Microwaves in advanced oxidation processes for environmental applications. A brief review

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 114–131

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Review

Microwaves in advanced oxidation processes for environmental applications. Abrief review

Nick Serponea,∗, Satoshi Horikoshib, Alexei V. Emelinec

a Dipartimento di Chimica Organica, Universita di Pavia, via Taramelli 10, Pavia 27100, Italyb Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japanc V.A. Fock Institute of Physics, Saint Petersburg State University, Saint Petersburg 198504, Russian Federation

a r t i c l e i n f o

Article history:Received 28 May 2010Received in revised form 25 July 2010Accepted 28 July 2010Available online 11 August 2010

Keywords:Advanced oxidation processesMicrowavesApplications to environment

a b s t r a c t

This review article focuses, albeit non-exhaustively, on the influence of microwave radiation on pho-toassisted processes often referred to as Advanced Oxidation Processes. In particular, we describe andillustrate the possible advantages of microwaves in TiO2-assisted photodegradations and photomin-eralizations of various organic pollutants such as herbicides and endocrine disruptors, among others.Described are also various reactor configurations involving UV/visible radiation and microwaves, withthe former being supplied either by traditional Hg lamps or alternatively by electrodeless lamps activatedby microwaves. To place the use of microwaves on processes occurring in aqueous TiO2 dispersions inperspective regarding environmental applications, we first introduce the various sources of pollutantsand subsequently describe in brief the various advanced oxidation processes such as UV/peroxidation,UV/ozonation and the photo-Fenton process(es) in addition to direct photolysis either by sunlight or byartificial light sources.

© 2010 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1151.1. Sources of pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1151.2. Conventional abatement of environmental pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

2. Solar and artificial light in direct photolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1173. Advanced oxidation processes in homogeneous phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

3.1. The UV/H2O2 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.2. The UV/O3 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.3. The traditional versus the novel ferrioxalate/H2O2 photo-Fenton system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4. Environmental photochemistry with TiO2: applications of heterogeneous photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1195. Microwave radiation in advanced oxidation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

5.1. General observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1205.2. Microwave effects on the decomposition of the RhB dye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1235.3. Degradation of the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide and microwave radiation effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1245.4. A simple microwave device to degrade pollutants in aqueous media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255.5. Microwave thermal versus non-thermal effects in TiO2-assisted photodegradation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

∗ Corresponding author.E-mail addresses: [email protected], [email protected] (N. Serpone).

1389-5567/$20.00 © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jphotochemrev.2010.07.003


N. Serpone et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 114–131 115

Nick Serpone did his undergraduate studies in honorschemistry at Sir George Williams University (Montreal,Canada; 1960–1964) and his PhD studies in physical-inorganic chemistry at Cornell University (1964–1968;Ithaca, New York). He joined Concordia University (Mon-treal) in 1968, was made associate professor in 1973,and professor in 1980. He spent sabbatic leaves at theUniversity of Bologna (Italy; 1975–76), at the Ecole Poly-technique Federale de Lausanne (Switzerland; 1983–84),at the Ecole Centrale de Lyon (France; 1990–91), and atthe University of Ferrara (Italy; 1997–98). He was a con-sultant for 3M’s Imaging Sector for nearly 10 years. In1998 he took early retirement from Concordia University

to become University Research Professor (1998–2004) and then Professor Emeritusin 2000. He was Program Director at the National Science Foundation (Washington;1998–2001) and has been a Visiting Professor at the University of Pavia, Italy, since2002. In July–August 2008 he was a Visiting Professor at the Tokyo University of Sci-ence, Noda Campus. His major research interests are currently in the photophysicsand photochemistry of semiconductor metal oxides, heterogeneous photocataly-sis, environmental photochemistry, photochemistry of sunscreen active agents, andapplication of microwaves to nanomaterials and to environmental remediation. Hehas co-authored over 390 articles and has co-edited four monographs. He was theco-recipient of the “Best Paper Award” from the Society of Imaging Science & Tech-nology (1997) and currently maintains an active collaboration with researchers atthe Fock Institute of Physics of St. Petersburg State University (Russia), at MeiseiUniversity (Tokyo) at the Tokyo University of Science, and at the University of Pavia.He was recently elected a Fellow Member of the European Academy of Sciences (July2010).

Satoshi Horikoshi was born in Saitama in 1971, Japan.He received his PhD degree in 1999 from Meisei Univer-sity, and subsequently was a postdoctoral researcher atthe Frontier Research Center for the Global EnvironmentScience (Frontier Research Program of the Ministry of Edu-cation, Science and Culture of Japan) between 1999 and2006. He joined Sophia University as Assistant Professorin 2006, and then moved to Tokyo University of Scienceas an associate professor in 2008. He is currently the Vice-President of the Japan Society of Electromagnetic WaveEnergy Applications (JEMEA; 2009–. . .), and a Member ofthe Board of the International Microwave Power Institute(IMPI, USA; 2010). His research interests include the appli-

cation of microwave radiation to catalytic chemistry, to the effects of microwaveson photocatalysts for environmental protection, to the microwave-assisted organicsyntheses, and to microwave effects on nanoparticles.

Alexei V. Emeline graduated from the Chemistry Depart-ment of Tomsk State University, Russia, in 1990, afterwhich he joined the V.A. Fock Institute of Physics at Saint-Petersburg State University and in 1995 defended his PhDin molecular physics. In 1997 he was granted a two-year NATO Science Post-doctoral Fellowship to conductresearch at Concordia University (Montreal, Canada) in thelaboratory of prof. Nick Serpone. Later he was promoted tothe Research Associate position at the same laboratory andcontinued research until 2004. In 2005 and 2008 he wonJSPS Fellowships for young North American researchersand for Advanced Researchers, respectively, to conductstudies at the Kanagawa Academy of Science and Technol-

ogy (Japan) in the research group led by prof. Akira Fujishima. In 2006 and 2007 hewas a visiting researcher in the same laboratory. In 2009 he defended his doctorateof science degree in condensed matter physics, and in 2010 was made a Univer-sity Professor at Saint-Petersburg State University, Russia. He is a co-author of morethan 60 research papers in the field of photostimulated processes in heterogeneoussystems for more than 20 years. He is a member of an international team of IUPAC todevelop a “Glossary of terms used in photocatalysis and radiocatalysis”, a memberof the International Organising Committee of IPS Conferences for the PhotoenergyConversion and Storage. His current research interests are in the areas of exper-imental and theoretical studies of factors affecting the activity and selectivity ofphotocatalysts, spectral sensitization of photocatalysts, superhydrophilic propertiesof photocatalyst surfaces, development of self-cleaning and bactericidal photoactivecoatings and development of photo-luminescence chemical sensors.

1. Introduction

Industrial and domestic activities inherently release many toxicagents into air, water and soil ecosystems with such conse-quences that Society needs to confront and resolve. Two majorissues in the environmental arena need serious considerations:(i) removal/disposal of undesirable substances from chemical andnuclear process effluents, and (ii) recovery of substances from pro-

gressively leaner initial sources. No need to emphasize the need forsome major efforts in pollution abatement and cleanup. As a start,engineering processes such as (a) trapping methods, (b) chemicalconversion of wastes, and (c) process changes have been developedto attenuate polluted sites.

By definition, every industrial process and usage of resultingproducts by industrial, domestic and agricultural sectors inevitablygenerate environmentally unacceptable hazardous wastes: e.g.chemical solvents in manufacturing processes, textile mill efflu-ents, acids from glass production, and herbicides and pesticidesfor agricultural and domestic use. The production, use and dis-posal of industrial chemicals have led to contamination of soils andwaters, particularly when such chemicals have not been properlyconserved during the manufacturing process [1]. Of the billions oftons of regulated hazardous wastes produced annually, less than0.5% is recycled and only about 1% is incinerated [2]. The rest isusually dumped, or otherwise directly discharged into local ecosys-tems. Thus, handling and disposal of hazardous chemicals fromthe moment of their manufacture to their ultimate disposal havebecome a major environmental issue.

Microwave radiation has become, in the last 25 years, an activeheat source used principally to drive numerous chemical reactions,particularly in organic syntheses. Such long wavelength (100–1 cm)microwave energy is not likely therefore to be used to drive photo-chemical reactions directly, which typically necessitate UV–Visibleradiation in the 200–700 nm wavelength range. A question recentlyentertained, therefore, was whether the role of microwaves wasto be relegated solely to being a source of heat [3]. It was arguedand illustrated that they do not have to be since photochemicalreactions can be driven, albeit indirectly, by microwaves using theUV light (180–400 nm) emitted from certain gas-fills excited bymicrowave radiation in microwave discharge electrodeless lamps(MDELs) [4].

To place the use of microwaves on processes occurring inaqueous TiO2 dispersions in perspective regarding environmentalapplications, we discuss first various sources of pollutants and sub-sequently describe various advanced oxidation processes such asUV/peroxidation, UV/ozonation and the photo-Fenton process(es)in addition to direct photolysis either by sunlight or by artificiallight sources.

1.1. Sources of pollution

A major source of contamination is accidental spillage, orotherwise, on soils and waters, as exemplified by (some times)questionable manufacturing practices. Disposal of toxic substances,residues, solvents and the like are the major issues that indus-tries and commercial establishments face continually. For instance,phosphate-containing detergents used in laundering of clothes areoften discharged in the sewage system and ultimately enter rivers,lakes and streams. Although it is practical to consider soil and wateras two distinct ecological systems, suspended soil particles in waterrepresent the interface between the two that provide a pathway bywhich contamination of one implies contamination of the other. Nocomponents or systems that make-up the real world can be consid-ered as being isolated from one another. They are all inter-related(Fig. 1) [1].

Agricultural practices are also not without problems. Pesticides(control of insects), herbicides (control of weeds) and fertilizers(control of plant growth and productivity) inevitably enter theaqueous ecosystem. Domestic and urban utilization of detergents,herbicides, and pesticides along with the disinfection of swimmingpools with chlorine-based chemicals inevitably lead to chlorinationof organic matter in nature, with formation of potential carcinogens(e.g. chloroform and other trihalomethanes, haloacetic acids andhaloacetonitriles [5]). This problem notwithstanding, chlorination


116 N. Serpone et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 11 (2010) 114–131

Fig. 1. Relationships between various environmental components.Adapted from ref. [1].

remains the most popular disinfection process for drinking waterbecause of its ability to kill pathogens.

Patterson has noted that some 10.2 million metric tons of toxicsubstances were released into the environment in 1987 [6]; thesewere partitioned as summarized in Table 1.

Unregulated and unsatisfactory disposal methods present long-term unknown consequences of pollutants to health and to Earth’secology. Drinking water and breathing air are the first lines ofcontact of people to polluting toxic substances. Relevant prod-ucts of some concern include solvents, volatile organics, dioxins,dibenzofurans, herbicides, pesticides, polychlorinated biphenyls,chlorophenols and chlorinated volatile organic compounds (VOCs).The latter are of particular significance since they are used widelyas metal degreasing agents, as refrigerants and aerosol propellants,as paint removers, as starting materials for the manufacture of fluo-rocarbons, as well as in extraction processes. Some chloroaliphaticsare toxic: e.g. CHCl3 and CCl4 are hepatotoxic and nephrotoxic sub-stances, and with CH2Cl2 are also teratogenic and embryotoxic; inaddition CHCl3, CCl4 and 1,2-dichloroethane are carcinogenic [7].

An additional and not insignificant source of environmentalpollution originates from consumers’ utilization of fuels for trans-portation and heating, and the use of pesticides, herbicides andfertilizers, detergents, aerosol sprays, paints and varnishes andtheir corresponding solvents. Because of people’s insatiable thirstfor potable drinking water, a broad range of compounds existswhich are transformed into potentially hazardous substances dur-ing water treatment, particularly when chlorination is used: e.g.formation of CHCl3 [8]. Of particular concern is the situation ofgroundwater because of the relatively rapid time normally associ-ated with the migration and subsequent appearance of pollutantsafter their initial entry into the environment [9,10].

1.2. Conventional abatement of environmental pollution

In any process designed to detoxify wastewaters, the useof alternative raw materials, newer processes and newer tech-nologies are typically considered to minimize the quantities ofpollutants discharged in waste effluents. Some of the classical non-

Table 1Example of the amount of toxic chemicals released in the United States in 1987 [6].

Route Million metric tons Percentage

Atmospheric 1.28 12Surface waters 4.49 43Underground injection wells 1.48 14Municipal wastewater treatment plants 0.46 8Off-site waste treatment/disposal 1.26 12Landfills 1.19 11

Totals 10.16 100

Table 2Principal classical physicochemical and chemical processes for the purification ofwaste-waters [11].

Physicochemical process mainly for Chemical process

Dispersed substances Substances in truesolutions

Flocculation Stripping Ion exchangeUltrafiltration Evaporation Precipitation

Electrophoresis Reverse osmosis HydrolysisElectrodialysis NeutralizationAdsorption Oxidation

Extraction Extraction Wastewater residueincineration;Electrolytic reduction

photochemical means of treating industrial wastewaters are givenin Table 2 [11].

The extent of water pollution depends on the behavior andfate of aromatic and aliphatic hydrocarbons, and inorganic saltsdischarged into the environment. These substances can either bemineralized to CO2 (organics), recovered (metals), or otherwisetransformed to less harmful forms. An interesting example is theconversion of the herbicide atrazine to cyanuric acid [12]. Treat-ment of a mixture of pollutants may well necessitate a combinationof processes (Table 2). Most often, phase transfer methods areemployed: e.g. air stripping, evaporation and adsorption. Unfor-tunately, the latter three methods do not lead to global pollutionabatement.

The more conventional methods of liquid/solid extraction,reverse osmosis, air stripping and bio-treatment or granular acti-vated carbon can remove several toxic organic substances frompolluted sites. However, despite claims to the contrary, these clas-sical methods simply dispose the problem only to create a waste ina different ecosystem. For example, (a) air stripping removes VOCsonly to discharge them into the atmosphere; (b) reverse osmo-sis generates a reject stream of concentrated contaminants thatrequire further treatment; (c) granular activated carbon requireseither costly regeneration (through incineration) or burial in land-fills; (d) liquid/solid separation methods generate sludge that alsorequires further disposal treatments [13].

Chemical oxidations with permanganate, chlorine and chlorineoxide are also popular in treating organic residues, e.g. phenolsand similar derivatives in wastewaters. Hydrogen peroxide in thepresence of Fe2+ ions (Fenton’s reagent used at pH ≤ 3) can oxidizephenols and other benzene derivatives.

Novel methods of water purification have moved from phasetransfer processes to chemical and photochemical destruction ofcontaminants through ozonation for example. In this regard, uti-lization of engineered microbubble systems with ozone has provento be a powerful method to decompose effectively chlorinatedethylenes (e.g. tetrachloroethylene, TCE) in situ to harmless HCl andCO2 [14]. The reaction involves a low molar ratio of ozone to con-taminant molecule. The gas-phase ozonation of TCE is depicted inScheme 1. As the reaction proceeds, the dichloroformaldehyde pro-duced in steps 1 and 2 is hydrolyzed to HCl and CO2 in step 3. Bycontrast, the aqueous phase reaction of ozone with TCE (or othercontaminants) is much more complex involving, as it were, cleav-age of the C C double bond of TCE through formation of dipolarzwitterionic complexes that rapidly collapse to ozonides [14].

In an earlier extensive report [15], we described and dis-cussed at some length the fundamental science that underliesHeterogeneous Photocatalysis in particular and Advanced Oxida-tion Processes in general. In the present review we consider firstsome photochemical methods of pollutant destruction in whichphoton absorption drives the photochemical processes. Among



Scheme 1.

these are (i) light + ozone; (ii) light + H2O2; (iii) light + ozone + H2O2;(iv) light + photocatalyst + O2 (air); and (iv) advanced oxidation pro-cesses that implicate a semiconductor nanomaterial (e.g. TiO2) asa potential photocatalyst. Accordingly, we examine the effect oflight on these processes before we examine more extensively howmicrowave radiation may be used to assist such processes.

2. Solar and artificial light in direct photolysis

Photochemical disposal of environmental pollutants can beachieved by various processes, some of which can occur under solarradiation [16]:

(1) Absorption of sunlight by a compound R produces the excitedstate R*, which can then react with ground-state molecular oxy-gen (i.e. 3O2) and/or H2O, followed by its conversion to smallermolecules or else mineralized to CO2 (reaction (1)).

Rh�−→R∗3O2/H2O→→ intermediates →→ Products (e.g. CO2) (1)

(2) Compound R is adsorbed on sediments, Rads, which whenexposed to sunlight produces the R*ads excited state that subse-quently reacts with 3O2 and/or H2O to yield products (reaction (2)).Interestingly, adsorption of R on a metal-oxide surface red-shiftsits light absorption properties as attested by the toxic dibenzo-p-dioxins which photodegrades in sunlight when adsorbed on soils[17,18] and on grass foliage [19], even though the latter two sup-ports may themselves be colorless.

Radsh�−→R∗

ads

3O2/H2O→→→ Products (2)

(3) A donor system D present in the environment (e.g. humicsubstances [20]) can sensitize production of either singlet oxy-gen, 1O2, and/or •OH radicals, both of which can oxidize organicsubstrates to CO2 (reactions (3)–(5)).

Dh�−→D∗ 3O2/H2O→→→ 1O2 (3)

1O2 →→→→ •OH (4)

1O2 (or •OH) + substrate →→→ Products (5)

(4) Sunlight can be absorbed by a suitable photocatalyst (e.g.TiO2, WO3, ZnO and iron oxides) to produce various reactive oxygenspecies (ROS; e.g. O2

•−, •OH, HOO• radicals, and 1O2), which drivethe oxidation of organic substrates.

(5) Sunlight absorbed by Fe3+/hydroxo complexes in photo-Fenton chemistry yields •OH radicals (reaction (6)), which oxidizethe organic substrate to its mineralized products (reaction (5)).

Fe(OH)aq2+ h�−→Feaq

2+ + •OH (6)

Despite these interesting processes, we must recognize that thesunlight reaching the Earth’s surface cuts-off around 290–295 nm(Fig. 2 [21]) so that only compounds that absorb light at wave-lengths longer than 290–295 nm undergo the kind of reactions thatlead to photodegradations [22].

Fig. 2. Solar irradiance and photon flow as the number of photons per square meterper second in the near-UV, visible, near-IR and infrared regions of the AM1 solarspectrum on the Earth’s surface.

Adapted from ref. [21].

Direct solar photolysis suffers from the lack of absorption of sun-light by most organic pollutants, in addition to which sunlight isattenuated, and together with its relatively shallow penetrationdepth in natural aquatic ecosystems makes direct photolysis ofpollutants somewhat moot. For example, common pollutants suchas phenol (PhOH), p-cresol (4-MePhOH), and p-chlorophenol (4-ClPhOH) are transparent to sunlight, whereas pentachlorophenol(Cl5PhOH) absorbs sunlight slightly at the solar UV threshold andthus can be degraded to some extent by direct solar photolysis,especially in alkaline aqueous media [23]; see Fig. 3 [24].

Legrini et al. [25] have reviewed the literature for direct photoly-sis of several organics, mostly chlorinated hydrocarbons, irradiatedat 210–230 nm, 254 nm, and 313–367 nm, and under polychro-matic irradiation. The 254-nm radiation has proven rather usefulin the degradation of substituted aromatics. Direct photolysis atthe latter wavelength, however, is inefficient at disposing of chlori-nated aliphatics, and in general is inefficient compared to processesinvolving •OH radicals.

Ridding aqueous and air ecosystems of organic pollutantsrequires a method that essentially burns off the pollutant in a man-ner that parallels high temperature combustion. In this regard,ozonation and peroxidation have not proven efficient at mineraliz-ing organic pollutants to carbon dioxide. The process necessitatesa more potent oxidizing agent (e.g. •OH radicals) that is capable ofdriving this combustion.

Fig. 3. Absorption spectra (arbitrary units) of various aqueous phenolic substrates.Also shown is the AM1 solar emission spectrum in the long-wavelength UV region.

After Serpone [24]. Copyright (1994) by The Electrochemical Society.



Fig. 4. Absorption spectra of H2O2 and O3 in aqueous media together with the AM1solar emission spectrum.

Adapted from Oriel [29].

3. Advanced oxidation processes in homogeneous phase

3.1. The UV/H2O2 system

Ultraviolet irradiation of H2O2 at suitable wavelengths photo-chemically cleaves the O O bond to yield hydroxyl radicals,even though the process is considerably less efficient at utilizing UVlight than is the corresponding UV/O3 process (see below) becauseof the much smaller UV absorption by H2O2 (ε254 = 19 M−1 cm−1

[26]) compared to ozone (ε254 = 2850–3000 M−1 cm−1)—see Fig. 4.Although cleavage of H2O2 should theoretically produce 2 •OHradicals per photon absorbed, the quantum yield of •OH radicalformation, �(•OH), is only 0.50 at 254 nm [27,28] because of cagerecombination of the newly formed radicals. Moreover, this sys-tem presents some disadvantages in high flow-rate reactors whereshort residence times of the organic substance(s) to be oxidized incontact with the oxidant requires high rates of •OH radical gener-ation [26].

Formation of •OH radicals in the UV/H2O2 system is pH- andtemperature-independent [30], yet the practical efficiency of thesystem is pH-dependent and also depends on the quantity of CO3

2−

present, since HCO3− and CO3

2− ions are good •OH-radical scav-engers (reactions (7) and (8) [31]). Evidently, oxidation of an organicsubstance to CO2 has the potential to produce hydroxyl radical

•OH + HCO3− → H2O + CO3

• k = 8 × 106 M−1 s−1 (7)

•OH + CO32− → HO− + CO3

− k = 3 × 109 M−1 s−1 (8)

scavengers and thus reduce the UV/H2O2 system efficiency. Fig. 5illustrates the pH dependence of the mineralization of total organiccarbon (TOC) to carbon dioxide of leachates from a landfill site(TOC = 1260 mg L−1) by the UV/H2O2 process [32]. Germane to this,Legrini et al. [25] have provided an extensive list of various organicsubstances on which the UV/H2O2 process has been tested. Forexample, the level of dimethylphthalate in an aqueous solution wasreduced by three orders of magnitude from 50 mg L−1 to less than50 �g L−1 by dosing with ca. 125 mg L−1 H2O2, whereas to decreasean isophorone level of 60 mg L−1 to less than 50 �g L−1 necessitated250 mg L−1 H2O2 under otherwise identical irradiation conditions[33].

3.2. The UV/O3 system

In neutral aqueous solutions, ozone is chemically unstable,decomposing through complex chain reactions that yield ulti-mately small levels of •OH radicals. The half-life of O3 in water atpH 7 is ca. 103 s increasing to 104 s in the presence of 2 M HCO3

−

Fig. 5. pH dependence of the TOC mineralization during the photooxidation of alandfill leachate by the UV/H2O2 process.

Adapted from Weichgrebe et al. [32]. Copyright (1993) by Elsevier Science B.V.

ions [34]. Although UV/O3 is more efficient at using light energy(Fig. 4) and quantitatively yields H2O2 [35], only ca. 5% of the pho-tolyzed ozone yields •OH radicals [26]. In alkaline aqueous media,ozone reacts with OH− ions to produce the HO2

• and O2•− radicals

(reaction (9)).

O3 + OH− → O2•− + HO2

• (9)

Two factors affect the kinetics of ozone oxidation of an organicsubstrate: (i) concentration of ozone, and (ii) reactivity of the sub-strate toward molecular O3 and •OH radicals. Many substances areoxidized slowly with O3, the rate of oxidation depending on therate of formation of •OH radicals.

Fig. 6 illustrates and compares the fractional decay of chemi-cal oxygen demand (as COD/CODo) against irradiation time for alandfill leachate using various combinations of light, O3 and H2O2at pH 3 [32]. UV light alone, or otherwise H2O2 and O3 systemsare far less efficient than the combination of UV light and one ofthese oxidizing agents. Fig. 6 also illustrates results for the com-bined O3 + H2O2 oxidizing agents and also when this combinationis UV-irradiated. Note the greater efficiency of the latter system. Thecomplex chemistry that summarizes the principal events is illus-trated in Scheme 2. The choice of whether to use the UV/H2O2 orUV/O3 or the combination UV/O3/H2O2 depends on the nature ofthe wastewaters to be treated.

Based on •OH radical involvement, UV/O3 has proven to be anadvanced water treatment method that effectively oxidizes andmineralizes a wide range of toxic and refractory organic substrates,bacteria and viruses in aqueous media. This AOP is commonly usedto decolorize bleaching waters in the pulp and paper industry [36].

Fig. 6. Decay of COD for a landfill leachate using various oxidation methods.After Weichgrebe et al. [32]. Copyright (1993) by Elsevier Science B.V.



Scheme 2.

An extensive list of substances that have been degraded by theUV/ozone method has been reported by Legrini et al. [25]. The ini-tial chemistry behind the UV/ozone method is also elaborated inScheme 2 (k in M−1 s−1).

3.3. The traditional versus the novel ferrioxalate/H2O2photo-Fenton system

Another process that generates •OH (and HO2•) radicals is the

Fenton reagent in which Fe2+ reacts with H2O2 (reaction (10)). Theresulting Fe3+ acts as a catalyst, decomposes H2O2, and regeneratesFe2+ (reaction (11)); the process is optimal in the range of pH 3–5[37].

Fe2+ + H2O2 → Fe3+ + •OH + OH (10)

Fe3+ + H2O2 → Fe2+ + HO2• + H+ (11)

A relatively novel homogeneous method which couplesUV/visible radiation and the Fenton system is the novel photo-Fenton advanced oxidation process [38], which uses ferrioxalateto harvest up to ca. 18% of the UV/visible sunlight to generate•OH radicals (reaction (12) followed by (13)). The optimal pHsto oxidize the organics lie in the range 1.2–7.4 [39], as attestedby the •OH radical formation in the degradation of the herbicide2,4-dichlorophenoxyacetic acid (2,4-D). This novel photo-Fentonsystem outperforms the heterogeneous system based on anataseTiO2 or its modified forms, at least in photo-oxidations of organicsubstances; e.g. photodegradation of toluene and trichloroethylene(TCE) in aqueous media [40]. This arises because TiO2 is somewhatlimited in absorbing sunlight to ca. 3–5%.

Fe(C2O4)33− + h� →→→ Fe2+ + 6 CO2 (12)

Fe2+ + H2O2 → Fe3+ + •OH + OH− (13)

The novel photo-Fenton process also outperforms the tradi-tional photo-Fenton process based on UV–vis/Fe2+/H2O2, or theUV/H2O2 process, by a factor of about 3–30 in the treatmentof moderate to highly contaminated waters that contain suchsubstances as chlorobenzene, benzene/toluene/xylenes (BTX), 1,4-dioxane, and a mixture of methanol, formaldehyde and formic acid[41].

Particularly appealing is the coupling of this novel photo-Fentonsystem with TiO2 particles in the sunlight-induced photodegra-dation of water contaminants. Tests with phenol have resulted ingreater efficiencies for the destruction of total organic carbon, i.e.complete mineralization of phenol, than was otherwise possiblewith the ferrioxalate/H2O2 system alone [42].

Fig. 7. Proposed mechanism of the Fenton degradation of aromatic compounds inthe presence of the AV dye under visible irradiation.

Adapted from Ma et al. [43]. Copyright (2005) by the American Chemical Society.

Another novel photo-Fenton process was proposed recentlyby Ma et al. [43] who examined the influence of dyes on theFenton reaction of organic compounds under visible light irra-diation alone (wavelength > 450 nm). The presence of such dyesas Malachite Green (MG), Alizarin Violet 3B (AV), Alizarin Red(AR), Acridine Orange and Rhodamine-B (RhB) greatly acceler-ated the Fenton reaction of organic compounds such as salicylicacid, trichloroacetic acid and the sodium benzenesulphonate andbenzyltrimethylammonium chloride surfactants under visible irra-diation to their complete mineralization. Alizarin dyes that possessan anthraquinone structural unit displayed a greater overall effecton the Fenton reaction than did dyes that lack the quinone unit, e.g.Malachite Green.

Fig. 7 displays the reaction mechanism [43] for the degrada-tion and mineralization of the AV dye by the photo-Fenton reactionunder visible irradiation and based on the Fe3+/Fe2+ cycle sensitizedby quinone species involving electron transfer from the exciteddye to Fe3+. The presence of dyes in the photo-Fenton process suc-cessfully led to two cycles: (a) the quinone/hydroquinone analogcycle, and (b) the Fe3+/Fe2+ cycle induced by the excited dye, bothof which promoted formation of •OH radicals. As such, the oth-erwise slow Fenton process to degrade organic compounds wassignificantly accelerated, but with the unfortunate consequencethat the AV photosensitizer also decomposed. Hence, by drivingthe iron(III)–iron(II) cycle and the continuous generation of •OHradicals, the AV dye in effect played the role of a sacrificial electrondonor in this novel photo-Fenton process.

Some workers have suggested that photo-Fenton processesare more efficient than the heterogeneous photocatalytic processinvolving TiO2 and oxygen from the air. Despite certain limitationsof photon efficiencies, however, the heterogeneous TiO2/air sys-tem has some other advantages that make it an attractive advancedoxidation technology in other niches, particularly where both pho-tooxidations and photoreductions are desirable. An example is thesimultaneous recovery of silver through photoreduction of silverions and mineralization of organic developers through photooxi-dation of the photographic effluents (see e.g. ref. [44]).

4. Environmental photochemistry with TiO2: applicationsof heterogeneous photocatalysis

As an example of the use of Heterogeneous Photocatalysis[15], reaction (14a) (X = a halogen atom) summarizes the oxidativemineralization of haloorganics. In homogeneous AOP processes,



Table 3Examples of the oxidation number of carbon in selected organic compounds [45].

nC Compound

+4 CO2, CCl4, CF2Cl2+3 HOOC COOH, CCl3 CCl3, CF2Cl CCl2F+2 HCOOH, CHCl3, CO, CF3 CHBrCl+1 OHC CHO, CCl2 CHCl

0 C, HCHO, C6H12O6, CH3CCl3, C6H4O2 (benzoquinone)−1 C6H6, HOCH2 CH2OH−2 CH3Cl, CH3OH−3 C2H6

−4 CH4

dioxygen and oxygen-based radicals play the role of reactantstoward organic molecules, whereas in the heterogeneous photo-catalysis case involving a metal oxide such as TiO2, ambient O2can act as a scavenger of the photogenerated electrons (reactions(14b) and (14c)) to yield the superoxide radical anions, O2

•−. Whilethis simplified picture has some merit in many of the organiccompounds examined, reductive pathways are also crucial in thedegradation of some organic and nitrogen-containing compounds.Involvement of these reductive pathways depends on the initialaverage oxidation number of the carbon and nitrogen atoms in thesubstrates (Table 3) [45].

CxHpOqXy +{

x + p − y − 2q

4

}O2

TiO2,h�−→ xCO2 + yH+ + yX−

+{

p − y

2

}H2O (14a)

TiO2h�−→e− + h+ (14b)

O2 + e− → O2•− (14c)

OH− + h+ → •OH (14d)

Where oxygen is present in excess, photoassisted or pho-tocatalyzed reductions are less frequently encountered relativeto oxidations because the reducing power of photogeneratedelectrons is significantly lower than the oxidizing power of pho-togenerated holes, and also because the most reducible substratesdo not compete effectively with the ubiquitous pre-adsorbed oxy-gen for the conduction band electrons on the metal-oxide particlesurface (see e.g. references in ref. [46]).

Table 3 reports the formal oxidation number, nC, of carbon atomsin various organic substrates [45]; where the substrates containtwo or more carbons, the value of nC was the average over all thecarbon atoms. Oxidation numbers for H, X (halogen), and O weretaken as +1, −1 and −2, respectively. For mixtures, nC was takenas the weighted average of carbon oxidation states over all thecompounds present in the system estimated through Eq. (15) withci denoting the concentration of species in which carbon has theoxidation state (nC)i.

nC =∑

ci(nC)i∑ci

x (15)

Except for the unreactive CCl4 toward •OH radicals, oxida-tion of compounds with low nC values by photogenerated TiO2valence band holes or by •OH radicals (reaction (14d)) predomi-nates (increase of nC), whereas reductive pathways are dominantfor compounds with carbon having the highest oxidation number[45]. For the photodegradation of CCl4, CHCl3 and CH2Cl2 [47,48], nCvalues resulting from different C1 compounds present in the reac-tion mixture were estimated as a function of time resulting in plotssuch as those depicted in Fig. 8. A similar result was reported fordodecane [49]. Since the final product in the presence of oxygen wasCO2, nC reached +4 as occurred for dodecane and CH2Cl2, whereas

Fig. 8. Average oxidation number nC of the carbon in some organic compounds asa function of irradiation time in aerated solutions and pH 5.

After Pelizzetti and Minero [49]. Copyright (1998) by Elsevier B.V.

nC for CCl4 was invariant (+4 in CCl4 and +4 in CO2). Nonetheless,an initial decrease of nC was observed for CCl4 inferring that thereduction pathway predominates largely in the early part of theprocess.

The ability of photocatalysis to achieve degradation of organicsby concurrent oxidative and reductive processes has been demon-strated amply by the degradation of chloromethanes in aqueousTiO2 suspensions, even in the absence of oxygen. In this case, theaverage oxidation number of carbon was expected to undergo lit-tle, if any change as may occur by consecutive redox reactions withe∼ and •OH, or with •OH and e∼ formally equivalent to (photo-induced) hydrolysis reactions. The latter have been demonstratedto be 6–8 orders of magnitude faster than the corresponding ther-mal hydrolyses of chloromethanes.

5. Microwave radiation in advanced oxidation processes

5.1. General observations

Microwave radiation has found its niche in several domes-tic, industrial and medical applications. Interesting studies haveexplored applications of microwave radiation (a) in organic syn-theses, as well as in polymerization and dehydration processes; (b)in inorganic syntheses by ceramic calcinations and solidification;(c) in environmental water treatments; (d) in safety and biologicalaspects; (e) in analyses and extraction processes; and (f) not leastin food sterilization. Microwaves can also decompose nondegrad-able materials from wastes. Microwave plasma has been used foretching materials, in chemical vapor deposition, and in surface pro-cessing of polymers and semiconductors, among others. It has evenfound a niche in the removal of NOx gases and in the degradation ofchloro- and nitro-aromatics in an electrohydraulic discharge (seee.g. ref. [50] and references therein).

The dynamics of chemical reactions (e.g. organic syntheses)that are thermally driven by conventional heating methods can beenhanced significantly by microwave irradiation [51–53]. The useof this microwave energy source to drive organic syntheses wasnot explored until the mid-1980s, i.e. not until the first two reportsby Gedye et al. [54] and Giguere et al. [55] in 1986 on microwave-enhanced organic chemistry. Since then, many organic chemistshave discovered the benefits of microwaves to drive synthetic reac-tions leading some industries to manufacture microwave ovensspecifically targeted to research laboratories. Since these early days,the number of published articles on the use of microwaves todrive chemical reactions has witnessed an astronomical growth[56]. As an example of the rapidity of microwave-assisted chemical



reactions, Mingos and Baghurst [57] estimated that under certainconditions a first-order process that would typically require 13.4 hto reach 90% conversion under reflux at 77 ◦C could be carried outin ca. 1.6 s when performed at 227 ◦C under microwave irradia-tion. Fast heating and high temperatures achievable in microwavechemistry has meant that most reported rate enhancements couldbe attributed to simple thermal/kinetic effects [58]. Of some rele-vance, but nonetheless not fully understood in the present state ofknowledge, are the rapid organic syntheses induced by microwaveradiation under cooled ambient conditions [59,60]. In this regard,essential features of microwave effects have yet to be clarified andfully understood. Hence, additional investigations are necessary toclarify how microwaves interact with reacting substrates as theymay be relevant in applications to environmental remediation pro-cesses.

Heterogeneous processes that occur in photoassisted andmicrowave-assisted reactions at solid/solid, solid/liquid andsolid/gas interfaces represent a novel and attractive utilizationof the two technologies. For instance, the floral and food storageindustries use ethylene as a ripening gas, which often leads to pre-mature aging of fruits, vegetables and flowers. Accordingly, suchindustries have been interested in how ethylene degrades at lowconcentrations in low-temperature environments. In this regard,Kataoka et al. [61] coupled microwave irradiation with UV-inducedheterogeneous photo-oxidation processes to degrade ethylene inthe gas phase on TiO2/ZrO2 mixed-oxide thin films examining thedynamics of the photo-oxidation as a function of water concen-tration by perturbing the presence of water on the surface of themixed metal-oxide photocatalyst using two types of reactor assem-blies. In the first assembly, which used microwave plasma lighting,microwave irradiation increased the rates of photooxidation by15–27% in studies conducted at 15% relative humidity; the effectwas negligible at 0 and 5% relative humidity. By contrast, with alight source external to the microwave chamber, the degradationof ethylene was 84% faster on injection of 3 �L of liquid waterin the presence of microwave irradiation relative to the photo-assisted treatment alone. At the solid/gas interface water on thesurface of the metal oxide had an impact on the dynamics of photo-oxidations that originated from two likely phenomena: (i) watermolecules scavenge the photogenerated holes before their recom-bination with photogenerated electrons, thereby producing •OHradicals that effectively oxidize the reactants, and (ii) in the pres-ence of sufficient quantities of water, the water molecules coverthe surface of the catalyst and compete with the reactants for theactive species (i.e. •OH radicals and/or reactive holes) [61]. Mea-surements of water adsorption revealed that microwaves facilitatethe removal of excess water from the metal-oxide surface treatedwith UV illumination at a relative humidity above 15%. However, noremoval of excess water was seen at a relative humidity below 5%.Contact angle measurements indicated that the microwave effectalone increased the wettability of the TiO2/ZrO2 catalyst surface(films), the contact angle decreasing from 15◦ to 4◦, i.e. the sur-face was made more hydrophilic by the microwaves. By contrast,UV irradiation of the mixed-oxide film led to a greater decreaseof the contact angle from 15◦ to less than 1◦. When the film wassubjected to both UV and microwave radiations, the contact anglewas 4◦, inferring that the involvement of microwaves under pho-tocatalytic conditions made the film surface slightly hydrophobic.A similar effect was seen for naked TiO2 particles.

In a series of parallel studies carried out in the early part of the2000–2010 decade, Horikoshi et al. [50] sought to gain a funda-mental understanding of the science that underlies the couplingof microwave radiation with heterogeneous photocatalysis witha particular emphasis on processes occurring at solid/liquid inter-faces. In particular, the objective was two-pronged: (i) examine themicrowave thermal effects arising from the temperature increase in

Fig. 9. Experimental reactor configuration for separate and coupled UV andmicrowave irradiation of aqueous TiO2 dispersions containing various substrates.

After Horikoshi et al. [50]. Copyright (2002) by the American Chemical Society.

the solid/liquid dispersion, and (2) examine possible non-thermaleffects in which the microwave radiation might cause changes tothe nature of the metal-oxide surface. Accordingly, a variety of sub-strates at solid/solution interfaces were examined using variousreactor configurations that coupled UV radiation with microwaveradiation. A simple reactor configuration system is illustrated inFig. 9.

Variance in reactor geometry allowed for an examination of thefate of various pollutant model substrates by no less than fourdifferent methods, so as to compare the photocatalytic methodwith the microwave technique: (1) microwave irradiation of theTiO2 dispersion alone (MW method); (2) UV irradiation of the TiO2dispersion alone (PD method); (3) coupled microwave and UV irra-diation of the TiO2 dispersion (PD/MW method); and (4) sincemicrowave irradiation typically leads to heating the dispersion,the effect of temperature (heating) on the photocatalytic degrada-tion process was also examined by applying an external source ofheat (PD/TH method), typically a silicone oil bath. An aqueous TiO2dispersion absorbs nearly 99% of the microwave radiation. Table 4summarizes results for the various substrates examined with theabove four methods. Taking the cationic rhodamine-B (RhB) dye asan example of a substrate in an aqueous TiO2 dispersion subjectedto the PD/MW method, the decomposition was more efficient thandegradation by the PD method alone. Fig. 10 illustrates variationsin the UV/visible absorption spectra of RhB under various condi-tions for the PD and the PD/MW methods, and in the presence andabsence of oxygen. Clearly, the PD method had little effect on thedye whether or not oxygen was present, whereas the integratedPD/MW coupled technique was far more efficient in discoloringthe dye solution initially through de-ethylation.

With few exceptions, the integrated PD/MW method alsoproved superior for other pollutant chemical systems. The greaterefficacy of the PD/MW technique was the result of at least threefactors: (i) enhanced formation of reactive oxygen species (e.g. •OHradicals) on the TiO2 surface, as attested by DMPO spin-trap ESRmethods and •OH radical attack on the dye; (ii) possible changesin the activity of bulk water, and (iii) changes that take placeat the TiO2 particle surface caused by the microwave radiation.



Table 4Survey of relative efficiencies between the UV/TiO2 photoassisted degradationmethod (PD) versus the integrated PD/MW method for the mineralization of varioussubstrates after 1–2 h monitored by total organic carbon assays [50].

Model substrates Relative degradation rates

Diethyl phthalate PD/MW � PDPhthalic acid PD/MW ∼= PDBenzoic acid PD/MW ∼= PDSuccinic acid PD/MW > PDPhenol PD/MW ≥ PDSodium benzenesulphonate PD/MW < PDPhenyltrimethylammonium chloride PD/MW ≥ PD4-Nonylphenol polyethoxylate (NPE-9) PD/MW < PDAtrazine PD/MW > PDBisphenol-A PD/MW < PDRhodamine-B cationic dye* PD/MW � PDMethyl orange (anionic dye)* PD/MW > PDPolyacrylamide** PD/MW > PDPolyacrylic acid** PD/MW < PDPolyvinyl alcohol** PD/MW � PDPolyethylene glycol** PD/MW < PDPolyvinyl pyrrolidone** PD/MW ≥ PD

* By visual discoloration of dye.** By gel permeation chromatography.

The PD/MW degradation method also displayed greater efficacyat low concentrations of O2 and at low irradiances. Evidently, ina concerted system of microwave radiation plus UV/TiO2 photo-catalysis, photodegradation of organics can take place even for asmall amount of TiO2, for a relatively weak UV light source, and forlow concentrations of oxygen. Thus, it is conceivable to treat largequantities of pollutants in wastewaters by a hybrid combination ofthe microwave method and the photocatalytic technology.

Another reactor configuration was investigated to improvethe efficacy of the process, in which UV radiation was sup-plied by an electrodeless mercury/neon light source powered bymicrowave radiation in a double-quartz cylindrical plasma pho-toreactor (DQCPP); see refs. [62,63]. Wavelengths emitted by theplasma of the electrodeless light source are reported in Fig. 11. Thetwo reactor configurations are illustrated in Fig. 12a and b.

The characteristic features of the DQCPP photoreactor wereassessed [63] by revisiting the photodegradation of the RhB dyein aqueous TiO2 dispersions irradiated simultaneously by bothmicrowaves and UV/visible radiation emitted from a microwave-powered (MW, 2.45 GHz) electrodeless mercury/neon lamp. Thenaked DQCPP and the water-cooled DQCPP reactor absorbed morethan 50% of the microwave radiation (50–88 and 50–75%, respec-

Fig. 10. Absorption spectral variations in the photodegradation of RhB by the PDand PD/MW methods with and without oxygen.

After Horihoshi et al. [50]. Copyright (2002) by the American Chemical Society.

Fig. 11. Ultraviolet and visible wavelengths emitted by the DQCPP light source.Intensity is given in arbitrary units (a.u.).

After Horikoshi et al. [62]. Copyright (2002) by Elsevier Science B.V.

tively), whereas the emitted irradiance scaled sublinearly withapplied microwave power. With respect to the naked DQCPP lamp,loss of irradiance by the water-cooled DQCPP lamp was ca. 28–46%at 250 nm and about 41–58% at 360 nm in the range of microwavepower 74–621 W. The smallest loss occurred at ∼179 W, at which

Fig. 12. (a) Experimental setup of a double-quartz cylindrical photoreactor (DQCPP)used in the photocatalytic decomposition of RhB dye in aqueous TiO2 dispersionsusing integrated UV/visible and microwave (MW) radiations. (b) Similar experimen-tal apparatus equipped with a UV/visible light source (250-W Hg lamp) and a sourceof MW radiation.




Fig. 13. Thermographic images indicating the effective temperatures in (a) a naked DQCPP device and (b) in a DQCPP system containing an aqueous TiO2 dispersion. Bothwere microwave-irradiated for 5, 15, and 60 min. Highly intense mercury lines were seen at 365, 404, 435, 547, and 579 nm (those below 365 nm were more than 10 timesweaker—see Fig. 11).


the degradation of RhB was subsequently examined. Fig. 13 illus-trates the thermographic images of the plasma from the nakedDQCPP light source (Fig. 13a) and from the water-cooled DQCPPreactor containing an aqueous TiO2 dispersion (Fig. 13b)—thesewere taken with the thermograph at the position indicated inFig. 12a.

Usage of the TiO2/DQCPP/MW setup of Fig. 12a enhanced therate of decomposition of RhB dye in aqueous TiO2 dispersions(Fig. 14). The faster dynamics of degrading RhB with this setupwas due to the greater absorption of incident UV and visible radi-ation by the entire 19-cm length of the cylindrical DQCPP quartzreactor illuminated at all possible angles (360◦). By contrast, theradiation from the external Hg lamp (Fig. 12b) irradiated the dis-persion through the fiber optic light guide, which focused radiationonly on a small portion of the dispersion. The quantity of disper-sion irradiated with the latter setup was significantly smaller. As aresult, the extent of degradation of RhB was greater when irradiatedwith the DQCPP light source, even though it emitted significantlyless radiation. Unfortunately, measurements of quantum efficien-cies for a quantitative comparison were precluded by the nature ofthe devices [63].

5.2. Microwave effects on the decomposition of the RhB dye

The effect of microwave radiation in the photo-oxidations notedabove tacitly implied the involvement of thermal and non-thermalinteractions of the microwave radiation with the TiO2 particlesurface. It is not unlikely that the greater efficacy of the photodegra-dation process implicated non-thermal effects through formationof a greater number of •OH radicals (or some equivalent reactiveoxygen species). Accordingly, Horikoshi et al. [64] subsequentlyexamined the formation of •OH radicals in aqueous TiO2 disper-sions irradiated by UV light alone (photocatalytic degradation, PD),

Fig. 14. (a) Temporal variations in UV/visible spectral features in the degrada-tion of RhB (0.05 mM, aqueous media) in aqueous TiO2 dispersions using theTiO2/DQCPP/MW system. (b) Decrease of UV/visible spectra in the degradation ofRhB with the TiO2/Hg-lamp and TiO2/Hg-lamp/MW systems.




by microwave radiation alone (MW), and by simultaneous irradia-tion with both UV light and microwave radiation (PD/MW) using asetup in which the microwave generator was connected to the ESRcavity. Radicals that formed were detected by the DMPO spin-trapESR methodology by the classical 1:2:2:1 spectral signature of theDMPO–•OH spin-adducts.

The influence of microwaves on the degradation of RhB wasalso examined using the reactor assembly shown in Fig. 12b: viz. aTiO2/Hg-lamp setup and a TiO2/Hg-lamp/MW system [63]. About80% of RhB was photomineralized after 60 min on exposing theaqueous RhB/TiO2 dispersion to the DQCPP emitted radiation. NoUV/visible spectral features of RhB were evident at wavelengthsgreater than 250 nm after 30 and 60 min of UV/MW irradiation(Fig. 14a). By comparison, 60 min of UV/Hg-lamp irradiation orUV/Hg-lamp/MW irradiation showed significantly less degradationof the RhB dye (Fig. 14b).

Assessment of the effect of temperature originating fromabsorption of microwave radiation (i.e. thermal effects) determinedusing the TiO2/Hg-lamp/MW setup and the constant-temperatureTiO2/Hg-lamp/MWcool system showed that after 2 h of irradiationa greater quantity of RhB degraded with the TiO2/Hg-lamp/MWcool(ca. 45% more) and TiO2/Hg-lamp/MW (ca. 60% more) systems incontrast to the TiO2/Hg-lamp assembly (∼10%). Also, there was aMW-induced heat effect (ca. 30%) in degrading RhB when compar-ing results from the TiO2/Hg-lamp/MW and TiO2/Hg-lamp/MWcoolsystems. At longer irradiation times (5 h), however, the extentsof degradation of RhB with microwave radiation coupled to theTiO2/Hg-lamp systems were even more significant: ca. 70%, 85%and 40%, respectively; the MW-induced heat effect accounted forca. 50% degradation.

Microwave irradiation of water alone produced no •OH radicals,as was also the case for non-irradiated TiO2 aqueous disper-sions, whereas exposing the dispersions to microwaves produceda small quantity of the highly oxidizing •OH radicals. By con-trast, the 1:2:2:1 spectral signature of the DMPO–•OH spin-adductswas rather significant when UV light was used to activate theTiO2/H2O/UV dispersions. The number of •OH radicals produced forthe P-25 TiO2 specimen was nearly twofold greater than for pureanatase and rutile specimens. At the constant temperature of 18 ◦C,irradiation of the dispersions by both UV light and microwaves gen-erated a greater quantity of such radicals for the P-25 specimen,whereas the quantity of •OH radicals decreased for pure anataseand rutile. A fivefold increase in incident microwave power from3 to 16 W caused only a slight increase (1.4 times) in the numberof •OH radicals produced. Nonetheless, the increase was significantenough to increase the efficiency of the photooxidation of phenol[64]. A slight increase in temperature from 18 to 22 ◦C caused onlya slight decrease in the number of •OH radicals formed, so thatthe increase of temperature (thermal effects) due to microwavescould not account for the increase in radicals produced when theUV-irradiated P-25 dispersion was also microwaved. Rather, theincrease was due to some non-thermal interaction(s) between themicrowave radiation field and the TiO2 particle surface. Results alsoshowed that the mixed-phase P-25 TiO2 specimen was more photo-active than were the anatase and rutile specimens, whereas puresamples of the latter two TiO2 polymorphs were equally active, atleast for the two specimens used.

5.3. Degradation of the 2,4-dichlorophenoxyacetic acid (2,4-D)herbicide and microwave radiation effects

After establishing proof of concept in the first three studies of theseries that used rhodamine-B to test the methodology, the photo-degradation/-mineralization of the 2,4-D herbicide in aqueous TiO2dispersions was examined next with the dispersions subjected tosimultaneous irradiation by UV light and microwave radiation [65].

The characteristics of the process were monitored for the effect(s)of microwave radiation by examining the fate of each substituentgroup, namely the carboxylic acid group, the chlorine substituentsand the benzene ring of the 2,4-D structure. The thermal effect ofthe microwave radiation on the system was investigated by com-paring results from microwave dielectrically generated heat versusexternally applied heat. The possible effects that dissolved oxy-gen and absence of oxygen might have on the degradative processwere also examined. To obviate the issue of unequal UV radia-tion incident on the double-quartz cylindrical plasma photoreactor(DQCPP), the experimental reactor setup of Fig. 13a was modi-fied having two otherwise equivalent Hg lamps albeit powereddifferently—see Fig. 15 inserts a (top left) and b (bottom left).

Significant variations in the three methods examined were seenunder irradiation for the PD, PD/TH and PD/MW methods, withthe quantity of 2,4-D (cleavage of aromatic ring) decreasing inthe order PD/MW > PD/TH > PD after 60 min of irradiation. The PDroute was the slowest of the three processes; the PD/TH processwas slower than PD/MW even though the temperatures of thedispersion from the heat dielectrically generated by microwaveirradiation and from heat externally applied were identical in bothcases. The same was observed for TOC decay and for the dechlo-rination of 2,4-D. However, the dynamics of dechlorination by thePD route were the same as for the PD/MW route in the formationof chloride ions. It seems that the microwaves had no impact onthe dechlorination events, which were rationalized by dechlorina-tion occurring mostly through a reductive path involving electronsphotogenerated by the UV irradiation of the TiO2 dispersion (seereaction (14b)).

Microwave radiation effects predominated on the oxidativeroute to degradation of the substrate, but had relatively little impacton the reductive events taking place at the TiO2 particle surface.Factors that influence the faster dynamics of the oxidative degra-dation of 2,4-D by the PD/MW route relative to those by the PD/THroute were attributed to microwave non-thermal effects. Nonethe-less, both thermal and non-thermal effects impacted the oxidativedegradation of 2,4-D. Germane to this discussion, extensive stud-ies by Stuerga and Delmotte [66] and by Baghurst et al. [67] onmicrowave-assisted photochemical syntheses of various organiccompounds in homogeneous media have shown that in such casesthe faster dynamics also originate from a microwave non-thermaleffect attributed to changes in the pre-exponential factor A of theArrhenius equation: k = A exp(−Ea/RT). In the current context, how-ever, specific interactions of the microwave radiation with theUV-illuminated TiO2 particle surface cannot be precluded. Suchinteractions can give rise to generation of additional surface defectsthat may increase directly the number of •OH radicals (and/or tosome other but equivalent reactive oxygen species) formed in theaqueous dispersion. In accord with this notion, the model proposedby Booske et al. [68] to explain non-thermal effects suggests thatmicrowave radiation can couple with low (MW) frequency elas-tic lattice oscillations of the crystalline solid, thus generating anon-thermal distribution. In this way, ion mobility, and thus dif-fusion of charge carriers (in the present case) to the surface, canbe enhanced leading to increased formation of surface •OH radicalsand to increased concentration of electrons at the surface.

The relative concentrations of the two charge carriers (electronsand holes) at the surface of TiO2 are very sensitive functions of thesurface potential [69]. The model of Booske et al. [58] also impliesresonant coupling of microwave photons to weak-bond surfacemodes and to point defect modes, as well as non-resonant cou-pling to zero-frequency displacement modes. Clearly, the surfaceof the TiO2 particles can be perturbed significantly by microwaveradiation with the net result (experimentally observed) that thephotodegradation of polluting organic substrates is more efficientfor the integrated UV/MW irradiation method than UV irradiation



Fig. 15. Experimental setup of the DQCPP photoreactor used in the photodecomposition of 2,4-D in aqueous TiO2 dispersions using integrated UV and microwave radiations.

After Horikoshi et al. [55]. Copyright (2003) by Elsevier Science B.V.

alone. The greater efficiency of the MW-assisted photoprocess wasascribed [65] to a non-thermal effect of microwave radiation on thebreakup of the aromatic ring of 2,4-D (oxidation), but evidently noton the dechlorination process (reduction) for which MW radiationhad only a negligible influence, if any.

Scheme 3 summarizes the mechanism [65] for the initial stagesof the photodegradation of 2,4-D by the PD and PD/MW routesunder air-equilibrated conditions on the basis of intermediatesidentified by LC/MS techniques. The initial step in the PD routeinvolves cleavage of the C C bond in the CH2COOH group of 2,4-Dyielding formic acid and 2,4-dichlorophenoxymethanol (species II)by addition of an •OH radical to the RCH2 fragment (step A). Forthe PD/MW route, degradation of 2,4-D initially forms 6-hydroxy-2,4-dichlorophenoxyacetic acid (species I) by the same mechanismas the PD route (step C). Differences in the initial degradations wereattributed to the initial cleavage positions of the 2,4-D molecularstructure. In other words, the C C bond (bond energy, 347 kJ mol−1)was cleaved in the PD route, whereas the O C bond (352 kJ mol−1)was cleaved in the PD/MW method. To achieve efficient degrada-tion of polluting agrochemicals such as the 2,4-D herbicide (andother environmental substrates) clearly necessitates three basicelements: light, oxygen, and water. Of these, UV/visible radiationis no doubt the most important element.

The above discussion has shown that the dynamics of decompo-sition of organic pollutants using the TiO2-mediated method (PD)can be improved by simultaneous irradiation of the aqueous dis-persion by UV light and by microwaves (PD/MW).

5.4. A simple microwave device to degrade pollutants in aqueousmedia

A practical process to photodegrade environmental pollutantsin aqueous media can be achieved using a modified domesticmicrowave oven (Fig. 16) [70]. The performance of this devicewas examined using the photodegradation of 2,4-D {0.04 mM or8.8 mg L−1 (ppm); 3.8 mg L−1 in TOC} as the test process driven

by the integrated PD/MW method in an aqueous TiO2 dispersioncontained in a high-pressure Teflon batch reactor. The reactorintegrated a double-glass cylindrical plasma lamp (DGCPL) as thesource of UV/visible radiation that contained mercury gas and aminute amount of neon gas. The lamp was powered solely bymicrowave radiation. The irradiance emitted by this naked elec-trodeless DGCPL light source was ca. 2 mW cm−2 compared to anirradiance of ca. 12 mW cm−2 emitted in the wavelength range310–400 nm (maximal emission, � = 360 nm) by the external elec-trode Hg lamp used for comparison. The UV radiation from theDGCPL source fell entirely on the Teflon batch reactor and thus onthe aqueous solution or dispersion.

UV absorption spectroscopy monitoring (� = 204 nm) the tem-poral decrease of the concentration of 2,4-D showed thatthe PD/MW method caused a relatively faster and completedegradation of 2,4-D within ca. 20 min via zero-order kinetics(k = 2 × 10−3 mM min−1) compared to the PD method alone, whichrequired ca. 20 min to degrade nearly 59% of the initial quantity of2,4-D (k = 1.1 × 10−3 mM min−1). Although the degradation rate of2,4-D by the PD/MW route was nearly twofold faster than by the PDroute, the UV irradiance in the latter method was six times greater.Clearly, microwave radiation in the coupled PD/MW technique hada significant impact (nearly 10-fold) on the overall degradationefficiency.

The rate of dechlorination of 2,4-D by the PD/MW route wasslightly greater than by the PD method. Only one chlorine sub-stituent was released to the bulk solution (expected 0.04 mM,observed 0.02 mM). By contrast, the quantity of Cl− ions obtainedfrom the PD/MW method was rather small, and none was obtainedby microwave irradiation of the 2,4-D solution alone. The moreextensive study described above of the microwave-assisted pho-todegradation of 2,4-D using the simple device of Fig. 16 confirmedthe earlier conclusion that the greater efficiency of the MW-assistedprocess was mostly due to a non-thermal effect of the microwaveradiation in the breakup of the aromatic ring of 2,4-D (oxidation),but not apparently in the dechlorination process (reduction). How-ever, under the conditions used to degrade the 2,4-D herbicide in



Scheme 3.

the modified domestic microwave oven, microwaves did have someinfluence, albeit small, on the extent of dechlorination.

The degree of mineralization (TOC loss; initial TOC, 3.8 ppm) of2,4-D was ca. 49% with the PD/MW method and ca. 35% for thePD method. The extent of mineralization of 2,4-D was relativelyinsignificant when a TiO2/2,4-D aqueous dispersion and an aqueoussolution of 2,4-D were irradiated by microwave radiation alone.

5.5. Microwave thermal versus non-thermal effects inTiO2-assisted photodegradation

The above notwithstanding, it was relevant, indeed important,to examine the details of microwave-assisted photodegradationsunder UV and/or visible irradiation, so as to obtain some evidenceof the initial mechanistic steps and to probe more fully the non-thermal versus thermal effects of microwave radiation since (webelieve) the electromagnetic radiation is more than a mere heatsource. Accordingly, RhB was re-examined [71] using either orboth in combination UV/visible and microwave radiations by thefollowing techniques: (1) PD/MW with UV/visible and MW irra-diation, with UV irradiation only (� < 400 nm; PD/MWUV), withvisible irradiation at � > 400 nm (PD/MWvis), and at � > 480 nm(PD/MW480); (2) the PD method with UV/visible irradiation alone,with UV irradiation at � < 400 nm (PDUV), and with visible irradia-tion at � > 400-nm (PDvis), and with visible irradiation at � > 480 nm(PD480); (3) MW irradiation with no TiO2 particles present; (4)PD/TH method with UV/visible irradiation, with UV irradiation at� < 400 nm (PD/THUV), and with visible irradiation at � > 400 nm(PD/THvis), and at � > 480 nm (PD/TH480). The maximal tempera-ture reached in both the PD/MW and PD/TH methods was 147 ◦Cafter 15 min.

Under all irradiation conditions, the extent of degradationof RhB dye occurred in the following sequence: (unfilteredUV/vis light) � (� < 400 nm) > (� > 400 nm) > (� > 480 nm). For casesinvolving UV, UV/visible irradiation and visible irradiation alone(� > 400 nm), the rate of degradation decreased in the orderPD/MW > PD/TH > PD, whereas for irradiation above 480 nm, theorder was PD/TH > PD/MW ∼ PD. Microwave radiation alone had noeffect on the degradation of RhB even after 180 min of irradiation.

Dyes can be photodegraded, indeed photomineralized, byUV irradiation of suitable TiO2 dispersions with the process(es)

occurring through reactions (16)–(21), or alternatively though pho-tosensitization through reactions (22)–(28):

TiO2 photo-assisted degradation/mineralization:

TiO2 + h� (< 387.4 nm) → TiO2 (eCB− + hVB

+) (16)

hVB+ + OH− (H2O) → •OH (+H+) (17)

eCB− + O2 → O2

•− (18)

O2•− + H+ → •OOH (19)

•OH (or •OOH) + RhB →→ mineralized products (20)

hVB+ + RhB → RhB•+ →→ mineralized products (21)

Photosensitization in the presence of TiO2:

RhBads + h� → RhB∗ads (22)

RhB∗ads + TiO2 (eCB

−) → RhBads•+ + TiO2(eCB

−) (23)

TiO2(eCB−) + O2 → O2

•− (24)

O2•− + H+ → •OOH (25)

•OOH + O2•− + H+ → O2 + H2O2 (26)

H2O2 + O2•− → OH− + •OH + O2 (27)

RhBads•+ + O2 (and/or O2

•− and/or •OH) → intermediates

→→→ mineralized products (28)

In the photosensitization process, excitation of chemisorbedRhB at wavelengths longer than 480 nm yields singlet and tripletexcited states (reaction (22); RhB*

ads). Electron transfer fromexcited RhB*

ads to TiO2 particles causes rhodamine-B to be con-verted to radical RhBads

•+ cations, whereas the photogeneratedelectrons involved in reactions (24)–(27) (similar to reactions(16)–(21)) generate •OH radicals. Consequently, degradation of RhBcan also take place at wavelengths greater than 400 nm throughphotosensitization in which the dye essentially ‘digs its own grave’.

Mineralization of RhB (loss of TOC) depends on the wave-length of irradiation (i.e. on the cutoff filter used to access certainwavelengths). When no filter was used, the decrease of TOC inthe degradation of RhB followed PD/MW > PD/TH > PD > MW. In all



Fig. 16. (a) Photograph of the modified domestic microwave oven showing theTeflon batch-type reactor with a double glass cylindrical plasma lamp (DGCPL) usedin the photocatalytic decomposition of 2,4-D through microwave irradiation cou-pled with UV/visible irradiation emitted by the DGCPL source. (b) Schematic of thesetup illustrating some of the details of the reactor.

After Horikoshi et al. [60]. Copyright (2004) by Elsevier B.V.

cases, MW radiation alone had no effect on the loss of TOC. Visi-ble irradiation (� > 480 nm) also had no effect on the time courseof TOC loss. Thermal effects on the degradation of RhB were mostpronounced after 60 min of irradiation by the PD and PD/TH meth-ods. Interestingly, the difference in the rate of degradation betweenPD and PD/MW methods was greater than the difference in ratebetween the PD and PD/TH methods. Since microwave radiationcaused the temperature to increase, it was deduced [71] thatmicrowave non-thermal effects also played a non-insignificant rolein the degradation process, with the UV-irradiation-led opening ofthe rings in RhB being facilitated by microwave radiation, in part bythermal and in part by non-thermal effects. Conventional heatingwas less effective. Non-thermal microwave effects also assisted inthe further oxidation of intermediates. Note that microwave effectswere apparent only when microwaves were coupled to the TiO2-assisted photodegradation process occurring under UV radiation(reactions (16)–(21)).

Measurements showed that when P-25 TiO2 particles wereimmobilized on a Pyrex glass plate and then placed in the waveg-uide of the microwave apparatus, followed by irradiation withmicrowaves (power, 120 W) and with UV/visible light (irradiance,ca. 10 mW cm−2) the contact angle was ca. 18◦ [61]. With UV/visibleillumination alone, changes in the average contact angle were lessthan 4◦. Irradiation with integrated microwaves and UV/visiblelight rendered the TiO2 particle surface more hydrophobic. Suchhydrophilic/hydrophobic changes in the morphology of the TiO2surface likely involved changes in the population of surface hydrox-yls (OH− and OH2

+ groups) as a result of microwave irradiation.The cause for the increase in hydrophobicity was attributed tothe formation of micro-/nano-scale hot spots on the TiO2 surface.Unfortunately such hot spots could not be detected by thermo-graphic and infrared radiation thermometric methods [71].

Considerations of adsorption modes of RhB on TiO2 suggestedthat for the PD method, RhB adsorbs on the positively charged TiO2surface through the two oxygen atoms in the carboxylate functionthat bear the greater negative charge, with further assistance pro-vided by the repulsion between the positively charged nitrogenatoms and the positive TiO2 surface [71]. De-ethylation of RhB bythe PD method tended to be rather inefficient. Dimethylamine wasformed by cleavage of the C N bond in the C N(C2H5)2 fragmentof RhB. For the PD/MW method, the increase of the hydropho-bic character of TiO2 through microwave irradiation facilitated theadsorption of RhB through the aromatic rings aided by the threeoxygen atoms; i.e., RhB lies flat on the particle surface.

The principal intermediates generated in the degradation ofRhB by the PD/MW route were N-de-ethylated species; the aminogroups were converted mostly into NH4

+ ions. Formation ofNO3

− ions was negligibly small in all cases. Kinetics of forma-tion of NH4

+ ions increased in the order PD < PD/TH < PD/MWand PD/THUV < PD/MWUV. By contrast, formation of NH4

+ ionsoccurred only after 2 h of irradiation with the PD/MWvis method.The more rapid formation of NH4

+ ions with the PD/MWUV relativeto PD/MWvis calls attention to a slower formation of •OH radicalsunder visible irradiation. In this regard, comparison of reactions(16)–(21) that occur by UV light irradiation with those from vis-ible light irradiation (reactions (22)–(28)) showed that electrontransfer sensitization of TiO2 by excited RhB* involved a few moresteps that may have retarded the formation of oxidizing radicalspecies.

Regardless of the light irradiation used, the PD method causedonly mono-de-ethylation of RhB; diethylamine was also observed.The PD/TH method under UV/visible irradiation produced all thepossible de-ethylated intermediates together with diethylamine.With UV irradiation alone, similar intermediates were produced,whereas with visible irradiation the PD/TH method yielded onlytwo de-ethylated rhodamine intermediates but no evidence ofdiethylamine. By contrast, the PD/MW method under UV/visibleirradiation led to de-ethylation and degradation of the interme-diates in relatively short time with diethylamine being the onlyspecies observed after 1 h. Under UV irradiation alone, this methodproduced all the de-ethylated species that also degraded in rela-tively short time (about 2 h); only diethylamine was still observableafter this time, whereas with visible irradiation at wavelengthslonger than 480 nm degradation by the PD/MW method yielded thefirst three de-ethylated intermediates, but only after 3 h. However,no diethylamine was evident even after this time.

Comparison of the results for the PD/MW and PD/TH methodsled Horikoshi et al. [71] to deduce that microwave non-thermaleffects had an impact on both the extent and the efficiency of thephotodegradation of the RhB dye. Microwaves can also induce addi-tional localized defects on the TiO2 photocatalyst (e.g. creation ofanion vacancies, Va) at or near the particle surface, defects thatcould have a significant influence on the degradation kinetics.



Scheme 4.

With the knowledge that non-thermal effects were signif-icant in the degradation of RhB, the photodegradation of theendocrine disruptor 4,4′-isopropyldiphenol (bisphenol-A, BPA;0.10 mM) was undertaken [72] in aqueous TiO2 dispersions toconfirm what the thermal versus non-thermal effects of themicrowave radiation might be on the photoprocess driven byUV radiation for this particular substrate. Near-quantitative min-eralization of BPA occurred after 90 min of irradiation by thePD/MW and PD/TH routes, whereas the PD route accounted foronly 67% mineralization, under otherwise identical conditions.Microwave radiation alone on a solution of BPA (no TiO2) causedno loss of TOC, but led to a slight increase in pH (6.7 → 7.2)and formation of a small quantity of formic acid (ca. 0.005 mM)originating from oxidation of the isopropyl methyl groups aftera 30-min irradiation period. Scheme 4 displays the major stepsin the degradation of this endocrine disruptor. In addition toacetic acid and formic acids, other intermediates produced bythe other three routes were 4-hydroxy-acetophenone (PD/MW,PD/TH, PD), 4-hydroxybenzaldehyde (PD/MW, PD/TH), 4-hydroxy-phenylisopropanol (PD/TH, PD), phenol and hydroquinone (PD/TH),and 3-hydroxy-1,3,5-hexatriene (PD/MW). These mechanistic dif-ferences were attributed to variations in the adsorption mode ofthe BPA on the TiO2 particle surface [62].

An examination of the endocrine disruptors dimethyl phtha-late {DMP; two methoxycarbonyl functions (−COOCH3)}and diethylphthalate {DEP; two ethoxycarbonyl moieties(−COOC2H5)}, showed that microwave radiation alone hadno consequence on DMP but did cause significant degradationand partial mineralization of DEP (ca. 36%). The dynamics of TOCloss by the PD and MW routes were nearly identical. Conventionalheating of the DEP solution for 20 min in the absence of TiO2 to ca.140 ◦C caused 50% loss of DEP through hydrothermal degradation.Accordingly, microwave thermal effects were significant in thedegradation of DEP. Important differences were seen in the tem-poral variations of UV absorption features for the DEP and DMPsubstrates by the MW and PD routes. Cleavage of the benzenering by the photoassisted process (PD) was more effective than bythe MW method alone. Comparison of the PD and PD/TH resultsfor DMP and DEP indicated that the thermal factor was indeed

significant in the oxidative degradation of these two endocrinedisruptors. Non-thermal effects also played a role, especially forTOC loss of DEP, albeit less so for the mineralization of DMP.Similar considerations in the oxidative degradation of benzoic acidby the PD/MW route relative to PD/TH suggested that non-thermaleffects also promoted the process at the TiO2 surface.

That the dynamics of the oxidative degradation of DEP by thePD/MW route were faster than the total sum of the thermal (MWroute) and photodegradation (PD route) indicate the existence ofsome synergy between thermal and non-thermal effects in thecoupled PD/MW route. Specifically, the degradation of DEP by thePD/MW route was nearly sevenfold more efficient than by the PDroute.

The DMP substrate was not affected by microwave radiation,which suggested that microwave thermal effects alone are of lit-tle consequence for this substrate. Little difference was seen inthe degradation dynamics between the PD/MW and PD/TH routes.However, significant variations were evident between the PD andPD/MW routes and between the PD and PD/TH methods, indicat-ing that conventional heating assistance of the photoprocess ledto more efficient degradations of such substrates as DMP. What-ever the specific differences between the PD/TH and PD/MW routes,microwave non-thermal factors also led to a greater loss of TOC onirradiation for about 1 h.

The above results have led to more questions than answers onthe role of thermal and non-thermal factors in the microwave-assisted photodegradation of pollutants. Accordingly, a studyof the characteristic features of the degradation and adsorp-tion/desorption of carboxylic, aldehyde, and alcoholic functionsembodied in phthalic acid, o-formylbenzoic acid, phthaldehyde,and phenol on the TiO2 particle surface was also undertaken [73].

Phthalic acid adsorbs strongly on the positively charged TiO2surface (56%) and degrades rather efficiently by the PD/MW routecontrary to the PD route. Variations between PD/MW and PD/THdegradation routes were rather small. For the degradation of o-formylbenzoic acid, variations in efficiency followed the orderPD/MW � PD/TH > PD≫MW. Loss of TOC showed the greatestdifference between the PD and the PD/MW routes indicating sig-nificant thermal and non-thermal effects of microwave radiation



in both the degradation and mineralization of this substrate [73].The oxidative degradation of phenol displayed the least differencesbetween the PD/TH and the PD/MW methods, both of which variedgreatly relative to the PD and MW routes. Evidently, most of themicrowave effects for phenol were thermal effects in nature, withsome non-thermal microwave component appearing at relativelylonger irradiation times. Under microwave radiation, phthalic acid,o-formyl-benzoic acid, phthaldehyde, and phenol desorbed to someextent from the TiO2 particle surface, in keeping with an increasein the hydrophobicity of TiO2.

To account for the adsorption of each of the above substrateson TiO2 a model was proposed [73] based on results of calcula-tions of point charges and frontier electron densities. Accordingto the model, since the position of •OH radical attack on thearomatic rings was the same (ortho and para carbons), the rel-evance of thermal and non-thermal effects of the microwaveradiation was due to both the extent and mode of initial adsorp-tion of the substrates on the TiO2 surface, as the substrates haveto compete with water molecules for the adsorption sites. Thedynamics of degradation of phthalic acid, o-formylbenzoic acid,and phthaldehyde should follow the order by which they approachthe TiO2 surface. Experimental results showed that the extentsof adsorption and degradation correlated with the order phthalicacid � o-formylbenzoic acid > phthaldehyde. Another factor thatcould also have influenced the degradation dynamics of these sub-strates is the dipole moment of the particular substrate. The rolethat this factor may play when a polar substrate is subjected to amicrowave radiation field may therefore be of some importance.

Many of the substances examined were strongly polar,their dipole moments decreasing in the order phthalicacid ≥ o-formylbenzoic acid > dimethyl phthalate > diethyl phtha-late > benzoic acid > phthaldehyde ∼= phenol. No simple correlationwas discerned, however, between dipole moments and extent ofdegradation because the reaction occurred in a heterogeneousmedium of water and TiO2 particles in which several competingevents can take place, and several kinds of intermediates can beproduced during the degradation process.

The principles that underlie the characteristic heating ofmicrowave irradiation of polar substrates are the dipole orien-tation (dipole rotation) and ionic conduction. The orientation ofdipoles changes with the alternating change(s) of the directions ofthe microwave radiation field. In the case of polarization of a highlydielectric substance, such as a polar water molecule, the dipole isconsidered to float in a viscous fluid and is forced to align with theelectric field (Fig. 17a). However, when the dipole is subjected toa high frequency alternating electric field of the microwave radi-ation, rotation (reversing) of the dipole cannot adequately followthe rate of change of direction of the electric field. This usually leadsto a time delay, causing a substantial quantity of energy to changeinto heat. In dielectric substances, further generation of heat bymicrowave irradiation is most pronounced for substances havinglarge dielectric losses.

Under the experimental conditions used by Horikoshi et al.[73], the polar substrate(s) and water taking part in the degrada-tion process undergo dipole rotation (i.e. orientation polarization).However, whether rotation of the substrate occurs through align-

Fig. 17. Rotation of dipoles of (a) water and (b) benzoic acid (BA) on absorption of MW energy. (c) Iionic conduction: movement of positively charged TiO2 particles under aMW radiation field. (d) Expected adsorption and desorption of BA on TiO2 by simple Coulombic forces and non-thermal MW effects.




ment of its dipole with the field at the underlying microwavefrequency in water (2.45 GHz) is unknown. Accordingly, whetherthe substrate was heated internally also remains unknown(Fig. 17b). For the TiO2 particles, ionic conduction can occur becauseof the positively charged surface (see Fig. 17c). However, the TiO2particles cannot undergo the rapid motion necessitated by the high-frequency microwave radiation field. It is nonetheless possible thationic conduction on TiO2 particles may have caused some peculiarmotion and aggregation of TiO2 particles, which for the PD routecould change the size of the effective reaction area on the TiO2surface.

Adsorption and desorption events of the substrates and wateron TiO2 particles are typically governed by simple Coulombicforces and by non-thermal effects of the microwave radiation field(Fig. 17d). Horikoshi et al. [63] have speculated that the cause ofdiffusion of the substrates may be due, in part, to the formationof micro-/nano-scale hot spots on the TiO2 surface and, in part,to the non-uniformity of temperature of the microwave irradiatedheterogeneous medium. Clearly then, microwave effects can havean impact on the mechanism through which a model substrateundergoes oxidative degradation.

6. Concluding remarks

The results presented in this, albeit non-exhaustive, reviewdemonstrate the usefulness of a simple modified microwave ovenfor degrading a large variety of polluting substrates typically foundin aquatic ecosystems. Because of its relative ease of use and safety,the microwave oven can find applications in minimizing domesticand agrochemical products otherwise discarded into ecosystems.However, membrane filtering will be needed to reclaim theTiO2 particles. The modified microwave oven can also handle aflow-through reactor for continuous operation of relatively largervolumes of polluted feed. Certainly, the 2,4-dichlorophenoxyaceticacid herbicide could be disposed of in reasonably short time (ca.20 min) compared to other methods. The cost of operating such asimple device is expected to be relatively small as the only powerneeded is the electric power to drive the domestic microwave oven(i.e. about 700 W).

Large-scale treatments of polluted aquatic ecosystems are rel-atively scarce because of several unusual challenges, not least ofwhich is the low photodegradation efficiency when using TiO2 as apotential photocatalyst relative to remediation of volatile organiccompounds in air. This may be the result of several factors: (i)the concentration of pollutants in wastewater treatment may betoo high, and their chemical structures may be too complex incomparison to pollutants encountered in air treatments; (ii) pene-tration of UV light tends to be shallow in turbid wastewaters; (iii)wastewater organic pollutants are poorly adsorbed on the TiO2surface; (iv) the essential need of dissolved oxygen in photocat-alytic degradations; (v) removal of TiO2 particles after treatmentof wastewaters; (vi) the UV light (<387 nm) component in thesolar irradiance is only about 3–5%, so that photodecompositionsare somewhat inefficient with respect to total incident solar irra-diance reaching the Earth’s surface. In this regard, factors (i–iv)could be resolved by an integrated PD/MW degradation technique,whereas (v) could be overcome by immobilizing the TiO2 on var-ious supports, e.g. buoyant glass beads, fiberglass cloth, opticalfibers, glass vessels of various shapes and sizes, silica, glass pel-lets, and (not least) paper. In a photocatalytic process, issue (vi)can be resolved, in part, using TiO2 specimens doped with nitro-gen, carbon, or with some suitable transition metals (see ref. [71]and references therein), and in part by using TiO2 coupled to co-catalysts such as CdS taking advantage of the interparticle electrontransfer process (IPET, [74]) and transition metal co-catalysts based

on ruthenium(II) polypyridyl complexes (or similar dye antennas)as a means to increase utilization of solar photons. Most impor-tant, however, degradations can be promoted with the assistance ofmicrowave radiation even under adverse conditions: e.g. for smallquantities of TiO2, low concentration of dissolved oxygen, and lowlight irradiance.

Acknowledgments

One of us (NS) is grateful to Prof. A. Albini and his group at theUniversity of Pavia, Italy, for their hospitality during several wintersemesters in their laboratory. We also wish to acknowledge themany students, associates and colleagues named in the referencesfor their invaluable contributions.

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Microwaves in advanced oxidation processes for environmental applications. A brief review

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