Chemical Degradation Methods for Wastes and Pollutants - Chapter 1 pps

Hydroxyl radicalshave the largest standard redox potential except for fluorine seeTable 1.In addition, they react very rapidly with almost all types of organicsubstances through reactions

Chemical Degradation Methods

Environmental and Industrial Applications

edited by

Matthew A Tarr

University of New Orleans

New Orleans, Louisiana, U.S.A

tion, shall be liable for any loss, damage, or liability directly or indirectly caused oralleged to be caused by this book The material contained herein is not intended toprovide specific advice or recommendations for any specific situation.

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I Toxic Metal Chemistry in Marine Environments, Muhammad Sadiq

2 Handbook of Polymer Degradation, edited by S Halim Hamid, Mohamed B Amin, and Ali G Maadhah

3 Unit Processes in Drinking Water Treatment, Wily J Masschelein

4 Groundwater Contamination and Analysis at Hazardous Waste Sites,

edited by Suzanne Lesage and Richard E Jackson

5 Plastics Waste Management: Disposal, Recycling, and Reuse, edited

by Nabil Mustafa

6 Hazardous Waste Site Soil Remediation: Theory and Application of

Innovative Technologies, edited by David J Wilson and Ann N Clarke

7 Process Engineering for Pollution Control and Waste Minimization,

edited by Donald L Wise and Debra J Trantolo

8 Remediation of Hazardous Waste Contaminated Soils, edited by

Donald L Wise and Debra J Trantolo

9 Water Contamination and Health: Integration of Exposure Assess-

ment, Toxicology, and Risk Assessment, edited by fihoda G M

Wang

10 Pollution Control in Fertilizer Production, edited by Charles A Hodge

and Neculai N Popovici

1 1 Groundwater Contamination and Control, edited by Uri Zoller

12 Toxic Properties of Pesticides, Nicholas P Cheremisinoff and John A King

13 Combustion and Incineration Processes: Applications in Environ-

mental Engineering, Second Edition, Revised and Expanded, Walter

R Niessen

14 Hazardous Chemicals in the Polymer Industry, Nicholas P Chere-

misinoff

15 Handbook of Highly Toxic Materials Handling and Management,

edited by Stanley S Grossel and Daniel A Crowl

16 Separation Processes in Waste Minimization, Robert B Long

17 Handbook of Pollution and Hazardous Materials Compliance: A

Sourcebook for Environmental Managers, Nicholas P Cheremisinoff

and Madelyn Graffia

1 8 Biosolids Treatment and Management: Processes for Beneficial Use,

edited by Mark J Girovich

20 Separation Methods for Waste and Environmental Applications,, Jack

S Watson

21 Handbook of Polymer Degradation: Second Edition, Revised and

Expanded, S Halim Hamid

22 Bioremediation of Contaminated Soils, edited by Donald L Wise, Debra J Trantolo, Edward J Cichon, Hilary 1 Inyang, and Ulrich Stottmeister

23 Remediation Engineering of Contaminated Soils, edited by Donald L

Wise, Debra J Trantolo, Edward J Cichon, Hilary 1 Inyang, and Ulrich Stottmeister

24 Handbook of Pollution Prevention Practices, Nicholas P Cheremisinoff

25 Combustion and Incineration Processes: Third Edition, Revised and

Expanded, Walter R Niessen

26 Chemical Degradation Methods for Wastes and Pollutants:

Environmental and Industrial Applications, edited by Matthew A ‘Tarr

Addition a 1 Volumes in Preparation

TM Copyright © 2003 by Marcel Dekker, Inc All Rights Reserved.

Human activities have a large and important impact on the environment.Naturally occurring elements or compounds are often concentrated andredistributed in the environment through industrial processes, power pro-duction, and consumer activity For example, lead, which is found innaturally occurring mineral deposits, has become a major pollutant throughits use in batteries, paints, and gasoline additives In addition, the production

of non-natural or anthropogenic substances, such as halogenated solvents,can also result in the eventual release of often toxic and biorecalcitrantsubstances into the environment Wide-scale redistribution of pollutants byhumans dates at as far back as the ancient Greek and Roman civilizations(2000–2500 years ago), during which time extensive smelting activitiesresulted in significant atmospheric pollution by heavy metals such as lead

In fact, heavy-metal contamination of Arctic and Antarctic ice has revealedevidence of global pollution from smelting and other human activities sincethese ancient times

Most certainly the people of ancient Greek and Roman times were notaware of the extent of their pollution In fact, only in the late twentieth centurydid widespread awareness and understanding of the degree of anthropogenicpollution begin to develop Unfortunately, large releases of contaminants intothe environment transpired without either knowledge of or concern for theconsequences Once contaminants have been introduced into the environ-ment, subsequent clean-up is extremely difficult, time consuming, and costly.Due to the existence of many contaminated sites, significant research anddevelopment efforts have been expended to develop effective means ofremediating these sites These methods must be both economically feasibleand environmentally sound For some sites, these challenges have beensuccessfully met, while other sites remain contaminated because of lack ofacceptable (economically and/or environmentally) technologies or becausethe sites pose a low risk

While cleaning up previous contamination is a high priority, developingnew technologies to prevent future contamination is equally important, if notmore so Without environmentally acceptable industrial processes, powerproduction, and consumer activity, the Earth’s environment will continue to

be threatened Development of inherently clean technologies as well asimplementation of effective waste stream treatment are viable routes topreventing future environmental contamination

Chemical Degradation Methods for Wastes and Pollutantsfocuses onchemical methods of destroying pollutants Chemical methods can be advan-tageous over biological methods because they are often faster, can treat highlycontaminated systems, and may be less sensitive to ambient conditions Incontrast, bacteria can be killed by contaminants or solvents and lose viabilityoutside relatively narrow pH and temperature ranges However, chemicalmethods are often more costly and labor-intensive than biodegradationtechnologies Despite their limitations, both biological and chemical tech-nologies are valuable tools that can be used successfully under appropriateconditions Furthermore, combinations of biological and chemical treatmentmethods can often provide advantages over the individual systems

The book covers several chemical technologies for remediation or wastestream treatment of predominantly organic contaminants Although notevery chemical technology has been included, ten common or potentiallyuseful methods are covered Each chapter presents the fundamentals behindeach technology and covers selected applications and practical issues relevant

to adaptation of the technique to real treatment systems

Continued research into both fundamentals and applications of ical treatment technologies will hopefully provide solutions to many currentpollution treatment problems, both for waste streams and for contaminatedsites Only through cooperation among scientists, engineers, industry, gov-ernment, and consumers can we maintain a healthy and productive environ-ment for the future

chem-Finally, I would like to thank those who served as reviewers for eachchapter

Matthew A Tarr

5 Sonochemical Degradation of Pollutants

Hugo Destaillats, Michael R Hoffmann, and Henry C

Wallace

6 Electrochemical Methods for Degradation of OrganicPollutants in Aqueous Media

Enric Brillas, Pere-Lluı´s Cabot, and Juan Casado

7 The Electron Beam Process for the Radiolytic

Degradation of Pollutants

Bruce J Mincher and William J Cooper

8 Solvated Electron Reductions: A Versatile Alternative

for Waste Remediation

Gerry D Getman and Charles U Pittman, Jr

9 Permeable Reactive Barriers of Iron and Other

Zero-Valent Metals

Paul G Tratnyek, Michelle M Scherer, Timothy L

Johnson, and Leah J Matheson

10 Enzymatic Treatment of Waters and Wastes

James A Nicell

Fernando J Beltra´n Departamento de Ingenieria Quimica y Energetica,Universidad de Extremadura, Badajoz, Spain

Enric Brillas Laboratori de Ciencia i Tecnologia Electroquimica de als, Departament de Quimica Fisica, Universitat de Barcelona, Barcelona,Spain

Materi-Pere-Lluı´s Cabot Laboratori de Ciencia i Tecnologia Electroquimica deMaterials, Departament de Quimica Fisica, Universitat de Barcelona, Barce-lona, Spain

Juan Casado Departamento de Investigacion, Carburos Meta´licos S.A.,Barcelona, Spain

William J Cooper Department of Chemistry, University of North lina–Wilmington, Wilmington, North Carolina, U.S.A

Caro-Hugo Destaillats Department of Environmental Science and Engineering,California Institute of Technology, Pasadena, California, U.S.A

Gerry D Getman Commodore Solution Technologies, Inc., Marengo,Ohio, U.S.A

Michael R Hoffmann Department of Environmental Science and ing, California Institute of Technology, Pasadena, California, U.S.A

Engineer-Indira Jayaweera SRI International, Menlo Park, California, U.S.A

Timothy L Johnson AMEC Earth & Environmental, Inc., Portland, gon, U.S.A

Ore-Leah J Matheson MSE Technology Applications, Inc., Butte, Montana,U.S.A.

Bruce J Mincher Radiation Physics Group, Idaho National Engineering &Environmental Laboratory, Idaho Falls, Idaho, U.S.A

James A Nicell Department of Civil Engineering and Applied matics, McGill University, Montreal, Quebec, Canada

Mathe-Pierre Pichat Laboratoire Photocatalyse, Catalyse et Environment, EcoleCentrale de Lyon, Ecully, France

Charles U Pittman, Jr Department of Chemistry, Mississippi State versity, Mississippi State, Mississippi, U.S.A

Uni-Michelle M Scherer Department of Civil and Environmental Engineering,University of Iowa, Iowa City, Iowa, U.S.A

Matthew A Tarr Department of Chemistry, University of New Orleans,New Orleans, Louisiana, U.S.A

Paul G Tratnyek Department of Environmental and Biomolecular tems, Oregon Health and Science University, Beaverton, Oregon, U.S.A

Sys-Henry C Wallace Ultrasonic Energy Systems Co., Panama City, Florida,U.S.A

to the high reactivity and redox potential of this free radical that reactsnonselectively with organic matter present in water In practical cases, theseprocesses present a high degree of flexibility because they can be usedindividually or in combination depending on the problem to be solved Forinstance, for phenols or substances with high UV molar absorption coef-ficients, ozone or UV radiation can be used alone, respectively, without theneed of any additional reagent, such as hydrogen peroxide Anotheradvantage of these AOTs is that they may be applied under mild exper-imental conditions (atmospheric ambient pressure and room temperature).The need for the application of these AOTs is based on different social,industrial, environmental, and even academic reasons The increasing aware-ness of society for the quality of drinking water has led to the establishment

of maximum contaminant levels of priority pollutants in drinking water [1,2].The preparation of ultrapure water is needed for some industrial activitiessuch as those derived from the pharmaceutical and electronic processes

Also, the release of wastewater into natural environmental reservoirs isanother concern; recycling of wastewater is already in progress in countrieswhere the lack of water is a national problem [4] Finally, academic interestexists because the study of these AOTs allows testing the application of somephysical and chemical laws and engineering theories (mass, energy, and/orradiation conservation equations, kinetic modeling, absorption theories, etc.)

to the environmental problems of water treatment

Because of the aforementioned reasons, the number of research worksand applications based on these AOTs in the treatment of water hasincreased considerably during the past 20 years Numerous publicationsthat refer to different aspects of these processes have so far been published injournals such as Ozone Science and Engineering, Water Research, OzoneNews, IUVA News, and the Journal of Advanced Oxidation Technologies Inaddition, several books on the subject are available, such as that edited byLanglais et al [5] on applications and engineering aspects of ozone in watertreatment and that of Dore´ [6] on the chemistry of oxidants Reviews arealso abundant, including those of Camel and Vermont [7] on ozone in-volving oxidation processes, Reynolds et al [8] and Chiron et al [9] on theoxidation of pesticides, Legrini et al [10] on photochemical processes, Yue[11] on kinetic modeling of photooxidation reactors, and Scott and Ollis [12]

on the integration of chemical and biological oxidation processes forwastewater treatment

In this chapter, AOTs based on ozone, UV radiation, and hydrogenperoxide are presented with special emphasis on their fundamental andapplication aspects Related literature of research studies and applications, es-pecially those appearing in the last decade, are also listed, and specific exam-ples of laboratory and scale-up studies are described in separate sections

II BACKGROUND AND FUNDAMENTALS OF O3/UV/H2O2

PROCESSES

O3/UV/H2O2processes are characterized by the application of a chemicaloxidant (ozone and/or hydrogen peroxide) and/or UV radiation Individualdescription of properties and reactivities of these oxidation technologies isnecessary to understand their synergism when used in combination for thetreatment of specific water pollutants or wastewaters However, becausecombined processes (O3/H2O2, UV/H2O2, or O3/UV) are usually recom-mended in real situations, a general description of the processes andfundamentals of the individual and integrated O3/UV/H2O2 technologies

is also presented in the following sections

A General Description

Ozone- or UV-radiation-based technologies (O3/UV/H2O2) are chemicaloxidation processes applied to water treatment for the degradation ofindividual pollutants or the reduction of the organic load (chemical oxygendemand, COD) and improved biodegradability of wastewaters In addition,ozone and UV radiation alone can be used for disinfection purposes; in fact,this was their first application in water treatment [13,14] In addition, theseAOTs, particularly ozonation, can be used to enhance the efficiency of otherprocesses such as Fe–Mn removal [15,16], flocculation–coagulation–sedi-mentation [17,18], biological oxidation [12], or biological degradation oforganic carbon in granular activated carbon [19–21]

O3/UV/H2O2AOTs are suitable for the treatment of water containingorganic pollutants in concentrations not higher than some tens of milligramsper liter However, these technologies can also be used to treat concentratedsolutions In addition to concentration, factors such as molecular structure

of pollutant, aqueous organic matrix, pH, etc are variables that affect theefficiency and applicability of O3/UV/H2O2AOTs for practical application.For wastewater treatment, O3/UV/H2O2 AOTs are used in combinationwith biological oxidation processes because of the enhancement achieved onthe biological oxygen demand (BOD) In fact, another feature of O3/UV/

H2O2 AOTs is that they steadily transform high molecular weight stances into more oxygenated lower molecular weight substances, whichinvolves an increase of BOD [22,23] Examples of studies on wastewatertreatment that give a general view of the application of O3/UV/H2O2AOTsare those of Rice and Browning [24] and, more recently, by Rice [25] on theuse of ozonation, or Zhou and Smith [26], Rivera et al [27], and Kos andPerkowski [28] for combined oxidation involving UV radiation

sub-O3/UV/H2O2AOTs, together with other processes treated in differentchapters (such as Fenton oxidation), can be named ambient (temperatureand pressure), advanced oxidation technologies, in contrast with otherAOTs such as hydrothermal oxidation processes that require pressuresand temperatures above 1 MPa and 150jC, respectively, and which aremore suitable for the treatment of concentrated wastewaters It is evidentthat appropriate ranges of concentrations for the different oxidation tech-nologies cannot be exactly established but some recommended values havebeen reported [29] Fig 1 shows some possible recommended ranges ofconcentrations for these types of AOTs

O3/UV/H2O2AOTs generally involve two oxidation/photolysis routes

to remove foreign matter present in water Thus, ozone, hydrogen peroxide,and/or UV radiation can react individually or photolyze directly the organic

in water However, when used in combination, they can degrade pollutants by

oxidation through hydroxyl free radicals generated in situ Hydroxyl radicalshave the largest standard redox potential except for fluorine (seeTable 1).

In addition, they react very rapidly with almost all types of organicsubstances through reactions whose rate constants vary from 107 to 1010

M
1s
1[30].Table 2gives a list of rate constant values of these reactions.Because of the high and similar values of the rate constants, it is saidthat these free radicals react nonselectively with the organic matter present

in water, although, as deduced from the above range of values, there arecompounds that react with them almost three orders of magnitude fasterthan others Among the most common water pollutants, phenols and somepesticides are substances that react rapidly with hydroxyl radicals, whereassome organochlorine compounds are less reactive

Another feature of these AOTs is that they are destructive types ofwater pollution removal processes because they eliminate compounds ratherthan transfer them to another medium Thus, carbon adsorption or strip-ping transfers pollutants from one phase (water) to another phase such as asolid phase (carbon) or a gas phase (air) In the latter case, purification of air

is required so that an additional step (i.e., carbon adsorption) is also needed,which implies higher processing costs

Figure 1 Oxidation process advisable according to COD of water (WAO, wet airoxidation SCWAO, supercritical wet air oxidation)

Table 1 Standard Redox Potential

of Some Oxidant Species

Table 2 Rate Constants of the Reaction Between the Hydroxyl Radical

and Organic Compounds in Water

Organic compound Rate constant10
9, M
1s
1 Reference no

At first sight, however, the main drawback of O3/UV/H2O2AOTs isthe high processing cost, mainly because both ozone and UV radiationrequire a continuous feed of energy for process maintenance, as well as highcapital costs for ozone generators and photoreactors However, the develop-ment of improved ozonators and UV lamp technologies has made theseprocesses more amenable in practice as can be deduced from their actualapplications (seeSec IV).

1 Background and Fundamentals

Ozone is an inorganic chemical molecule constituted by three oxygen atoms

It is naturally formed in the upper atmosphere from the photolysis ofdiatomic oxygen and further recombination of atomic and diatomic oxygenaccording to the following reactions:

In this way, ozone forms a stratospheric layer several kilometers widethat protects life on earth by preventing UV-B and UV-C rays from reachingthe surface of the planet Ozone may arise from combustion reactions inautomobile engines, resulting in pollutant gases These gases usually containnitric oxide that is photolyzed by sunlight in the surrounding atmosphere toyield nitrous oxide and atomic oxygen Atomic oxygen, through reaction (2),finally yields ozone In this sense, ozone is a contaminant of breathing air;the maximum level allowed during an 8-hr exposure is only 0.1 ppm.However, despite the importance of ozone as a tropospheric pollutant, thefate of ozone in the atmosphere is beyond the scope of this chapter.Ozone was discovered in 1840 and the structure of the molecule astriatomic oxygen was established in 1872 The first use of ozone wasreported at the end of the 19th century—as a disinfectant in many water-treatment plants, hospitals, and research centers such as the University ofParis where the first doctoral thesis on ozonation was presented [36].Although the number of water-treatment plants using an ozonation step

increased steadily during the 20th century, it was at the end of the 1970s thatthe use of ozone significantly increased This increase came about when tri-halomethanes and other organohalogenated compounds were identified indrinking water as disinfection by-products arising from chlorination [37].This discovery gave rise to an enormous research effort to look foralternative oxidants to replace chlorine Additional research aimed atdiscovering mechanisms of organochlorine compound formation estab-lished that these substances are formed from the electrophilic attack ofchlorine on nucleophilic positions of natural humic substances present insurface water [38] Because ozone is a powerful electrophilic agent, it wasfound that, generally, the application of ozone before chlorine significantlyreduced trihalomethane formation Subsequent study of ozone reactions inwater led to a wide array of applications (presented in a further section) thatcan be summarized in the following: use as a disinfectant or biocide, use as

an oxidant for micropollutant removal, and use as a complementary agent

to improve other unit operations in drinking and industrial water andwastewater treatments (sedimentation, cooling water treatment, carbonadsorption, iron and manganese removal, biological oxidation, etc [5]).The role of ozone in medical applications has also increased over the pasttwo decades [39] In the mid-1980s, the need to comply with environmentalregulations on allowable levels of refractory substances such as pesticides [2]gave rise to another class of ozone water treatment for drinking water:ozone advanced oxidations These processes are based on the combined use

of ozone and hydrogen peroxide and/or UV radiation to generate hydroxylradicals as indicated above [1]

Ozone is known as a very reactive agent in both water and air Thehigh reactivity of the ozone molecule is due to its electronic configuration.Ozone can be represented as a hybrid of four molecular resonance structures(seeFig.2) As can be seen, these structures present negative and positivelycharged oxygen atoms, which in theory imparts to the ozone molecule thecharacteristics of an electrophilic, dipolar and, even, nucleophilic agent.Because of this reactivity, the ozone molecule is able to react throughtwo different mechanisms called direct and indirect ozonation Thus, ozonecan directly react with the organic matter through 1,3 dipolar cyclo-addition, electrophilic and, rarely, nucleophilic reactions [40,41] In water,only the former two reactions have been identified with many organics[42] On the contrary, the nucleophilic reaction has been proposed in only

a few cases in non-aqueous systems [43] (see examples of these mechanisms

Another group of ozone direct reactions are those with inorganicspecies such as Fe2+, Mn2+, NO2
, OH
, HO2
, etc [44] These could bedefined as redox reactions because in the overall process ozone acts as a true

oxidizing agent by taking electrons whereas the other species act as truereducing agents by losing electrons Ozone has the highest standard redoxpotential among conventional oxidants such as chlorine, chlorine dioxide,permanganate ion, and hydrogen peroxide (see Table 1) At acid pH, theredox reaction for ozone is as follows:

O3þ 2H
þ 2e

!O2þ H2O Ej ¼ 2:07 V ð3Þ

Figure 2 Resonance structures of the ozone molecule

Figure 3 Direct pathways of ozone reaction with organics (A) Criegge mechanism.(B) Electrophilic aromatic substitution and 1,3-dipolar cycloaddition (C) Nucleo-philic substitution

However, these reactions can actually be considered as electron transfer

or oxygen atom transfer reactions, as in the case of the ozone reactions withthe hydroxyl and hydroperoxide ions or with the nitrite ion, respectively:

O3þ HO2 
k¼210!6 M
1 s
1

O3þ NO2

k¼3:710!5 M
1 s
1 NO3
þ O2 ð6ÞReactions (4) and (5) are extremely important because they are the initiatingsteps of the radical mechanism leading to the formation of hydroxyl radicalswhen ozone decomposes

On the other hand, the indirect type of ozonation is due to the reactions

of free radical species, especially the hydroxyl radical, with the organic matterpresent in water These free radicals come from reaction mechanisms of ozonedecomposition in water that can be initiated by the hydroxyl ion or, to bemore precise, by the hydroperoxide ion as shown in reactions (4) and (5).Ozone reacts very selectively through direct reactions with compounds withspecific functional groups in their molecules Examples are unsaturated andaromatic hydrocarbons with substituents such as hydroxyl, methyl, aminegroups, etc [45,46]

The mechanism of decomposition of ozone in water has been thesubject of numerous studies, starting from the work of Weiss [47] Amongmore recent studies, the mechanisms of Hoigne´ et al [48] and Tomiyashu

et al [49] are the most accepted in ozone water chemistry The main clusion that can be drawn is that ozone stability in water is highly depen-dent on the presence of substances that initiate, promote, and/or inhibit itsdecomposition The ozone decomposition mechanism usually assumed isgiven inFig 4 [50]

con-As observed from Fig 4, ozone decomposition generates hydrogenperoxide that reacts with ozone [reaction (5)] to yield free radicals,initiating the propagation steps of the mechanism It should be notedthat hydrogen peroxide has been detected during ozonation reactions inwater in the presence and absence of organics such as humic substances

or aromatic compounds [51] From this mechanism, it is also deducedthat ozonation alone, or single ozonation, can be included under thegroup of AOTs, especially when the pH is increased Notice that in themechanism presented in Fig 4 other possible reactions of ozone notshown are those corresponding to the direct pathway (see later) that leads

to molecular products

Ozone decomposition is usually a first-order process, where theapparent pseudo first-order rate constant depends on the concentration of

promoters, P, inhibitors, S, and initiators, I, of ozone decomposition as wasreported by Staehelin and Hoigne´ [48] with the equation given below:

as initiator, promoter, or scavenger (see also Fig 4); CMi is the tration of any other species i present in water other than the initiators, whichreact with ozone directly to yield molecular products; kiand kIirepresent therate constants of the reactions between ozone and the hydroxyl ion and anyinitiator species i, respectively; kPiand kSirepresent the rate constants of thereactions between the hydroxyl radical and any promoter and inhibitor i ofozone decomposition, respectively; and kDi represents the rate constant ofthe direct reaction of ozone with any other species i present in water otherthan the initiators As can be deduced from Eq (7) the half-life of ozone inwater is highly dependent on the pH and matrix content of the water Forexample, the half-life of ozone in distilled water can vary from about 10
2sec at pH 12 to 105 sec at pH 2 or from 10 sec for secondary wastewatereffluents to 104 sec for certain ground and surface waters as reported inthe literature [50,52]

concen-2 Kinetics of Ozonation

The design of ozonation contactors requires knowledge of kinetic tion (see later), that is, the rate at which pollutants or matter present inwater react with ozone, both directly and/or indirectly, and hence the rate ofozone absorption Reaction rates can be calculated if rate constants of thesereactions are known Thus, the determination of rate constants represents a

informa-Figure 4 Scheme of ozone decomposition mechanism in water P=promoter (e.g.,ozone, methanol) S=scavenger or inhibitor (i.e., t-butanol, carbonate ion).I=initiators (e.g., hydroxyl ion and hydroperoxide ion)

crucial point in contactor design In practice, ozonation is a heterogeneousprocess involving ozone transfer from air or oxygen to the water phase andsimultaneous chemical reactions in the aqueous medium The kinetics of thistype of processes can be established if the kinetic regime of ozone absorption

is known This process requires knowledge of the relative importance ofboth physical and chemical rates (diffusion of ozone and chemical reac-tions), which can be quantified from the dimensionless number of Hatta[53] For any ozone–organic substance reaction in water, second-orderirreversible reactions normally occur (first-order with respect to ozone andcompound M) [41,44–46,54]:

Eq (9), kD and kL are parameters representing the chemical reaction andphysical diffusion rate constants, that is, the rate constant of the ozone–compound reaction and water phase mass transfer coefficient, respectively.Their values are indicative of the importance of both the physical andchemical steps in terms of their rates However, two additional parameters,

as shown in Eq (9), are also needed: the concentration of the compound,

CM, and the diffusivity of ozone in water, DO3 The ozone diffusivity in watercan be calculated from empirical equations such as those of Wilke andChang [55], Matrozov et al [56], and Johnson and Davies [57]; from theseequations, at 20jC, DO3 is found to be 1.6210
9, 1.2510
9, and1.7610
9m2s
1, respectively

The value of Ha determines the rate of the ozone reaction Thus, for

Ha <0.3 ozone reactions are slow reactions, whereas for Ha > 3 they arefast reactions There is also an intermediate kinetic regime defined as mod-erate, which is rather difficult to treat kinetically [53] However, for mostcommon situations, reactions of ozone in drinking water are considered asslow reactions This does not mean that the time needed to carry out theozonation is high (time needed to have high destruction of pollutants), butthat the mass transfer rate is faster than the chemical reaction rate For in-stance, in most cases, ozonation of micropollutants, which are found in verylow concentrations (mg L
1orAg L
1), lies in this kinetic regime In othercases, where the concentration of pollutants is higher (i.e., wastewaters

containing compounds that react very fast with ozone such as phenols in highconcentration), the chemical reaction rates are equal to or even much fasterthan the mass transfer rate and the kinetic regime is fast or instantaneous[58] To distinguish between kinetic regimes of fast reactions, anotherdimensionless number, the instantaneous reaction factor, Ei, should bedetermined [53]:

Ei¼ 1 þ zDMCM

In Eq (10) z is the stoichiometric coefficient of the ozone-compound reaction[reaction (8)], DMis the diffusivity of compound M in water (which can becalculated from the Wilke and Chang equation), and C*O 3 is the ozonesolubility (or properly defined, the ozone concentration at the gas–waterinterface) If the parameters of Eqs (9) and (10) are known, the kineticregime can be established, and hence the kinetics of ozonation can bedetermined Table 3 gives the kinetic equations corresponding to differentkinetic regimes found in ozonation processes As can be deduced from theequations in Table 3, the rate constant, mass transfer coefficients, and ozonesolubility must be previously known to establish the actual ozonationkinetics The literature reports extensive information on research studiesdealing with kinetic parameter determination as quoted below

Table 3 Kinetic Equations and Absorption Kinetic Regimes for Second-OrderIrreversible Ozone–Organic Gas–Liquid Reactionsa

Kinetic regime Kinetic equation ConditionsVery slow NO3=kLa(C*O3
CO 3) =dCO3

dt +S

iri Ha<0.02

CO 3p 0Diffusional NO3=kLaC*O3 0.02<Ha<0.3

CO3=0Fast NO3=kLa Ha

CO3=0Fast pseudo first order NO3=aC*O3

CO3=0

a

Equations according to film theory, see also Ref 53 For stoichiometry, see reaction (8).

N O3=ozone absorption rate, Ms
1; Ha according to Eq (9); E i , according to Eq (10); n=function (Ha, E i ) In the fast, pseudo first-order kinetic regime equation, a represents the specific interfacial area.

3 Ozone Solubility, Rate Constants, and Mass Transfer

Coefficients

Similar to ozone decomposition, ozone solubility has been the subject ofmultiple studies These studies usually propose an empirical equation forthe Henry’s law constant as a function of pH, ionic strength, and temper-ature [59,60] For example, Sotelo et al [60] found the following equa-tion valid for phosphate buffer aqueous solutions at temperatures between

0 and 20jC, pH range of 2 to 8.5, and ionic strength varying from 10
3

Theoret-of pH according to the following equation:

log He=Hejð Þ ¼X

where Hej is Henry’s constant in ultrapure water and h is the salting-outcoefficient, a function of the different ionic and dissolved gas species inwater [61] Thus, in a more recent paper, Andreozzi et al [62] studied thisproblem and tried to develop an equation of this type The authors did notarrive at this equation, but they concluded that the change in He with pHshould be due to the salting-out coefficients of the different ionic speciesthat also change with pH

For the experimental determination of He, a mass balance of ozone in

a system where ozone is absorbed in ultrapure buffered water in a semibatchreactor is usually applied:

concen-PO3¼ HeC

Depending on the disappearance rate of the reacting compound orozone, rate constants of direct ozone reactions can be obtained from bothhomogeneous and heterogeneous ozonation systems Thus, for very slow re-actions, homogeneous ozonation has the advantage of the absence of a masstransfer step In these cases, the concentration of one of the reactants (ozone

or compound M) can be considered constant throughout the reaction period,and the kinetics are determined by measuring the concentration of the othersubstance with time When the reaction is very fast (of the order of micro-seconds or milliseconds) homogeneous ozonation can also be followed, butspecial equipment is needed to stop the reaction at very short times, forexample, with stopped flow spectrophotometers [63] For kinetic studies inthese cases, heterogeneous ozonation reactions are recommended because thevariation of concentration with time is much slower than in homogeneousprocesses Consequently, conventional methods, such as gas or liquid chro-matography or even classical spectrophotometry, can be used For heteroge-neous kinetics, the equations given inTable 3will be needed InTable 4,a list

of rate constant values for ozone direct reactions is given together with themethod of calculation In other cases, to avoid the interferences of ozoneconsumption from by-products, the rate constants are deduced from com-petitive ozonation kinetics of two compounds: the compound whose kineticswith ozone is being determined and the reference compound Obviously, theozone kinetics of the reference substance must be well known In this way,Gurol and Nekouinaini [71] and Beltra´n et al [72] have determined the rateconstants of ozone fast reactions with some phenolic compounds

Ozonation processes can also be used for determination of mass fer coefficient In fact, both ozone absorption in organic-free water, which

trans-is a slow gas–liquid reaction, and other ozone gas–liquid reactions havebeen used for this purpose For example, Roth and Sullivan [59] and Sotelo

et al [60] determined the mass transfer coefficient from ozone absorption inorganic-free water, whereas Ridgway et al [73] and Beltra´n et al [67] carriedout similar calculations from ozone absorption in water at pH 2 containingindigo and p-nitrophenol, respectively

4 Kinetic Modeling

Kinetic models utilize a set of algebraic or differential equations based onthe mole balances of the main species involved in the process (ozone in waterand gas phases, compounds that react with ozone, presence of promoters,inhibitors of free radical reactions, etc) Solution of these equations providestheoretical concentration profiles with time of each species Theoreticalresults can be compared with experimental results when these data areavailable In some cases, kinetic modeling allows the determination of rateconstants by trial and error procedures that find the best values to fit the

experimental and calculated concentrations.Table 5presents a list of studieswhere kinetic modeling of ozonation processes were carried out.

C Hydrogen Peroxide Oxidation

Similar to ozone, hydrogen peroxide can react with organic matter present

in water through direct and indirect pathways In direct mechanisms,hydrogen peroxide participates in redox reactions where it can behave as

Phenoxide ion 1.4109 10 EX 46p-Nitrophenol 4.5106 6.5 AHEK 67o-Chlorophenol 1600 2 CHEK 68

4.3105 7 CHOK 544.7105 7 AHEK 70

a AHOK=absolute rate constant by homogeneous kinetics; CHOK=competitive neous kinetics; AHEK=absolute rate constant by heterogeneous kinetics; CHEK=compe- titive heterogeneous kinetics EX=by extrapolation of values at lower pH.

homoge-or as a reductant:

Indirect reactions are due to the oxidizing action of free radicals that areformed from the decomposition of aqueous hydrogen peroxide when itreacts with other inorganic compounds, such as ozone or Fe2+, or when it

is photolyzed

Examples of direct reactions are mainly with inorganic compoundssuch as cyanides and sulfides or ozone and Fe2+ Both reactions of ozoneand Fe2+with hydrogen peroxide represent the initiating steps of advancedoxidation processes: O3/H2O2, treated later in this chapter, and the Fentonoxidation, presented in another chapter, respectively Hydrogen peroxide,

on the other hand, does not significantly react with most organic pounds, at least at appreciable rates for water treatment [6]

com-Hydrogen peroxide was discovered in 1818 by Tenard; the molecularstructure forms an oxygen bridge, with each oxygen bonded to one hydro-gen atom In water, it is a weak acid, which dissociates to yield thehydroperoxide ion, HO2
:

Table 5 Studies Dealing with AOP Kinetic Modeling Involving Ozone,

Hydrogen Peroxide, and UV Radiationa

Compounds treated AOP system Reacting system Reference no.1,2-Dibromo-3-

chloropropane

UV/H2O2 CMBPR 74Chlorobutane UV/H2O2 CSTPR 75Trichloroethene,

a CMBPR=completely mixed batch photoreactor; CSTR=continuous stirred tank reactor; CBDR=continuous bubble reactor with dispersion; PFHPR=plug flow homoge- neous photoreactor; BHR=batch homogeneous tube reactor; CMSBPR=completely mixed semibatch photoreactor.

photo-radiation, another advanced oxidation system commented on later In thischapter, discussion of hydrogen peroxide reactions will be limited only tothose of the O3/H2O2and UV/H2O2systems.

D UV Radiation

UV radiation is also the basis of several chemical oxidation technologieswhere the action of radiation and free radicals generated in the process allowfor a high degree of micropollutant degradation and/or disinfection Similar

to ozonation or hydrogen peroxide oxidation, UV radiation may act on thematter present in water in two different ways: direct photolysis or indirectphotolysis (e.g., free radical oxidation)

1 Background and Fundamentals

UV radiation comprises energies from about 300 kJ Einstein
1(UV-A tion, 1 einstein=1 mol of photons), up to 1200 kJ Einstein
1(vacuum UV).Table 6 shows the wavelength and energy of different UV radiation types.For disinfection and oxidation purposes, UV-C radiation is normallyused although the application of other types of UV radiation has also beenreported in the literature [10] For example, the use of UV-A or even visibleradiation to treat natural organic matter present in surface water has beenreported with and without the presence of catalysts [82,83] Concerning theutilization of UV-C radiation, the most common use is 254-nm radiationdue to the development of low-pressure vapor mercury lamps by Hewitt in

radia-1901 [13] For this reason, in this chapter the information presented mainlyfocuses on the use of 254-nm UV-C radiation

Similar to ozonation processes, since the discovery of the germicidaleffects of solar UV radiation by Downes and Blount in 1877 [13], UV radia-tion was first used for disinfection The development of reaction mecha-nisms in photochemistry led to the discovery of the advantages of UVradiation as an oxidation technology At room temperature, most molecules

Table 6 Radiation Type and Associated Energy

Radiation Wavelength range, nm Energy range, kJ Einstein
1aInfrared >780 <155

reside in their lowest-energy electronic state, known as the‘‘ground state.’’When UV radiation (or any other type of radiation with sufficient energyper photon) is incident upon a molecule, the radiation can be absorbed,promoting the molecule to an excited state That is, one electron of themolecule goes to a higher-energy state or excited state Depending on thedirection of the electron spin, for most organic molecules, the excited state iseither a singlet (all electron spins cancel) or a triplet state (two unpairedelectrons with parallel spins) Overall the most probable transition occursfrom the ground state to the singlet state The energy difference between theground and excited states corresponds to the absorbed energy, hm, m beingthe frequency of the absorbed radiation and h the Planck constant Themolecule in the excited state has a very short lifetime (10
9to 10
8sec) [84],after which it returns to the ground state by one of several mechanisms(fluorescence, phosphorescence, internal conversion, collisions, etc.) or de-composes to yield a different molecule; that is, it undergoes a photochemicalreaction A simple mechanism of photochemical reaction already used insome studies [85,86] is given below:

a carbon–hydrogen bond followed by reaction with oxygen to yield organicperoxyl radicals [10,87]:

to the indirect photolysis of M In fact, Faust and Hoigne´ [82] reported thatthere are four possible routes of the excited photosensitizing action:

reactions of the photosensitizer with any compound M, with natural oradded solvents, with itself, and unimolecular decay as shown below:

Another possible mechanism of photolysis is through the formation ofsecondary photooxidants that can be formed from one of the photosensi-tizer routes shown above For example, a possible mechanism with humicsubstances as photosensitizers [90] could involve the formation of hydrogenperoxide and, subsequently, hydroxyl radicals:

In addition to humic substances, nitrites and nitrates usually found innatural water also act as indirect photosensitizers to produce secondaryoxidants such as hydroxyl radicals [91] A simplified scheme of the mech-anism is as follows [92]:

NO
3 þ hm!NO

The use of nitrate to improve the photodegradation rates of pollutantshas been reported For example, So¨rensen and Frimmel [92] observed thatthe rate of photolysis of xenobiotic compounds such as EDTA and somephenyl and naphthalene sulfonates was significantly increased in the presence

of nitrates

Finally, another possibility of photolytic reaction is due to geneous processes, that is, photocatalysis In these processes, a metaloxide surface is irradiated to yield surface hole–electron pairs [93] Forexample, TiO2 suspensions are often used for this aim to generate thesespecies [94]:

hetero-TiO2þ hm!TiO2þ h þþ e


ð40ÞThe electron and hole may react at the surface with adsorbed compounds toinitiate oxidation or reduction reactions:

where V is the reaction volume andl [96] is a function of molar absorptioncoefficients of species present in water,ei, defined as follows:

and ka, kb, and kcare the rate constants of steps (18) to (20), respectively.Notice that the first minus sign on the right side of equation (45) is due to thestoichiometric coefficient of M which is
1 As a rule, stoichiometriccoefficients of reacting products are negative

In Eq (45) qi, is the flux of incident radiation, which varies according

to the geometrical configuration of the photoreactor and photochemicalmodel used, and F is the fraction of absorbed radiation that M absorbs:

FM¼ MCM

P

whereeMandeiare the molar absorption coefficient and molar absorptivity

or optical density of M and any compound i, respectively, present insolution that also absorbs radiation The term [ka(1
kb/(kb+kc)] in Eq (45)can be defined as the quantum yield of M, /M The quantum yield isperhaps the most important parameter in UV radiation kinetics because itmeasures the fraction of the excited molecules that are transformed intoproducts This parameter is defined as the moles of M decomposed perEinstein absorbed (1 Einstein being 1 mol of photons, 6.0231023photons).Substances with high quantum yields that are constant over a wide range

of wavelengths are usually called actinometers, and are used to measure theintensity or flux of incident radiation as shown later In any case,compounds of high quantum yield are prone to decomposition through

UV radiation In Table 7 values of quantum yield and molar absorptioncoefficients for different compounds and oxidants in water are shown

as examples

Equation (45) can be solved by applying different photoreactormodels The literature reports several photochemical reactor models forboth homogeneous and heterogeneous reactors [11,108,109] In practice,annular photoreactors are often used (seeFig 5); therefore, models for thistype of reactor are considered here For other types of reactors, attentionshould be given to other publications [109]

Here Eq (45) is solved for three models: the linear source withemission in parallel planes to the lamp axis (LSPPM) model, the pointwith spherical emission (PSSE) model, and a semiempirical model based

on Lambert’s law (LLM) The first two models come from the solution of

a radiation balance equation throughout the photorreactor assumingdifferent hypotheses

e

e

e

Table 7 Values of Quantum Yields and Molar Absorption Coefficients

for Different Water Pollutants and Oxidants in Water

Pollutant or oxidant

e, M
1cm
1(k, nm) /, mol Einstein

1

(k, nm) Reference no.Ozone 3300 (253.7) 0.62 (253.7) 97

H2O2 18.7 (253.7) 0.5d 98,99

Phenol 516 (213–400) 0.05 (213–400) 1012-Chlorophenol 1920 (272) 0.03 (296) 1022-Chlorophenolate 3760 (293) 0.30 (296) 1023-Chlorophenol 1750 (273) 0.09 (254 or 296) 1023-Chlorophenolate 3000 (292) 0.13 (254 or 296) 1024-Chlorophenol 1650 (278) 0.25 (254 or 296) 1024-Chlorophenolate 2400 (296) 0.25 102Nitrobenzene 5564 (254) 0.007 (254) 242,6-Dinitrotoluene 6643 (254) 0.022 (254) 24Fluorene 16654 (254) 0.0075 (254) 73Phenanthrene 40540 (254) 0.0069 (254) 73Acenaphthene 1333 0.0052 73Acenaphthalene 26941 0.004 1031,3-Dichlorobenzene 0.06 (213–400) 1011,3,5-Trichlorobenzene 0.043 101Trichloroethylene 18.3 (254) 0.88 (254) 95Atrazine 2486 (254) 0.05 104Simazine 2512 0.06 105

Alachlor 540 0.177 107Parathion 0.0076 (240–320)e 89

The LSPPM assumes that the lamp can be represented as a utive line of points, each one emitting radiation in all directions contained in

consec-a plconsec-ane perpendiculconsec-ar to the lconsec-amp consec-axis An consec-approximconsec-ate equconsec-ation for qi

The PSSE model considers each point of the lamp emitting radiation in allspace directions The expression for qiis as follows [111,112]:

rUV¼dCM

dt ¼
FMfMIo½1
exp
Ay½ L ð53Þwhere Iois the intensity of incident radiation and yLis the effective path ofradiation through the photoreactor

In all these models, knowledge of parameters such as qo(LSPP model),

Eo (PSSE model), or Io and yL (LL model) are necessary to determine thephotolysis rate of M These parameters are determined experimentally byactinometry experiments [86] It is noteworthy to mention that the use of thesetheoretical models (LSPP or PSSE models) implies that all radiation incidentinto the solution is absorbed without end effects, reflection, or refraction Inexperimental photoreactors, it is not usual to fulfill all these assumptionsbecause of the short wall distance of the photoreactor For instance, toaccount for such deviations, Jacob and Dranoff [114] introduced a correctingequation, as a function of position Another important disadvantage is thepresence of bubbles that leads to a heterogeneous process as, for example, inthe case of O3/UV oxidation In this case, photoreactor models should be used[109] This is the main reason for which the LL model is usually applied in thelaboratory for the kinetic treatment of photochemical reactions In the LLM,

the effective path of radiation can be considered as the correction functionaccounting for deviations from ideality.

E Combined Oxidations: O3/H2O2, UV/H2O2, and O3/UV

The reaction of ozone and hydrogen peroxide in its ionic form and photolysis

of both oxidants constitute the initiation reactions leading to a mechanism ofhydroxyl radical formation in water This mechanism is basically the samefor all these advanced oxidation systems, whereas the main differences lie inthe initiating steps These oxidation technologies have been applied for thetreatment of pollutants in water for more than two decades

1 Background and Fundamentals

Photolysis of hydrogen peroxide was first studied by Baxendale and Wilson[99] They reported that the decomposition of 1 mol of hydrogen peroxideneeded one Einstein of incident 254 nm UV radiation:

although the latter reaction is negligible

These authors [99] also showed that the quantum yield of bothforms of hydrogen peroxide, H2O2 and HO2
, remained constant over awide range of concentrations and UV radiation intensity Baxendale andWilson [99] also carried out experiments in the presence of organic sub-stances, such as acetic acid (a well-known scavenger of the hydroxylradicals) so that the measured rate of hydrogen peroxide disappearancecorresponded to the rate of its direct photolysis [reaction (54)] From theseexperiments, they found that the rate was half of that of the process in theabsence of hydroxyl radical scavengers Consequently, they concluded thatthe quantum yield of reaction (54) was 0.5 mol of hydrogen peroxide perEinstein This value is called the primary quantum yield of hydrogenperoxide photolysis

As reported by Staehelin and Holgne´ [115], ozone reacts only with theionic form of hydrogen peroxide, the hydroperoxide ion, HO2
Theseauthors studied this reaction at different hydrogen peroxide concentrations

and in the presence of methylmercury hydroxide, another hydroxyl radicalscavenger At pH values below the pKaof hydrogen peroxide (pKa=11.7),these authors observed that the rate of reaction (5) increased one order ofmagnitude per unit increase of pH They found a second-order reaction rateconstant of 2.8106M
1 s
1 A similar behavior can be noticed with theozone decomposition rate in organic-free water in the absence of hydrogenperoxide [reaction (4)], although the rate constant is several orders ofmagnitude lower (70 M
1s
1) as found by the same authors and by Forni

et al [116]

Taube [97] studied the photolysis of aqueous ozone and postulated theformation of hydrogen peroxide, which he found to be formed with almostexact stoichiometry Taube [97] reported a quantum yield for ozone of 0.62

at 254 nm Later, Prengle [117] claimed that ozone photolysis yields atomicoxygen, which directly leads to hydroxyl radicals To elucidate whichmechanism is the correct one, Peyton and Glaze [118] later studied thisreaction and concluded that hydrogen peroxide is first formed from ozonephotolysis without formation of atomic oxygen From these studies [115–118] and others reported by Staehelin and Hoigne´ [48] and Tomiyasu et al.[49], the mechanism of any type of advanced oxidation system involvingozone, hydrogen peroxide, and UV radiation can be established A sim-plified scheme of this mechanism, applied to the oxidation of a potentialpollutant M, is presented inFig 6

The main reactions of the mechanism are given below:

vs 2.8106 M
1 s
1 for reaction (51) [115] The relative importance,however, is pH dependent For the conditions usually applied in watertreatment, reaction (61) is faster An extensive study of these reactions can

be seen in the work of Staehelin and Hoigne´ [115] Reaction (61) alsopredominates against the direct photolysis of hydrogen peroxide [reaction(58)] This is due to the low value of the molar absorption coefficient ofhydrogen peroxide (see Table 7)and the very high rate constant value ofreaction (61) (see above) Furthermore, the direct photolysis of hydrogenperoxide competes, among others, with the direct photolysis of ozone, which

is also faster This can be deduced from the values of the product betweenthe quantum yield and molar absorption coefficient of each photolysisreaction The product is 2046 L Einstein
1cm
1for the case of ozone and

105 L Einstein
1 cm
1 for the case of hydrogen peroxide at its mostfavorable conditions (see Table 7), that is, at alkaline pH when hydrogenperoxide is present in the ionic form A thorough study of the competitionbetween these initiation reactions can be followed from the studies ofBeltra´n [119,120]

For chemical structures refractory to direct reactions with ozoneand UV photolysis, free radical reactions are fundamental Among freeradicals, the hydroxyl radical shows a high oxidizing power, and it is ge-nerally accepted as the main oxidant in these advanced oxidation systems

Figure 6 Scheme of O3/UV/H2O2 oxidation processes Key: 1: From directozonation 2: From direct photolysis 3: From free radical oxidation 4: Intermediatepathway if M is inhibitor 5: Intermediate pathway if M is promoter

The reactions through which hydroxyl radicals participate are shownbelow [121]:

CO¼3 þ HO!k¼3:7108 M
1 s
1 CO
3 þ OH
ð68ÞHCO
3 þ HO!k¼2107 M
1 s
1 CO
3 þ H2O ð69Þwhich is also known to react with organic compounds in water Table 8

presents a list of rate constants of these reactions In addition, thecarbonate ion radical also reacts with hydrogen peroxide, if present inwater, to regenerate the carbonate ion and the hydroperoxide ion radicalthat eventually leads to the hydroxyl radical in the presence of ozone.Consequently, there could be a fraction of carbonates that do not inhibitthe ozone decomposition in water Notice that in some cases when ozone isapplied, hydroxyl radical oxidation is negligible or does not developbecause direct ozonation is very fast This is, for example, the case in theozonation of phenolic compounds under neutral or alkaline conditions,where the rate constants of the direct ozone reactions vary between 106and

1010 M
1 s
1 As observed, the order of magnitude is similar to or evenhigher than that of the reactions with the hydroxyl radical An extensivestudy of this situation can be seen in a previous paper [124].

2 Chemical Kinetics

Once the mechanism of a reaction is established, kinetic studies constitutethe next step to determine the rate of pollutant degradation Kinetic lawsmust be deduced experimentally following well-established rules [125] Forthe degradation of a compound M through O3/H2O2/UV oxidation, the rate

of M disappearance is given by Eq (70):

where the four terms of the right-hand side of Eq (70) refer to thecontribution of direct reactions (photolysis, ozonation, reactions withhydrogen peroxide) and free radical oxidation, respectively By assuming

an irreversible first-order reaction for every reactant (a global second-orderreaction), the last three rates are as follows:

Table 8 Rate Constants of the Reaction Between the Carbonate Ion Radicaland Organic Compounds in Water

Compound Rate constant, M
1s
1 pHBenzene <5104 11.7

Phenoxide ion 4.7108 11.2p-Chlorophenoxide ion 1.9108 12.2

Acetone 1.6102 12.1–12.7Formate ion 1.1105 6.4

Source: Ref 123.

the steady state approximation is accounted for, it is easy to show that theconcentration of hydroxyl radicals is given by Eq (74):

of ozone into water [76] Some investigations have also reported [76,126,127]that during ozonation, hydrogen peroxide plays a double role as initiatorand inhibitor of ozone decomposition Thus, for concentrations usually up

to 10
3or 10
2M, the increase of hydrogen peroxide concentration leads to

an increase of the ozonation rate of M, but for concentrations above thesevalues hydrogen peroxide inhibits the ozonation rate In these latter cases,reactions of ozone are so fast that ozone is not detected dissolved in waterand the process becomes mass transfer controlled According to absorptiontheories [61], a complex kinetic equation can be deduced from the solution

of microscopic mass balance equations of ozone, hydrogen peroxide, and

M, but a simplified equation is obtained from the macroscopic mass balanceequations as previously reported for trichloroethylene and tetrachloroethy-lene oxidation with O3/H2O2 in a semibatch system [76] Thus, the finalequation for the concentration of hydroxyl radical is as follows [76,128]:

The double role as scavenger and initiator, observed for hydrogenperoxide in the O3/H2O2 system, has also been reported in the UV/H2O2

system It should be noted that hydrogen peroxide does not inhibit theozone decomposition and Eq (75) is valid only in the cases that ozone ispresent in the reaction mixture and the process is chemically controlled (lowconcentrations of hydrogen peroxide) This is because reactions of hydrogenperoxide with the hydroxyl radical release the superoxide ion radical that