Advances in Heterogeneous Photocatalytic Degradation

Advances in Heterogeneous Photocatalytic Degradationof Phenols and Dyes in Wastewater: A Review

Saber Ahmed & M. G. Rasul & Wayde N. Martens &Richard Brown & M. A. Hashib

Received: 7 January 2010 /Accepted: 21 April 2010# Springer Science+Business Media B.V. 2010

Abstract The heterogeneous photocatalytic waterpurification process has gained wide attention dueto its effectiveness in degrading and mineralizingthe recalcitrant organic compounds as well as thepossibility of utilizing the solar UV and visiblelight spectrum. This paper aims to review andsummarize the recently published works in the fieldof photocatalytic oxidation of toxic organic com-pounds such as phenols and dyes, predominant inwastewater effluent. In this review, the effects ofvarious operating parameters on the photocatalyticdegradation of phenols and dyes are presented.Recent findings suggested that different parameters,

such as type of photocatalyst and composition, lightintensity, initial substrate concentration, amount ofcatalyst, pH of the reaction medium, ionic compo-nents in water, solvent types, oxidizing agents/electron acceptors, mode of catalyst application,and calcinations temperature can play an importantrole on the photocatalytic degradation of organiccompounds in water environment. Extensive re-search has focused on the enhancement of photo-catalysis by modification of TiO2 employing metal,non-metal, and ion doping. Recent advances in TiO2photocatalysis for the degradation of various phenolsand dyes are also highlighted in this review.

Keywords Phenols . Azo dyes . Photocatalysis .

Water purification

1 Introduction

The reuse and recycling of wastewater effluent israpidly growing and becoming a necessity for waterutilities both in Australia and in other parts of theworld to augment our limited fresh water supply,which is currently under pressure due to rapidpopulation growth (Radcliff 2006; Mitchell et al.2002; DEH 2002). Heightened concerns over publichealth and associated environmental hazards due tothe presence of toxic organic compounds such asphenols and dyes in wastewater have been reported

Water Air Soil PollutDOI 10.1007/s11270-010-0456-3

S. Ahmed (*) :M. G. RasulFaculty of Science, Engineering and Health, CQ University,Rockhampton QLD 4702, Australiae-mail: [email protected]

W. N. MartensDiscipline of Chemistry, Faculty of Science and Technology,Queensland University of Technology,Brisbane, Australia

R. BrownSchool of Engineering System,Queensland University of Technology,Brisbane, Australia

M. A. HashibDepartment of Ecological Engineering,Toyohashi University of Technology,Toyohashi, Japan

(Eriksson et al. 2007). Phenols and azo dyes are well-known for their bio-recalcitrant and acute toxicity.Phenols and azo dyes are being continuously intro-duced into the aquatic environment through variousanthropogenic inputs. The presence of toxic organiccompounds in wastewater effluent is still a majorhindrance as regards widespread acceptance of waterrecycling (Mahmoodi et al. 2007; DEC 2006).Furthermore, their variety, toxicity, and persistencecan directly impact the health of ecosystem andpresent a threat to humans through contamination ofdrinking water supplies, e.g., surface and groundwater. The response has been the drive to achieveeffective removal of persistent organic pollutants fromwastewater effluent to minimize the risk of pollutionproblems from such toxic chemicals to enable itsreuse. Consequently, considerable efforts have beendevoted to developing a suitable purification methodthat can easily destroy these bio-recalcitrant organiccontaminants. Because of incomplete removal duringprimary and secondary treatment processes, they areubiquitous in secondary wastewater effluents at lowconcentration. Despite their low concentration, thesecontaminants have raised substantial concern in thepublic and regulatory agencies due to their extremelyhigh endocrine disrupting potency and geno toxicity(Arques et al. 2007). Moreover, the conventional end-of-pipe techniques also generate wastes during thetreatment of contaminated water, which requiresadditional steps and cost. Heterogeneous photo-catalytic oxidation (HPO) process employing catalystsuch as TiO2, ZnO, etc., and UV light has demon-strated promising results for the degradation ofpersistent organic pollutants producing morebiologically degradable and less toxic substances(Garcia et al. 2006; Garca et al. 2007; Garcia et al.2008; Hincapie et al. 2006; Maldonado et al. 2007;Malato et al. 2002; Oller et al. 2006).This process islargely dependent on the in situ generation ofhydroxyl radicals under ambient conditions whichare capable of converting a wide spectrum of toxicorganic compounds including the non-biodegradableone into relatively innocuous end products such asCO2 and H2O. In HPO process, destruction ofrecalcitrant organics is governed by combined actionof semiconductor photocatalyst, an energetic radiationsource and an oxidizing agent. Moreover, the processcan be driven by solar UV/visible light. Near the earthsurface, the sun produces 0.20.3 mol photons m-2h-1

in the range of 300400 nm with a typical UV flux of2030 Wm-2.This suggests the feasibility of usingsunlight as an economically and ecologically sensiblelight source (Goslich et al. 1997). As a result,development of efficient photocatalytic waterpurification process for large-scale applications is arecent research challenge in order to bring forwarda new and efficient end-of-pipe technology.Compounds studied using photocatalytic oxidationsprocess in aqueous medium belong to various types ofpesticides, phenolic compounds, and cationic andanionic dyes (Shifu and Yunzhang 2007; Echavia etal. 2009; Bahnemann et al. 2007). In light of the basicand applied researches reviewed, photocatalyticoxidation method appears to be a promising routefor the treatment of wastewater contaminated withphenols and dyes. However, low quantum efficiencydue to inefficient visible light harvesting catalyst (Sunand Bolton 1996), the design of photoreactor(Mukherjee and Ray 1999), the recovery and reuseof titanium dioxide (Moctezumaa et al. 2007), thegeneration of toxic intermediates (Konstantinou andAlbanis 2003), as well as concern over catalystdeactivation (Legrini et al. 1993; Rincon and Pulgarin2004; Doll and Frimmel 2005) are reported to be themajor drawbacks. Information from various inves-tigations suggest that photocatalytic degradation ofpesticides, phenols, and dyes is largely dependent onthe solution pH, types of catalyst and composition,organic substrate type and concentration, lightintensity, catalyst loading, ionic composition ofwastewater, types of solvent, oxidant concentration,and calcinations temperature (Shakthivel et al. 2004;Byrappa et al. 2006). Understanding the impacts ofvarious process parameters that govern the photo-catalytic degradation efficiency is of paramountimportance from the design and the operational pointsof view when choosing a sustainable, efficienttechnique for the treatment of wastewater. This paperaims to review and summarize the role of importantoperating parameters on the photocatalytic de-gradation of phenolic compounds and dyes togetherwith recent achievements. Recent developments inTiO2 photocatalysis for the degradation of variousphenols and dyes by means of metal, non-metal, andion doping are also highlighted in this review.The existing limitation and future research needsassociated with the treatment technology are alsodiscussed.

Water Air Soil Pollut

2 Principle of Photocatalytic Oxidation Process

In the photocatalytic oxidation process, organicpollutants are destroyed in the presence of semi-conductor photocatalysts (e.g., TiO2 and ZnO), anenergetic light source, and an oxidizing agent such asoxygen or air. Only photons with energies greater thanthe band gap energy (E) can result in the excitationof valence band (VB) electrons which then promote thepossible reactions. The absorption of photons withenergy lower than E or longer wavelengths usuallycauses energy dissipation in the forms of heat. Theillumination of the photocatalytic surface withsufficient energy leads to the formation of a positivehole (hv

+) in the valence band and an electron (e-) inthe conduction band (CB). The positive hole oxidizeseither pollutant directly or water to produce OHradicals, whereas the electron in the conduction bandreduces the oxygen adsorbed on the photocatalyst(TiO2). The activation of TiO2 by UV light can berepresented by the following steps.

TiO2 + h e- + h+ 2:1

e-+ O2 O2 - 2:2

In this reaction, h+ and e- are powerful oxidizingand reductive agents, respectively. The oxidative andreductive reaction steps are expressed as,

Oxidative reaction:

h+ + Organic CO2 2:3

h+ + H2O .OH +H+ 2:4

Reductive reaction:

OH +Organic CO2 2:5Hydroxyl radical generation by the photocatalytic

oxidation process is shown in the above steps. In thedegradation of organic pollutants, the hydroxylradical, which is generated from the oxidation of

adsorbed water where it is adsorbed as OH-, is theprimary oxidant, and the presence of oxygen canprevent the recombination of an electronhole pair. Inthe photocatalytic degradation of pollutants, when thereduction process of oxygen and the oxidation ofpollutants do not advance simultaneously, there is anelectron accumulation in the CB, thereby causing anincrease in the rate of recombination of e- and h+.Thus, it is of paramount importance to prevent electronaccumulation in efficient photocatalytic oxidation. Inphotocatalysis, TiO2 has been studied extensivelybecause of its high activity, desirable physical andchemical properties, low cost, and availability. Ofthree common TiO2 crystalline forms, anatase andrutile forms have been investigated extensively asphotocatalysts. Anatase has been reported to be moreactive as a photocatalyst than rutile. Similar oxidationpathways to those of TiO2 are confirmed in ZnOphotocatalyst including the formation of OH radicaland the direct oxidation by photogenerated holes, etc.ZnO is reported to be as reactive as TiO2 underconcentrated sunlight, since the band gap energy ofZnO is equal to that of TiO2, i.e., 3.2e V. Differentlight sources such as UV lamps and solar radiationhave been used in previous investigations into thephotocatalysis of various organic contaminants dom-inant in wastewater effluent. The following sectionsdescribe the influence of parameters on the photo-catalytic degradation of various phenols and dyes.

3 Influence of Parameters on the PhotocatalyticDegradation of Phenols and Dyes

3.1 Crystal Composition and Catalyst Type

The photocatalytic activity of TiO2 is dependent onsurface and structural properties of semiconductor suchas crystal composition, surface area, particle sizedistribution, porosity, band gap, and surface hydroxyldensity. Average crystal size is of primary importancein heterogeneous catalysis because it is directly relatedto the efficiency of a catalyst through the definition ofits specific surface area. A number of commerciallyavailable catalysts have been investigated for thephotocatalytic degradation of phenolic compoundsand dyes in aqueous environment. The photocatalysttitanium dioxide Degussa P25 has been widely used inmost of the experimental conditions; other catalyst


powders, namely, Hombikat UV100, PC 500, andtravancore titanium products (TTP) were also used fordegradation of toxic organic compounds. P25 contains75% anatase and 25% rutile with a specific Brunauer-Emmett-Teller (BET) surface area of 50 m2/g and aprimary particle size of 20 nm (Bahnemann et al.2007). Hombikat UV 100 consists of 100% pure andsmaller anatase with a specific BET surface area of250 m2/g and a primary particle size of 5 nm(Bahnemann et al. 2007).The photocatalyst PC 500has a BET surface area of 287 m2/g with 100% anataseand primary particle size of 510 nm (Bahnemann etal. 2007), and TiO2 obtained from TTP, India, has aBET surface area of 9.82 m2/g. It has been demon-strated that the degradation rate of dyes proceeds muchmore rapidly in the presence of Degussa P25 ascompared with other photocatalysts. The efficiency ofphotocatalysts was shown to follow the order:P25>UV100>PC500>TTP for the degradation of var-ious dyes (Qamar et al. 2005a, b; Tariq et al. 2005,2008; Faisal et al. 2007). In some cases, the order ofdegradation was found to be UV100>P25>PC500(Saquib and Muneer 2002; Saquib et al. 2008a, b).The differences in the photocatalytic activity are likelyto be related to the variations in the BET surface,impurities, existence of structural defects into crystal-line frame work, or density of hydroxyl groups on thecatalyst's surface. These factors could influence theadsorption behavior of a pollutant or degradationintermediates and the lifetime and recombination rateof electronhole pairs (Qamar et al. 2005a, b). Lu et al.2007 compared the photocatalytic degradation ofmethyl orange (MO) using natural rutile (rutile 93%)and P25 (20% rutile) under visible light. After 2 hirradiation, the degradation efficiency was reported tobe 82.33% and 94.85%, respectively. A possible reasonwas the particle size of P25 (30 nm) which issignificantly smaller compared with the natural rutilesample (7080 m). In suspended form, Lachheb et al.(2008) reported that, in comparison to PC 500, P25was more efficient for the degradation of phenols andpoly nitrophenols (4-NP, 2,4-DNP,2,4,6-TNP) in thepresence of either artificial or solar light. The photo-catalytic degradation of the tested compounds wasshown in the following order: 2,4,6-TNP>2,4-DNP>4-NP>phenol. For PC 500 supported on Ahstrom paper1048, the order is different: phenolTiO2>CdS.CdS was reported to be less efficient compared withZnO and TiO2, due to its smaller band gaps.Muruganandham et al. (2006) studied the effect ofvarious photocatalyst on the decolorization and degra-dation of Reactive Yellow 14 using UV and solar light(Muruganandham and Swaminathan 2006a, b). Forboth processes, the order of activities of the photo-catalysts is shown to be ZnO>P25>TiO2 anatase after40 min irradiation. CdS, Fe2O3, and SnO2 werereported to have negligible activity on the degradationof RY 14 decolorization and degradation. An order ofP25>ZnO>TiO2 anatase has been reported for thesolar photocatalytic degradation of reactive orange 4(Muruganandham and Swaminathan 2004). Guettaand Ait Amar (2005) compared the performance offive catalysts for the degradation of MO. The orderof efficiency was shown to be P25>Pt-UV100>UV100>Mikro anatase>Prtios AV01. The photo-catalytic activity of Degussa P25 was reported to behigher due to slow recombination between electron andholes where as Hombikat UV 100 has a high photo-reactivity due to fast interfacial electron transfer rate.The higher photoactivity of P25 has been attributed toits crystalline composition of rutile and anatase. Thesmaller band gap of rutile absorbs the photons andgenerates electronhole pairs. Then, the electrontransfer takes place from the rutile CB to electron trapsin anatase phase. Recombination is thus inhibited,allowing the hole to move to the surface of the particleand react (Bahnemann et al. 2007). Guillard et al.(1999) compared the photocatalytic efficiency ofdifferent catalyst with various surface areas, crystallite,


particle size, and their chemical surface for thedegradation of 4-chlorophenol. The observed order ofefficiency based on the rate of degradation upon solarexposure was shown to be: TiONA PC 10>P25>TiLCOM HC 120>Hombikat UV 100. In the presence ofP25, Selvam et al. (2007) indicated that completedegradation of 4-flurophenol with P25 was achieved in60 min while complete degradation occurred in 90 minin the presence of ZnO. Zheng et al. 2007 comparedthe photocatalytic degradation of reactive brilliant redX-3B in water with pure ZnO, ZrO2ZnO, and P25TiO2. The degradation of reactive brilliant red overcoupled ZrO2ZnO was shown to be higher comparedwith pure ZnO and also slightly better than that of P25and TiO2. Sobana and Swaminathan (2007) comparedthe efficiencies of various catalysts (ZnO, TiO2anatase, ZnS, SnO2, Fe2O3, and CdS for the degrada-tion of Acid Red 18 under UV irradiation. The highestdegradation was achieved with ZnO over TiO2 anatasecatalyst due to higher surface area of ZnO (10 m2/g)over TiO2 anatase (8.9 m

2/g). SnO2, Fe2O3, CdS, andZnS have negligible actively on Acid Red 18 decolor-ization due to smaller band gap which permits rapidrecombination of hole and electron. Talebian andNilforoushan 2009 compared the photocatalytic degra-dation efficiency of methylene blue (MB) under UVirradiation. The observed order of photocatalytic effi-ciency was shown to be SnO2SnO2.

3.2 UV Light Intensity

The light intensity determines the extent of lightabsorption by the semiconductor catalyst at a givenwavelength. The rate of initiation for photocatalysis,electronhole formation in the photochemical reactionis strongly dependent on the light intensity (Cassanoand Alfano 2000). Light intensity distribution withinthe reactor invariably determines the overall pollutant

conversion and degradation efficiency (Pareek et al.2008). Consequently, the dependency of pollutantdegradation rate on the light intensity has beenstudied in numerous investigations for various organicpollutants; while in some cases, the rate of reactionexhibited a square root dependency on the lightintensity, others observed a linear relationshipbetween the two variables (Terzian and Serpone1991).Ollis et al. (1991) reviewed the effect of lightintensity on the organic pollutant degradation rate. Ithas been reported (Hermann 1999) that the rate isproportional to the radiant flux for

intensity of 20, 100, and 400 W are 8.310-3, 0.012,and 0.031 min-1, respectively. Under the conditionstested, an acceptably good linear correlation existsbetween the apparent first-order-rate constant andlight intensity. The photocatalytic degradation of AcidRed 88 using immobilized ZnO was shown toincrease with increase in light intensity (Behnajadyet al. 2009). At higher light intensity, the catalystabsorbs more photons producing more electronholepairs in the catalyst surface, and this increases thehydroxyl radical concentration and consequentlyincreases the degradation rate.

3.3 Pollutant Type and Concentration

The successful application of photocatalytic oxidationsystem requires the investigation of the dependence ofphotocatalytic degradation rate on the substrateconcentration. A summary of various phenols anddyes studied under different initial concentration ispresented in Table 1. In the presence of TiO2 andZnO, the degradation rate of 4-flurophenol wasshown to decrease with the increase in initial

concentration from 0.022 to 0.09 mM (Selvam et al.2007). Similar trends have been observed for thephotocatalytic degradation of phenol and m-nitrophenol (Chiou et al. 2008b), Acid Red 114(Nikazor et al. 2008), methyl red (Sahoo et al.2005), Acid Blue 80 under solar irradiation (Su etal. 2008), Amido Black (Qamar et al. 2005b), andAcid Orange 7 (Daneshvar et al. 2007). The removalof bisphenol Awas reported to decrease from 100% to97% as the initial substrate concentration increasedfrom 0.001 to 0.018 mM, and the removal efficiencydecreased from 97% to 67% with further increase inBPA concentration to 0.044 mM (Tsai et al. 2009).

Venkatachalam et al. (2007a) observed that thephotocatalytic degradation of 4-CP increases withincrease in initial concentration up to 0.194 mM andthen decreases with further increase in 4-CP concen-tration from 0.194 to 0.233 mM. This effect has beenassociated with the screening effects at 4-CP concen-tration greater than 0.194 mM. Consistently similarresults have been reported for the photocatalyticdegradation of Chromotrope 2B, Xylenol Orange,Bromothymol, Acridine Orange, Acid Orange 8,

Table 1 Influence of initial pollutant concentration on the photocatalytic degradation of various pollutants

Pollutant type Lightsource

Photocatalyst Range of initialconcentration, mM

Optimum concentration,mM

Reference

Phenol UV TiO2 0.130.71 0.13 Chiou et al. 2008b

m-Nitrophenol UV TiO2 0.130.71 0.13 Chiou et al. 2008b

Chrysoidine Y UV TiO2 0.131.0 0.75 Qamar et al. 2004

Acid Orange 7 UV ZnO 0.0030.009 0.003 Daneshvar et al. 2007

Remazol Brilliant Blue R Solar TiO2 0.120.5 0.12 Saquib and Muneer 2002

Disperse Blue 1 UV TiO2 0.130.5 0.125 Saquib et al. 2008b

Amaranth UV TiO2 0.30.6 0.5 Tariq et al. 2005

Bismarck UV TiO2 0.30.6 0.6 Tariq et al. 2005

Reactive Orange 4 UV F-TiO2 0.0150.035 0.3 Vijayabalan et al. 2009

Acid Blue 45 UV TiO2 0.30.6 0.3 Tariq et al. 2008

Xylenol Orange UV TiO2 0.30.6 0.5 Tariq et al. 2008

Acridine Orange UV TiO2 0.10.5 0.25 Faisal et al. 2007

Ethidium bromide UV TiO2 0.10.4 0.1 Faisal et al. 2007

Bromothymol UV TiO2 0.150.5 0.35 Haque and Muneer 2007

Fast green FCF UV TiO2 0.0310.125 0.031 Saquib et al. 2008a

Phenol UV Pr-TiO2 0.111.7 0.11 Chiou and Juang 2007

Acid Blue 80 Solar TiO2 0.030.2 0.03 Su et al. 2008

Acid Red 18 UV ZnO 0.21.0 0.2 Sobana and Swaminathan 2007

Chromotrope 2B UV TiO2 0.250.75 0.35 Qamar et al. 2005b

Amido Black 10B UV TiO2 0.250.75 0.25 Qamar et al. 2005b


Amaranth, and Remazol Brilliant Blue R (Saquib andMuneer 2002). At high pollutant concentration, theadsorbed reactant molecules may occupy all the activesties of catalyst surface, and this leads to decrease indegradation rate (zero-order kinetics). As explained inseveral investigations that, as the concentration oftarget pollutant increases, more and more molecules ofthe compound get adsorbed on the surface of thephotocatalyst. Therefore, the requirement of reactivespecies (OH and O2

-) needed for the degradation ofpollutant also increases. However, the formation ofOH and O2

- on the catalyst surface remains constantfor a given light intensity, catalyst amount, andduration of irradiation. Hence, the available OHradicals are inadequate for the pollutant degradationat higher concentration. Consequently, the degradationrate of the pollutant decreases as the concentrationincreases (Bahnemann et al. 2007). In addition, anincrease in substrate concentration can lead to thegeneration of intermediates, which may adsorb on thesurface of the catalyst. Slow diffusion of the generatedintermediates from the catalyst surface can result in thedeactivation of active sites of the photocatalyst andconsequently, a reduction in the degradation rate. Incontrast, at low concentration, the number of catalyticsites will not be a limiting factor, and the rate ofdegradation is proportional to the substrate concentra-tion, in accordance with apparent first-order kinetics(Selvam et al. 2007). Under the conditions invest-igated, the degradation rate of Bismarck was reportedto increase with the increase in substrate concentration(Tariq et al. 2005). This is in accordance with the L-Hlaw. Several investigations adequately described thedependence of photocatalytic degradation rates on theconcentration of various organic compounds by theL-H kinetics model (Galindo et al. 2000; Wenhua et al.2000; Alaton and Balcioghu 2001). The L-H modelwas established to describe the dependence of theobserved reaction rate on the initial solute concen-trations (Turchi and Ollis 1990).

3.4 Influence of Catalyst Loading

A number of studies (Daneshvar et al. 2003, 2004,2006) have shown that the photocatalytic rate initiallyincreases with catalyst loading and then decreases athigh dosage because of light scattering and screeningeffects. The tendency toward agglomeration (particleparticle interaction) also increases at high solid

concentration, resulting in a reduction in surface areaavailable for light absorption and hence, a drop inphotocatalytic degradation rate. Although the numberof active sites in solution will increase with catalystloading, a point will appear to be reached where lightpenetration is to be compromised because of exces-sive particle concentration. The trade-off betweenthese two opposing phenomena results in an optimumcatalyst loading for the photocatalytic reaction(Adesina 2004). A further increase in catalyst loadingbeyond the optimum will result in non-uniform lightintensity distribution, so that the reaction rate wouldindeed be lower with increased catalyst dosage.Table 2 summarizes the effect of catalyst concentrationon the photocatalytic degradation of various phenolsand dyes in numerous studies. Venkatachalam et al.(2007a, b) observed that the degradation rate of 4-CPincreases linearly with catalyst concentration up to 2 g/l and then decreases due to increase in solutionturbidity. Selvam et al. (2007) indicated that thedegradation rate constant of 4-fluorophenol increasesfrom 0.0152 to 0.0358 min-1 as the concentration ofTiO2 increases from 1 to 3 g/L in the presence of P25.Furthermore, increase in catalyst loading from 3 to 4 g/L results in a decrease in the rate constant from 0.0358to 0.0296 min-1. In the case of ZnO, a similarobservation has been made. The optimum concentra-tions were found to be 3 g/L TiO2 and 4 g/L ZnO forefficient removal of 4-fluorophenol. The highestdecolorization and degradation rate of Reactive Orange16 was achieved at the concentration of 0.4 g/L ofTiO2 (Mahvi et al. 2009).

Sobana and Swaminathan (2007) studied the effectof ZnO concentration on the decolorization anddegradation rate of Acid Red 18. The optimumconcentration of ZnO was found to be 4 g/l. Theobserved low degradation rate at high catalyst loadingmay also be due to deactivation of activated moleculesby collision with ground state molecules of titania. Thephotocatalytic degradation of Acid Red 88 was foundto increase as the AgTiO2 loading increases from 0.2to 1.8 g/l (Anandan et al. 2008). Under the conditionstested, Muruganandham and Swaminathan (2006a)reported that the decolorization and degradation ofReactive Orange 4 (RO4) increased as the catalystloading increases from1 to 4 g/l underUVA irradiation.Consistent results have been observed for the solarphotocatalytic degradation of RO4 (Muruganandhamand Swaminathan 2004). The degradation rate was


found to increase with increasing catalyst concentrationup to a level and, on subsequent addition of catalyst,leads to the leveling off the degradation rate (Kaneco etal. 2004; Tsai et al. 2009). Regardless of static, slurry,or dynamic flow reactors, the initial reaction rates weredemonstrated to be directly proportional to catalystconcentration, indicating a heterogeneous regime(Bahnemann et al. 2007). However, several researchershave noticed that, above a certain concentration, thereaction rate even decreases and becomes independentof the catalyst concentration. Earlier studies indicatedthat this limit depends on the nature of the pollutants,reactor geometry, and operating conditions of thephotoreactor and the amount of illuminated surface ofTiO2 particles. In the case of high catalyst concentra-tion, turbidity impedes penetration of light within thereactor. It is assumed that the increase in the number ofTiO2 particles will result in increased number ofphotons absorbed and the number of the pollutant

molecule absorbed. Consequently, the degradationefficiency will be enhanced with increasing TiO2concentration due to the increase in total surface areaavailable for contaminant adsorption. Excessive TiO2dosage leads to opacity of the suspension, whichprevents the catalyst farthest in solution from beingilluminated (Haque and Muneer 2007; Saquib et al.2008a, b). At high catalyst concentration, particleparticle aggregation may also reduce the catalyticactivity due to the scattering and screening effects(Adesina 2004). When the concentration of TiO2 isincreased above a certain level, the number of activesites on the TiO2 surface may become almost constantbecause of the decreased light penetration, the increasedlight scattering, and the loss in surface area occasionedby aggregation (particleparticle interactions) at highsolid surface (Hermann 1999). In an immobilizedreactor, McMurray et al. (2006) reported an optimumTiO2 loading of 1.1 mg/cm

2 for the degradation of

Table 2 Influence of catalyst loading on the photocatalytic degradation of various pollutants

Pollutant type Lightsource

Photocatalyst TiO2 (g/L) Optimum TiO2concentration, g/L

Reference

Reactive Yellow 14 UV TiO2 1.06.0 4 Muruganandham and Swaminathan 2006a, b

BPA Solar TiO2 01.0 0.5 Kaneco et al. 2004

Chrysoidine R UV TiO2 0.55.0 5.0 Qamar et al. 2005a

Cibacron Yellow LS-R UV TiO2 0.14.0 4.0 Kositzi et al. 2007

Phenol UV TiO2 04.0 2.0 Chiou et al. 2008a

Supra Blue BRL Visible K+TiO2 0.252.0 1.5 Chen et al. 2007

Reactive Yellow 14 Solar TiO2 1.06.0 4.0 Muruganandham et al. 2006

Acid Red 88 UV ZnO 212 12 Behnajady et al. 2009

Remazol Brilliant Blue R UV/Solar TiO2 0.55.0 5.0 Saquib and Muneer 2002

Disperse Blue 1 UV TiO2 0.54.0 3.0 Saquib et al. 2008b

Acridine Orange Visible ZnO 0.0.35 0.25 Pare et al. 2008



Acid Orange 8 UV TiO2 0.55.0 2.0 Saquib and Muneer 2003


Xylenol UV TiO2 0.53.0 3.0 Tariq et al. 2008

Acridine Orange UV TiO2 0.53.0 2.0 Faisal et al. 2007



Reactive Orange 4 UV FTiO2 1.05.0 4.0 Vijayabalan et al. 2009


Rhodamine B Solar ZnO 0.050.4 0.3 Byrappa et al. 2006

Chromotrope 2B UV TiO2 0.55.0 5.0 Qamar et al. 2005b

Amido Black 10B UV TiO2 0.55.0 5.0 Qamar et al. 2005b


atrazine. For any application, the optimum catalystconcentration has to be determined in order to avoidexcess catalyst (Hermann 1999) and to ensure efficientabsorption of photons.

3.5 Solution pH

Organic compounds in wastewater differ greatly inseveral parameters, particularly in their speciationbehavior, solubility in water, and hydrophobicity. Whilesome compounds are uncharged at common pHconditions typical of natural water or wastewater, othercompounds exhibit a wide variation in speciation (orcharge) and physico-chemical properties. At pH belowits pKa value, an organic compound exists as a neutralspecies. Above this pKa value, organic compoundattains a negative charge. Some compounds can existin positive, neutral, as well as negative forms in aqueoussolution. This variation can also significantly influencetheir photocatalytic degradation behavior. pH of thewastewater can vary significantly. pH of aquaticenvironment plays an important role on the photo-catalytic degradation of organic contaminants since itdetermines the surface charge of the photocatalyst andthe size of aggregates it forms( Bahnemann et al. 2007).The surface charge of photocatalyst and ionization orspeciation (pKa) of an organic pollutant can beprofoundly affected by the solution pH. Electrostaticinteraction between semiconductor surface, solventmolecules, substrate, and charged radicals formedduring photocatalytic oxidation is strongly dependenton the pH of the solution. In addition, protonation anddeprotonation of the organic pollutants can take placedepending on the solution pH. Sometimes, protonatedproducts are more stable under UV radiation than itsmain structures (Saien and Khezrianjoo 2008). There-fore, the pH of the solution can play a key role in theadsorption and photocatalytic oxidation of pollutants.The ionization state of the surface of the photocatalystcan also be protonated and deprotonated under acidicand alkaline conditions, respectively, as shown in thefollowing reactions:

pHPzc TiOH + OH- TiO-+H2O5:2

The point of zero charge (Pzc) of the TiO2(Degussa P25) is widely investigated/reported atpH6.25 (Zhu et al. 2005). Table 3 presents asummary of phenols and dyes investigated undervarious solution pH. While under acidic conditions,the positive charge of the TiO2 surface increases asthe pH decreases (Eq. 5.1); above pH 6.25, thenegative charge at the surface of the TiO2 increaseswith increasing pH. Moreover, the pH of the solutionaffects the formation of hydroxyl radicals by thereaction between hydroxide ions and photo-inducedholes on the TiO2 surface. The positive holes areconsidered as the major oxidation steps at low pH,whereas hydroxyl radicals are considered as thepredominant species at neutral or high pH levels(Shifu and Gengyu 2005; Mathews 1986). It wouldbe expected that the generation of OH radicals arehigher due to the presence of more availablehydroxyl ions on the TiO2 surface. Thus, thedegradation efficiency of the process will be logi-cally enhanced at high pH. The degree of elec-trostatic attraction or repulsion between thephotocatalyst's surface and the ionic forms oforganic molecule can vary with the change insolution pH, which can result in enhancement orinhibition on the photocatalytic degradation oforganic pollutants in the presence of TiO2. In thepresence of P25 and ZnO, the effect of pH on thephotocatalytic degradation of 4-fluorophenol wasobserved to be pH7>pH9>pH4 (Selvam et al.2007). As shown elsewhere in (Venkatachalam etal. 2007a), that the degradation rate of 4-CP wasfaster in the acidic pH range compared with thealkaline pH which was attributed to enhancedadsorption of 4-CP on the surface of nano-TiO2.Minimization of electronhole recombination in theacidic pH was also indicated to be an importantfactor for the enhancement of degradation.Mahmoodi and Arami 2009 tested the effect of pH(3.510.5) on the photocatalytic degradation of AcidBlue 25. Under the conditions examined, the optimumpH was reported to be 10.5 for the degradation of AcidBlue 25. Using Z2S, Cun et al. 2002 examined theinfluence of pH (4.2212.57) on the photocatalyticdegradation of MO and noticed an optimum pH of 4.2.

Soutsas et al. (2009) studied the effect of pH (39)on the decolorization rates and efficiency of RemazolRed RR, Remazol Yellow, Procion Crimson H-exl, andProcion Yellow H-exl. At pH 3, nearly complete


decolorization (>90%) of all dyes was achieved in only15 min UV irradiation. Sobana and Swaminathan(2007) suggested that the decolorization and degrada-tion of Acid Red 18 by ZnO increases as the pHincreases from 3 to 11. At acidic pH, the removalefficiency is less due to the dissolution of ZnO. AR18 was efficiently removed at pH 11 due to theabundance of OH- ions in the particle surfaces as wellas in the reaction medium. Using ZnO and solar light,the degradation of Rodhamine B was reported to favorat acidic and basic pH (Byrappa et al. 2006).Muruganandham and Swaminathan (2004) observedthat the photocatalytic decolorization and degradation ofRO4was efficient/faster in alkaline pH in comparison toacidic pH range in both UVand solar irradiation. At lowpH value, TiO2 particle agglomeration reduces the dyeadsorption as well as photon absorption. In RO4 dye,the azo linkage (-N=N-) was indicated to be suscep-tible to electrophilic attack by OH radical. Talebian andNilforoushan 2009 examined the effect of pH (28) onthe photocatalytic degradation of MB using SnO2,

ZnO, TiO2, and In2O3 as catalysts. For all the catalyststested, an observed order of pH: pH 8>pH 6>pH 4>pH2 was found to exist in the degradation of MB.

4 Co-occurring Substances

4.1 Ionic Components

The amount of UV absorption is influenced by watertransmittance over the spectral UV range of interest.Some common constituents that affect water transmit-tance are dissolved organic matter, nitrate, and ferrous/ferric ions. The presence of spectator's components inwater can adversely affect the contaminant degradationrates. Inorganic anions, such as phosphate, sulfate,nitrate, and chloride, have been reported to limit theperformance of solar-based photocatalysis (Abdullah etal. 1990). Bicarbonate, in particular, is detrimental toreactor performance as it acts as hydroxyl radicalscavenger (Abdullah et al. 1990). Long term experi-

Table 3 Influence of pH on the photocatalytic degradation of phenols and dyes

Pollutant type Light source Photocatalyst pH range Optimum pH Reference

Phenol UV TiO2 4.112.7 7.4 Chiou et al. 2008a, b

Chrysoidine Y UV TiO2 3.09.0 9.0 Qamar et al. 2004

m-Nitrophenol UV TiO2 4.112.7 8.9 Chiou et al. 2008b

Reactive Blue 4 UV NdZnO 3.013.0 11 Zhou et al. 2009

BPA Solar TiO2 2.010.0 6 Kaneco et al. 2004

Methylene Blue Visible La3+TiO2 210 10 Parida and Sahu 2008

Supra Blue BRL Visible K+TiO2 4.511.8 7.2 Chen et al. 2007

Reactive Orange 4 UV FTiO2 1.09.0 3.0 Vijayabalan et al. 2009

Acid Red 88 Visible AgTiO2 0.21.8 1.8 Anandan et al. 2008

Remazol Brilliant Blue R Solar TiO2 3.011.0 3.0 Saquib and Muneer 2002

Disperse Blue 1 UV TiO2 311 3.0 Saquib et al. 2008b

Methyl Orange UV PtTiO2 2.511.0 2.5 Huang et al. 2008



Acid Orange 8 UV TiO2 3.011.0 9.0 Saquib and Muneer 2003


Acridine Orange Visible ZnO 2.97.1 7.1 Pare et al. 2008



Methyl Red UV AgTiO2 3.013.0 310.0 Gupta et al. 2006


Acid Red 29 UV TiO2 310.5 10.5 Qamar et al. 2005a


ence with photocatalytic oxidation system showed thathumic substances in contaminated water can adsorbstrongly titanium dioxide particles and reduce activitytoward the target substances (Goswami 1997). There-fore, the incorporation of pre-treatment process hasbeen indicated to have a beneficial effect on theperformance of photocatalytic oxidation process. Guptaet al. (2006) studied the influence of various ioniccomponents on the degradation of crystal violate (CV)and methyl red (MR) dyes using AgTiO2 as photo-catalyst. Cl-, NO3

-, SO42-, HPO4

2-, and humic acidwere shown to have negligible effect on the degrada-tion rate, whereas Ca2+ and Fe2+ interfered significant-ly. In case of the MR, Cl-, Ca2+, and Fe2+ werereported to affect the degradation with the exception ofNO3

-, SO42-, and HPO4

2-(Sahoo et al. 2005). Theeffect of transition metal ions on the degradation of4-flurophenol was shown to be in the order of Mg2+>Fe3+>Fe2+>Cu2+, and the inhibition of inorganic anionson the degradation of 4-FP was demonstrated to beCO3

2->HCO3->Cl->NO3

->SO42- (Selvam et al. 2007).

Muruganandham et al. (2006) observed that theaddition of 0.471.89 mM Na2CO3 decreases thedegradation of Reactive Yellow 14 from59.2% to48.2% due to hydroxyl radical scavenging property ofcarbonate ion.

CO32-+.OH OH-+CO3.- 6:1

HCO3-+.OH H2O+CO3.- 6:2

As a result, the primary oxidant hydroxyl radicaldecreases gradually with the increase in carbonateion, and consequently, there is significant decrease inphotocatalytic degradation. Addition of Cl- ion from0.47 to 1.89 mM Na2CO3 to reaction decreases thedegradation from 80.1% to 72.3% by time of 80 min.The decrease in degradation efficiency is due to thehole scavenging and hydroxyl radical scavengingproperties of chloride ion.

Cl-+hVB+ Cl. 6:3

Cl. +Cl- Cl2.- 6:4

The inhibitive effect of CO32- ion is shown to be

greater than the inhibitive effect of Cl- ion. Lin and Lin

(2007) observed that the presence of humic acid causeda significant retardation on the photocatalytic degrada-tion of 4-chlororphenol. The observed retardations ofhumic acids were related to the inhibition (surfacedeactivation), competition, and light attenuationeffects. Moreover, the presence of humic acid in thereaction mixture has been reported to significantlyreduce light transmittal and therefore, the photooxida-tion rate. Humic acid can also compete with organo-halides for the active sites on the TiO2 surface. Mahviet al. (2009) studied the effect of anions on thephotocatalytic decolorization Reactive Orange 16.The observed order of negative impact of anions wereshown to be as SO4

2-sodiumdodecyl sulfonate>sodium dodecyl sulfate (SDS). Thepresence of anions is reported to alter the ionic strengthof the solution, and therefore, influence the catalyticactivity, and hence the photocatalytic degradation(Calza and Pelizzetti 2001). The reaction of hvb+ and.OH with anions which behave as scavengers and thusinhibit the degradation. Pare et al. (2008) investigatedthe effect of carbonate ions on the photocatalyticdegradation of Acridine Orange for a range ofconcentration from 2.010-3 to 810-3 mM. Thedegradation rate constant was shown to decrease from0.023 to 0.015 min-1 under the above concentrationrange. In the presence of NaCl, similar degradationtrends have been reported for a certain concentrationrange of 2 to 10 mM due to the hole scavengingproperties of these ions. The presence of anionic (SDS)and cationic surfactants (C16TAB) retarded the rate ofdegradation due to the preferential adsorption of theamphiphilic surfactant molecules on the photocatalystsurface. Epling and Lin (2002) examined the inhibitioneffect of humic substances and inorganic anions on thedegradation of several cationic and anionic dyes. Ofthe ionic species (Cl-, NO3

-, PO43-, and HCO3

-)studied, Cl- showed the strongest inhibition effect onboth cationic and anionic dyes, followed by PO4

3-. Theoverall retardation has been attributed to the surfacedeactivation mechanism. A significant retardation on


the degradation of MB in the presence of either Aldrichhumic acid or natural humic substances has beenreported. Guillard et al. (2003) studied the inhibition ofinorganic salts on the photocatalytic degradation ofMB at neutral and basic pH. Under the conditionstested, the order of inhibition was reported to beCO3

2->PO43->SO4

2->Cl->NO3-. In the presence of

200 M nitrate salt, the effect of metal ions on thephotocatalytic degradation of Azure Awas observed tobe Cu2+>Al3+>Co2+>Zn2+, and the order was shown tobe Cu2+>Co2+>Fe3+ in the presence of 200 Mchloride salt (Aarthi et al. 2007). Papadam et al.(2007) examined the effect of water matrix on thedecolorization of Acid Orange 20. The presence of17.1 mM NaCl or 7.04 mM Na2SO4 led to decreaseddecolorization which was attributed to the trapping ofphotogenerated valence band holes and the hydroxylradicals by the respective anions. Aarthi and Madras(2007) investigated the effect of metal ions (Cu2+,Fe3+,Zn2+, and Al3+) on the photocatalytic degradation ofRhodamine B and was shown to follow the order ofCu2+>Al3>Zn2>Fe3+. In the presence of Cu2+ andZn2+, the degradation of Malachite Green was reducedby 76% and 66%, respectively.

4.2 Solvent Types

Organic solvents are mostly present in many industrialwastewaters. Therefore, their effects on the photo-catalytic degradation system have been investigated inseveral studies. Lin and Lin 2007 examined the effectsof acetonitrile or isopropanol on the photocatalyticoxidation of 4-chlorophenol. The degradation rate of4-chlorophenol was shown to decrease from 1.0436 to0.0525 h-1 as the acetonitrile content increases in thereaction mixture. A similar retardation effect was alsoobserved when 5% isopropanol was added into theacetonitrile/water (1:1) reaction mixture. Behnajady etal. (2009) observed that the pseudo first-order-rateconstants of Acid Red 88 and water ethanol were0.0257 and 0.0271 min-1, respectively. The use ofethanol instead of water as a solvent did not show anysignificant effect on loading morphology of ZnO onglass plate. Shohrabi et al. (2009) studied the effect ofvarious solvents on the photocatalytic degradation ofBenzidene Yellow. The observed order of solventseffects on the degradation efficiency are reported to bet-butyl alcohol>n-butanol>2-propanol>acetonitrile>N-hexane. Of the alcohols, t-butyl alcohol was shown to

have the most inhibitive effect due to the spatialhindrance effect of the t-butyl alcohol. In the presenceof ethanol and methanol, Aarthi et al. (2007) investi-gated the effect of solvents and mixed-solvents systemson the photocatalytic degradation of dye Sudan III. Asthe ethanol content in the system increases, thedegradation rate was reported to decrease. This effectwas attributed to the scavenging of OH radicals bythese solvents. Liu et al. (2009) studied the effect ofsolvent type on the photocatalytic activity of N-dopedTiO2 under visible light irradiation for the degradationof MO. Under the conditions tested, it was observedthat the anatase gradually changed into rutile withincrease in carbon chain of methanol, ethanol,1-propanol, and 1-butanol as the solvent undersolvothermal conditions. The bicrystalline TiO2(19.5% rutile and 80.5% anatase) prepared in methanolshowed the highest photocatalytic activity for thedegradation of MO. Aarthi and Madras (2007) exam-ined the effect of solvent effects on the photocatalyticdegradation of rodamine B (RB) using combustion-synthesized TiO2. In the mixture of ethanol and water,when ethanol was 10% by volume, the rate decreasedby 91.5%, and when the ethanol increased to 50% byvolume, no photodegradation was observed. Similarly,the rate decreased by 76%, 96%, and 100% whenacetonitrile in the mixture of water and acetonitrile wasincreased from 10%, 20%, and 50% by volume,respectively. This effect is attributed to the lessersolvation of excited electrons in the organic solventscompared with the aqueous solution.

5 Influence of Oxidants/Electron Acceptor

The electronhole recombination is one of the maindrawbacks in the application of TiO2 photocatalysisas it causes waste of energy. In the absence of suitableelectron acceptor or donor, recombination step ispredominant, and thus, it limits the quantum yield.Thus, it is crucial to prevent the electronholerecombination to ensure efficient photocatalysis.Molecular oxygen is generally used as an electronacceptor in heterogenous photocatalytic reactions.Addition of external oxidant/electron acceptors intoa semiconductor suspension has been shown toimprove the photocatalytic degradation of organiccontaminants by (1) removing the electronholerecombination by accepting the conduction band


electron; (2) increasing the hydroxyl radical concen-tration and oxidation rate of intermediate compound;and (3) generating more radicals and other oxidizingspecies to accelerate the degradation efficiency ofintermediate compounds (Qamar et al. 2005a, b;Saquib and Muneer 2002; Saquib et al. 2008a, b;Tariq et al. 2005, 2008; Faisal et al. 2007; Haque andMuneer 2007). Since hydroxyl radicals appear to playan important role in the photocatalytic degradation,several researchers have investigated the effect ofaddition of electron acceptors such as H2O2, KBrO3,and (NH4)2S2O8 on the photocatalytic degradation ofvarious dyes and phenolic compounds (Qamar et al.2005a, b; Saquib and Muneer 2002; Saquib et al. 2008a,b; Tariq et al. 2005, 2008; Faisal et al. 2007; Haque andMuneer 2007) to enhance the formation of hydroxylradicals as well as to inhibit the electron/hole(e-/h+) pairrecombination. In all cases, the addition of oxidants hasresulted in higher pollutant degradation rate comparedwith the molecular oxygen. In most of the cases, theorder of enhancement is reported to be UV/TiO2/H2O2>UV/TiO2/BrO3

->UV/TiO2/S2O82-. The enhance-

ment of degradation rate is due to the reaction betweenBrO3- and conduction band electron. This reactionreduces the recombination of electronhole pair.

BrO3-+6e-CB+6H+ Br-+3H2O 7:1

S2O82- can generate sulfate radical anion (SO4

-)both thermally and photolytically in aqueous solution.SO4

- then reacts with H2O to produce.OH radicals.

S2O82- 2SO4.- 7:2

SO4.-

+ H2O .OH+ SO42

-

+ H+ 7:3

With the addition of H2O2, the enhancement ofdegradation is due to the increase in the hydroxylradical concentration as shown by Eqs. 7.4 and 7.5:

H2O2+eCB- .OH+OH- 7:4

7:5

The degradation efficiency of UV/TiO2/oxidantprocess is slightly more in acidic medium than in

basic medium. Muruganandham et al. (2006) examinedthe effect of H2O2, KBrO3, and (NH4)2S2O8 additionon the solar photocatalytic degradation of ReactiveYellow 14 by varying the amount of oxidant con-centration. The optimum concentrations of the oxidantswere 15 mM, 0.90 mM, and 0.88 mM for H2O2,KBrO3, and (NH4)2S2O8, respectively. Of the oxidantsstudied, (NH4)2S2O8 was found to be efficient tooxidize the Reactive Yellow 14 azo dye. Under visiblelight irradiation, addition of optimum amount of H2O2and K2S2O8 was shown to increase the degradation rateof Acridine Orange in the presence of ZnO photo-catalyst (Pare et al. 2008). Sobana and Swaminathan2007 observed that addition of KBrO3 and (NH4)2S2O8was shown to increase the photocatalytic degradationof Acid Red 18 by ZnO. However, the addition of H2O2was reported to decrease the photocatalytic degradationrate which was attributed to its low adsorption on theZnO surface. Both decolorization and degradation ofReactive Yellow 14 by UV/TiO2 system were reportedto increase in the presence of H2O2, (NH4)2S2O8, andKBrO3, up to a certain dosage beyond which theenhancement was negligible (Muruganandham andSwaminathan 2006b). The effect of oxidants on thedegradation of 4-flurophenol was shown to be in theorder of IO4

->BrO3->S2O82

->H2O2>ClO3- (Selvam et

al. 2007). After 15 min irradiation at pH 4, thedegradation of 4-flourophenol were observed to be79.86%, 66.89%, 60.88%, 53.85%, and 46.35% in UV/TiO2/IO4

-, UV/TiO2/BrO3-, UV/TiO2/H2O2, UV/TiO2/

S2O82-, and in UV/TiO2/ClO3

- processes, respectively.IO4- was found to be most efficient oxidants due tothe generation of a number of highly reactiveintermediate radicals such as IO3

., OH., and IO4.. The

optimum concentration of H2O2 addition is found to beabout 1.2 mM/L for the degradation of MO in thepresence of 0.5 wt.% PtTiO2/Zeolite systems (Huanget al. 2008). Under visible light irradiation, theinfluence of oxidants on the degradation of Acid Red88 by AgTiO2 was found to be peroxomonosulfate>peroxodisulfate>hydrogen peroxide (Anandan et al.2008). Chiou et al. (2008a) investigated the effect ofH2O2 addition from 1.77 to 88.2 mM on thephotocatalytic degradation of phenol. The addition ofH2O2 from 1.77 to 8.82 mM leads to an increase inremoval efficiency from 58% to 84% within 3 h. Incontrast, phenol is completely degraded within 2.5 and1 h when the level of H2O2 increases to 44.1 and88.2 mM, respectively.


6 Combined Approach

In recent years, photocatalysis has been combined withother physical and chemical means of treatment toimprove the efficiency of photocatalysis. Generally,integration of several methods provides high treatmentefficiency compared with individual treatment. TiO2 hasbeen combined with ozonation, membrane filtration,and ultrasound technology for the treatment of variousorganic compounds. Chen and Smirniotis (2002)indicated the enhancement of photocatalytic degrada-tion of phenol and chlorophenol by ultrasound, UVirradiation with photocatalysis results in 76% destruc-tion in 3 h of treatment. Combination of ultrasound andphotocatalysis results in almost complete destruction ofphenol in about 150 min. Synergistic effects observedin combined approach are attributed to desorption oforganic substrate and degradation by products from thecatalyst surface and de-agglomeration of the catalyst.Mrowetz et al. (2005) reported that the degradation rateconstant of Acid Orange 8 in ultrasonic irradiation andthe combined technique was 0.1, 1.81, and 3.32910-4 s-1, respectively, clearly indicating the synergisticeffects. For Acid Red 1 and 2 chlorophenol, the rateconstants in same order are 0.038, 1.22, and 2.89(10-4s-1) and 0.59, 0.88, and 1.58(10-4s-1), respec-tively. Li et al. (2003) reported that the degradation rateconstants of reactive Brilliant Red X-3B using TiO2/UV/O3 system is shown to be 2.563.2 times higher incomparison to the maximal rate constants of TiO2/UV/O2, 4.689.8 times of maximal rate constants withTiO2/UV. For the degradation of catechol, the order ofremoval efficiencywas demonstrated to be O3>UV/TiO2/O3>UV/TiO2/O2>UV/TiO2. Within 6 min reaction time,the decolorization efficiency of Reactive Red 2 wassignificantly higher in the case of UV/TiO2/O3 systemcompared with UV/TiO2 system (Wu et al. 2008).Oyama et al. (2009) indicated significant mineralizationof bisphenol A in UV/TiO2/O3 system as compared witheither ozonation or UV/O3 and to the UV/TiO2/O2system. Fernando et al. (2005) reported that thedegradation rate constant of photocatalytic ozonationprocess was higher compared with TiO2 photocatalysisfor phenol, p-chlorophenol, and p-nitrophenol. Photo-catalytic ozonation process exhibited the highest miner-alization efficiency of phenols. Zou and Zhu (2008)investigated the comparative efficacy of UVA/TiO2and UVA/TiO2/O3 system for the removal of color,A254, and total organic carbon (TOC). By means of

TiO2/UVA process, color removal was 33% after40 min treatment. For UVA/TiO2/O3 system, colorremoval was greater than 50% with a significantremoval within first 10 min. The highest color removalrate (70.2%) was achieved by the combined treatment ofphotocatalytic oxidation and ozonation. The co-treatment of ozonation and photocatalytic oxidationincreased TOC removal rate by nearly 50% comparedwith the process when ozone was used as a pre-treatment for photocatalysis. Mozia et al. (2009)compared the effectiveness of photocatalysis and hybridprocess photocatalysis-membrane distillation (MD) forthe degradation of Acid Red 18. The degradation rateconstants were higher for 10% or less than during thehybrid process. This effect was attributed to theconcentration of feed solution during hybrid process.Grzechulska-Damszel et al. (2009) studied the effective-ness of TiO2 photocatalysisnanofiltration (NF) mem-brane system for the degradation of Acid Red 18.However, the integration of post-NF membrane systemdid not result in complete removal of organic matterfrom solution when catalyst was used in immobilizedform. Suspended TiO2/UF/MD system was reported tobe efficient for the removal of organic matter from thesolution treated by TiO2 photocatalysis compared withthe TiO2 photocatalysisNF system. The ion rejectionefficiency was reported to be similar for both systems.Gonzalez and Martinez (2008) compared the efficiencyof sonophotocatalytic system using nano-size anataseand rutile TiO2 powders with ultrasound alone for thedegradation of Methyl Orange. Under the conditionsstudied, the pseudo first-order-rate constants of the threecases were shown to be 0.0179, 0.0068, and0.0017 min-1, respectively. The sonophotocatalyticactivity of nano-size anatase was found to be efficientcompared with nano-rutile TiO2 powder. Bejarano-Perezand Suarez-Herrera (2007) reported that the kineticconstant of the Congo Red oxidation is 2.6 timeshigher than the one obtained in the absence ofultrasound and in the presence of UV light and TiO2P25. It was also observed that the degradation rate ofMO in the presence of both ultrasound and UV lightis significantly higher compared with either sonoca-talytic or photocatalysis. Zhang et al. (2006) studiedthe combined photocatalytic membrane technique forthe removal of Direct Black 168 dye. The removal ofDirect Black 168 with UV irradiation alone wasachieved at 70%, and the retention of the dye withmembrane filtration alone was reached at 67% after


300 min in the continuous system. However, whenthe photocatalysis system was combined with mem-brane techniques, the removal efficiency of the dyewas increased to 82% after 300 min under theconditions investigated. Wang et al. (2008) comparedthe sonophotocatalytic degradation of Methyl Orangeusing Degussa P25 (55.07 m2/g), Yilli TiO2(10.45 m2/g), and AgTiO2 (Ag loaded on YiliTiO2). The degradation rate constants obtained withcombined approach were reported to be 5.810-3,1.1710-2, and 3.5810-2 min-1, respectively for YillTiO2, P25, and AgTiO2. However, the degradationrate constants were shown to be 3.210-3, 9.810-3,and 2.5910-2 min-1 in photocatalysis in the sameorder. Reported results indicated that the combinationof sonolysis and photocatalysis enhanced the MOdegradation. Comparatively, the photocatalytic activ-ity of AgTiO2 nanoparticles was much higher thanthat of Degussa P25 and Yilli TiO2. At pH 7, thedecolorization rates of Reactive Red 2 were shown tofollow the order of UV/US/TiO2(0.94 h

-1)>UV/TiO2(0.85 h

-1)>US/TiO2(0.25 h-1)>US (Wu and Yu

2009).

7 Film Fabrication Technique and CalcinationTemperature

Nanocrystalline titanium oxide powder for catalyticpurposes can be applied by a variety of techniquesincluding solgel method, solvothermal process, reversemiscellar, hydrothermal method, and electrochemicalmethods. There have been several investigations onphotocatalytic degradation of various dyes by titaniananoparticles; however, information on the titania filmfabrication routes on the photocatalytic degradation ofdyes is still desirable. Song et al. (2009) investigated theeffect of titania films with different nanostructures ofnanorods (NR), solgel film, nanotubes (NT), andnanoparticle aggregates on the photocatalytic degrada-tion of RB, Methylene Blue, and Methyl Orange. Inthe case of RB degradation, the observed order wasreported to be NR>DP>SG>NT, and the order wasshown to be NR>SG>DP>NT for the degradation ofMB. Nonetheless, an order of NT>DP>SG>NR wasfound to observe for the degradation of MO.Bessekhouad et al. (2004) studied the effect of alkaline(Li, Na, and K) doped TiO2 prepared by solgel routeand impregnation method on the photocatalytic degra-

dation of malachite green oxalate. The crystallinitylevels of catalysts are found to be largely dependent onboth the nature and the concentration of alkaline. Thebest crystallinity is obtained for Li-doped TiO2 and islowest for K-doped TiO2. For a given alkalineconcentration, the catalyst prepared by the impregna-tion technique was shown to be more efficient thanthose prepared by solgel route. For 5% Li-doped TiO2catalyst prepared by impregnation method, the half lifeof malachite green is reported to be twice shorter thanthat observed for P25 TiO2. However, the catalystprepared by solgel route, the presence of Li reducesthe catalyst photoactivity. Indeed, the photoactivity ofundoped catalyst is shown to be better than Li-dopedTiO2. Structure and size of TiO2 crystallites aresignificantly dependent on calcinations temperature.Thermal treatment of TiO2 gels at higher temperaturepromotes phase transformation from thermodynamical-ly metastable anatase to more stable and condensedrutile phase. As the dehydration process occurs duringheat treatment, crystallites grow to dimensions largerthan those of the original particles. Pecchi et al. (2001)studied the effect of calcinations temperature on thephotocatalytic degradation of pentachlorophenol usingsolutiongel method under different pH. Increasingcalcinations temperature results in reduced surface areaand increased crystallinity and particle size. Thedegradation rate constant was shown to increase asthe calcinations temperature increased from 300C to500C for pH values 3 and 5. Nonetheless, the rateconstant was higher at 300C and 500C comparedwith 400C at pH 9. Peng et al. (2005) noticed thatcompared with undoped sample and P25, highestdegradation of RB was achieved using La-dopedTiO2 at optimum calcinations temperature of 300C.This effect was ascribed to larger surface area due tothe presence of lanthanum ion in the high thermalstable mesostructures TiO2. However, the degradationrate was shown to decrease as the calcinationstemperature increases from 300C to 700C. Thereduction of photocatalytic activity was reported to berelated with the decrease in specific surface area from460.6 to 102.1 m2/g. After calcinations at 900C, itwas also indicated that the anatase crystallites begin togrow extensively and transform to rutile phase andthen segregate from mesostructures. Subsequently, thewhole mesostructure is completely destroyed (Chen etal. 2007). The surface areas are in the decreasing orderof TiO2>4.6% K

+ doped>14.3% K+ doped, 4.6% K+


doped>TiO2>14.3% K+ doped, 4.6% K+ doped>14.3%

K+ doped>TiO2, and 14.3%K+ >4.6%K+>TiO2 when

the samples were calcined at 400C, 600C,700C, and850C, respectively. Similar trends of the dependenceof pore volume, micropore volume, external surfacearea, and micropore surface area on the K+ content andcalcination temperature are reported. Of all the samplestested, 4.6% K+ doped TiO2 calcined at 700C wasshown to be efficient for the degradation of Supra Bluedye under the conditions studied. Behnajady et al.(2009) investigated the effect of furnace temperatureon the photoactivity of immobilized ZnO for thedegradation of Acid Red 88 for range of 400C to500C. Degradation rate was shown to increase from0.0257 to 0.0208 min-1 with increasing temperaturefrom 400C to500C, which is attributed to theincrease in particle size from 42 to 67 nm and therebyreduced availability of surface area. Zheng et al. (2007)studied the effect of calcinations temperature on thephotocatalytic degradation of reactive Brilliant RedX-3B for a range of 250550C. Under the conditionsinvestigated, the highest degradation was achieved at350C. Using a carbon doped-TiO2, Xiao and Ouyang(2009) observed that the reduction in surface area from20 to15.5 m2/g is due to the increase in calcinationtemperature from 500800C. This would lead to thedecrease of active sites on which reactants couldadsorb. Under the conditions tested, decolorization ofMB was reported to decrease from 49.16% to 39.16%with increasing calcination temperature. The order ofphotocatalytic activity and the formation rate of .OHradicals per unit surface area was shown to be closelyrelated. An optimum temperature of 600C wasalso noticed for the photocatalytic degradation ofMethylene Blue and the formation rate of .OH radicals.Wen et al. (2009) indicated that no significant changeson the photocatalytic degradation of MB were ob-served as the calcination temperature increases from500C to 600C. The degradation rates were higherin the case of Co doped TiO2 calcined at 500C and600C, compared with that of one calcined at 700C.This effect was attributed to the presence of largersurface area. It is well-known from the literature thatcalcination temperature is an important factor thatprobably influences the crystalline sizes, morphology,and surface area of TiO2 which can clearly affect thephotocatalytic activity. Lim et al. (2007) reported thatthe BET surface area of C-doped TiO2 decreased from200 m2/g (200C) to 80 m2/g (400C). A rapid

decrease of specific surface area from 300C onwardsis a consequence of crystallization and growth ofanatase. At calcination temperature above 700C, thespecific surface area was

STiO2 was the visible light catalyst (Tian et al. 2009).Talebian and Nilforoushan (2009) studied the effect ofannealing temperature (250550C) on the photocata-lytic activity of In2O3 film for the degradation of MB.The optimum annealing temperature was 550C, andthe activity of In2O3 film was directly related to degreeof crystallinity at higher temperature. Gorska et al.(2009b) studied the effect of calcinations temperature(350750C) on the photocatalytic degradation of0.21 mM phenol under UV (25400) in the presence of acatalyst-obtained hydrolysis, followed by calcination at250C for 3 h. Using a carbon doped TiO2 prepared byTiCl4 hydrolysis in tetrabutylammonium hydroxide(calcinations at 400C, 1 h), 70% TOC reduction of0.25 mM 4-CP was reported by Shakthivel et al.(2004). Porter et al. (1999) examine the microstructuralchanges in Degussa P25 due to heat treatment. TheTiO2 powder was annealed from 600C to 1,000C.Under UV irradiation, the apparent crystallite size andrutile content of catalyst increased with increasingcalcination temperatures, whereas the specific surfacearea and the rate of phenol degradation was reported todecrease. Sonawane and Dongare (2006) tested theeffect of calcinations temperature (275500C) on thephotocatalytic activity of 2%-Au/TiO2 for the degrada-tion of phenol under solar light. The catalyst calcinatedat 275C exhibited the highest activity compared withthe sample calcinated at higher temperature. Using ZnOupon solar irradiation, the photocatalytic degradation of4-nitrophenol increases from 52.9% to 74.4% as thecalcinations temperature increases from 110C to 300Cdue to higher surface area and lowest crystallite sizes(Parida et al. 2006). Zhang et al. (2004) studied theeffect of calcinations temperature (300900C) on thephotocatalytic degradation of MO using ZnOSnO2.

At 350C, the higher photocatalytic activity of coupledoxides was reported due to variation in phasecomposition and particle size. Using Z2S, an optimumtemperature of 600C was found for the degradation ofMO (Cun et al. 2002).

8 Doping and Mixed Semiconductor

The recombination of photogenerated electrons andholes are believed to be reason for the low photo-activity of TiO2. Structural imperfections in the TiO2lattice generate trap sites which may act as recombi-nation centers, leading to a decrease in the levels ofelectrons and holes. In order to enhance the TiO2photocatalysis as well as the response into the visiblespectrum of solar light, TiO2 has been doped withcertain transition metals, non-metals, and ionic com-ponents. Doped ions can also act as charge trappingsites and thus reduce electronhole recombination.The effect of doping on the activity of photocatalyst isgoverned by several factors, e.g., the type andconcentration of dopant, preparation method, thestructure and the initial concentration of the pollutants(dyes and phenols), and physico-chemical propertiesof the catalyst. Both positive and negative resultshave been reported from doping with metal ions. Theincrease in charge separation efficiency will enhancethe formation of both free hydroxyl radicals andactive oxygen species (Kato et al. 2005). The amountsof doping concentration along with a summary of thedyes and phenolic compounds degraded using dopedcatalyst are shown in Table 4. Upon 45 min UVirradiation, >99% degradation of Methyl Red wasreported with Ag doped TiO2 whereas only 85% wasdegraded in the presence of TiO2 (Sahoo et al. 2005).Liu et al. (2005) compared the photocatalytic activi-ties of N-doped TiO2 and Degussa P25 for thedegradation of three azo dyes: AO7, MX-5B, andRB5 in the presence of UV and solar light. Some 95%decolorization of AO7 was reported to occur in 1 hunder UV illumination and the doped TiO2 wasreported to be four times more efficient comparedwith bare TiO2. In addition, 70% dye removal wasachieved within 3 h whereas no detectable dyedecolorization was reported to obtain under solarirradiation. Under the tested conditions, 0.2 mmolfluoride ion is shown to be the optimum concentrationfor the efficient removal of Reactive Orange 4


(Vijayabalan et al. 2009). Ag and Pd modified TiO2were shown to have improved photocatalytic activityfor the degradation of p-chlorophenol compared withDegussa P25 which was coupled with better chargeseparation and therefore, to slower recombination(Kirilov et al. 2006). Under the conditions tested,Gupta et al. (2006) reported that more than 90%degradation of CV and MR mixtures was achieved inthe presence of Ag doped TiO2 compared with onlyabout 70% with TiO2. Anandan et al. (2008)compared the effectiveness of 8 wt.% AgTiO2 forthe degradation of Acid Red 88 in the presence ofelectron acceptors such as under visible lightillumination. In comparison to TiO2, the nano-sizedAgTiO2 exhibited seven times higher photodegrada-tion rate in the presence of peroxomonosulfate while2.7 times higher in the case peroxodisulfate. Andronicand Duta (2007) noticed that optimum photocatalyticbehavior of Cd doped-TiO2 was found to be at 0.1 at.% for cadmium acetate and 0.5 at.% for cadmiumchloride for the degradation of MO and MB,respectively. Carbon doped TiO2 was reported to beefficient for the degradation phenol under visible lightirradiation (Lee et al. 2008). This effect was attributedto the shifting of absorption edge to a lower energy,thus improving the photocatalytic activity in thevisible region.

Ren et al. (2007) observed a high surface area andsignificant absorption in the visible light (>420 nm)for a carbon doped TiO2 compared with undoped

counterpart. The doped TiO2 was reported to showhigher photocatalytic activity for the degradation ofRhB compared with undoped TiO2. In comparison toundoped TiO2, 0.5% AgTiO2 was shown to achieve1.53 times higher degradation of 2,4,6-trichlorophenolby Rengaraj and Li (2006). Nonetheless, Ag contentbeyond the stated level was reported to have adetrimental effect on the photocatalytic activity ofTiO2. Under the conditions tested, Silva et al. (2009)noticed an optimum Ce content of 0.6% (w/w) intoTiO2 matrix for enhanced degradation of 4-chlorophenol in the visible light. In addition to theretardation of phase transformation, this behavior wasrelated to the shift of TiO2 absorption edge towardslonger wavelengths, by reducing the band gap oforiginal material. Subramanian et al. (2008) examinedthe effect of Co doping on the structural and opticalproperties of TiO2 to obtain a nanocrystalline films ofpure and doped TiO2 with grain size 10 to 25 nm. Thefilms were reported to be transparent in the visibleregion, and the absorption edge shifted towardsthe higher wavelength with increasing Co content.The observed decrease in band gap was attributed tothe formation of Co impurity band into the TiO2energy bands. Venkatachalam et al. (2007a) observedthat under identical and optimal experimental condi-tions, 1 mol% Mg2+ and Ba2+ doped nano-TiO2, nano-TiO2, and P25 required 300,360, and 450 min,respectively, for the complete mineralization of4-chlorophenol. The photocatalytic degradation of 4-

Table 4 The influence of dopant concentration on photocatalytic activity of photocatalyst

Pollutant Light source Photocatalyst Doping (%) Optimum doping (%) Reference

Methylene Blue Visible C-TiO2 03.2 1.25 Wong et al. 2008

Supra blue BRL dyes Visible K+TiO2 014.3 4.6 Chen et al. 2007

XRG dyes UV/Visible Fe3+/Fe2+TiO2 0.030.15 0.09 Zhu et al. 2004

Methyl Orange Solar Eu2+TiO2 0.51.0 1.0 Yi et al. 2007

Methyl orange Solar Gd3+TiO2 0.11.0 0.5 Yi et al. 2007

p-chlorophenol UV Ag+TiO2 0.10.5 0.5 Kirilov et al. 2006

p-chlorophenol UV Pd2+TiO2 0.10.5 0.5 Kirilov et al. 2006

Methelyne Blue Visible Pd2+TiO2 05.0 5.0 Chan et al. 2009

Phenol Visible Fe3+TiO2 0.45.1 1.0 Adan et al. 2007

Methyle Orange UV Mo6+TiO2 01.5 1.0 Yang et al. 2004

4-Chlorophenol Visible Ce4+TiO2 01.0 0.6 Silva et al. 2009

Methylyne Blue UV V5+TiO2 0.255.0 0.5 Tian et al. 2009

2,4-Dichlorophenol Visible V5+TiO2 0.255.0 1.0 Tian et al. 2009

Methelyne Orange UV Pt6+TiO2 03.0 1.5 Huang et al. 2008


CP over TiO2 and Mg2+ and Ba2+ doped TiO2

indicated the higher activity for doped TiO2. Enhancedadsorption of 4-CP on the catalyst surface and smallerparticle size due to Mg2+ and Ba2+ loadings areindicated to be the cause for higher activity of thecatalysts. The BET surface area of both S-doped TiO2and undoped TiO2 is shown to decrease significantly asthe calcinations temperature increases from 300C to500C (Raileanu et al. 2009). The presence of S dopantonto TiO2 matrix slightly improve their photocatalyticactivities and led to better chlorobenzene removalcompared with undoped TiO2. The photocatalyticactivity is also increased slightly as the sinteringtemperature increases. Regardless of the organicchloride pollutant, the removal has been reported tovary between 96% and 100% under the conditionsstudied. Lattice defects induced by dopant wereindicated to strongly influence the photocatalyticactivity. Irrespective of the number of coatings andthe irradiation times, the Ag and S-doped TiO2exhibited higher removal compared with undopedTiO2. The S-doped TiO2 were shown to be moreefficient than the Ag doped-TiO2 which was attributedto the presence of anatase crystalline phase of TiO2 andto a more significant surface concentration of dopant.Xie and Zhao (2008) indicated that increasing reactivetemperature from 40C to 140C was reported toenhance the crystalline size from 3.8 to 6.0 nm. F-Nand S-N co-doping resulted in a new strong absorptionband in the visible range of 400620 nm, and with thedecrease in reactive temperature, the absorption edgeshifted toward higher wavelength. In the visible region(420 nm), photocatalytic activity of doped-TiO2 forthe degradation of MO was reported to be three timesmore effective than that of bare P25. Sheng et al.(2008) noticed that N-Br-codoped TiO2 readilyphotodegrade MB under visible light irradiation incomparison to N-doped TiO2. This behavior wasattributed to its anatase crystalline framework, lowelectronhole recombination rate, and high absorbancein the visible light. In contrast to P25 TiO2, photo-catalytic activity of N, C-doped TiO2 was four timeshigher for the degradation of phenol under visible light(>400 nm) irradiation (Gorska et al. 2009a). Thedegradation rate of RB by La-doped-TiO2 was highercompared with undoped TiO2 for all the calcinationtemperature investigated (Peng et al. 2005). TiO2doped with 4.6% K+ and calcined at 700C wasdemonstrated to exhibit higher activity compared

with other samples when the doping level of K+ andcalcinations temperature are 0143.% and 4001,000C, respectively (Chen et al. 2007).

Wong et al. (2008) studied the influence ofcrystallinity and carbon content on the photocatalyticdegradation of MB under visible light by varying thecarbon content from 03.2%. Under the conditionstested, the highest degradation was achieved at 1.25%carbon content. Zhu et al. (2004) observed that TiO2doped with 0.09% FeCl3 and 0.09% FeCl2 exhibitedhigher photocatalytic activity than undoped TiO2 forthe decolorization of Active Yellow XRG dye dilutedin water under both UV and visible light irradiation.Fe2+/Fe3+ doped TiO2 was reported to have highspecific surface areas and small crystal sizes. TiO2doped with FeCl3 was found to have better catalyticactivity for the degradation of XRG than those dopedwith FeCl2. Yi et al. (2007) studied the effect ofEu2+/Gd3+-co-doping onto TiO2 for the degradation ofMO under natural sunlight. The suitable dopingcontent is reported to be 0.51.0% for Eu2+ and0.11.0% for Gd3+; the optimum doping content forEu2+ is 1.0% and for Gd3+ 0.5%. Coleman et al.(2005) noticed that the addition of Ag or Pt had noeffect on the photocatalytic degradation of endocrinedisrupting chemicals at levels found in water. Thiswas attributed to the high concentration of holes andhydroxyl radicals in the systems compared with thelow level of organic matters to be degraded. At highconcentration, a significant increase in the reactionrate observed for bisphenol A and resorcinol over Pt/TiO2. Under UV and visible light irradiation, themesoporous PdTiO2 film with a molar ratio of Pd/Ti=0.05 exhibited the highest activity for the degradationof MB due to smaller crystallite size and lowercrystallinity in mesoporous TiO2 film that that ofnonporous TiO2 film (Chan et al. 2009). The photo-catalytic activity for aqueous phenol degradation ofanatase titania is shown to be enhanced upon dopingwith Fe3+ up to ca. 1 wt.% (Adan et al. 2007). Inparticular, maximum activity is observed for the0.7 wt.% Fe3+ sample; in contrast, doping above ca.3 wt.% does not produce any significant enhancementin the photocatalytic activity. Under solar radiation,La3+-TiO2 containing 0.4 mol % lanthanum showedhighest surface area(124.8 m2/g), lowest crystallitesize (8 nm), and exhibited 50% more degradation ofMethylene Blue compared with pure TiO2(Parida andSahu 2008). Zheng et al. (2007) noticed that the


coupled ZrO2ZnO system with 2.5% Zr was moreefficient compared with ZnO for the degradation ofreactive Brilliant Red X-3B. Wen et al. (2009)investigated the effects of I and F codoped TiO2(prepared by solgel-impregnation method) on thephotocatalytic degradation of MB. Co doping with Iand F was shown to enhance the degradation of MBunder simulated sunlight irradiation. The photocata-lytic activity of as prepared I-F codoped TiO2 wasreported to be much higher than that of pure, I-doped,and F-doped TiO2 when the molar ratios of I and F toTi were maintained at 10. The difference in thephotocatalytic activity was ascribed to larger surfacearea and stronger absorbance in the visible light rangeby doping with I and F. The photocatalytic activity ofco-doped TiO2 was higher than pure TiO2 under UVand visible lights.

Bouras et al. (2007) compared the photocatalyticdegradation of Basic Blue in the presence of pure andthree cationic doped (Fe3+, Cr3+, and Co2+) TiO2under UV-vis and visible light irradiation. In all threestudied cases, the efficiency of UV-vis photodegrada-tion dramatically decreased in the presence ofdopants. The decrease was faster in the case of Fe3+

and Co2+ dopants and slower in the case of Cr3+. Li etal. (2003) indicated that the photocatalytic activity ofCarbon-Black-modified nano-TiO2 (CBTiO2) thinfilms was 1.5 times higher than that of TiO2 thinfilms in degrading Reactive Brilliant Red X-3B. Tianet al. (2009) studied the photocatalytic activity of Vdoped-TiO2 for the degradation of MB and 2,4-dichlorophenol using UV and visible light. In thepresence of UV light, 0.5% V-TiO2 was reported to beefficient for the degradation of MB relative to pureTiO2. Under visible light irradiation, the degradationrate of 2,4-dichlorophenol over 1% V-TiO2 is shownto be two times higher compared with undoped TiO2.Baiju et al. (2007) tested the photocatalytic activity ofTa2O5 doped TiO2 for the degradation of MB byvarying the amount of Ta2O5 from 1 to 10 mol %.Under the conditions examined, 2 mol % of tantalumwas shown to be efficient compared with pure TiO2.Zhou et al. (2009) studied the photocatalytic activityof Nd-doped ZnO for the degradation of ReactiveBlue 4 in aqueous suspension by varying the dopantconcentration from 13 mol%. Some 2.5 mol %Nd-doped ZnO was demonstrated to be efficient forthe enhanced photocatalytic activity under the ex-perimental conditions. Naeem and Ouyang (2009)

studied the effect of Fe3+ doping (03.0 mol% Fe3+)on the photocatalytic activity of TiO2 for thedegradation of phenol under UV light. Fe3+-dopedTiO2 was reported to possess the anatase structurewith a range of crystal size 811 nm. The highestdegradation efficiency was found at 0.5 mol% Fe3+-doped TiO2. After 2-h calcination at 600C, the orderof photocatalytic activity of La-doped TiO2 for phenoldegradation was demonstrated to be 1>1.5>3>0.5>5>0 mol% La (Liqiang et al. 2004). Mohammad andAl-Esaimi (2006) observed that 98% degradation ofMB was achieved after 75 min UV light irradiation inthe presence of 2V/TiO2SO4 catalyst, whereas only78% degradation was obtained using 2V/TiO2 in thesame time. The variation in degradation efficiencywas related to the difference in vanadium and sulfatecontent. Under visible light irradiation, the photo-catalytic activity of S-doped TiO2ZrO2 was shown tobe higher compared with unmodified STiO2 andDegussa P25 for the degradation of Rhodamine B(Tian et al. 2009). The observed effect has beenattributed to the larger surface area, smaller crystalsize, porous structure, and more surface hydroxylgroups in the catalyst. Ghasemia et al. (2009)tested the effect of transition metal ions on thephotocatalytic degradation of Acid Blue 92 underUV light. The effect of transition metals ions on thephotocatalytic degradation rate and efficiency wasshown to be Fe-TiO2>CoTiO2>CrTiO2>MnTiO2>CuTiO2>NiTiO2>ZnTiO2>TiO2. The photo-catalytic degradation of MB by various TiO2/SnO2thin films photocatalyst was investigated by varyingthe amount of tin. For all TiO2/SnO2 thin films, thedegradation efficiency was higher compared withpure TiO2 films, exhibiting the highest for TiO2/SnO2 films with 9% Sn. The behavior was related togreater porosity and smaller band gap energiescompared with pure TiO2 films (Martinez et al. 2005).

Yuan et al. (2002) studied the influence ofco-doping of Zn2+ and Fe3+ on the photocatalyticdegradation of phenol under solar light irradiation.The co-doping of 0.5 mol% Zn2+ and 1 mol % Fe3+

onto TiO2 was found to be two times more efficientcompared with pure TiO2. This behavior was attrib-uted to the coupled influenced of the co-dopant andtitania energy bands. Qu et al. (2009) studied theeffects of the co-addition of Zn2+ and sodiumdodecylbenzenesulfonate (DBS) on photocatalyticdegradation of RB relative to the undoped anatase


TiO2 nanoparticle films. In comparison to undopedTiO2 film, the addition of Zn

2+ was shown to improveboth the photocatalytic activity and the hydrophilicity,which was attributed to surface oxygen vacancies.However, the co-addition of Zn2+ and DBS resulted ina film with super hydrophilic behavior. The degrada-tion efficiency was reported to decrease from 43.5%to 35% due to the co-addition of Zn2+ and DBS. Leeet al. (2005) compared the photocatalytic degradationof p-nitrophenol over TiO2 and TiO2/SiO2 nano-particles prepared by the micro-emulsion methodusing PFPE-NH4 surfactant. TiO2/SiO2 (80:20)nanoparticles were reported to have a higher photo-catalytic activity than pure TiO2 and the TiO2/SiO2(90:10) particles. This effect has been attributed to thedecrease of crystallite size from 17 to 133 nm with anincrease in silica content. Chen et al. (2009) studiedthe effect of Cu2+ doping (00.2 wt.%) on thephotocatalytic degradation of Methyl Orange usingTiO2/SiO2. The optimum dopant content was about0.10 wt.%. The photocatalytic activity of Cu2+-dopedTiO2/SiO2 system was higher than that of TiO2/SiO2.This behavior was attributed to the trapping of bothphotogenerated holes and electrons, and inhibition ofthe electronhole recombination, leading to theincrease of photocatalytic activity. Arabatzis et al.(2003) observed a two times faster degradation ofMethyl Orange in the presence of Au/TiO2 comparedwith that obtained with the original TiO2 material.Sonawane and Dongare (2006) tested the effect ofAu doping (12% Au/TiO2) on the photocatalyticdegradation of phenol in solar light irradiation of4.55 Wm2/day. In comparison to undoped TiO2,TiO2 doped with 12% Au showed 22.3 timeshigher photocatalytic activity. Using solgel method,Venkatachalam et al. (2007b) examined the effect ofZr4+ doping(0.55.0 mol % Zr4+) onto TiO2 matrixfor the photocatalytic degradation of 4-chlorophenol.The photocatalytic activity of 3.0 mol% doped TiO2samples were reported to be higher than that of nano-TiO2 and Degussa P25. Enhanced adsorption of 4-CP over the catalyst surface and decrease in particlesize are indicated to be the reason for high activity ofthe catalyst. Bellardita et al. (2007) investigated theeffect of metal loading (W, Co, and Sm) onto TiO2matrix on the photocatalytic degradation of 4-nitrophenol. In comparison to Degussa P25, TiO2loaded with 1% W and Sm caused a significantimprovement in the photocatalytic activity under UV

irradiation whereas Co was beneficial only undervisible light irradiation. This effect was ascribed toan increased charge separation of the photogeneratedelectronhole pairs.

9 Conclusion

Based on the recent representative studies, this reviewfocuses on the role of various operating parameters onthe photocatalytic degradation of various dyes andphenols, and reports the main advances.TiO2 has beensuggested to be efficient for the degradation andmineralization of various toxic organic pollutants, e.g., phenols and dyes in water in the presence of UV,visible or solar light, and oxygen. The findings alsosuggest that various operating parameters such as typeof photocatalyst, light intensity, pollutant types, andinitial concentration, amount of catalyst, initial pH ofthe reaction medium, mode of catalyst application,oxidizing agents/electron acceptors, and presence ofionic components in solution can significantly influ-ence the photocatalytic degradation rate of phenolsand dyes. Optimization of degradation parameters iscrucial from the perspective of efficient design andapplication of photocatalytic oxidation process toensure sustainable operation. The potential of thistechnique under multi-component pollutants needsfurther attention to yield stable pollutant removalthrough the optimization of process parameters. Metaland non-metal doped TiO2 have been reported to beefficient for the improved degradation rate. In spite ofextensive investigations, the commercial exploitationof photocatalysis has been hindered by the lack ofefficient and low-cost visible light harvesting catalyst,a relatively poor understanding of the reactor design,and inadequate scale-up strategies. Future researchshould focus on the development of a more reliablephotocatalyst that can be activated by visible andsolar light or both. In addition, more work is requiredon the modeling of photoreactor to optimize its designfor pollutant degradation. In the literature, there iscurrently little information on the modeling of photo-reactor to optimize its performance. Present researchactivities at CQ University and QUT, Australia focuson the computational fluid dynamics modeling of aflat plate reactor to optimize its design and to predictits performance. Although this review is not exhaus-tive in the scope of photocatalytic degradation of


organic pollutants; however, it addresses the funda-mental principles and recent applications in this area.

Acknowledgement This study is supported under an Austra-lian Research Council (ARC) linkage grant in collaborationwith CM Concrete Private limited and Department of PublicWorks, QLD Government. The authors gratefully acknowledgethe financial support of ARC project. One author is alsograteful for the financial support of the Queensland Govern-ment through the Smart State fellowship scheme.

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