vis

ibl

e

40

0-

77

0

MicrowavesX-rays

Wavelength , nanometers

a)

b)

100 1000 10000

Figure 2.1The optical portion of the electromagnetic spectrum (a)

and light wave front modelled as a straight-line (b).

Like all electromagnetic waves, light waves can interfere with each other, becomedirectionally polarised, and bend slightly when passing through an edge. These properties

allow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry,lights propagating wave front is modelled as a ray travelling in a straight line (See Figure2.1b). Lenses and mirrors redirect these rays along predictable paths. Wave effects areinsignificant in a large-scale optical system, because the light waves are randomly distributedand there are plenty of photons.

2.1.1 Ultraviolet light

Short wavelength UV-light exhibits more quantum properties than its visible or infraredcounterparts. Ultraviolet light is arbitrarily broken down into three bands, according to itsanecdotal effects. UV-A (315-400 nm), which is the least harmful type of UV light, because ithas the least energy (recall Eq. 1.1), is often called black light, and is used for its relative

harmlessness and its ability to cause fluorescent materials to emit visible light thusappearing to glow in the dark. UV-B (280-315 nm) is typically the most destructive form of

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UV light, because it has enough energy to damage biological tissues, yet not quite enough tobe completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since theatmosphere blocks most of the extraterrestrial UV-B light, a small change in the ozone layercould dramatically increase the danger of skin cancer. UV-C (100-280 nm) is almostcompletely absorbed in air within a few hundred meters. When UV-C photons collide with

oxygen atoms, the energy exchange causes the formation of ozone. UV-C is never observed innature, however, since it is absorbed so quickly. Germicidal UV-C lamps are often used topurify water because of their capability to kill bacteria.

2.1.2 Visible light

Visible light is concerned with the radiation perceived by the human eye. The lumen (lm) isthe photometric equivalent of the watt, weighted to match the eye response of the standardobserver. Yellowish-green light receives the greatest weight because it stimulates the eyemore than the blue or red light of equal radiometric power (1 W at 555 nm = 683.0 lumens).To put this into perspective: the human eye can detect a flux of about 10 photons per secondat 555 nm; this corresponds to a radiant power of 3.58 x 10-18 W (or J s-1). Similarly, the eye

can detect a minimum flux of 214 and 126 photons per second at 450 nm and 650 nm,respectively.

400 500 600 700 nm

Blue Green Yellow Red

Figure 2.2Visible light colours distribution

2.1.3. Infrared lightInfrared light contains the least amount of energy per photon of any other band and is uniquein that it has primarily wave properties. This can make it much more difficult to manipulatethan ultraviolet and visible light. Infrared is more difficult to focus with lenses, refract withlenses, diffracts more, and is difficult to diffuse. Since infrared light is a form of heat, farinfrared detectors are sensitive to environmental changes such as a person moving in the

field of view. Night vision equipment takes advantage of this effect, amplifying infrared todistinguish people and machinery that are concealed in the darkness.

2.2 THE SOLAR SPECTRUM

All the energy coming from that huge reactor, the sun, from which the earth receives1.7x1014 kW, meaning 1.5x1018 kWh per year, or approximately 28000 times worldconsumption for one year (Figure 2.3a). Radiation beyond the atmosphere has a wavelengthof between 0.2m and 50m, which is reduced to between 0.3 m and 3.0 m when reachingthe surface due to the absorption of part of it by different atmospheric components (ozone,oxygen, carbon dioxide, aerosols, steam, clouds). The solar radiation that reaches the groundwithout being absorbed or scattered is called direct radiation; radiation that reaches the

ground but has been dispersed is called diffuse radiation, and the sum of both is called globalradiation. In other words, it is the direct radiation that produces shadow when an opaque

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object blocks it; diffuse radiation does not. In general, the direct component of globalradiation on cloudy days is minimum and the diffuse component is maximum, and theopposite on clear days.

0.6 1.2 1.8 2.4 3.0 3.6 4.20

500

1000

1500

2000

0

500

1000

1500

2000

Extraterrestrial

Direct Air Mass 1.5

Global 37 Air Mass 1.5

Irradia

nce,Wm

-2m

-1

Wavelength, m

Figures 2.3a and 2.3b(a) World solar irradiance, MWh m-2 year-1)

(b) Spectral solar radiation plotted from 0.2 to 4.5 m

Figure 2.3b shows the standard solar radiation spectra (Hulstrom et al., 1985) at ground levelon a clear day. The dotted line corresponds to the extraterrestrial radiation in the samewavelength interval. When this radiation enters the atmosphere, it is absorbed and scattered

by atmospheric components, such as air molecules, aerosols, water vapor, liquid waterdroplets and clouds. The spectral irradiance data are for the sun at a solar zenith angle of48.19. This zenith angle corresponds to an air mass of 1.5, which is the ratio of the direct-

beam solar-irradiance path length through the atmosphere at a solar zenith angle of 48.19 tothe path length when the sun is in a vertical position. AM =1 when the sun is directlyoverhead (zenith). As air mass increases, the direct beam traverses longer path lengths in theatmosphere, which results in more scattering and absorption of the direct beam and a lower

percentage of direct-to-total radiation (for the same atmospheric conditions).

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48.2

60

Sunrise

Sunset

Zenith

Diffuseradiation

Directradiation

Global radiation E ART H

A

M2.0

AM

1.5

Atmosphere

Figure 2.4

Air mass and solar components

The AM 1.5 global irradiance is shown for a flat surface facing the sun and tilted 37 from thehorizontal. The 37 tilt angle is used because it is the latitude of the Plataforma Solar deAlmera, where most of the research presented here was done. The scarce part of the solarspectrum that can be used in photocatalysis with TiO2 may be clearly seen (See Table 1.4)

but, as the energy source is so cheap and abundant, even under these limitations its use is ofinterest.

2.3 SOLAR ULTRAVIOLET IRRADIATION

Solar ultraviolet radiation is, as explained above, only a very small part of the solar spectrum,between 3.5% and 8% of the total of the solar spectrum, as demonstrated by measurement,although this ratio may be different for a given location on cloudy and clear days. The

percentage of global UV radiation (direct + diffuse) generally increases with regard to totalglobal when atmospheric transmissivity decreases mainly because of clouds, but also becauseof aerosols and dust. In fact, the average percentage ratio between UV and total radiation oncloudy days is up to two percentage points more than values on clear days.

The efficiency of a chemical reaction is calculated from the ratio between the products and thedeparting reactants. In photochemistry, it is very common to use the quantum yield concept,which is calculated from a known amount of photons absorbed in the reaction. Quantum yield

() is defined as the ratio between the number of reacting molecules (n) and the quantity ofphotons absorbed by the system (Na):

=n

Na(2.1)

Experimentally, the quantum yield is expressed as the number of moles of reactant in aninterval of time t, divided into the number of moles of photons absorbed during the same

period. Knowledge of the quantum yield is rather important for an understanding of themechanism of a photochemical reaction. If every absorbed photon produces a moleculartransformation, = 1. If it is less than 1, it means that deactivation processes or otherreactions competing with the one studied exist. Over 1 indicates a series of reactions the

promoter of which has been excited by a photon. In the case of photocatalysis by UVradiation, the number of photons that reach the reacting mixture and are thereby susceptible to

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being absorbed, will be in relation to the UV solar spectrum. For reactors using solarradiation, knowledge of the solar UV spectrum is important for the following reasons: The radiation (sunlight) that reaches them is not constant. This prevents correct

comparison between experiments carried out at different times of the day or seasons of theyear or under different atmospheric conditions.

The extensive bibliography on photocatalytic decomposition of organic compoundsindicates that the majority of the experiments in which the photon flux is known arecarried out in laboratory reactors illuminated by lamps. In order to compare these resultswith solar radiation or to use the information contained in those reports, it is necessary toknow the photon flux inside the solar reactor.

The quantum yield of the reaction tested under a given experimental condition providesinformation on the optimum conditions for decomposition of the contaminant. Knowledgeof the photon flux in this situation is basic to the determination of the efficiency of thesolar reactor components (reflective surface, absorber tube, control system, concentrationfactor, etc.) and any possible modification, in each case, to improve photodegradationconditions.

Any economic comparison between solar radiation and electric lamps as the UV photonsource requires knowledge of the photon flux incident on the solar reactor.

The two spectra shown in Figure 2.5 correspond to the same spectra shown in Figure 2.3 forthe solar UV spectrum range at ground level. The shorter of them (direct UV) reaches 22 Wm-2 between 300 and 400 nm, the longer (global UV) reaches 46 W m-2.

The number of photons, N, supplied by a monochromatic source of light with wavelength and energy Q is related to the energy of one photon, U, by Plancks equation (Eq.1.1):

N =

Q

W= Q

hc(2.2)

When a source of light is polychromatic as is solar radiation, the number of photons is givenby an integral covering the whole range of wavelengths of that source:

N = N )d =1

hcQ( ) d

1

2

1

2(

(2.3)

0.30 0.32 0.34 0.36 0.38 0.400

5

10

15

20

0

5

10

15

20

UVD

UVG

Wavelength, m

Photonflux,10-20photonsm-2s

-1m-1

8.4 x 1019 photons m-2 s-1

3.6 x 1019 photons m-2 s-1

Figure 2.5Ultraviolet spectra on the earth surface (standard ASTM)

Equation 2.3 gives the ratio between photonic and radiometric quantities, defining from here

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the photon flux density I [Einstein s-1 m-2] as the number of incident photons per unit ofsurface and time:

I = dN

N dt dA

2

0

(2.4)

where N0 is Avogadros number (6.023 x 1023). 1 Einstein = 1 mol of photons = 6.02 x 1023

photons.

Using the spectrum data and the above equations in congruent units [S.I], it is possible todetermine the photon flux density I (ID = 3.6 x 10

19 photons m-2 s-1 = 6 x 10-5 Einstein m-2 s-1,IG = 8.4 x 10

19 photons m-2 s-1 = 14 x 10-5 Einstein m-2 s-1). These two values give an idea ofthe energy coming from the sun and available for photocatalytic reactions with TiO2, whichonly uses the part of the UV spectrum up to 390 nm, as explained below. In any case, the UVradiation values described vary from one location to another, and obviously, at different hoursof the day and in different seasons, making it necessary to know these data for the particularlocation and in real time. This data will be very useful in those cases where this is not

possible.2.4 ATMOSPHERIC ATTENUATION OF SOLAR RADIATION

A general expression for transmittance (T) of direct-bean solar radiation under cloudless skiesat a specific wavelength () is (Iqbal, 1983):

T T T T T T R a o g v = , , , , , (2.5)

TR, is the spectral transmittance resulting from the dispersion produced by molecules of air(dimensions of many of which are 1, Raleigh dispersion). Ta, is the spectral transmittancerelated to absorption and dispersion by aerosols (solid or liquid particles suspended in the air).To, corresponds to the effect of the ozone layer. Tg,, is the transmittance resulting fromabsorption of atmospheric gases (such as carbon dioxide and oxygen). Tv,,

corresponds to the

absorption by water vapour. The effect of each of these parameters within the range inquestion (300 nm-400 nm) would be the following (Riordan et al., 1990):

TR, = exp (-0.008735 -4.08 M), where M is the air mass corrected according to itsdensity, which depends on the pressure and, therefore, the altitude. In agreement withthis, this factor would be practically constant for a given site.

Ta, = exp (-- M) where M is the mass of air, is the coefficient of turbidity,which usually varies between 0 and 0.5 and is a reflection of the amount of aerosolsin the air and is an index of the size of the aerosol molecules. Depending on theatmospheric contamination at the site, varies differently. Where there is nocontamination it varies only slightly. The same reasoning is valid for.

To, is constant for a specific site since the ozone layer has a practically constantthickness (for now).

Tg, only influences wavelengths over UV. Tv, does not affect the UV spectrum.

Keeping in mind then, the different transmittances, it may be assumed that the solar UVspectrum does not vary substantially within a specific site throughout the year, unlessatmospheric conditions (except clouds) do so. The dominating attenuator of solar radiation isclouds. Under overcast skies there is no direct-beam radiation, and under partly cloudy skiesthere is intermittent direct-beam radiation when clouds are not obscuring the suns disk.Clouds are often assumed to have a wavelength-independent attenuation function in the UVrange; but in the near-infrared region (See Figure 2.6) they cause increased absorption due towater vapour and liquid water (Tv,).

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400 600 800 1000 12000.0

0.2

0.4

0.6

0.8

1.0

1.2

Clear sky

Clouds

Wm

-2nm

-1

Wavelength, nm

300 325 350 375 4000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Clear sky

Clouds

Wm

-2nm

-1

Wavelength, nm

Figure 2.6Solar spectra on the earth surface (Plataforma Solar de Almera) between 300 and 1100 nm.

Clouds modify the total UV energy reaching the earths surface, but the wavelengthdistribution is not affected. This cannot be guaranteed, however, if the data for all the spectrashown in Figure 2.6 are not represented in a standardized manner as in Figure 2.7. This can bedone for any wavelength interval by the following operation. Summations have been used totreat the discrete values nm to nm:

1fthereforeUV

UV

f

nm

nmnm

nm

=,

400

300400

300

=

==

=

=

(2.6)

where f, is the fraction of power associated with wavelength and UV is the irradiance,W m-2 nm-1 corresponding to each wavelength and measured with a spectroradiometer. InFigure 2.7, the homogeneity of all the spectra recorded may be observed. If the spectrum ofUV radiation is assumed to have a fixed form, then standardized spectrum can be consideredas standard for each site. Therefore, the number of photons corresponding to this range ofwavelengths is only a function of the intensity (Measurable in real time with the radiometers,see the following section in this chapter).

300 325 350 375 4000.00

0.01

0.02

0.03

0.04

0.05

0.06

f,nm

-1

Wavelength, nm

Area below each curve = 1

1ftherefore

UV

UVf

nm

nmnm

nm

=,400

300400

300

=

==

=

=

Figure 2.7

Normalised solar UV spectra shown in Figure 2.6.

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2.4.1 Annual available ultraviolet radiation

A general index of atmospheric transmittance due to all the processes described above is theso-called cloud factor (fn). For the calculation of fn the annual average of ultravioletradiation must be found. Knowledge of this factor enables the amount of energy that reachesthe earths surface to be predicted for a given place at any time of the year. It should be noted

that the cloud factor for global radiation is always lower than the direct, as the diffusecomponent of solar radiation is maximum when direct radiation is absent (global = direct +diffuse). This factor is calculated (Eq. 2.7) from the ratio between average radiation (affected

by all atmospheric phenomena) and the highest attainable radiation at all times of the year.This is usually calculated for each month separately. To find out the highest UV radiationavailable each month, a typical best day (completely clear sky during all daylight hours) isselected for each month (TBDUV) from among all the days for which average radiation iscalculated. The parameters taken into account in selecting the TBDUV are absence of cloudsand proximity to the 15th day of each month (See Figure 2.8). Several days per month(corresponding to each year for period to be analyzed) must be chosen and compared to findthe day with the maximum TBDH .

=

TBDn H

Hf 1 (2.7)

where TBDH (kJ m-2) is the TBDUV radiance exposure (direct or global). It has been obtained

integrating the UV irradiance along the day. In this case, being discrete values, it has beencalculated integrating numerically the irradiance from sunrise to sunset. H (kJ m-2) is theaverage of the month. This was calculated by multiplying the monthly average irradiance bythe monthly average of hours of sunlight. The cloud factor for UV radiation has to becalculated from data collected with radiometers which are UV-specific. The UV-radiationdata base must be large enough to be considered statistically correct (at least 4-5 years).

SOLAR HOUR

UV,Wm

-2

6 8 10 12 14 16 18 200

10

20

30

40

50

October

June

April

January

Figure 2.8TBDUVof different periods of the year at Plataforma Solar de Almera (37 N)

2.5 SOLAR RADIATION MEASUREMENT

Detectors translate light energy into an electrical current. Light striking a silicon photodiodecauses a charge to build up between internal P and N layers. When an external circuit isconnected to the cell, an electrical current is produced. This current is linear with regard to the

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incident light over a dynamic 10-decade range. A wide dynamic range is a prerequisite formost applications. The radiometer should be able to cover the entire dynamic range of anydetector that will be plugged into it. This usually means that the instrument should be able tocover at least 7 decades of dynamic range with minimal linearity errors. The current orvoltage measurement device should be the least significant source of error in the system. The

second thing to consider when choosing a radiometer is the type of features offered. Ambientzeroing, integration ability, and a hold button should be standard. The ability to multiplexseveral detectors to a single radiometer or control the instrument remotely may also bedesired. Lastly, portability and battery life may be an issue for measurements made in thefield.

Light is all around us every day, yet it remains the most elusive form of energy to measureaccurately. A single photon of light travels in a straight line in one direction, at a givenwavelength. A light bundle consists of a jumbled mixture of billions and billions of photons atdifferent wavelengths, going in different directions, at different moments in time. The watt(W), the fundamental unit of optical power, is defined as a rate of energy of one joule (J) per

second. Optical power is a function of both the number of photons and the wavelength. Each photon carries an energy that is described by Plancks equation (Eq. 1.1). All lightmeasurement units are spectral, spatial or temporal distributions of optical energy. The

biggest hurdle in light measurement is the very spatial nature of light. Irradiance is a measureof the energy density received from a light source. Since light expands outward from a pointsource, the irradiance decreases with distance. The irradiance also decreases with incidentangle. Carefully designed input optics cannot prevent measurement errors caused by laxattention to the measurement geometry upon which the units systems are based. Spectralresponsivity and detectivity present a very different problem. Many properties of light aredependent on wavelength and the energy of one photon is inversely proportional to itswavelength. Since a detector measures only absorbed light, it cannot differentiate between 1

photon (200 nm) and 10 photons (2000 nm). The light must be filtered by wavelength beforeit reaches the detector.

Sensitivity to the band of interest is a primary consideration when choosing a detector. Youcan control the peak responsivity and bandwidth through the use of filters, but you must havean adequate signal to start with. Filters can suppress out-of-band light but cannot boost signal.Another consideration is blindness to out-of-band radiation. If you are measuring solarultraviolet in the presence of massive amounts of visible and infrared light, for example, youwould select a detector that is insensitive to the long wavelength light that you intend to filterout. Lastly, linearity, stability and durability are considerations. Some detector types must be

cooled or modulated to remain stable. High voltages are required for other types. In addition,some can be burned out by excessive light, or have their windows permanently ruined by afingerprint.

2.5.1 DetectorsPlanar-diffusion-type silicon photodiodes are perhaps the most versatile and reliable sensorsavailable. The P-layer material at the light sensitive surface and the N material at the substratefrom a P-N junction which operates as a photoelectric converter, generating a current that is

proportional to the incident light. Silicon cells operate linearly over a ten-decade dynamicrange, and remain true to their original calibration longer than any other type of sensor. Forthis reason, they are used as transfer standards at the NIST (National Institute of Standards

and Technology, USA).

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The phototube is a light sensor that is based on the photoemissive effect. The phototube is a bipolar tube which consists of a photoemissive cathode surface that emits electrons inproportion to incident light, and an anode which collects the electrons emitted. The anodemust be biased at high voltage (50 to 90 V) in order to attract electrons to jump through thevacuum of the tube. Some phototubes use a forward bias of less than 15 volts, however. The

cathode material determines the spectral sensitivity of the tube. Solar-blind vacuumphotodiodes use Cs-Ta cathodes to provide sensitivity only to ultraviolet light, providing asmuch as a million to one long wavelength rejections. A UV glass window is required forsensitivity in the UV down to 185 nm, with fused silica windows offering transmission downto 160 nm.

The thermopile is a heat sensitive device that measures radiated heat. Infrared light containsthe least amount of energy per photon of any other band. Because of this, an infrared photonoften lacks the energy required to pass the detection threshold of a quantum detector. Infraredis usually measured using a thermal detector such as a thermopile, which measurestemperature change due to absorbed energy. While these thermal detectors have a very flat

spectral responsibility, they suffer from temperature sensitivity, and usually must beartificially cooled. The sensor is usually sealed in a vacuum to prevent heat transfer except byradiation. A thermopile consists of a number of thermocouple junctions in series, whichconvert energy into a voltage using the Peltier effect. Thermopiles are convenient sensors formeasuring the infrared, because they offer adequate sensitivity and a flat spectral response ina small package. More sophisticated bolometers and pyroelectric detectors need to be choppedand are generally used only in calibration labs.

The best method of operating a thermal detector is by chopping incident radiation, so that driftis zeroed out by the modulated reading. The quartz window in most thermopiles is adequatefor transmitting from 200 to 4200 nm, but for long wavelength sensitivity out to 40 microns,Potassium Bromide windows are used. Another strategy employed by thermal detectors is tomodulate incident light with a chopper. This allows the detector to measure differentially

between the dark (zero) and light states. Quantum-type detectors are often used in the nearinfrared, especially below 1100 nm. Specialized detectors such as InGaAs offer excellentresponsivity from 850 to 1700 nm. Typical silicon photodiodes are not sensitive above 1100nm. These types of detectors are typically employed to measure a known artificial near-IRsource without including long wavelength background ambient. Most radiometric IRmeasurements are made without lenses, filters, or diffusers, relying on just the bare detector tomeasure incident irradiance.

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200 400 600 800 1000 12000

10

20

30

40

50

60

70

80

90

100

So

lar

-bli

nd

vacuum

p

ho

todiod

e

Therm

opile

Silicon photodiode

%

Wavelength, nm

220 240 260 280 300 3200.0

0.2

0.4

0.6

0.8

1.0

Combined responsivity

DetectorFilter

Wavelength, nm

Figure 2.9Responsivities of three detectors.

In the inset is shown a schematic of the effect of a filter on detector responsivity.

2.5.2 FiltersA detectors overall spectral sensitivity is equal to the product of the responsivity of thesensor and the transmission of the filter. Given a desired overall sensitivity and a knowndetector responsivity, you can then solve a transmission curve for the ideal filter. Filter

bandwidth decreases with thickness according to Lambert-Beers law (see Eqs. 1.2 and 1.3),so by varying filter thickness, you can selectively modify the spectral responsivity of a sensorto match a particular function. Multiple filters cemented in layers give a net transmissionequal to the product of the individual transmissions. Filters operate by absorption orinterference. Colored glass filters are doped with materials that selectively absorb light bywavelength, and obey Lambert-Beers law. The peak transmission is inherent to the additives,while bandwidth is dependent on thickness. Sharp-cut filters act as long pass filters, and areoften used to subtract out long wavelength radiation in a secondary measurement. Interferencefilters rely on thin layers of dielectric to cause interference between wave-fronts, providingvery narrow bandwidths. Any of these filter types can be combined to form a composite filterthat matches a particular photochemical process.

2.5.3 Input Optics

When selecting input optics for a measurement application, consider both the size of thesource and the viewing angle of the intended real-world receiver. Suppose, for example, thatyou were measuring the erythemal (sunburn) effect of the sun on human skin. While the sunmay be considered very much a point source, skylight, refracted and reflected by theatmosphere, contributes significantly to the overall amount of light reaching the earthssurface. Sunlight is a combination of a point source and a 2 steradian area source. The skin,since it is relatively flat and diffuse, is an effective cosine receiver. It absorbs radiation in

proportion to the incident angle of the light. An appropriate measurement system should alsohave a cosine response. If you aimed the detector directly at the sun and tracked the suns

path, you would be measuring the maximum irradiance. If, however, you wanted to measurethe effect on a person lying on the beach, you might want the detector to face straight up,

regardless of the suns position. This example can be extended to solar collectors (see Chapter6).

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Different measurement geometries necessitate specialised input optics. Radiance andluminance measurements require a narrow viewing angle (< 4) in order to satisfy theconditions underlying the measurement units. Power measurements, on the other hand,require a uniform response to radiation regardless of input angle to capture all light. There

may also be occasions when the need for additional signal or the desire to exclude off-anglelight affects the choice of input optics. A high-gain lens, for example, is often used to amplifya distant point source. A detector can be calibrated to use any input optics as long as theyreflect the overall goal of the measurement.

100%

100%

50%

50%

30 30

30 30

60 60

60 601.5%

Figure 2.10Relative spatial response of an ideal cosine diffuser (up)

and a radiance lens barrel (down).

Cosine Diffusers. A bare silicon cell has a near perfect cosine response, as do all diffuseplanar surfaces. As soon as you place a filter in front of the detector, however, you change thespatial responsivity of the cell by restricting off-angle light. Fused silica or optical quartz witha ground (rough) internal hemisphere makes an excellent diffuser with adequate transmissionin the ultraviolet. Teflon is an excellent alternative for UV and visible applications, but is notan effective diffuser for infrared light.

Figure 2.11Solar Global UV detector (tilted 37 and facing south) with a cosine diffuser

Radiance Lens Barrels. Radiance and luminance optics frequently employ a dual lens systemthat provides an effective viewing angle of less than 4. The trade-off of a restricted viewingangle is a reduction in signal. Radiance optics merely limit the viewing angle to less than the

extent of a uniform area source. This input optic is used to measured direct sunlight, but

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mounted on a mobile sun-tracking platform (one loop per day) to follow the sun from sunriseto sunset.

Figure 2.12Solar Direct UV detector installed on a solar tracking system

Fibre Optics. Fibre optics allow measurements in tight places or where irradiance levels andheat are very high. Fibre optics consist of a core fibre and a jacket with an index of refractionchosen to maximise total internal reflection. Glass fibres are suitable for use in the visible, butquartz or fused silica is required for transmission in the ultraviolet. Fibres are often used tocontinuously monitor UV curing ovens, due to the attenuation and heat protection they

provide. Typical fibre optics restrict the field of view to about 20 in the visible and 10 inthe ultraviolet.

Integrating Spheres. An integrating sphere is a hollow sphere coated inside with BariumSulfate, a diffuse white reflectance coating that offers greater than 97% reflectance between450 and 900 nm. The sphere is baffled internally to block direct and first-bounce light.Integrating spheres are used as sources of uniform radiance and as input optics for measuringtotal power. Often, a lamp is place inside the sphere to capture light that is emitted in any

direction.

High Gain Lenses. In situations with low irradiance from a point source, high gain inputoptics can be used to amplify the light by as much as 50 times while ignoring off angleambient light. Flash sources such as tower beacons often employ fresnel lenses, making nearfield measurements difficult. With a high gain lens you can measure a flash source from adistance without compromising signal strength. High gain lenses restrict the field of view to8, so cannot be used in full immersion applications where a cosine response is required.

SUMMARY OF THE CHAPTER

The three principal components of light (ultraviolet, visible and infrared) and their

wavelength distribution have been described. Typical solar spectra and air mass effect (sunposition) have been shown. The calculation of photonic fluxes (Einstein) from radiometricmeasurement (W) and spectral data (nm-1) has been introduced. The attenuating componentsof the atmosphere and their effect on UV radiation have been discussed to achieve a finalconclusion: UV spectrum is constant at a definite emplacement under certain circumstances.This characteristic permits the standardisation of the solar-UV spectrum, which is veryhelpful for finding a standard photon flux. The cloud factor index has been described and itscalculation from an UV-radiation database has been explained. Finally, solar radiationmeasurement systems have been described, with special emphasis on their main components.Their correct combination will permit accurate analysis of solar radiation and correctevaluation of the quantum yield of photochemical reactions.

BIBLIOGRAPHY AND REFERENCES

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Hulstrom, R.; Bird, R.; Riordan, C. Spectral Solar Irradiance Data Sets for SelectedTerrestial Conditions. Solar Cells, 15, 365-391, 1985.

Iqbal, M.An Introduction to Solar Radiation. Academic Press, Canada. 1983.Riordan, C.J.; Hulstrom, R.L.; Myers, D.R.. Influences of Atmosferic Conditions and Air

Mass on the Ratio of Ultraviolet to Total Solar Radiation. Solar Energy Research

Institute (SERI)/TP-215-3895. 1990.

SELF-ASSESSMENT QUESTIONS

PART A. True or False?

1. The Visible portion of the light is more powerful than the UV portion.2. The most important portion of Solar UV light at earths surface is between 100 and 280

nm.3. The solar radiation that reaches the ground level without being absorbed or scattered, is

called direct radiation.4. Any economic comparison desired between solar radiation and electric l

solar detoxification - for cleaning toxic sites and materials

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