2009.Heterogeneous Photocatalytic Degradation of Organic Contaminants Overtitanium Dioxide a Review of Fundamentals,

Journal of Photochemistry and Photobiology C: Photochemistry Reviews 9 (2008) 112

Contents lists available at ScienceDirect

Journal of Photochemistry and Photobiology C:Photochemistry Reviews

journa l homepage: www.e lsev ier .com/ locate / jphotochemrev

Review

Heterogeneous photocatalytic degradation of organic contaminants overtitanium dioxide: A review of fundamentals, progress and problems

Umar Ibrahim Gayaa, Abdul Halim Abdullaha,b,

a Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor D.E., Malaysiab Advanced Materials and Nanotechnology Laboratory, Institute of Advanced Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor D.E., Malaysia

a r t i c l e i n f o

Article history:Received 6 August 2007Received in revised form27 September 2007Accepted 15 December 2007Available online 18 March 2008

Keywords:SemiconductorTitaniaDegradationPhotocatalysisEcotoxicity

a b s t r a c t

Even though heterogeneous photocatalysis appeared in many forms, photodegradation of organic pol-lutants has recently been the most widely investigated. By far, titania has played a much larger rolein this scenario compared to other semiconductor photocatalysts due to its cost effectiveness, inertnature and photostability. Extensive literature analysis has shown many possibilities of improving the

Contents

1. Introduction . . . . . . . . . . . . . . . . . . .2. Basic principles of photocataly3. Mechanism of titania-assisted4. Titania versus existing photoc5. Effect of operational paramete

5.1. Light intensity . . . . . . . . .5.2. Nature and concentrati5.3. Nature of the photocat5.4. Photocatalyst concentr5.5. pH . . . . . . . . . . . . . . . . . . . . .5.6. Reaction temperature .

6. Methods of utilization of titania photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77. Trends in improving the activity of titania . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7.1. Novel photocatalyst preparations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.2. Combined operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

8. Miscellaneous . . . . . . . . . . . . . . . .8.1. Ecotoxicity of titania ph8.2. Future trends . . . . . . . . . .

9. Conclusion . . . . . . . . . . . . . . . . . . . .Acknowledgement . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . .

Corresponding author at: Departmenfax: +60 389435380.

E-mail address: [email protected]

1389-5567/$20.00 2008 Elsevier B.V. Adoi:10.1016/j.jphotochemrev.2007.12.003. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10otoprocesses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

t of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor D.E., Malaysia. Tel.: +60 389466777;

du.my (A.H. Abdullah).

ll rights reserved.efciency of photodecomposition over titania by combining the photoprocess with either physical orchemical operations. The resulting combined processes revealed a exible line of action for wastew-ater treatment technologies. The choice of treatment method usually depends upon the compositionof the wastewater. However, a lot more is needed from engineering design and modelling for success-ful application of the laboratory scale techniques to large-scale operation. The present review paperseeks to offer an overview of the dramatic trend in the use of the TiO2 photocatalyst for remediationand decontamination of wastewater, report the recent work done, important achievements and prob-lems.

2008 Elsevier B.V. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2sis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2photocatalytic degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2atalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4rs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5on of the substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5alyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6ation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 9 (2008) 112

1. Introduction

As recalcitrant organic pollutants continue to increase in air andwastewater streams, environmental laws and regulations becomemore stringent [1,2]. As a response, the development of newereco-friendly methods of destroying these pollutants became animperative task.Ultimately, researchactivities centredonadvancedoxidation processes (AOPs) for the destruction of synthetic organicspecies resistant to conventionalmethods. AOPs rely on in situ gen-eration of highly reactive radical species,mainlyHO by using solar,chemical or other forms of energy [3,4]. Themost attractive featureof AOPs is that this highly potent and strongly oxidizing radicalallows thedestructionof awide rangeof organic chemical substratewith no selectivity.

Among AOPs, heterogeneous photocatalysis has proved to beof real interest as efcient tool for degrading both aquatic andatmospheric organic contaminants [5]. Heterogeneous photocatal-ysis involve the acceleration of photoreaction in presence ofsemiconductor photocatalyst. One of themajor applications of het-erogeneous catalysis is photocapartial or total mineralisation of ginants to benign substances [6].with a partial degradation, the tusually refers to complete photoeralisation, essentially to CO2, H[7].

Titania photocatalysis also refeeffect was rst unfolded by theandHonda [8]. Theseworkers revetingbyphotoelectrochemical celltitania anode. Consequently, theysis extended to environmentalthe rst time reported the apploxidation of CN and SO32 in aSubsequent reports of photocatalal. [10] attracted more interest to

As part of the dawn, the photbutane reported by Izuml et al. [1tion mechanism based on hydronew chapter in organic synthesistodecomposition of organic comreaction parameters was reported

Heterogeneous photocatalysissince its infancy considering the hand books devoted bymany reseacations the basic photophysical prphotocatalysis is largely the same.the fundamentals of the heterogeof organic contaminants and repments. Although photocatalytic dfor destruction of both organic anof this paper is on organic contam

2. Basic principles of photocata

Heterogeneous photocatalysislarge variety of reactions: organitoreduction, hydrogen transfer, Oisotopic exchange, metal deposittherapy,water detoxication, gaseAmong these appearances titanicatalytic oxidation has received malternative method for puricatio

The basic photophysical and ping photocatalysis are already est

in many literatures [15,16]. Vinodgopal and Kamat [17] reportedthat the dependence of the rate of 1,3-diphenylisobenzofuran pho-todegradation on the surface coverage. In other words, only themolecules that are in direct contact with the catalyst surfaceundergo photocatalytic degradation.

Photocatalytic reaction is initiatedwhenaphotoexcitedelectronis promoted from the lled valence band of semiconductor photo-catalyst (SC) to the empty conduction band as the absorbed photonenergy, h, equals or exceeds the band gap of the semiconductorphotocatalyst leaving behind a hole in the valence band. Thus inconcert, electron and hole pair (eh+) is generated. The followingchain reactions have been widely postulated.

Photoexcitation : TiO2/SC+h e +h+ (1)Oxygen ionosorption : (O2)ads + e O2 (2)Ionizationof water : H2O OH +H+ (3)Protonationof superoxides : O2 +H+ HOO (4)

l forrolo

ctionctor ple oc

rm sual (H

siste

seding rfor tto adinguct:

nistiationme feratuiIIIOHurfacce-buivaer a

nd Seare inationd the lasbed bctroemp

iO2/Sd drers what ftalytic oxidation (PCO) to effectas phase or liquid phase contam-Even though degradation beginserm photocatalytic degradationcatalytic oxidation or photomin-2O, NO3, PO43 and halide ions

rred to as the HondaFujishimapioneering research of Fujishimaaled thepossibilityofwater split-having an inert cathode and rutileapplication of titania photocatal-frontiers. Frank and Bard [9] forication of TiO2 in photocatalyticqueous medium under sunlight.ytic reduction of CO2 by Inoue ettitania photocatalysis.o-kolbe decarboxylation route to1] and the suggestion of its reac-xyl radical generation opened a. The earliest description of pho-pounds and studies of effects ofby Kraeutler and Bard [12].has attracted constant researchigh number of excellent reviewsrchers [7,13]. Despitemany appli-inciple and physical chemistry inThepresent reviewsheds lightonneous photocatalytic degradationorts the important accomplish-egradation has broad generalityd inorganic compounds the focusinants.

lysis

is a discipline which includes ac synthesis, water splitting, pho-218O216 and deuteriumalkane

ion, disinfection and anti-cancerous pollutant removal, etc. [7,14].a-assisted heterogeneous photo-ore attention for many years as

n of both air and water streams.hotochemical principles underly-ablished and have been reported

The hydroperoxyl radicaproperty as O2 thus doubly p

HOO + e HO2

HOO +H+ H2O2Both the oxidation and reduthe photoexcited semicondution between electron and hoscavenge the electrons to foform the hydroperoxyl radic

3. Mechanism of titania-asdegradation

Titania has been widely ucharge carriers thereby inducrespectively [18]. Generally,alytic reaction as opposednegative [7]. The corresponstituent is formed as by prod

Many elementary mechain the photocatalytic degradsurface. The characteristic tibeen reported in previous lit

The {>TiIVOH+} and {>Tvalence band electron and strons, respectively. The surfa{>TiIVOH+} is chemically eqallowing the use of the form[20]. According to Lawless aa surface-bound OH radical

There exist a good correlics, their surface densities andegradation over TiO2. In thsions of TiO2 have been profemtosecond absorption speelectron scavenger has beenond spectroscopic study of Tand Bowman [23] indicatetion of trapped charge carriThe results also conrmed tmed in (4) also has scavengingnging the lifetime of photohole:

(5)

(6)

can take place at the surface ofhotocatalyst (Fig. 1). Recombina-curs unless oxygen is available toperoxides (O2), its protonatedO2) and subsequently H2O2.

d photocatalytic

as a photocatalyst for generatingeductive and oxidative processes,itania-assisted aerobic photocat-photosynthetic reaction G isacid HA of the non-metal sub-

(7)

c processes have been describedof organic compounds over TiO2or each elementary reaction hasre (Table 1).} represent the surface-trappede-trapped conduction band elec-ound OH radical represented bylent to the surface-trapped holend latter terms interchangeablyrpone [22] the trapped hole anddistinguishable species.between charge carrier dynam-e efciency of the photocatalytict two decades, aqueous suspen-y picosecond and more recentlyscopies [23,24]. Traditionally, anloyed in such study. A femtosec-CN aqueous system by Colomboamatic increase in the popula-ithin the rst few picoseconds.or species adsorbed to TiO2, the

U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 9 (2008) 112 3

Fig. 1. Schematic photophysical and photochemical processes over photon activated semiconductor cluster (p) photogeneration onation, (r) recombination in the bulk, (s) diffusion of acceptor and reduction on the surface of SC, and (t) oxidation of donor on the

hole-transfer reaction can successfully compete with the picosec-ond electronhole recombination process. The following interfacialphotochemical reactions were described:

Photoexcitation : TiO2 +h eCB +h+VB (8)Charge carrier trapping : eCB eTR (9)Charge carrier trapping : h+VB h+TR (10)

Electronhole recombination :

eTR +h+VB(h+TR) eCB +heBahnemann et al. [25] provide

carriers using ash radiolysis. Porganic molecules is due to the(Ti3+) and h+TR (presumably OHIn agreement with the foregoingtrapped carriers mainly exist nea

Table 1Primary processes and time domains in tipollutants [1921]

Primary process

Charge carrier generationTiO2 +h e +h+

Charge carrier trappingh+ + >TiIVOH {>TiIVOH+}e +>TiIVOH>TiOHIII}

e +>TiIV TiIII

Charge carrier recombinatione + {>TiIVOH+}>TiIVOHh+ +>TiIIIOHTiIVOH

Interfacial charge transfer{>TiIVOH+}+organic molecule

>TiIVOH+oxidized molecule{>TiIIIOH}+O2 >TiIVOH+O2

undergo rapid (1ps) recombinattation. The important consequencand electrons (eTR) to the phothave been highlighted by Serpon

In most applications, photocacarried out in presence of waterthe photocatalyst. The presencephotocatalysis. Earlier work oncated that the reaction did not p[29]. Fig. 2 shows the stages in thphotomineralisation of organic co

otenno

with

onp

dativtro-leasuH [3bsen

ons (holeadicaat (11)

d evidence for the trapped chargerimarily, the ssion of bonds ininteraction of the trapped eTR) pairs near TiO2 particle [26].

, Furube et al. [27] observed thatr the particle surface and do not

tania-catalyzed mineralisation of organic

Characteristic time

fs (very fast)

10ns (fast)100ps (shallowtrap; dynamicequilibrium)

Photoholes have great pdirectly (although mechanismrectly via the combinationsolution [15,30]:

H2O + h+ OH + H+

RH + OH R + H2O

R + h+ R+ Degradati

The mediation of radical oxievidencedbyphoto- andelectrode in aqueous solutions mpotential and the solution poxidative species originally aunder anodic bias.

The primary photoreactiof charge carriers (electrontion. Essentially, hydroxyl r10ns (deep trap)

100ns (slow)10ns (fast)

100ns (slow)

ms (very slow)

ions (O2) and hydroperoxyl radintermediates that will act concety of organic pollutants includ(VOCs) and bioaerosols [32,33]. Itally that the oxidative reactionoccurs mainly via the formationof 5.7102) not hydroxyl rad7105) [34].f electron/hole pair, (q) surface recombi-surface of SC particle.

ion immediately after photoexci-es of surface trapped holes (h+TR)oxidation of organic compoundse et al. [28].talytic degradation reactions are, air, the target contaminant andof water is indispensable in TiO22-propanol photooxidation indi-roceed in the absence of watere photoinduced processes of thentaminant in presence of TiO2.tial to oxidize organic speciest proven conclusively [6]) or indi-

OH predominant in aqueous

(12)

(13)

roducts (14)

e species in photooxidation wasuminescence spectraof TiO2 elec-red as functions of the electrode1]. It was found that the radicalt accumulated after illumination

1)(11) indicate the critical rolepair) in photooxidative degrada-ls (OH), holes (h+), superoxide

icals (OOH) are highly reactiveomitantly to oxidize large vari-ing volatile organic compoundst is however argued experimen-on titania photocatalyst surfaceof holes (with quantum yield

icals formation (quantum yield


Fig. 2. Conceptual diagram for the primary processes involved in photomineralisation of organic compounds.

As aphotochemical application, TiO2 photocatalysis is invariablyaffected by the surface properties of the TiO2 particle. The photoin-duced phenomenon is affected by quantum size. Anpo et al. [35]observed blue shift and increase in reaction yield and photocat-alytic activity as the diameter of the TiO2 particles becomes smallerespecially below 100 A. This observation was attributed to the sup-pression of radiationless transfer and the concurrent enhancementof the activities of the charge carriers.

The reaction mechanism for the photooxidative degradation ofmany organic pollutants over titania particle has been extensivelyreviewed [19,20,36]. The number of intermediates in the reac-tion and ease of decomposition depends upon the nature organiccontaminant studied. The photocatalytic degradation of methanaland phenol are interesting mechanistic examples on the role ofhole, superoxide and hydroxyl radicals in titania-assisted pho-tomineralisation of aliphatic and aromatic organic templates. Themechanisms are illustrated in Figs. 3 and 4, respectively.

In the degradation of phenol,reported (Fig. 4). The OH radicaphenol (a), yielding catechol (b), r(d) andhydroquinone (e), then thebreak up to givemaleic acid (f), thas, 3-hydroxy propyl carboxylic2-hydroxy-ethanoic acid glycol athoughH produced during the at

Fig. 3. Photocatalytic oxidation o

in the process, it is scavenged bwhich nally convert to OH radi

The kinetics of photocatalytpounds usually follows the L[16,19,20].

r = dCdt

= kKC1 + KC

where r represents the initial ratetration of the reactant, t the irraof the reaction and K is the adsorAt mM concentrations C1, theapparent rate order equation [39,

ln C0C

= kKt = Kapptor

rstC vellutanationrst o

deraoved]. Ator thd nondedound

hoto

photes [7several intermediates have beenl attacks the phenyl ring of theesorcinol (c), benzene-1,2,3-triolphenyl rings in thesecompoundsen short-chain organic acids suchacid (g), 2-hydroxy propanal (i),cid (j), nally CO2 and H2O. Eventack of bonds by OH participates

Ct = C0 eKapp t

where Kapp is the apparentslope of the graph of lnC0/centration of the organic pocondition, the initial degradconforming to the apparent

r0 = KappCA quasi-exhaustive consi

of organic contaminants prtion above holds true [4146model serves as a basis fcompounds even if it coul[47]. Nevertheless, for suspedegradationof organic comp[48].

4. Titania versus existing p

An ideal photocatalyst forized by the following attribu

(1) Photo-stability.f formaldehyde over titania [37].

(2) Chemically and biologically in(3) Availability and low cost.y oxygen to form HO2 radicals,cals.ic degradation of organic com-angmuirHinshelwood scheme

(15)

of photooxidation, C the concen-diation time, k the rate constantption coefcient of the reactant.equation can be simplied to the40]:

(16)

order rate constant given by thersus t and C0 is the initial con-t. Consequently under the samerate could be written in a formrder rate law:

(17)

tion of photodestruction studiesthat the rst order rate equa-

any rate, LangmuirHinshelwoode photodegradation of organict directly give adequate ttingtitania-mediated photocatalytics pseudo-zerothorder is reported

catalysts

tocatalytic oxidation is character-]:ert nature.


(4) Capability to adsorb reactanttion (h Eg).

Many chalcogenide semicondCdS, MoS2, Fe2O3 and WO3 havetocatalysts for the degradation ominimum band gap energy requireration of charge carriers over TiOis 3.2 eV corresponding towavelenTiO2, photoactivation takes place

The photoinduced transfer oadsorbed speciesover semiconduband-edge position of the semicoof the adsorbates [30].

In spite of the constant vigordecades in search for an ideal phmodication has remained a bencing material candidate will be metitania is reported to give the beand photostability [6]. Nearly alltalline forms of titania namely anand Maggard [52] reported theamorphous titania with wider basignicant photocatalytic activitypositions for various semiconduc

5. Effect of operational parame

5.1. Light intensity

Photocatalytic reaction rate dabsorption of the photocatalyst [5in the degradation rate with incretocatalytic degradation. The natuaffect the reaction pathway [56sensitization mechanism does nodation.

Unfortunately, only5%of the tosufcient energy to cause effectiveenergy loss due to light reectionheat is inevitable in the photoproinvited more research in the app

ght athe q

actioof

2 iniationdeteid r

ic efle [6

eld orevio

terna3] beld cheredisag De

of t

can are suradatted tphenoroapheenerbenzdark

idatiFig. 4. Degradation of phenol in wastewater over nanomaterial titania [38].

s under efcient photonic activa-

uctors such as TiO2, ZnO, ZrO2,been examined and used as pho-f organic contaminants [49]. Theed for photon to cause photogen-2 semiconductor (anatase form)gth of 388nm [50]. Actuallywithin the range 300388nm.f electrons that take place withctorphotocatalyst dependson thenductor and the redox potentials

ous research activities over twootocatalyst, titania in its anatasehmark against which any emerg-asured [51]. The anatase form ofst combination of photoactivity

studies have focused on the crys-atase and rutile. However, Zhangpreparation of hydrated form ofnd energy gap than anatase and. The schematic diagram of bandtors is shown in Fig. 5.

ters

epends largely on the radiation3]. Refs. [54,55] revealed increasease in light intensity during pho-re or form of the light does not]. In other words, the band-gapt matter in photocatalytic degra-

tal irradiatednatural sunlighthas

tion. The overall quanta of lireactant is given by overall,

overall =rate of re

rate of absorption

as metal oxide such as TiOabsorb all the incident radexperimentally difcult tolight scattering in solidliquAnother factor limits photontion between electron and hothat reference to quantum yitem is ill-advised despite preferences [61,62].

A practical and simple alciencies was suggested [6efciency r. A quantum yifrom r, as = r phenol, wthe photocatalyzed oxidativesecondary actinometer) usincatalyst material.

5.2. Nature and concentration

Organicmoleculeswhichthe photocatalyst will be moThus the photocatalytic degsubstituent group. It is reporan adsorbing substrate than[65]. In thedegradationof chlout that mono-chlorinatedtri-chlorinated member. In gdrawing group such as nitroto adsorb signicantly in thedonating groups [67].

During photocatalytic ox

photosensitization [57]. Besides,, transmission and energy loss ascess [58]. This limitation largelylication of TiO2 to decontamina-

substrate over time is dependenAt high-substrate concentrationsdiminishes and the titanium dioleading to catalyst deactivation [6bsorbed by any photocatalyst oruantum yield:

nradiation

(18)

a heterogeneous regime cannotdue to refraction, it has been

rmine quantum yield [59]. Theegime particularly is signicant.ciency is the thermal recombina-0]. For these reasons, it is arguedr efciency in heterogeneous sys-us use of the term by previous

tive for comparing process ef-y dening a relative photonican subsequently be determinedphenol is the quantum yield forppearance of phenol (a standardgussa P-25 TiO2 as the standard

he substrate

dhere effectively to the surface ofsceptible to direct oxidation [64].ion of aromatics depends on thehat nitrophenol is much strongerol and therefore degrades faster

romatics,Huqulet al. [66]pointednol degrades faster than di- oral, molecules with electron with-ene and benzoic acid were foundcompared to those with electron

on the concentration of organic

t upon photonic efciency [68].however, the photonic efciencyxide surface becomes saturated9].


Fig. 5. The conduction and valence band sentsThe right hand scale is the normal hydrog

5.3. Nature of the photocatalyst

There is direct correlation betwface coverage of TiO2 photocatalythat the number of photons strcontrols the rate of the reactionthe reaction takes place only inconductor particle. A very impoperformance of photocatalyst in pface morphology, namely the pa[72].

Numerous forms of TiO2 havmethods to arrive at a photocataproperties, activity and stability foEvidently, there is a clear connecties, the rational development of ipossible usefulness of the materiaFor instance, smaller nano-particconversion in gaseous phase phopounds over nano-sized titanium

5.4. Photocatalyst concentration

The rate of photocatalytic reconcentration of the photocatalyreactions are known to showtodegradation with catalyst loadphotocatalytic application, themust be determined, in order tototal absorption of efcient phunfavourable light scatteringandthe solution is observed with exc

in this thpertIn tulskhaveia cafollo

ll remativeis reesspositions of selected metal oxide semiconductors at pH 0. The left hand scale repreen electrode scale which allows predictions based on reduction and oxidation.

een of organic pollutant and sur-st [70]. Kogo et al. [71] reportediking the photocatalyst actually. The latter is an indication thatthe adsorbed phase of the semi-rtant parameter inuencing thehotocatalytic oxidation is the sur-rticle size and agglomerate size

e been synthesized by differentlyst exhibiting desirable physicalr photocatalytic application [73].tion between the surface proper-mproved synthesis routes and thel prepared in application [74,75].le size is reported to give highertomineralisation of organic com-

5.5. pH

An important parameterplace on particulate surfacestates the surface charge proof aggregates it forms [80].of zero charge (pzc) by Kosmand 20% rutile is reported tocondition the surface of titanrespectively according to the

TiOH + H+ TiOH2+

TiOH + OH TiO +H2OThus, that titania surface wimedium (pH6.9). Titanium dioxideactivity at lower pH but excreaction rate [82].dioxide [76].

action is strongly inuenced byst. Heterogeneous photocatalyticproportional increase in pho-ing [77]. Generally, in any givenoptimum catalyst concentrationavoid excess catalyst and ensureotons [78]. This is because anreductionof lightpenetration intoess photocatalyst loading [79].

The effect of pH on the phocompounds and adsorption on Tstudied [83,84]. Change in pH caefciency of photoremoval of organiumdioxidewithout affecting thconditions better degradation ha

5.6. Reaction temperature

Experimental studies on theof degradation of organic compocarried out since 1970s [12]. Manimental evidence for the depenon temperature [8791]. Generathe internal energies to the vacuum level.

e photocatalytic reactions takinge pH of the solution, since it dic-ies of the photocatalyst and sizehe current update of the pointsi [81] Degussa P-25, 80% anatasepzc 6.9. Under acidic or alkalinen be protonated or deprotonatedwing reactions:

(19)

(20)

ain positively charged in acidicly charged in alkaline mediumported to have higher oxidizingH+ at very low pH can decreasetocatalytic reactions of organiciO2 surface has been extensivelyn result in enhancement of thenic pollutants in presence of tita-e rate equation [85]. At optimizeds been reported [86].

dependence of the reaction rateunds on temperature have beeny researchers established exper-dence of photocatalytic activitylly, the increase in temperature


enhances recombination of charge carriers and desorption processof adsorbed reactant species, resulting in decrease of photocatalyticactivity. This is in conformity with Arrhenius equation, for whichthe apparent rst order rate constant Kapp should increase linearlywith exp(1/T).

6. Methods of utilization of titania photocatalyst

Titania photocatalyst can be used either as free-standing par-ticulate or as coating on a substrate. Most experiments utilizednely powdered TiO2 particles suspended in contaminated water,which provides large surface area and makes recovery easy aftertreatment [92]. Larger particulates may prove useful even in thecase of gaseous organic contaminants but are rather commerciallyunavailable and may be costly [6].

Coated catalyst congurations, on the other hand, eliminate theneed for catalyst ltration and centrifugation but generally result ina signicant reduction in system efciency. A reduction of 6070%reduction inperformance is reporbilized TiO2 as compared to the u

Many kinds of support have balyst which include soda lime glatiles [95] and coated glass [96].actual active surface area of the phall volume is low. Despite aforemephotocatalysts and immobilisatioIn many of these cases TiO2 coateciency in organic compound rem

7. Trends in improving the activ

Despite drawbacks of titania ppounds have been successfullyviewpoint of air and water puriccontaminants have been studiedcompounds (EDCs) [97]. By far, tconstantly explored mainly to sution and to enhance the photoseapplication [58].

7.1. Novel photocatalyst preparatio

Due to the constraints involvetivation, there has been growingwavelengthof 388nmwhich correThe principal foci of these activiti

incorporation of energy levels i changing the life time of charge substitution of the Ti4+ with cat shifting the conduction band an

photoexcitation at lower energthe preparation method.

The major practices involvemetal coating, surface sensitizatidesign and development of secoface sensitization has been inteRecent studies indicate enhancetion of gaseous organic contamiphenol over uorinated titania susubstrate and reaction conditioncan be positive or negative [99].

Fig. 6. Modication of titania photocatalyst by metal doping.

Since 1980s TiO2 has been modied mainly by metal loadingor platinization to achieve better photocatalytic activity [100102].Successful doping can be achieved with either transition metal ionor with non-metal resulting in enhanced efcacy of the photocata-lyst system (Fig. 6). The last 4 years has attracted growing interestin doping of titania with Pt due to promising improvement in pho-tooxidation rate especially in gas phase. PtTiO2 has been foundto improve the photooxidation rate of ethanol, acetaldehyde andacetone in gaseous phase [103]. Nitrogen doped into substitutional

eforte

hephut thTi4+

typed Ni,um dcasee ba

sts Tandt beylighas t

hes a9,11aredshowcenthotolyticTiO2in thiO2 he phoy crylyticensiinteescrto de

h+)T

)

iO2 c136]ted in aqueous systems for immo-nsupported catalyst [39].een explored for TiO2 photocat-ss [93], aluminium [94], ceramicSince coatings are very thin, theotoreactor compared to the over-ntioned drawbacks, more coatedn techniques are still investigated.d on support assumed more ef-oval than uncoated TiO2 [18].

ity of titania

hotocatalysis many organic com-investigated largely from the

ation. A large number of organicincluding endocrine disrupting

he following avenues have beenppress electronhole recombina-nsitivity of titania for successful

ns

d in ensuring effective photoac-quest to go beyond the thresholdsponds to thebandgapof titania.es include [57,98]

nto the band gap of the titania,carriers,ion of the same size andd/or valence band so as to enableies; success of which depends on

catalyst modication by doping,on, increase in surface area or byndary titania photocatalyst. Sur-nsively reviewed elsewhere [7].ment of photocatalytic degrada-nants such as acetaldehyde andrface. Depending on the kind ofthe effect of surface uorination

sites of TiO2 has proven verydoped zirconia has been repeven titania P-25 [105,106]. TTiO2 is not yet understood bvalencies lower than that ofcentres as opposed to the p-ions such as V, Cr, Mn, Fe anthe absorption band of titaniThere is no red shift in the[107] which indicates that thimplantation itself.

Unlike many photocatalyform of sun or room lightbe improved to absorb lighThe assertion to use visibleNozik [108] has been as oldsis itself. Numerous researcphotocatalytic oxidation [10photocatalyst have beenprepmetal or non-metal. Table 2photocatalyst made in the re

Inmixed semiconductor phas marked effect on the catalyst as reported in the case ofoxides have great potentialcase of TiO2ZrO2 andTiO2Sture has marked effect on threcombination is increased bachieved folds of photocatadegradation withmixed suspprimary processes leading to(Fig. 7) were conjectured to dtheory has beenwidely usedphotocatalysis [135].

CdS+h CdS(h+ + e)TiO2 +h TiO2(h+ + e)CdS(h+ + e) + TiO2 CdS(

CdS(h+ + e) + TiO2(h+ + e

The excess electrons on the Tdiatomic molecular oxygen [cient for photocatalysis [104]. Fed to show lower efciency thanotophysicalmechanismof dopede p-type metal ion dopants (with) are believed to act as acceptor[7]. Metal ion implantation withwas found to cause large shift inioxide towards the visible region.of TiO2 implanted with Ar or Tithocromic shift is not due to the

iO2 can absorb UV-A light in theas reported in this review canond the wavelength of 388nm.t in photoinduced processes byhe phenomenon of photocataly-imed at investigating solar TiO20] and novel visible responsivebydoping the photocatalystwiths promising preparation of TiO2years.catalysts the synthesis procedureactivity of the hybrid photocata-

SiO2 [130,131]. TiO2SiO2 binarye removal of VOCs [132]. In theybrid oxides the crystallite struc-toactivity [133] perhaps becausestallite defects. Doong et al. [134]performance in 2-chlorophenolon of CdS and TiO2. The followingr-particle electron transfer (IPET)ibe the phenomenon. Today, IPETscribe promotion effect in similar

(21)

(22)

iO2(e) (23)

CdS(h+ +h+) + TiO2(e + e)(24)

an be scavenged by chemisorbed.


Table 2Some novel preparations of UV and visible light responsive titania photocatalysts

Photocatalyst Highlights on synthesis and comments on performance Reference

Nd3+TiO2 sol The sol prepared by coprecipitation had anatase crystalline structure and showed higherphotocatalytic activity with reactive dye X-3B than titania under visible light illumination

[111]

N-doped TiO2 Nitridation of TiO2 extended absorption to visible region. The addition of PdCl2 further extendedabsorption to near IR with high-photocatalytic activity

[112]

TiO2xNy The photocatalyst was prepared by low-temperature process involving mechanical doping and oxygenplasma treatment

[113]

Highly active bicrystalline TiO2 (anatase-brookite) Nanocrystalline TiO2 was prepared at 100 C by hydrolysis of Ti(C3H5O)4. High specic area of750m2/g and higher photoactivity over Degussa P-25 was obtained

[114]

Mesoporous TiO2 Titanium isopropoxide was the Ti source in the solgel method. Co-polymer surfactant was used todirect structure. The polymeric template was removed by solvent extraction

[115]

Cr-doped TiO2 Cr doped anatase TiO2 was prepared by the combination of solgel and hydrothermal methods. Crdoping improved photocatalytic activity for the degradation of XRG dye

[116]

Ce-doped mesophorous TiO2 Doping inhibited mesophores collapse and anataserutile phase transformation. Doped mesophorousanatase nanoparticles exhibited higher photocatalytic activity than commercial Degussa P-25

[117]

Ag-doped nanocrystalline TiO2 order Photocatalyst was prepared by solgel method and subsequent ultrasonication. Photocalalytic activitywas evaluated with methylene blue in presence and in absence of NO32 , SO42 and CH3COO . Theanions caused signicant increase in the photocatalytic degradation of the dye which followedpseudo-rst order

[118]

C-modied nano-TiO2 The photocatalyst was prepared by heating TiO2 at high temperatures in an atmosphere of hexane.without C deposition the latter

[119]

Fe3+-, Cr3+- and Co2+-doped nano-crystall increasing dopant [120]

Titanium oxocluster-derived nanocrystall than the one obtained from [121]

Zn2+-doped TiO2 ethods and evaluated withsulted

[122]

NF codoped TiO2 tetrabutyl titanate precursor.ced p-chlorophenol

[123]

TiO2/WO3 multilayer thin lm tocatalytic activity towardsn

[124]

V-doped TiO2 tocatalytic activity in the [125]

TiO2SiO2 beads te by dip coating hollow glassf silica inhibited anataserutile

[126]

TiO2ZnO ing citric acid as complexingards methyl orange improved

[127]

TiO2-carbide is loaded in the former. Thisncentration

[128]

Ball-milled TiO2/SnO2 O was used as disperser. Thend by

[129]

7.2. Combined operations

For deriving an effective decocost theUV/TiO2 version of photoobeen combined with either phyThe rise in the hyphenated metnumber of novel water treatmening high-treatment costs. The chocomposition of water in terms ofpollutant level [38].

In combined TiO2/inorganicscavengers such as O3 [137], Fe2+

and BrO3 [139] are added to trap[50].

H2O2 + e OH+ OHS2O82 + e SO42 + SO4

SO4 +H2O SO42 + OH + HBrO3 +2H+ + e BrO2 + H2BrO3 +6H+ +6e [BrO2,HOO3 + e O3Even though lower photocatalytic activity was obtained than the TiO2showed less turbidity after photocatalyst sedimentation

ine TiO2 Under UVvis excitation anataserutile transformation increased withconcentration

ine TiO2 Photocatalyst sourced from Ti7O4(OEt)20 precursor had higher activitytitanium tetraisopropoxideThe photocatalyst was prepared by solgel and solid phase reaction mRhodamine B. Signicant enhancement of the photoactivity of TiO2 reAnatase NF doped TiO2 was prepared by solvothermal method usingThe photocatalyst showed very high activity towards visible light induphotooxidative degradationMultilayer prepared by pulse laser deposition (PLD) showed high-phophotodecomposition of methylene blue under visible light illuminatioPhotocatalyst prepared by modied solgel method showed high-phodegradation of crystal violet and methylene blue under visible lightThe photocatalyst was prepared from [Ti(iso-OC3H7)4] and ethyl silicamicrobeads and calcination preferably at 650 C for 5h. The addition otransitionThe novel photocatalyst was prepared by modied solgel method usagent. By sulfating, the degradation efciency of the photocatalyst towsignicantlyCorrugated shapes of carbide were prepared on metal mesh and TiO2preparation has commercial potentials for photodegradation at low coPhotocatalyst was prepared by ball milling through doping of TiO2. H2crystal faces of TiO2 were not changed and new crystal faces were fou

O3 + H+ HO3ntamination at relatively lowerxidative degradation process has

sical or chemical operation [13].hods paved way to the growingt technologies thereby overcom-ice of method depends upon theclass of the organics [1] and the

additive photoprocess electron/Fe3+ +H2O2 [138], H2O2, S2O82

electrons and generatemore OH

(25)

(26)

+ (27)

O (28)

Br] Br +3H2O (29)(30)

HO3 HO + O2

Fig. 7. Band diagram illustrating charge caball milling

(31)(32)

rrier transfer in coupled semiconductors.

U.I.G

aya,A.H

.Abdullah

/JournalofPhotochemistry

andPhotobiology

C:Photochemistry

Reviews9(2008)

1129

Table 3Photocatalytic degradation of organic compounds by TiO2 under UV irradiation

Class of organic contaminants Target compound Highlights on experimentals and ndings Reference

Aldehydes Acetaldehyde Photomineralisation efciency over TiO2 lm exceeds that over F-TiO2. Surface uorination causes adsorptioninhibition to a large extent

[99]

Formaldehyde 30W UV lamp (297) and silica support were used in the study. CO and CO2, found as product of degradationadsorb on the TiO2 photocatalyst. This could cause catalyst deactivation

Carboxylic acids Phenoxy acetic acid and 2,4,5-phenoxyacetic acid The effect of pH, catalyst, BrO3 and H2O2 to degradation was signicant in all cases. Degussa P-25 was moreefcient photocatalyst than Hombicat UV 100, Millenium Inorganic PC500 and Travancore

[145]

Oxalic acid Synergetic effect of combined sonolysis photocatalysis was conrmed in Ar atmosphere. H2 and CO wereobtained in addition to CO2. H2O2 played signicant role in the process

[146]

Chloroanilines 2-Chloroaniline Slower degradation resulted at low pH in the UV/TiO2/H2O2 system. Excess H2O2 retarded the degradation rate [147]Chlorocarboxylic acids Monochloro-acetic acid In the ozonation-photocatalysis, O3 showed high-electron afnity thereby improving the removal rate. O3 was

not decomposed by UV light in the system[137]

Chlorophenols 2-Chlorophenol Photoelectrocatalytic degradation carried out in a batch reactor using TiO2 coated Ti sheets as anode and Ptcathode was more effective in acidic medium and rate increases linearly with light intensity

[148]

4-Chlorophenol Photomineralisation studied with different samples of TiO2. Degussa P-25 proved more effective aphotocatalyst. Both solar pilot plant and laboratory experiment indicated apparent rst order kinetics. Fewerintermediates and faster TOC disappearance was observed in the solar pilot plant which worked with smalleroptimum titania concentration

[5]

2,4-Dichlorophenol Two kinetic models for photocatalytic degradation of 2,4-dichlorophenol over Degussa P-25TiO2 suspensionwere proposed based on the inuence of different variables; pH, radiation and TiO2 concentration

[149]

Mixture of 4-chlorophenol, 2,4-dichlorophenol,2,4,6-trichlorophenol and pentachlorophenol

Sequential photochemical-biological degradation proved useful. There was no removal of chlorophenol withH2O2 or TiO2 alone

[150]

Dyes Acid orange 8 and Acid red 1 Sonophotocatalytic degradation was faster than photocatalytic degradation followed by sonolysis [151]Chrysoidine Y Degussa P-25 was found to be more effective than ZnO in photomineralisation of the dye at laboratory scale.

UV analysis was used in studies[152]

Acridine orange and Ethidium bromide Degussa P-25 showed superior photocatalytic activity than PC300. Degradation rate was affected by inorganicadditives

[153]

Methylene blue, methyl orange, indigo carmine, Chicagosky blue, mixed dye (mixture of the four dyes)

TiO2 photocatalyst was immobilised on glass and used for dye removal. Chicago sky blue was the mostresistant to the photodegradation. Methyl orange with t1/2 85.6min was removed faster

[154]

Ethers Methyl tert-butyl ether (MBTE) H2O2-enhanced photocatalysis had additive effects apart from synergetic effect but hydrogen peroxidephotolysis had higher degradation rate. Acetone, tert-butyl formate and tert-butyl alcohol were determined asintermediates

[155]

Flourophenols 4-Fluorophenol TiO2-P25 was found to be more efcient than ZnO under the study conditions. The efciency of anion oxidantsand cations is respectively in the following order IO4 >BrO3 >S2O82 >H2O2 >ClO3 andMg2+ > Fe3+ > Fe2+ >Cu2+

[156]

Fungicides Fenamidone Coated optical bre photoreactor was used in the study. Slow photocatalytic degradation of fenamidone overTiO2 was observed. COO and SO42 were identied in the reactor

[157]

Herbicides Isoproturon Degradation rate over Degussa TiO2 was faster than Hombicat 100 and was increased by the addition ofelectron acceptors. Degradation was slower under solar illumination

[158]

Ketones Acetone Vibrouidized and multiple xed bed photoreactors were compared. The comparison was based on thequantum efciency for the photooxidation of acetone using TiO2 (Hombicat UV 100). Vibrouidized-bedshowed higher activity for photooxidation. Application of ultrasound does not inuenced the rate ofphotooxidation of acetone

[159]

Perouroaliphatics Triouroacetic acid, sulfonic acid of nonauorobutane andheptadecaourooctane

Peruorosulphonic acids were persistent under the experimental conditions studied. However,phosphotungstic acid enhanced mineralisation at extreme acidic pH. CO2 evolution was dependent uponmolecular oxygen availability for the process

[160]

Phenolics Phenol TiO2 does not favour degradation at concentrations higher than 100ppm. Active radicals in mechanism wereconrmed to be H and HO

[36]

Pharmaceuticals Tetracycline Compound was resistant to photolysis. Photocatalysis over 0.5 g/l TiO2 suspension showed rapid rate ofdegradation. The irradiated solution inhibits the activity of microorganisms

[161]

Lincomycin Solar photocatalysis and membrane separation was used to study the degradation of the antibiotic.Photocatalysis was coupled with membrane separation to remove catalyst. The photooxidation of lincomycinfollowed pseudo-rst order rate kinetics

[162]

Polymers Polyvinylpyrollidone (PVP) Three major steps were identied in the photomineralisation of PVP; Adsorption, cleavage of main ring andlactam ring caused by OH and OOH attack and nally the conversion of ammine to NH4+ and NO3 . DegussaP-25 with particle size 2030nm was used for the study. The higher the intensity from 1 to 4mWcm2 thehigher the CO2 yield resulting from mineralisation

[163]


Fig. 8. Photocatalytic system development cycle.

The UV/TiO2/O3 version of the TiO2/inorganic additive pho-toprocess is based on attractive features of ozone for organiccontaminant removal which include relatively higher scaveng-ing effect provided by ozone, ability to decrease the rate ofde-excitation of electrons and elimination of residues [133,140].The combined ozononation-phott shared in the scenario of disinforganic contaminants. Applicatiocial to remedy the failureof theozof recalcitrant N-nitrosodimethyl

Coupled methods of operationand water purication. Suryamanbiological treatment with photocthe removal of phenol to reducetocatalytic process. The mineralito the single biological treatmecompared to the photocatalysis. Isonophotocatalytic degradationmineralisation of the popular heapplication of photocatalytic membine slurry with membrane ltraof separation of photocatalyst ingives an overview of pollutantsover titania.

8. Miscellaneous

The stages involved in the devform. The cycle, shown in Fig. 8sis of needs by the end-user. Eacis critical for successful operatioproblem identied with stage (a)treated are in a form of mixturelaboratory studies have been carryears [164168]. Additionally, labon degradation kinetics of targeting the toxicity of degradation inthe intermediate of photodegradcoli than the starting material. Mfor successful industrial applicattor conguration is mainly depeas much catalyst per unit volumfor scale-up [170]. Due to the higduced during the photocatalyticcontaminants it is difcult tomakwork is needed in the fundamentation of heterogeneous photoreactout the development of faulty moreactors by many researchers [1[140,149] stressed the need to dethis promising method of decont

8.1. Ecotoxicity of titania photoprocesses

The nal products of photocatalytic degradationmay not purelybe innocuous substances. Harmful by-productsmay cause decreasein reaction rate and secondary pollution [35]. There has been scantyliterature on the toxicity of the photocatalyst or the overall photo-catalytic process. Although TiO2 is found in toothpaste due to itssafety nano-scale TiO2 water suspension has been reported toxic toE. coli and antibacterial inhibition increased with particle concen-tration during photocatalytic experiment [174]. Maness et al. [175]attributed this toxicity to the attack of polysaturated phospholipidsin the E. coli by radical oxidizing species.

8.2. Future trends

At the present infancy stage of new century future trends fordevelopment would include:

the preparation of photocatalyst material capable of selectiveof ory m

ationies botocar bot

tic dativeth aprobon-pcicotocobictes stoden-exganirincip

Uniahim

nvirotandar

e, Res77 (20rrman

shraeEnginr, Proe (Lonhem.shi, K.. Physhem.lmisa

53 (19ocatalysis provides double bene-ection and removal of recalcitrantn of this method could be bene-onation-photolysis in the removalamine (NDMA) from water [141].have proven their efcacy in airet al. [142] considered couplingatalysis degradation method foroperational cost on single pho-

sation time shortened comparednt and the electrical cost savedt is reported that H2O2-enhancedmethod is efcient in completerbicide propyzamide [143]. Thebrane reactors designed to com-tion is a solution to the problemaqueous systems [144]. Table 3successfully photodecomposed

elopment of AOPs assume cyclicstarts and ends with the analy-h stage of the development cyclen of the next immediate step. Aand (b) is most of the pollutants[1] and only a limited number ofied on mixtures mainly in recentoratory studies concentrate onlyorganic contaminants disregard-termediates. Ref. [169] reportedation more toxic to Escherichiaodelling and design are criticalion. Design efciency of a reac-ndent upon its ability to installe as possible a factor necessaryh number of intermediates pro-oxidation of even simple organice complete kinetic analysis. Morels of reactor design andoptimiza-ors [171]. Several reports pointeddels for photocatalytic oxidation72,173]. Hernandez-Alonso et al.velop accurate kinetic models foramination.

photocatalytic degradation novel preparation of terna

idative degradation; novelphotocatalystprepar

more member of the famil design of more reliable ph

by visible and solar light o

9. Conclusion

Heterogeneous photocatalylyst remains a viable alternorganic contaminants in boing technology solves theorganic contaminants to n

Despite its moderate speremains a very efcient ph

Environmental friendly aerproduce fewer intermediaaccurate development of m

Though this review is nocatalytic degradation of orregarding fundamental paddressed.

Acknowledgement

The authors acknowledgelowship granted to Umar Ibr

References

[1] P.R. Gogate, A.B. Pandit, Adv. E[2] US EPA, National Emission S

part 63, 2006.[3] T. Kudo, Y. Nakamura, A. Ruik[4] D. Bahnemann, Solar Energy[5] C. Guillard, J. Disdier, J.-M. He

Catal. Today 54 (1999) 217.[6] W.A. Zeltner, D.T. Tompkin, A

Heating and Air-Conditioning[7] O. Carp, C.L. Huisman, A. Relle[8] A. Fujishima, K. Honda, Natur[9] S.N. Frank, A.J. Bard, J. Phys. C

[10] T. Inoue, A. Fujishima, S. Koni[11] I. Izuml, F-R.F. Fan, A.J. Bard, J[12] B. Kraeutler, A.J. Bard, J. Am. C[13] V. Augugliaro, M. Litter, L. Pa

(2006) 127.[14] J.-M. Herrmann, Catal. Todayrganic pollutants;ixed oxide systems for photoox-

s fromtitaniumoxo families sinceecame available;talyst that can be photoactivatedh.

egradation using TiO2 photocata-for the degradation of persistent

ir and water. At least this promis-lem of transforming recalcitrantersistent substance.area of 50m2/g Degussa P-25

atalyst.photocatalysis ormethodswhichill need to be developed to allowls and successful application.haustive in the scope of photo-c contaminants, important facetsles and application have been

versiti Putra Malaysia for the fel-Gaya.

n. Res. 8 (2004) 501.ds for Hazardous Air Pollutants, 40 CFR,

. Chem. Intermed. 29 (2003) 631.04) 445.n, C. Lehaut, T. Chopin, S. Malato, J. Blanco,

Transactions, vol. III, American Society ofeers Inc., 2005, part 2, p. 532.g. Solid State Chem. 32 (2004) 33.don) 238 (1972) 37.81 (1977) 1484.Honda, Nature 277 (1979) 637.. Chem. 85 (1981) 218.Soc. 100 (1978) 5958.no, J. Soria, J. Photochem. Photobiol. C 7

99) 115.


[15] J. Zhao, X. Yang, Build. Environ. 38 (2003) 645.[16] N.T. Dung, N.V. Khoa, J.-M. Herrmann, Inter. J. Photoenergy 7 (2005) 11.[17] K. Vinodgopal, P.V. Kamat, J. Phys. Chem. 96 (1992) 5053.[18] H. Kim, S. Lee, Y. Han, J. Park, J. Mater. Sci. 40 (2005) 5295.[19] A. Mills, S. Le Hunte, J. Photochem. Photobiol. A 108 (1997) 1.[20] I.T. Horvath, Encycl. Catal. 5 (2003) 577.[21] J. Clarke, R.R. Hill, D.R. Roberts, J. Chem. Tech. Biotechnol. 68 (1997) 397.[22] D. Lawless, N. Serpone, J. Phys. Chem. 95 (1991) 5166.[23] D.P. Colombo Jr., R.M. Bowman, J. Phys. Chem. 100 (1996) 18445.[24] X. Yang, N. Tamai, Phys. Chem. Chem. Phys. 3 (2001) 3393.[25] D. Bahnemann, A. Henglein, J. Lilie, L. Spanhel, J. Phys. Chem. 88 (1984) 709.[26] M. Anpo, N. Aikawa, S. Kodama, Y. Kubokawa, J. Phys. Chem. 88 (1984) 2569.[27] A. Furube, T. Asahi, H. Masuhara, H. Yamashita, M. Anpo, J. Phys. Chem. B 103

(1999) 3120.[28] N. Serpone, D. Lawless, R. Khairutdinov, E. Pelizzetti, J. Phys. Chem. 99 (1995)

16655.[29] K. Domen, S. Naito, T. Onishi, K. Tamaru, Chem. Lett. 4 (1982) 555.[30] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem. Photobiol. C 1 (2000) 1.[31] Y. Nakato, A. Tsumura, H. Tsubomura, J. Phys. Chem. 87 (1983) 2402.[32] W.A. Jacoby, D.M. Blake, J.A. Fennell, J.E. Boulter, Le-A.M. Vargo, M.C. George,

Suzanne K. Dolberg, Air Waste Manage. Assoc. 46 (1996) 891.[33] M. Sleiman, P. Conchon, C. Ferronato, J.-M. Chovelon, Appl. Catal. B: Environ.

71 (2006) 279.[34] K. Ishibashi, A. Fujishima, T.Watanabe, K. Hashimoto, J. Photochem. Photobiol.

A 134 (2000) 139.[35] M. Anpo, T. Shima, S. Kodama, Y. Ku[36] E. Pelizzetti, C. Minero, Electrochim[37] H. Liu, X. Ye, Z. Lian, Y. Wen, W. Shan[38] Z. Guo, R. Ma, G. Li, Chem. Eng. J. 11[39] K. Kabra, R. Chaudhary, R.L. Sawhne[40] I.K. Konstantinou, T.A. Albanis, App[41] J.P.S. Valente, P.M. Padilha, A.O. Flor[42] J.-Q. Chen, D. Wang, M.-X. Zhu, C.-J.[43] A.F. Caliman, C. Cojocaru, A. Antonia

265.[44] H. Lachheb, E. Puzenat, A. Houas, M

rmann, Appl. Catal. B: Environ. 39 ([45] N. Guetta, H. Ait Amar, Desalinatio[46] J. Beltran DeHeredia, J. Torregrosa, J

B 83 (2001) 255.[47] K. Demeestere, A. De Visscher, J. Dew

Appl. Catal. B: Environ. 54 (2004) 2[48] M.F.J. Dijkstra, A. Michorius, H. Buw

A.A.C.M. Beenackers, Catal. Today 6[49] I.K. Konstantinou, V.A. Sakkas, T.A. A[50] J. Perkowski, S. Bzdon,A.Bulska,W.K

457.[51] K. Rajeshwar, C.R. Chenthamaraks

Chem. 73 (2001) 1849.[52] Z. Zhang, P.A. Maggard, J. Photochem[53] D. Curco, J. Gimenez, A. Addardak, S

76 (2002) 177.[54] M. Qamar, M. Muneer, D. Bahneman[55] C. Karunakaran, S. Senthilvelan, Cat[56] M. Stylidi, D.I. Kondarides, X.E. Ver

189.[57] K. Wilke, H.D. Breuer, J. Photochem[58] L. Yang, Z. Liu, Energy Convers. Man[59] N. Serpone, G. Sauv, R. Koch, H. Ta

Hisao, J. Photochem. Photobiol. A 94[60] C. Minero, D. Vione, Appl. Catal. B: E[61] M.A. Aguado, M.A. Anderson, C.G. H[62] C.G. Silva, W. Wang, J.L. Faria, J. Pho[63] N. Serpone, J. Photochem. Photobio[64] M. Abu Tariq,M. Faisal,M.Muneera,

231.[65] D.S. Bhatkhande, S.P. Kamble, S.B. S

(2004) 283.[66] M. Huqul, E. Ercaq, R. Apak, J. Enviro

37 (2002) 365.[67] G. Palmisano, M. Addamo, V. Augug

V. Loddo, G. Marca, L. Palmisano118.

[68] D.A. Friesen, L. Morello, J.V. HeadleyA 133 (2000) 213.

[69] J. Arana, J.L.M. Nieto, J.A.H. Melian,Bergasa, J. Mendez, Chemosphere 5

[70] C. Guillard, H. Lachheb, A. Houas, Mtochem. Photobiol. A 158 (2003) 27

[71] K. Kogo, H. Yoneyama, H. Tamura, J.[72] H. Dinga, H. Suna, Y. Shan, J. Photoc[73] Y. Gao, H. Liu, Mater. Chem. Phys. 9[74] M.R. Mohammadi, M.C. Cordero-Ca

B 120 (2006) 86.[75] U. Diebold, Surf. Sci. Rep. 48 (2003)

[76] A.J. Maira, K.L. Yeung, J. Soria, J.M. Coronado, C. Belver, C.Y. Lee, V. Augugliaro,Appl. Catal. B: Environ. 29 (2001) 327.

[77] J. Krysa, M. Keppert, J. Jirkovsky, V. Stengl, J. Subrt, Mater. Chem. Phys. 86(2004) 333.

[78] M. Saquib, M. Muneer, Dyes Pigments 56 (2003) 37.[79] H. Chun, W. Yizhong, T. Hongxiao, Chemosphere 41 (2000) 1205.[80] M.M. Haque, M. Muneer, J. Hazard. Mater. 145 (2007) 51.[81] M. Kosmulski, J. Coll. Inter. Sci. 298 (2006) 730.[82] J. Sun, X. Wang, J. Sun, R. Sun, S. Sun, L. Qiao, J. Mol. Catal. A. 260 (2006) 241.[83] W.-Y. Wang, Y. Ku, Coll. Surf. A: Physicochem. Eng. Aspects 302 (2007) 261.[84] M. Mrowetz, E. Selli, J. Photochem. Photobiol. A: Chem. 180 (2006) 15.[85] Z. Shourong, H. Qingguo, Z. Jun, W. Bingkun, J. Photochem. Photobiol. A 108

(1997) 235.[86] H.D. Mansilla, C. Bravoa, R. Ferreyra, M.I. Litter, W.F. Jardim, C. Lizama, J. Freer,

J. Fernandez, J. Photochem. Photobiol. A 181 (2006) 188.[87] S. Tunesi, M.A. Anderson, Chemosphere 16 (1987) 1447.[88] E. Evgenidou, K. Fytianos, I. Poulios, Appl. Catal. B: Environ. 59 (2005) 81.[89] N.Z. Muradov, a. T-Raissi, D. Muzzey, C.R. Painter, M.R. Kemme, Solar Energy

56 (1996) 445.[90] X. Fu, L.A. Clark, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A:

Chem. 97 (1996) 181.[91] E.T. Soares, M.A. Lansarin, C.C. Moro, Brazilian J. Chem. Eng. 24 (2001) 29.[92] D. Gumy, A.G. Rincon, R. Hajdu, C. Pulgarin, Solar Energy 80 (2006) 1376.[93] S. Hunoh, J.S. Kim, J.S. Chung, E.J. Kim, Chem. Eng. Comm. 192 (2005) 327.[94] S.-Z. Chen, P.-Y. Zhang, W.-P. Zhu, L. Chen, S.-M. Xu, Appl. Surf. Sci. 252 (2006)

aterl

Moor

. Trot

Electro: Envs. Lettjishim

oto, T, A.A.ron. 7aki, K.idalgo

: Gen.emist611.of wa

A. Arqosph

699.. A: PhGoleaQ. Zha

a, Z. Zittma(2006ng, H29.a, C. YR. Go6) 22.ki, A.W

os, Apang,

L. Zheu, Z. D

T. Ikeg

chem. Tech, G. Yu

an, Cagyuc,J. Lope

hena,

ub, Cht. 58 (bokawa, J. Phys. Chem. 97 (1987) 4305.. Acta 38 (1993) 47.gguan, Res. Chem. Intermed. 32 (2006) 9.9 (2006) 55.y, Indian Eng. Chem. Res. 43 (2004) 7683.l. Catal. B: Environ. 49 (2004) 1.entino, Chemosphere 64 (2006) 1128.Gao, Desalination 207 (2007) 87.dis, I. Poulios, J. Hazard.Mater. 144 (2007)

. Ksibi, E. Elaloui, C. Guillard, J.-M. Her-2002) 75.n 185 (2005) 439..R. Dominguez, J.A. Peres, J. Hazard.Mater.

ulf, M. Van Leeuwen, H. Van Langenhove,61.alda, H.J. Panneman, J.G.M. Winkelman,

6 (2001) 487.lbanis, Water Res. 36 (2002) 2733.. Jozwiak, Polish J. Environ. Stud.15 (2003)

han, S. Goeringer, M. Djukic, Pure Appl.

. Photobiol. A 186 (2007) 8.. Cervera-March, S. Esplugas, Catal. Today

n, J. Environ. Manage. 80 (2006) 99.al. Comm. 6 (2005) 159.ykios, Appl. Catal. B: Environ. 47 (2004)

. Photobiol. A 121 (1999) 49.age. 48 (2007) 882.hiri, P. Pichat, P. Piccinini, E. Pelizzetti, H.(1996) 191.nviron. 67 (2006) 257.ill Jr., J. Mol. Catal. 89 (1994) 165.tochem. Photobiol. A 181 (2006) 314.l. A 104 (1997) 1.D. Bahnemann, J.Mol. Catal. A. 265 (2007)

awant, V.G. Pangarkar, Chem. Eng. J. 102

n. Sci. A: Tox. Hazard Subst. Environ. Eng.

liaro, T. Caronna, A. Di Paola, E.G. Lopez,, M. Schiavello, Catal. Today 122 (2007)

a, C.H. Langford, J. Photochem. Photobiol.

J.M.D. Rodriguez, O.G. Diaz, J.P. Perna, C.A.5 (2004) 893.. Ksibi, E. Elaloui, J.-M. Herrmann, J. Pho-.Phys. Chem. 84 (1980) 1705.hem. Photobiol. A 169 (2005) 101.2 (2005) 604.brera, D.J. Fray, M. Ghorbani, Sens. Actuat.

53.

7532.[95] T. Kemmitt, N.I. Al-Salim,M.W

Phys. 4 (2004) 189.[96] L.C. Macedo, D.A.M. Zaia, G.J.

A 185 (2007) 86.[97] O.K. Dalrymple, D.H. Yeh, M.A

121.[98] A.G. Agrios, P. Pichat, J. Appl.[99] H. Kim, W. Choi, Appl. Catal. B

[100] S. Sato, J.M. White, Chem. Phy[101] R. Baba, S. Nakabayashi, A. Fu

2273.[102] B. Ohtani, H. Osaki, S.-i. Nishim[103] G.N. Kryukova, G.A. Zenkovets

K. Richter, Appl. Catal. B: Envi[104] R. Asahi, T. Morikawa, T. Ohw[105] S.G. Botta, J.A. Navio, M.C. H

Photobiol. A 129 (1999) 89.[106] H. Zou, Y.S. Lin, Appl. Catal. A[107] J.L.G. Fierro, Metal Oxides: Ch

Francis Group, 2006, pp. 597[108] A.J. Nozik, Photoelectrolysis

Nature 257 (1975) 383.[109] A. Garca-Ripoll, A.M. Amat,

Maldonado, W. Gernjak, Chem[110] D. Ljubas, Energy 30 (2005) 1[111] Y. Xie, C. Yuan, X. Li, Coll. Surf[112] S. Kumara, A.G. Fedorova, J.L.[113] S. Yin, K. Ihara, M. Komatsu,

Comm. 137 (2006) 132.[114] A.R. Liu, S.M. Wang, Y.R. Zhao[115] T. Alapi, P. Sipos, I. Ilisz, G. W

Dombi, Appl. Catal. Gen. 303[116] J. Zhu, Z. Deng, F. Chen, J. Zha

Catal. B: Environ. 62 (2006) 3[117] J. Xiao, T. Penga, R. Lia, Z. Peng[118] S. Senthilkumaar, K. Porkodi,

mani, Dyes Pigments 69 (200[119] M. Janus, B. Tryba, M. Inaga

(2004) 61.[120] P. Bouras, E. Stathatos, P. Lian[121] L. Wu, J.C. Yu, L. Zhang, X. W

2584.[122] G. Liu, X. Zhang, Y. Xu, X. Niu,[123] D.-G. Huang, S.-J. Liao, J.-M. Li

184 (2006) 282.[124] H. Shinguu, M.M.H. Bhuiyan,

(2006) 111.[125] J.C.-S. Wu, C.-H. Chen, J. Photo[126] S. Chen, C. Gengyu, Surf. Coat[127] S. Liao, H. Donggen, D. Yu, Y. Su

7.[128] Q. Zhang, J. Liu, W. Wang, L. Ji[129] C. Shifu, C. Lei, G. Shen, C. Gen[130] J. Aguado, R. van Grieken, M.-

312 (2006) 202.[131] J. Yang, J. Zhang, L. Zhua, S. C

Mater. B 137 (2006) 952.[132] L. Zou, Y. Luo, M. Hooper, E. H[133] K.Y. Jung, S.B. Park, Mater. Letand, V.J. Kennedy, A. Markwitz, Curr. Appl.

e, H. de Santana, J. Photochem. Photobiol.

z, J. Chem. Technol. Biotechnol. 82 (2007)

chem. 35 (2005) 655.iron. 69 (2007) 127.. 72 (1980) 83.a, K. Hondas, J. Am. Chem. Soc. 109 (1987)

. Kagiya, J. Am. Chem. Soc. 108 (1986) 308.Shutilov, M.Wilde, K. Gunther, D. Fassler,1 (2006) 169.Aoki, Y. Taga, Science 293 (2001) 269., G.M. Restrepo, M.I. Litter, J. Photochem.

265 (2004) 35.ry and Applications, CRC press, Taylor and

ter using semiconducting TiO2 crystals,

ues, R. Vicente, M.F. Lopez, I. Oller, M.I.ere 68 (2007) 293.

ysicochem. Eng. Aspects 252 (2005) 87., Appl. Catal. B: Environ. 57 (2005) 93.ng, F. Saito, T. Kyotani, T. Sato, Solid State

heng, Mater. Chem. Phys. 99 (2006) 131.nn, Z. Ambrus, I. Kiricsi, K. Mogyorosi, A.) 1.. Chen, M. Anpo, J. Huang, L. Zhang, Appl.

an, J. Solid State Chem. 179 (2006) 1161.mathi, A. Geetha Maheswari, N. Manon-

. Morawski, Appl. Catal. B: Environ. 52

pl. Catal. B: Environ. 73 (2007) 51.W. Ho, J. Solid State Chem. 177 (2004)

ng, X. Ding, Chemosphere 59 (2005) 1367.anga, L. Petrik, J. Photochem. Photobiol. A

ami, K. Ebihara, Thin Solid Films 506/507

. Photobiol. A: Chem. 163 (2004) 509.nol. 200 (2006) 3637.an, J. Photochem. Photobiol. A 168 (2004)

tal. Comm. 7 (2006) 685.Mater. Chem. Phys. 98 (2006) 116.z-Munoz, J. Marugan, Appl. Catal. A: Gen.

Y. Zhang, Y. Tang, Y. Zhua, Y. Li, J. Hazard.

em. Eng. Process. 45 (2006) 959.2004) 2897.


[134] R.A. Doong, C.H. Chen, R.A. Maithreepala, S.M. Chang, Water Res. 35 (2001)2873.

[135] S.-L. Kuo, C.-J. Liao, J. Chin. Chem. Soc. 53 (2006) 1073.[136] W.A. Jacoby, D.M. Blake, J.A. Fennell, J.E. Boulter, L.M. Vargo, M.C. George, S.K.

Dolberg, Air Waste Manage. Assoc. 46 (1996) 891.[137] D. Mas, P. Pichat, C. Guillard, F. Luck, Ozone: Sci. Eng. 27 (2005) 311.[138] M. Pera-Titus, V. Garca-Molina, M.A. Banos, J. Gimenez, S. Esplugas, Appl.

Catal. B: Environ. 47 (2004) 219.[139] M. Faisal, M.A. Tariq, M. Muneer, Dyes Pigments 72 (2007) 233.[140] M.D. Hernandez-Alonso, J.M. Coronado, A.J. Maira, J. Soria, V. Loddo, V.

Augugliaro, Appl. Catal. B: Environ. 39 (2002) 257267.[141] A.A.Modi, J.H.Min, L.S. Palencia, B.M. Coffey, S. Liang, J.F. Green,Metropolitan

Water District of Southern California, La Verne, California, California EnergyCommission, Sacramento, CA, 2002, pp. 193.

[142] D. Suryaman, K. Hasegawa, S. Kagaya, Chemosphere 65 (2006) 2502.[143] J. Yano, J.-I. Matsuura, H. Ohura, S. Yamasaki, Ultrasonics Sonochem. 12 (2005)

197.[144] S. Mozia, M. Tomaszewska, A.W. Morawski, Appl. Catal. B: Environ. 59 (2005)

131.[145] H.K. Singh,M. Saquib,M.M. Haque,M.Muneer, D.W. Bahnemann, J. Mol. Catal.

A: Chem. 264 (2007) 66.[146] H. Harada, H. Tanaka, Ultrasonics 44 (2006) 385.[147] W. Chu, W.K. Choy, T.Y. So, J. Hazard. Mater. 141 (2007) 86.[148] Y. Ku, Y.-C. Lee, W-.u. Wang, J. Hazard. Mater. B138 (2006) 350.[149] B. Bayarri, J. Gimenez, D. Curco, S. Esplugas, Catal. Today 101 (2005)

227.[150] T. Essam, M. Aly Amin, O. El Tayeb, B. Mattiasson, B. Guieysse, Chemosphere

66 (2007) 2201.[151] M. Mrowetz, C. Pirola, E. Selli, Ultrasonics Sonochem. 10 (2003) 247.[152] M. Qamar, M. Saquib, M. Muneer, Desalination 171 (2004) 185.[153] M. Faisal, M. Abu Tariq, M. Muneer, Dyes Pigments 72 (2007) 233.

[154] Z. Zainal, L.K. Hui, M.Z. Hussein, Y.H. Tauq-Yap, A.H. Abdullah, I. Ramli,Removal of dyes using immobilized titanium dioxide illuminated by uores-cent lamps, J. Hazard. Mater. B 125 (2005) 113.

[155] M. Bertelli, E. Selli, Appl. Catal. B: Environ. 52 (2004) 205.[156] K. Selvam, M. Muruganandham, I. Muthuvel, M. Swaminathan, Chem. Eng. J.

128 (2007) 51.[157] A. Danion, J. Disdier, C. Guillard, O. Passe, N. Jaffrezic-Renault, Appl. Catal.

B: Environ. 62 (2006) 274.[158] M.M. Haque, M. Muneer, J. Environ. Manage. 69 (2003) 169.[159] A.V. Vorontsov, E.N. Savinov, P.G. Smirniotis, Chem. Eng. Sci. 55 (2000) 5089.[160] R. Dillert, D. Bahnemann, H. Hidaka, Chemosphere 67 (2007) 785.[161] C. Reyes, J. Fernandez, J. Freer,M.A.Mondaca, C. Zaror, S.Malato, H.D.Mansilla,

J. Photochem. Photobiol. A: Chem. 184 (2006) 141.[162] V. Augugliaro, E. Garca-Lopez, V. Loddo, S.Malato-Rodrguez, I. Maldonado,

G. Marca, R. Molinari, L. Palmisano, Solar Energy 79 (2005) 402.[163] S. Horikoshi, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A 138 (2001) 69.[164] R. Molinari, F. Pirillo, V. Loddo, L. Palmisano, Catal. Today 118 (2006) 205.[165] I. Arslan, I.A. Balcioglu, D.W. Bahnemann, Appl. Catal. B: Environ. 26 (2000)

193.[166] B. Toepfer, A. Gora, G.L. Puma, Appl. Catal. B: Environ. 68 (2006) 171.[167] A.F. Martins, M.L. Wilde, C. da Silveira, J. Environ. Sci. Health A. 41 (2006) 675.[168] F.V. Santos, E.B. Azevedo, G.L. SantAnna Jr., M. Dezotti, Braz. J. Chem. Eng. 23

(2006) 451.[169] W.F. Jardim, S.G. Moraes, M.M.K. Takiyama, Water Res. 31 (1997) 1728.[170] H.K. Ray, A.A.C.M. Beenackers, Environ. Anti Ener. Engin. 44 (1998) 477.[171] J. Peral, X. Domenech, D.F. Ollis, J. Chem. Technol. Biotechnol. 70 (1997) 117.[172] R. Yang, Y.-P. Zhang, R.-Y. Zhao, Air Waste Manage. Assoc. 54 (2004) 1516.[173] G. Camera-Roda, F. Santarelli, C.A. Martin, Solar Energy 79 (2005) 343.[174] L.K. Adams, D.Y. Lyon, P.J.J. Alvarez, Water Res. 40 (2006) 3527.[175] P.-C. Maness, S. Smolinski, D.M. Blake, Z. Huang, E.J. Wolfrum, W.A. Jacoby,

Appl. Environ. Microbiol. 65 (1999) 4094.

Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals, progress and problemsIntroductionBasic principles of photocatalysisMechanism of titania-assisted photocatalytic degradationTitania versus existing photocatalystsEffect of operational parametersLight intensityNature and concentration of the substrateNature of the photocatalystPhotocatalyst concentrationpHReaction temperature

Methods of utilization of titania photocatalystTrends in improving the activity of titaniaNovel photocatalyst preparationsCombined operations

MiscellaneousEcotoxicity of titania photoprocessesFuture trends

ConclusionAcknowledgementReferences

Documents

2009.Heterogeneous Photocatalytic Degradation of Organic Contaminants Overtitanium Dioxide a Review of Fundamentals,