review article -based photocatalytic process for purification of...

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2013 Article ID 319637 14 pageshttpdxdoiorg1011552013319637

Review ArticleTiO2-Based Photocatalytic Process for Purification of PollutedWater Bridging Fundamentals to Applications

Chuan Wang Hong Liu and Yanzhen Qu

Key Laboratory of Reservoir Aquatic Environment Chinese Academy of Sciences Chongqing Institute of Green andIntelligent Technology Chongqing 401122 China

Correspondence should be addressed to Hong Liu liuhongcigitaccn

Received 19 June 2013 Revised 6 July 2013 Accepted 7 July 2013

Academic Editor Jiaguo Yu

Copyright copy 2013 Chuan Wang et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Recent years have witnessed a rapid accumulation of investigations on TiO2-based photocatalysis which poses as a greatly

promising advanced oxidation technology for water purification As the ability of this advanced oxidation process is welldemonstrated in lab and pilot scales to decompose numerous recalcitrant organic compounds and microorganism as well in waterfurther overpass of the hurdles that stand before the real application has become increasingly importantThis review focuses on thefundamentals that govern the actual water purification process including the fabrication of engineered TiO

2-based photocatalysts

process optimization reactor design and economic consideration The state of the art of photocatalyst preparation strategies forprocess optimization and reactor design determines the enhanced separation of photo-excited electron-hole (e-h) pairs on the TiO

2

surface For the process optimization the kinetic analysis including the rate-determining steps is in need For large-scale applicationof the TiO

2-based photocatalysis economics is vital to balance the fundamentals and the applied factors The fundamentals in this

review are addressed from the perspective of a bridge to the real applicationsThis reviewwould bring valuably alternative paradigmto the scientists and engineers for their associated research and development activities with an attempt to push the TiO

2-based

photocatalysis towards industrially feasible applications

1 Introduction

Heterogeneous photocatalysis based on semiconductors haswitnessed rapid progress in the last decades [1ndash3] The semi-conductor photocatalyst of which the electrons in the valenceband can be promoted to the conduction band when beingexcited by adequate photoenergy possesses photogeneratedelectron-hole (e-h) pairs The e-h pairs enable a series ofreductive and oxidative reactions [4ndash6] and some of themfurther result in valuable reactions This method has beeninitially put forward for the extraction of hydrogen energyfrom water via the conversion of photoenergy in the 1970swhen the energy crises has emerged [7] Later in the 1980s ithas been tested as an environmental purification alternativefor water [8 9] and gas [10] as wellThis review focuses on thewater purification by addressing the fundamentals that servesas a connection with the real-time application

Many reports have evidenced that numerous organictoxic compounds often present in water can be removed bythe photocatalyticmethod Chlorinated compounds alkenes

alkanes aromatics dyes and so forth have been tested asthe model pollutants [2 3 11 12] The rationale for TiO

2

photocatalytic method is predominantly based on the oxida-tion of pollutants by means of hydroxyl radicals (HO∙) thatfeature the advanced oxidation technologies (AOTs) [13 14]The HO∙ production can be expressed as follows

H2O + h ℎ]997888rarr ∙OH+H+ (1)

The h (hole) is highly oxidative and the oxidation of water byh leads to a formation of HO∙ radicals which are extremelyactive and nonselective in attacking the substrates in aqueoussolution Meanwhile the electron (e) needs to be scavengedby an electron acceptor Accordingly the photogeneratede-h pairs are separated leading to the transformation ofpollutants Otherwise the photogenerated e-h pairs will beself-combined with an undesired release of thermal energyAs desired the photogenerated e-h pairs should be separatedas far as possible to improve the process performance ofphotocatalysis

2 Journal of Nanomaterials

The TiO2photocatalysis has attracted great attention as

a promising water treatment technology due to quite a fewintrinsic advantages (i) Powerful ability to decompose thepollutants the organic pollutants can be decomposed andeven mineralized due to the dramatic powerful oxidationability of HO∙ Consequently this technique can be utilizedwidely in the cases once the advanced treatment of waterparticularly containing the recalcitrant organic compoundsis demanded (ii) Ambient operating conditions the processcan be realized under ambient operating conditions and thusis acknowledged as quite safe (iii) Low cost the sunlightcan be employed as the energy source and the oxidant isambient O

2 Meanwhile as the TiO

2photocatalyst can be

recycled the cost can be further cut off to run the process(iv) Environmentally friendliness TiO

2are chemically sta-

ble nearly non-toxic or relatively safe even in the harshconditions Thus the TiO

2photocatalysis is thought as an

environmentally-friendly approach even if recent concernsarise on its environmental risks as it is released into theenvironment [15]

Many reports available in recent years have evidencedthat numerous organic contaminants can be decomposed oreven mineralized [16] by photocatalysis and the mechanismgoverning the photocatalytic process on the TiO

2surface has

been disclosed in depth [17] Also efforts for preparationof photocatalytic materials [7ndash9 18ndash22] which include themodification of the commonly used TiO

2photocatalyst

and the design and preparation of new photocatalysts areintensively exerted These efforts inclusively have attemptsto accelerate the degradation process of target substrates byenhancing the separation efficiency of e-h pairs or to extendthe wavelength of excitation light from ultraviolet (UV)regime to visible regime

Along with the lab-scale investigation of simulated waste-water some pilot-scale tests have been performed [23 24]It could be understood that although the results served todemonstrate the technical feasibility for water purificationtheTiO

2photocatalysis is still facing a series of technical chal-

lenges for a large-scale installment in wastewater treatmentplants (WWTPs) It must be noted that challenges such asfabrication of active TiO

2photocatalyst rapid separation and

recycling of TiO2after use and optimization of the overall

process are determined by the fundamentals In particularthe connection between the fundamentals and the actualscenario should be well understood by both the scientistsand engineers as they are performing their own individualtasks

However the fundamentals which have been well estab-lished in the literature are far from being fully understoodor sophisticatedly applied in the actual application Also theoperating parameters optimized in laboratory often suffera mismatching with the actual scenario In view of theproblem this review highlights the connection between thetwo tasks which is expected to bring valuable informationfor the academia and industry Along with this highlight theeconomic consideration was covered which eventually leadsto a balance between the various variables working in theTiO2photocatalytic process

2 Water Purification of TiO2

Photocatalysis

With the increasing demand of the water sources novel tech-nologies which serve to purify the polluted water with pow-erful ability to decompose the pollutants in energy-saving andenvironmentally-benignmanners are becoming essential Aswell known conventional technologies such as filtrationcoagulation adsorption and precipitation principally serveto transfer the pollutants from one phase to another Thenewly emerging membrane separation technology [25] alsobelongs to such category Following them further steps mustbe taken to treat or dispose the transferred pollutants In thisregard chemical oxidation featured by their ability to destroythe molecular structures of pollutants is an indispensibleoption During the processes the toxicity of the involvedorganic moieties is likely to extenuate or aggravate and thusthe degree of pollutant degradation should be in controlMoreover the concept of wastewater will become out of datesince the so-called wastewater is just staying at a stage duringthe overall recycling of water as a resource The TiO

2-based

photocatalytic process harnessing the sunlight as an energysource and ambient O

2as an oxidant for water purification

is capable of exhaustively mineralizing the organic pollutantsand eliminating the toxicity Thus the TiO

2-based photo-

catalysis is accepted as an effective tool to purify the pollutedwater

As listed in Table 1 the water which is polluted by textilepesticides medicine and so forth can be treated by theTiO2-based photocatalysis It should be pointed out that the

TiO2-based photocatalysis is just employed as an individ-

ual method herein Actually however effluents dischargedfrom industrial processes commonly contain recalcitrant ornonbiodegradable pollutants and thus an integrated processcontaining biological and physicochemical units will bemoreapplicable Basically in the front of integrated process theinfluent can be subjected to a physical unit such as filtration orgrilling to separate the solids Following is a physicochemicalunit such as coagulation to remove the colloidal substancesSubsequently a biological unit including aerobic or inaero-bic oxidation serves to degrade most of the biodegradableorganic molecules and thus the water quality is usuallyimproved in terms of the indexes such as chemical oxygendemand (COD) and NH

3-N Finally a chemical means such

as the TiO2-based photocatalytical process is incorporated to

purify the water by finally removing the recalcitrant organicpollutants

From the technical point of view the TiO2-based pho-

tocatalytical process is applicable as one prerequisite issatisfied which is the transparency of water The transpa-rency ensures the transmission of the light The TiO

2-based

photocatalytical process can be installed as a pretreatmentunit or an advanced unit If the effluent is in need of furtherpurification to destroy the nondegradable organic pollutantsthe TiO

2-based photocatalytical process follows the bio-

logical unit Alternatively the TiO2-based photocatalytical

process is put in front of the biochemical unit to improvebiodegradability for a better biological oxidization of theorganic substrates

Journal of Nanomaterials 3

Table 1 Engineered TiO2 the associated treated targets reactors and principal operation parameters

TiO2 Water Reactor Principal operation parametersinvestigated

References

TiO2 P25 Pharmaceutical wastewater Sequencingbatch biological reactor

H2O2 dosage TiO2 dosage pHirradiation time and hydraulic

retention time[73]

TiO2carbonaerogel

High-concentration dyewastewater

Photocatalysis-enhancedelectrosorption reactor

Electrochemical adsorptionpotential temperature initial pH

and wavelength[74]

TiO2 P25Wastewater treatment plant

effluentsLab-scale solarphotoreactor

Temperature pH and lightwavelength

[75]

TiO2 nanofiber Hospital wastewater Batch annular slurryphotoreactor (ASP)

Fine bubble aeration lightwavelength and pH

[76]

Nano TiO2 Paper mill wastewater Solar reactor TiO2 dosage pH [59]

C-TiO2 Antibiotics wastewaterA computerized LuzchemCCP-4V photochemical

reactorpH wastewater flow rate [77]

TiO2 P25 Antibiotics wastewater Capacity cylindrical acrylicphotoreactor

pH gas rate light intensity andTiO2 dosage

[78]

TiO2SiO2 beadsOrganic

compounds-containingwastewater

Batch recirculation typephotocatalytic reactor pH wastewater flow rate [79]

TiO2 P25 Textile wastewater Pebble bed photocatalyticreactor

TiO2 pebbles water treatment afterabsorption of reflow exhaust gas

[80]

TiO2 P25 Industrial wastewater A hermetically sealedphotoreactor

Pressure adsorption time andtemperature

[81]

PFTTiO2Toxic heavy metals

wastewaterA column glassphotoreactor Wavelength [82]

TiO2 P25 Secondary effluent A cylindrical glass reactor Amount of TiO2 loaded maximumemission peak

[83]

TiO2Ti anode Textile wastewaterA thin-film

photoelectrocatalyticreactor

pH lamp position and lightintensity

[84 85]

TiO2 P25Pharmaceutical andcosmetic wastewater Common photoreactor pH time catalysts and H2O2

concentration[86]

TiO2 P25 Olive mill wastewater Common photoreactor Light wavelength pH [87]

TiO2 P25Textile dyehouse

wastewaterImmersion well batch-type

photoreactorpH light wavelength temperature

and H2O2 concentration[22]

TiO2SiO2DNP and municipal

wastewaterFixed bed circulation type

reactor Water flow rate residence time [88]

AgndashTiO2 Seawaterpoly (methyl methacrylate)

reactors temperature dosage of bacterialsuspension and O3

[89]

SndashTiO2 MC-LR in surface water Borosilicate glass dish pH light wavelength [90]

nano TiO2 Drinking water Tank-style reactor Permeate flux with a membrane [91]

TiO2 P25 Surface water Photovoltaic reactor Temperature [92]

TiO2 P25 Benzalkonium chloride Annular glass reactor Light wavelength TiO2 dosage [93]NF-TiO2-P25films Drinking water Glass

vessel reactor Light intensity pH [94]

TiO2microsphere

Organiccompounds-containing

wastewaterCylindrical photoreactor Light wavelength concentration of

TiO2 suspensions[66]

TiO2 films MC-LR in surface water Sealed round pyrex reactorTemperature water flow rate pHTiO2 coating surface area and

thickness[95]


Table 1 Continued


References

TiO2(P25)-AGSToxic heavy metal

wastewater A photocatalytic bath pH dosage of catalyst andtemperature

[96]

TiO2 P25 Drinking water Cylindrical Pyrex reactionvessel Catalyst loading light wavelength [97]

TiO2 P25Organic


A thermostated reactor Temperature dissolved oxygenconcentration

[60]

TiO2-modifiednanofiltrationmembranes


wastewaterCell reactor Light intensity temperatures [98]

TiO2 P25 Textile wastewater Immersion well reactor Dosage of TiO2 and H2O2 pH [99]

TiO2 filmsupported on aporous nickelnet

Pesticides Reactor with ozonegenerator

pH ozone dosage treatment timeand temperature

[100]

TiO2nanotubulararrays

Swimming pool water Cylindrical glass reactor pH potential [101]

VB

Self-combination

Ligh

t

OrganiccompoundEnergy

level

O2

CB

HOO∙

HO∙

HO∙

CO2 + H2OH2O OHminus

O2

H2O2

H+

Eg

h+

h+

eminus

∙O2

minus

Figure 1 The scheme of TiO2photocatalytic process

3 Fundamentals

To develop a real TiO2-based photocatalytical process opti-

mization of the operating parameters is of importanceHowever the fundamentals that have been well establishedin laboratory are not utilized sophisticatedly or even over-looked in real application Also the parameters that areoptimized in the laboratory test which has only one substrateare not applicable in the full-scale process The presenceof cosubstrates in wastewater may significantly influencethe degradation of target substrate C Lin and K S Linhave observed that the presence of humic acid (HA) reta-rded the photocatalytic degradation of 4-chlorophenol [26]

Consequently the fundamentals that closely connect the realscenario should be focused on Herein the e-h pair gener-ation and separation adsorption of organic substrate andrate determining step (RDS) of the photocatalytic reactionare addressed These fundaments are analyzed on how todefine amaximized efficiency in real application whereas anysuccessful and competitive real process should get a balancebetween efficiency and economics

31 Importance of Photogenerated e-h Pair Separation TheTiO2photocatalytic process starts with the generation and

separation of e-h pairs which are illustrated in Figure 1 Based


on the mechanism of photocatalytic oxidation proposed byHoffmann et al [27] the reactions on the TiO

2can be

correspondingly expressed as follows

TiO2+ ℎ]lArrrArr (TiO

2minus h) + (TiO

2minus e) (2a)

Redinterface1198961

lArrrArr

119896minus1

(TiO2minus Red)surface (2b)

∙OH+ (TiO2minus h)

1198962

lArrrArr

119896minus2

(TiO2minus∙OH) (2c)

(TiO2minus∙OH) + (TiO

2minus Red)surface

1198963

lArrrArr

119896minus3

(TiO2minusO119909)surface + e

(2d)

It could be understood that upon the photo-excitation theelectrons on the valence band (VB) jump to the conductionband (CB) generating h left on the VB As the electrons andholes initially separate and interact with substrates in thesolution and then combine together an effective conversionof substances is thus resulted in As recombination occursthere is no any conversion of substances

Following the generation of holes and electrons radicalsof HO∙ are generated and considered to work as the principaloxidative species during the photocatalytic process Xiang etal have proposed a concept of ldquoOH-indexrdquo to quantitativelycharacterize the production of HO∙ on different photocat-alytic systems [28] Such index which can bemeasured easilyby photoluminescence technique is using coumarin as aprobe molecule It appears that this index is quite adequateto explain the activity difference in different systems Forexample P25 TiO

2 which is a most available commercial

photocatalyst has the highest ldquoOH-indexrdquo as illustrated inFigure 2

The photoenergy plays an important role in the e-hseparation The energy that is sufficient to generate the e-h pairs must surpass the energy of TiO

2band gap The

equation 119864 = h120582 designates the relationship between theenergy and the associated light wavelength TiO

2(119899-type)

has a band gap of 32 eV Accordingly the light wavelengthmust be shorter than 380 nm which falls into the range ofUV light This requirement determines that a full illumi-nation of the photocatalyst is an indispensible factor whendesigning a real photocatalyticmatrix First an artificial lampmust be installed Lamps with 254 nm and 365 nm as themain wavelength are well commercialized A more powerfulillumination of the photocatalyst certainly accelerates thee-h pair generation High-pressure mercury lamp is alsopowerful whereas its illumination is not stable and thusnot recommended for real application Many lamps can beinstalled together in the photocatalytic reactor to ensurea full illumination while the heat release should also betaken into account as the illumination time lasts a long timeSecond the target water must be sufficiently transparentfor the transmission of the light Accordingly following thepretreatment that serves to remove the particles or speciesthat shadow the light the TiO

2-based photocatalytic process

can bewell employedThird only theUV lamps that aremade

Photocatalysts

OH

-inde

x

100

60

40

80

20

0 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) P25

(2) Am

(3) A

(4) R

(5) ZnO

(6) SnO2

(7) SrTiO3

(8) BaTiO3

(10) V2O5

(13) CdS

(14) CuS

(15) BiOCl

(16) BiOI

(17) ZnS

(19) BiOBr

(20) CuO

(26) NiO

(11) Bi2WO4

(12) CeO2

(18) Cr2O3

(21) MnO2

(22) Fe2O3

(23) BiVO4

(24) ZrO2

(25) La2O3

(9) WO3 (27) Bi2O3

Figure 2 Comparison of OH-index of various photocatalystsArrows represent OH-index = 0 Am A and R denote amorphousanatase and rutile of TiO

2 respectivelyThis figure is taken from the

article of Xiang et al [28]

from quartz glass can be immersed in water Fourth even ifan immobilization of the TiO

2photocatalyst serves to recover

the TiO2 it may bring about a decrease in the e-h generation

due to the decrease in the illumination intensity caused by thewater shadow As a result in real application the suspendedTiO2instead of the immobilized TiO

2is more efficient

Li et al have reported a type of suspended TiO2micro-

sphere which allows both the TiO2suspension and recovery

[29]

32Measure for e-h Pair Separation Following the photogen-eration of e-h pairs a fast recombination of charge carries isthermodynamically favorable and thus yields a low quantumyield of the reactive species for organic degradation Obvi-ously any effort to facilitate the e-h separation and inhibitits recombination is encouraged Many measures includingTiO2modification scavenge of electrons and imposition of

external force are allowed to enhance the e-h separation

321 TiO2Preparation The TiO

2is a cornerstone of the

photocatalytic process In laboratory it is often prepared bymethods such as sol-gel method and hydrothermal methodThe physical morphology is controllable through hydrother-mal preparation while the TiO

2is chemically composed of

two crystalline types of anatase and rutile The rutile type


is more stable and the thermal treatment of anatase typeleads to a transformation to rutile The difference in thedistribution of the two types is considered to determine theassociated catalytic activity Basically pure rutile is found toexhibit a poor activity while a suitable composition may bemore active than the pure anatase type [30] The preassumedcorrelation between the composition of crystal phase andthe e-h pair separation has been clearly evidenced Recentlyhowever Xu and coworkers have found that the anataseand rutile have the same activity in the degradation of 4-chlorophenol and the difference just lies in their differentadsorption ability towards O

2[31 32] Thus as the TiO

2

with different crystal phases is fabricated the O2adsorption

ability on the individual crystal phase needs to be takeninto account The composition of TiO


adjusted through thermal treatment by means of the increasein temperature

322 TiO2Doping To overcome the drawback of low photo-

catalytic efficiency brought by the e-h self-combination thebare TiO

2can be further modified by doping a foreign ion

Table 2 shows that various metal ions metal oxides and afew nonmetal elements such as nitrogen sulfur fluorine andsilicon can be employed as the dopants

Gurkan et al [33] have reported the density functiontheory (DFT) calculation of the Se(IV)-doping of TiO

2

sample showing that the doping does not cause a significantchange in the positions of the band edges whereas producesadditional electronic states originating from the Se 3p orbitalsin the band-gap Accordingly the electrons can be excitedfrom the defect state to the conduction band by a relativelylower energy and thus visible light can be utilized Furtherthe benefit of the doped transition metal lies in the improvedtrapping of electrons to inhibit e-h recombination duringirradiation

In real application for water purification special attentionshould be paid to the potential leakage of the doped ionswhich may result in a secondary pollution in the treatedwater In this regard the N-doped TiO

2appears to be a

promising method due to the cleanness brought by thenitrogen and has attracted great attention in recent yearsIn particular the photocatalytic activity under visible lightillumination has been well evidenced [34ndash40] Howeverthe unanimity exists for the photocatalytic mechanism Forexample Irie et al have stated that the irradiation with UVlight excites the electrons in both the VB and the impurityenergy levels but illumination with visible light only exciteselectrons in the impurity energy level [41] Ihara et al haveconcluded that the oxygen-deficient sites formed in the grainboundaries are importantly attributed to the visible lightactivity while the doped nitrogen in part of the oxygen-deficient sites is important as a blocker for reoxidation[42]

Also for the application of the fabricated TiO2 the

intrinsic advantages brought by the preparation methodsshould be accurately analyzed so as to appropriately ascribethe accelerated activities to the e-h separation For exampleLiu et al [43] have reported that a heat treatment of TiO

2

by hydrogen does not lead to any change in the physical

Table 2 Doping of TiO2 for the enhanced photocatalytic activity

Doped TiO2 Doping method References

N-doped TiO2Sol-gel method microwave

hydrothermal method [34ndash36]

Si-doped TiO2Hydrolysis of titanium

isopropoxide [102 103]

Si-doped TiO2ZrO2 Sol-gel method [37]S-dopedTiO2 nanoparticles

Hydrothermal method [38]

SiO3

2minus doped TiO2films Hydrothermal method [39]

NS co-doped TiO2Manual grinding ofthiourea and urea [104]

S-doped TiO2Tielectrode Anodization method [105]

S-doped TiO2-ZrO2nanoparticles Sol-gel method [40]

CndashN-doped TiO2nanotube

Chemical vapor deposition(CVD) [61]

Se(IV) on DegussaP25 Wet impregnation [33]

Al(III)-doped TiO2 Sol-gel method [106]V-doped TiO2 Sol-gel method [107]Fe-doped TiO2 Hydrothermal method [62]PolypyrroleFe-dopedTiO2

Sol-gel method emulsionpolymerization [108]

Er3+ YAlO3Fe- andCo-doped TiO2

Sol-gel method [109]

Cu2+-dopedTiO2SiO2


ZnFe2O4 on TiO2 Sol-gel method [111]

Ag doped TiO2Photoreduction using the

sacrificial acid [112]

AgF-TiO2Sol-gel method

photoreduction method [113]

Ag-TiO2-xNx Sol-gel method [114]Sn-doped TiO2 Sol-gel method [115]Lanthanideions-doped TiO2


La3+Zr4+ co-dopedTiO2

Sol-microwave methodoil-bath condition synthesis [117]

Ce-doped TiO2 Sol-gel method [118]Ce-doped TiO2microspheres solvothermal method [67]

NdF doped TiO2 Sol-gel method [119]W-doped TiO2 Solvothermal method [120]

W-TiO2 films Liquid phase deposition(LPD) [121]

WN co-dopedTiO2 nanoparticles

Sol-hydrothermal method [122]

Bi-doped TiO2 Sol-gel method [123]C-dope TiO2 powders Oxidative annealing of TiC [41]

properties including the crystal phase specific surface areaand particle size However results of electron paramagnetic


resonance (EPR) detection show that the H2treatment leads

to formation of oxygen vacancy and Ti3+ species on theTiO2 Moreover the kinetic constants of phenol degradation

increased with the H2treatment temperatures lower than

800∘CTherefore the production of oxygen vacancy and Ti3+species which are relatively stable as exposure to ambientair is considered to extend the lifetime of the e-h pairsThe oxygen vacancy and Ti3+ species act as holes traps andoxygen vacancy acts as an electron scavenger Further thetrapped holes transfer to the organic substrate leading to adegradation reaction and charged defects recover to theiroriginal states of oxygen vacancy and Ti3+

323 Scavenging of the Photogenerated Electrons To effec-tively separate the e-h pairs the electrons need to be scav-enged along with the trapping of holes by H

2O to form ∙OH

Otherwise the electrons will recombine together with theholes without any transformation of pollutants The mostcommonly available scavenger is the dissolved oxygen (DO)and thus the TiO

2-based photocatalytic reactions are most

often performed in an aerated aqueous system It has beenobserved that the reaction rate increases toward a plateauwith the partial pressure of O

2in a gas phase [44] Actually

before the electron scavenging an adsorption process of DOoccurs and the adsorption model may be described by atypical Langmuir adsorption

Besides DO H2O2is often added as an environmental

benign electron scavenger to aid the DO H2O2 being a

stronger electron acceptor than oxygen reacts with theelectrons in the valence band of the photocatalyst to generate∙OH Chu et al have observed that the degradation rateof chlorinated aniline can be improved by adding 001mMH2O2to the reaction solution [45] However as the H

2O2

is overdosed at 100mM a retardation of approximately 26rate is caused The rate retardation can be accounted for asfollowsThe H

2O2does scavenge the valuable HO∙ while the

H2O2in excess consumes the oxidative holes on the TiO

2

catalyst surface (3) where the overall oxidation capabilitiesof the system are significantly reduced [46]

HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)

Additionally alternative oxidative species such as Cr(VI) ionscan be employed as the electron scavenger in the TiO

2-based

photocatalysis [47] The Cr(VI) ions are also a type of pollu-tant and the photocatalysis is sometimes employed to removethe Cr(VI) ions as a reductive process using the electrons[48] As the Cr(VI) ions coexist with organic pollutant aspecial supply of DO as the electron scavenger appears to beunnecessary [49] Compared to the conventional process ofCr(VI) removal using ferrous reduction the competition ofTiO2-based photocatalysis may be impeded as the economics

is taken into account because the former appears to be a quiterapid process

324 Imposition of Bias to Separate the e-h Pairs Externalforces such as electric bias can be employed to accelerate

the separation of e-h pairs which is realized in an electrodesystem In this system the TiO

2is employed as the anode

where the organic substrate is oxidized by the radicalsgenerated from the interaction between the holes and H

2O

The photogenerated electrons flow through the externalcircuit and arrive at the cathode where they are scavengedby oxidative species In this system an externally imposedanodic bias serves to drive the electrons away from theanode to the cathode and thus the e-h self-combination ofTiO2can be efficiently inhibited on the anode Some reports

have shown that a bias of around 05V is efficient to leadto a significant increase in the organic degradation on theanode by means of enhancing the e-h separation [50 51]Additional advantage brought by this system leads to the easyrecovery of TiO

2compared to the conventional suspended

systemThe lab-scaled prototype of this system however may

find difficulty in the scale-up for actual water purificationlikely because of the utilization of the electrode As the TiO

2

powder is attached to the anode support a binder withpowerful binding strength is indispensible to anchor the TiO

2

particles on the electrode surface The binder making ofpolymer may be degraded after a long-time photocatalyticprocess which decreases the lifetime of electrode Despitethis challenge the photoelectrochemical system can beemployed as a powerful platform for the probing of photocat-alytic mechanism In essence the TiO

2-based photocatalytic

process is such one involving electron transfer which canbe exactly investigated by electrochemical means Liu et alhave employed electrochemical impedance spectroscopy toascertain the TiO

2-based photocatalytic process and thus the

reaction phenomenon can be accounted for [52] Bymeans ofthis powerful tool the rate-determining steps have been wellascertained

33 Adsorption of Organics onto TiO2Surface and Organic

Degradation Reaction Accompanied with the successful e-h separation is the organic degradation reaction Majorityof the reports on the water purification are to investi-gate the degradation reaction of an individual substrateThe substrate is initially adsorbed onto the TiO

2surface

and the adsorption ability is often observed to determinethe overall degradation efficiency [1] A large specific sur-face area of TiO

2photocatalyst benefits the accumulation

of organic substrate on the TiO2 and thus more active

sites are available for the subsequent degradation reaction[53] Generally the adsorption behavior can be describedby the Langmuir adsorption isotherm and the kineticmodel of organic degradation rate can be described asfollows

119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)

In (5)119870119903designates the contribution of organic degradation

and 119870119886is the contribution of organic adsorption [54]

Obviously large values of 119870119886and 119870

119904are beneficial for the

acceleration of reaction rate As the119870119886value is small enough


to be neglected (5) can be considered as the first-orderreaction rate as follows

Ln(1198620

119862

) = 119870ap119905 (6a)

where119870ap equals119870119903119870119886 as the apparent reaction rate constantIn real application it is noteworthy that the chemisorp-

tion occurring through a formation of chemical bond con-tributes to the acceleration of degradation reaction In somecases a great number of the organic substrate is adsorbedwhile the degradation reaction of organic substrate doesnot become more rapid just because of the physisorptioninstead of chemisorption It has been disclosed by FT-IR thatthe organic substrate is chemically adsorbed in the form ofcomplex with the TiO

2[55] This complex likely leads to a

deviation of the first-order rate model of the degradationreaction because the large 119870

119886value cannot be neglected in

(5) In this scenario the reaction rate can be described asfollows [29 56 57]

Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)

Certainly in real application the reaction kinetics involvingthe adsorption should be kept in mind to solve someproblems For example as the water containing textile as apollutant is treated a great number of textile molecules areadsorbed while physically onto the TiO

2 Obviously such

type of adsorption does not contribute to the degradationOn the other hand as the chemisorption predominates witha large 119870

119886value the first-order rate reaction model that is

commonly applied to describe the reaction is not applicableand (6b) is more adequate to fit the degradation data of targetpollutants

34 Rate-Determining Steps Besides the steps of adsorptionand degradation reaction mass transfer and desorption ofthe reaction products are also the main steps involved inthe heterogeneous catalytic reaction Among these steps theslowest step determines the overall reaction rate while whichone is the rate-determining step (RDS) As the aqueoussystem is commonly fully stirred by means of air bubblingto form a suspension the step of mass transfer is generallyconsidered as a fast step and the adsorption and degradationreactions are potentially the RDS

In real application the ascertainment of RDS is ofimportance Clearly only manipulation of the RDS leads toan alteration of the overall reaction rate For example as theadsorption step is RDS increase in the specific surface areato improve the adsorption ability enables the acceleration ofthe overall reaction rate Otherwise the associated efforts arein vain Liu et al have employed electrochemical impedancespectroscopy (EIS) to ascertain the RDS of TiO

2photo-

catalytic process [52] Their results show that the surfacedegradation reaction of organic pollutant is commonly anRDS while the adsorption step is another RDS once thesurface degradation reaction becomes quite rapid In essencethe acceleration of organic degradation reaction can beconsidered as the acceleration of the e-h separation To date

the TiO2photocatalysis is suffering from the low quantum

yield and TiO2that has a sufficiently rapid e-h separation is

not available Accordingly any effort in the acceleration of thee-h separation is valuable

4 Balance of the Efficacy and Economics

41 Economic Estimation of TiO2Photocatalysis for Water

Purification The organic pollutants experience an oxidationpathway during the photocatalytical process and thus a typ-ical TiO

2-based photocatalytical matrix is composed of four

indispensible elements light source photocatalyst oxidant asan electron acceptor and reactor The light source suppliesenergy that excites the electrons on the valence band of TiO

2

to jump to the conduction band and then holes are left onthe conduction bandThe TiO

2-based photocatalyst works as

a catalyst which adsorbs organic substrate onto the surfaceand then the degradation reaction occurs As the oxidativeholes are consumed to generate ∙OH via the interaction withH2O the electrons are left and must be scavenged by an

oxidant mostly by ambient O2 The ambient O

2has an

additional function to buoyant the TiO2particles to form a

suspension and thus themass transfer limit can be precludedThe reactor is the place where the photocatalytic process isrealized

Todevelop an effectiveTiO2-based photocatalytic process

for water purification the operating parameters should bewell optimized for the full utilization of the light illuminationability of catalyst and ambient O

2in the reactorWhen doing

this work the fundamentals that have been established in thelaboratory should be used sophisticatedly In addition sincethe solution chemistry varies significantly the experiencein one place even if quite successful is not applicable inanother place Particular the optimized parameters cannot beimplemented in some real applications due to the limitationof operating cost

Basically the operation cost of photocatalytic reactionmatrix includes the cost of photocatalytic materials elec-tricity and aging of equipment while the cost of TiO

2

photocatalyst shares the major part and is discussed hereinThe cost of TiO

2photocatalyst depends on its dosage For

example 01ndash2wtTiO2 as usually dosed inwater treatment

costs too much if it is used one time It is estimated that thecost of the TiO

2photocatalyst in treating 1m3 of water at this

dosage level varies from 3 to 60USD under the conditionthat the price of TiO

2powders is as low as approximately

3000USD per ton This cost is undoubtedly unaffordablefor most consumers particularly in the developing countries[58]

42 Recovering of the TiO2Photocatalyst This economic

estimation implies that the progress obtained in the reactionefficiency must be balanced by the economic considerationThus the stability of TiO

2photocatalyst may outweigh its

temporary activity even if quite high Also the recycling ofthe TiO

2powders which are commonly employed is also

vital In particular the TiO2particles are often prepared in

a scale of nanosize and it has been well recognized that thenanosizedTiO

2particles exhibit unique photocatalytic ability


(a)

(a)

(b)

(b)Figure 3 SEM image of the TiO

2microspheres samples ((a) has 600 multiple and (b) has 50000 multiple magnifications) Taken from [57]

[3 13 59ndash63] Obviously these nanosized TiO2powders

can be well suspended to form a fine contact between thecatalyst particles and the substrate and thus the illuminationof the particles is ensured Accordingly the organic substratescan be rapidly destroyed in the photocatalytic matrix withnanosized TiO

2powders

However this nanosized TiO2

is often synthesizedthrough hydrothermal method that demands high tempera-ture and pressure and thus is quite costly once the preparationis practiced in a large scale In view of the economic consi-deration as previously mentioned these TiO

2particles will

encounter a difficulty in separation from the aqueous phaseand recycling after use An addition of a chemical such ascoagulant enables the TiO

2sedimentation and separation

whereas the reuse of TiO2becomes impossible due to the

TiO2fouling Mostly the TiO

2powders are immobilized

onto a support [16] to preclude the postseparation and theTiO2can be reused Moreover as the TiO

2powders are

immobilized onto a support such as porous nickel [64] or Timesh [65] to form an electrode a bias can be convenientlyapplied to the anode TiO

2to enhance the e-h separation

It is noted that in both cases however a significant loss inthe contact area between the immobilized photocatalyst andthe light source exists which lowers the global efficiency ofphotocatalytic degradation of organic substrate Thereforethe efficacy sometimes must be sacrificed for the sake of costcutting and a balance between the efficacy of TiO

2and the

running cost is of significanceThis balance requires a novel concept of design of the

photocatalyst matrix in which the advantage of the suspen-sion system should be kept while the easy recycling of thephotocatalyst should be simultaneously realized Thereforea microsphere photocalyst appears to be an ideal option Ifthe size of microspheres can be designed at a suitable scalein size they can be well suspended by the air bubbling Theambient oxygen works as an electron scavenger and thus isinevitably supplied This way the suspended TiO

2can be

finely illuminated Upon the task of photodegradation the airbubbling stops Consequently the microspheres settle at thereactor bottom quickly by the aid of gravity and thus can bereadily reused

Such microsphere photocatalysts have been successfullyfabricated [29 56 57 66ndash70] For example as illustrated

in Figure 3 a TiO2microsphere photocatalyst with a size

regime of 30ndash160 120583m and an average size of around 80 m hasbeen prepared by reconstructing the mixture of TiO

2sol and

TiO2nanosized powders with a spray drier [29 57] After

that the as-prepared microspheres are thermally treated toobtain the anatase-dominant sample TiO

2microspheresThe

experimental results showed that the photoreaction rate usingthe TiO

2microspheres was comparable to that using the

TiO2powder counterparts Moreover the TiO

2microsphere

samples could be reused in the photocatalytic oxidationreaction for more than 50 times without obvious destructionof the physical structure and significant loss in its activity

43 Coupling of Diverse Technologies Effluents dischargedfrom industrial processes usually contain various pollutantsAccordingly diverse technologies including physical bio-logical and chemical ones are needed to be chosen totreat them in light of such properties Pollutants in theform of colloids or biodegradable solutes are treated byconventional approaches such as coagulation or biologicalprocess The TiO

2photocatalyst is specially designed to

destroy the recalcitrant pollutants that are nonbiodegradableThe ∙OH involved in the TiO

2photocatalytic process is

extremely powerful to destroy the molecular structures andlead to partial or complete mineralization Generally theTiO2photocatalysis is particularly fitful for the advance

treatment of water with an attempt of final discharge of theeffluent or reuse of the purified water Clearly it appearsinfeasible to purify the seriously contaminated water solelyby the TiO

2photocatalytic process Without surprise the

TiO2photocatalysis has its intrinsic drawbacks in treating

actual water For example the water should be transparentfor the light transmission and the extremely polluted wateroften shadows the light Second the biological processes arecost effective in the decomposition of a great number ofbiodegradable pollutants at low concentrations Thereforethe balance between the efficacy of TiO

2photocalytic process

and economics requites a smart coupling of it with otherconventional process

In real application a successful process for water purifi-cation is commonly an integrated one including quite a fewtechnologies for the sake of both technical efficacy and costeffectiveness Basically a physical unit such as filtration or


grilling unit goes first to separate the solids Then a physic-ochemical process such as coagulation follows to remove thecolloidal substances Further a biological process is installedto degrade most of the biodegradable organic moleculesFinally if the effluent requires further purification to destroythe nondegradable organic pollutants the TiO

2photocalytic

process is desirable In addition TiO2photocatalytic process

is expected to improve the biodegradability of pollutedwater as a pretreatment unit by decomposing the recalcitrantpollutants Anyway any pilot-scale test is usually extremelynecessary to verify the technique and economic feasibility[71] and more fundamental research is also in need tounderstand the enhancing mechanism of the photocatalyticactivity [72] while associated reports are very limited

5 Concluding Remarks

With the rapid accumulation of reports on the TiO2-based

photocatalysis for water purification its application becomesa task with great importance In view of the fact that thefundaments of the TiO

2-based photocatalysis have been

well established a link with the actual application in waterpurification should be followed Analysis in this review showsthat it is of significance to keep in mind that the fundamentsclarified in laboratory sometimes find it difficult to solvethe problems that are encountered in practice Consequentlysome TiO

2-based catalysts which possess high activity in

the degradation of model pollutants in the laboratory are ingreat need of further investigation to improve their applicableability such as separation and recycling During the furtherinvestigation apart from the fundamentals economics isbelieved to play a vital role in actual application indi-cating that a balance between the reaction efficiency andthe operating cost should also be considered Moreover aflexible coupling of TiO

2-based photocatalysis with other

technologies is equally important to push it towards realapplication of water purification

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (nos 21067004 and 51208539)and Chongqing Science and Technology Commission (nocstc2011ggC20013)

References

[1] J Zhao T Wu K Wu K Oikawa H Hidaka and N SerponeldquoPhotoassisted degradation of dye pollutants 3 Degradation ofthe cationic dye rhodamine B in aqueous anionic surfactantTiO2dispersions under visible light irradiation evidence for the

need of substrate adsorption on TiO2particlesrdquo Environmental

Science and Technology vol 32 no 16 pp 2394ndash2400 1998[2] J G Yu M Jaroniec and G X Lu ldquoTiO

2photocatalytic

materialsrdquo Internation Journal of Photoenergy vol 2012 ArticleID 206183 5 pages 2012

[3] J Yu J Xiong B Cheng and S Liu ldquoFabrication and character-ization of Ag-TiO

2multiphase nanocomposite thin films with

enhanced photocatalytic activityrdquo Applied Catalysis B vol 60no 3-4 pp 211ndash221 2005

[4] S Wen J Zhao G Sheng J Fu and P Peng ldquoPhotocatalyticreactions of phenanthrene at TiO

2water interfacesrdquo Chemo-

sphere vol 46 no 6 pp 871ndash877 2002[5] H Chun W Yizhong and T Hongxiao ldquoInfluence of adsorp-

tion on the photodegradation of various dyes using surfacebond-conjugated TiO

2SiO2photocatalystrdquoApplied Catalysis B

vol 35 no 2 pp 95ndash105 2001[6] S Kaneco Y Shimizu K Ohta and T Mizuno ldquoPhotocatalytic

reduction of high pressure carbon dioxide using TiO2powders

with a positive hole scavengerrdquo Journal of Photochemistry andPhotobiology A vol 115 no 3 pp 223ndash226 1998

[7] F Moellers H J Tolle and R Memming ldquoOn the origin of thephotocatalytic deposition of noble metals on TiO

2rdquo Journal of

the Electrochemical Society vol 121 no 9 pp 1160ndash1167 1974[8] H J Hovel ldquoTiO

2antireflection coatings by a low temperature

spray processrdquo Journal of the Electrochemical Society vol 125no 6 pp 983ndash985 1978

[9] R W Matthews ldquoSolar-electric water purification using pho-tocatalytic oxidation with TiO

2as a stationary phaserdquo Solar

Energy vol 38 no 6 pp 405ndash413 1987[10] W Behnke F Nolting and C Zetzsch ldquoA smog chamber study

on the impact of aerosols on the photodegradation of chemicalsin the troposphererdquo Journal of Aerosol Science vol 18 no 1 pp65ndash71 1987

[11] Y Tominaga T Kubo and K Hosoya ldquoSurface modificationof TiO

2for selective photodegradation of toxic compoundsrdquo

Catalysis Communications vol 12 no 9 pp 785ndash789 2011[12] C Hu J C Yu Z Hao and P K Wong ldquoPhotocatalytic

degradation of triazine-containing azo dyes in aqueous TiO2

suspensionsrdquo Applied Catalysis B vol 42 no 1 pp 47ndash55 2003[13] J Saien Z Ojaghloo A R Soleymani and M H Rasoulifard

ldquoHomogeneous and heterogeneous AOPs for rapid degradationof Triton X-100 in aqueous media via UV light nano titaniahydrogen peroxide and potassium persulfaterdquo Chemical Engi-neering Journal vol 167 no 1 pp 172ndash182 2011

[14] Z Ai J Li L Zhang and S Lee ldquoRapid decolorization of azodyes in aqueous solution by an ultrasound-assisted electrocat-alytic oxidation processrdquo Ultrasonics Sonochemistry vol 17 no2 pp 370ndash375 2010

[15] W Fan M Cui H Liu et al ldquoNano-TiO2enhances the toxicity

of copper in natural water to Daphnia magnardquo EnvironmentalPollution vol 159 no 3 pp 729ndash734 2011

[16] B Tryba ldquoImmobilization of TiO2and Fe-C-TiO

2photocata-

lysts on the cotton material for application in a flow photocat-alytic reactor for decomposition of phenol in waterrdquo Journal ofHazardous Materials vol 151 no 2-3 pp 623ndash627 2008

[17] J T Yates Jr ldquoPhotochemistry on TiO2 mechanisms behind the

surface chemistryrdquo Surface Science vol 603 no 10ndash12 pp 1605ndash1612 2009

[18] C Liang C Liu F Li and F Wu ldquoThe effect of Praseodymiumon the adsorption and photocatalytic degradation of azo dye inaqueous Pr3+-TiO

2suspensionrdquo Chemical Engineering Journal

vol 147 no 2-3 pp 219ndash225 2009[19] Y Yu J Wang and J F Parr ldquoPreparation and properties of

TiO2fumed silica composite photocatalytic materialsrdquo Proce-

dia Engineering vol 27 pp 448ndash456 2012[20] L F Qi J G Yu and M Jaroniec ldquoEnhanced and suppressed

effects of ionic liquid on the photocatalytic activity of TiO2rdquo

Adsorption vol 19 pp 557ndash561 2013[21] H Zhang R Zong J Zhao and Y Zhu ldquoDramatic visible pho-

tocatalytic degradation performances due to synergetic effect of


TiO2with PANIrdquo Environmental Science and Technology vol

42 no 10 pp 3803ndash3807 2008[22] P A Pekakis N P Xekoukoulotakis and D Mantzavinos

ldquoTreatment of textile dyehouse wastewater by TiO2photocatal-

ysisrdquoWater Research vol 40 no 6 pp 1276ndash1286 2006[23] M N Chong B Jin C W K Chow and C Saint ldquoRecent

developments in photocatalytic water treatment technology areviewrdquoWater Research vol 44 no 10 pp 2997ndash3027 2010

[24] H Xu Z Zheng L Z Zhang H L Zhang and F Deng ldquoRecentdevelopments in photocatalytic water treatment technology areviewrdquo Journal of Solid State Chemistry vol 181 no 9 pp 2516ndash2522 2008

[25] A A Merdaw A O Sharif and G A W Derwish ldquoMasstransfer in pressure-driven membrane separation processespart Irdquo Chemical Engineering Journal vol 168 no 1 pp 215ndash228 2011

[26] C Lin and K S Lin ldquoPhotocatalytic oxidation of toxicorganohalides with TiO

2UV the effects of humic substances

and organic mixturesrdquo Chemosphere vol 66 no 10 pp 1872ndash1877 2007

[27] M R Hoffmann S T Martin W Choi and D W BahnemannldquoEnvironmental applications of semiconductor photocatalysisrdquoChemical Reviews vol 95 no 1 pp 69ndash96 1995

[28] Q Xiang J Yu and P K Wong ldquoQuantitative characterizationof hydroxyl radicals produced by various photocatalystsrdquo Jour-nal of Colloid and Interface Science vol 357 no 1 pp 163ndash1672011

[29] X Z Li H Liu L F Cheng andH J Tong ldquoPhotocatalytic oxi-dation using a new catalystmdashTiO

2microspheremdashfor water and

wastewater treatmentrdquo Environmental Science and Technologyvol 37 no 17 pp 3989ndash3994 2003

[30] T Ohno K Tokieda S Higashida and M Matsumura ldquoSyner-gismbetween rutile and anatase TiO

2particles in photocatalytic

oxidation of naphthalenerdquo Applied Catalysis A vol 244 no 2pp 383ndash391 2003

[31] S Cong and Y Xu ldquoExplaining the high photocatalytic activityof a mixed phase TiO

2 a combined effect of O

2and crys-

tallinityrdquo Journal of Physical Chemistry C vol 115 no 43 pp21161ndash21168 2011

[32] Q Sun and Y Xu ldquoEvaluating intrinsic photocatalytic activitiesof anatase and rutile TiO

2for organic degradation in waterrdquo

Journal of Physical Chemistry C vol 114 no 44 pp 18911ndash189182010

[33] Y Y Gurkan E Kasapbasi and Z Cinar ldquoEnhanced solar pho-tocatalytic activity of TiO

2by selenium(IV) ion-doping char-

acterization and DFT modeling of the surfacerdquo ChemicalEngineering Journal vol 214 pp 34ndash44 2013

[34] J Ananpattarachai P Kajitvichyanukul and S Seraphin ldquoVisi-ble light absorption ability and photocatalytic oxidation activityof various interstitial N-doped TiO

2prepared from different

nitrogen dopantsrdquo Journal of Hazardous Materials vol 168 no1 pp 253ndash261 2009

[35] J Senthilnathan and L Philip ldquoPhotodegradation of methylparathion and dichlorvos from drinking water with N-dopedTiO2under solar radiationrdquo Chemical Engineering Journal vol

172 no 2-3 pp 678ndash688 2011[36] J A Cha SHAnHD Jang C S KimD K Song andT Kim

ldquoSynthesis and photocatalytic activity of N-doped TiO2ZrO2

visible-light photocatalystsrdquo Advanced Powder Technology vol23 no 6 pp 717ndash723 2012

[37] X Xu D Yin SWu JWang and J Lu ldquoPreparation of orientedSiO3

2minus-doped TiO2film and degradation of methylene blue

under visible light irradiationrdquo Ceramics International vol 36no 2 pp 443ndash450 2010

[38] Y Liu J Liu Y Lin Y Zhang and Y Wei ldquoSimple fabricationand photocatalytic activity of S-doped TiO

2under low power

LED visible light irradiationrdquo Ceramics International vol 35no 8 pp 3061ndash3065 2009




[40] G Tian K Pan H Fu L Jing and W Zhou ldquoEnhancedphotocatalytic activity of S-doped TiO

2-ZrO2nanoparticles

under visible-light irradiationrdquo Journal of Hazardous Materialsvol 166 no 2-3 pp 939ndash944 2009

[41] H Irie Y Watanabe and K Hashimoto ldquoCarbon-dopedanatase TiO

2powders as a visible-light sensitive photocatalystrdquo

Chemistry Letters vol 32 no 8 pp 772ndash773 2003[42] T IharaMMiyoshi Y IriyamaOMatsumoto and S Sugihara

ldquoVisible-light-active titanium oxide photocatalyst realized byan oxygen-deficient structure and by nitrogen dopingrdquo AppliedCatalysis B vol 42 no 4 pp 403ndash409 2003

[43] H Liu H T Ma X Z Li W Z Li M Wu and X H BaoldquoThe enhancement of TiO

2photocatalytic activity by hydrogen

thermal treatmentrdquoChemosphere vol 50 no 1 pp 39ndash46 2003[44] A Mills and S Le Hunte ldquoAn overview of semiconductor

photocatalysisrdquo Journal of Photochemistry and Photobiology Avol 108 no 1 pp 1ndash35 1997

[45] W Chu W K Choy and T Y So ldquoThe effect of solution pHand peroxide in the TiO

2-induced photocatalysis of chlorinated

anilinerdquo Journal of Hazardous Materials vol 141 no 1 pp 86ndash91 2007

[46] J Marsh and D Gorse ldquoA photoelectrochemical and acimpedance study of anodic titaniumoxide filmsrdquoElectrochimicaActa vol 43 no 7 pp 659ndash670 1997

[47] H Yu S Chen X Quan H Zhao and Y Zhang ldquoFabrication ofa TiO

2-BDD heterojunction and its application as a photocata-

lyst for the simultaneous oxidation of an azo dye and reductionof Cr(VI)rdquo Environmental Science and Technology vol 42 no10 pp 3791ndash3796 2008

[48] J J Testa M A Grela and M I Litter ldquoHeterogeneousphotocatalytic reduction of chromium(VI) over TiO

2particles

in the presence of oxalate involvement of Cr(V) speciesrdquo Envi-ronmental Science and Technology vol 38 no 5 pp 1589ndash15942004

[49] L Wang N Wang L Zhu H Yu and H Tang ldquoPhotocatalyticreduction of Cr(VI) over different TiO

2photocatalysts and

the effects of dissolved organic speciesrdquo Journal of HazardousMaterials vol 152 no 1 pp 93ndash99 2008

[50] Y S Sohn Y R Smith M Misra and V (Ravi) Subrama-nian ldquoElectrochemically assisted photocatalytic degradation ofmethyl orange using anodized titanium dioxide nanotubesrdquoApplied Catalysis B vol 84 no 3-4 pp 372ndash378 2008

[51] R K Quinn R D Nasby and R J Baughman ldquoPhotoassistedelectrolysis of water using single crystal 120572-Fe

2O3anodesrdquo

Materials Research Bulletin vol 11 no 8 pp 1011ndash1017 1976[52] H Liu X Z Li Y J Leng andW Z Li ldquoAn alternative approach

to ascertain the rate-determining steps of TiO2photoelectro-

catalytic reaction by electrochemical impedance spectroscopyrdquoJournal of Physical Chemistry B vol 107 no 34 pp 8988ndash89962003


[53] H Tada A Hattori Y Tokihisa K Imai N Tohge and S ItoldquoA patterned-TiO

2SnO2bilayer type photocatalystrdquo Journal of

Physical Chemistry B vol 104 no 19 pp 4586ndash4587 2000[54] C S Turchi and D F Ollis ldquoPhotocatalytic degradation of

organic water contaminants mechanisms involving hydroxylradical attackrdquo Journal of Catalysis vol 122 no 1 pp 178ndash1921990

[55] M Ivanda S Music S Popovic and M Gotic ldquoXRD Ramanand FT-IR spectroscopic observations of nanosized TiO

2syn-

thesized by the sol-gel method based on an esterificationreactionrdquo Journal of Molecular Structure vol 480-481 pp 645ndash649 1999

[56] C Wang X Zhang H Liu X Li W Li and H Xu ldquoReactionkinetics of photocatalytic degradation of sulfosalicylic acidusing TiO

2microspheresrdquo Journal of Hazardous Materials vol

163 no 2-3 pp 1101ndash1106 2009[57] X Z Li H Liu L F Cheng and H J Tong ldquoKinetic behaviour

of the adsorption and photocatalytic degradation of salicylicacid in aqueous TiO

2microsphere suspensionrdquo Journal of

Chemical Technology and Biotechnology vol 79 no 7 pp 774ndash781 2004

[58] H Liu C L Yu and C Wang ldquoEconomics and its implicationof photocatalyst recycling for water purificationrdquo in Handbookof Photocatalysts pp 527ndash534 Nova Science Publishers 2009

[59] M Y Ghaly T S Jamil I E El-Seesy E R Souaya and RA Nasr ldquoTreatment of highly polluted paper mill wastewaterby solar photocatalytic oxidation with synthesized nano TiO

2rdquo

Chemical Engineering Journal vol 168 no 1 pp 446ndash454 2011[60] J G Yu Q Li S W Liu and M Jaroniec ldquoIonic-liquie-

assisted synthesis of uniform fluorinated BC-codoped TiO2

nanocrystals and their enhanced visble-light photocatalyticactivityrdquo Chemistry A European Journal vol 19 pp 2433ndash24412013

[61] S Liu L Yang S Xu S Luo and Q Cai ldquoPhotocatalyticactivities of C-N-doped TiO

2nanotube arraycarbon nanorod

compositerdquo Electrochemistry Communications vol 11 no 9 pp1748ndash1751 2009

[62] H Zhang andH Zhu ldquoPreparation of Fe-doped TiO2nanopar-

ticles immobilized on polyamide fabricrdquo Applied Surface Sci-ence vol 258 no 24 pp 10034ndash10041 2012

[63] S H Othman S A Rashid T I M Ghazi and N AbdulahldquoDispersion and stabilization of photocatalytic TiO

2nanoparti-

cles in aqueous suspension for coatings applicationsrdquo Journal ofNanomaterials vol 2012 Article ID 718214 10 pages 2012

[64] H Liu S Cheng J Zhang C Cao and S Zhang ldquoTitaniumdioxide as photocatalyst on porous nickel adsorption and thephotocatalytic degradation of sulfosalicylic acidrdquo Chemospherevol 38 no 2 pp 283ndash292 1999

[65] X Z Li H L Liu P T Yue and Y P Sun ldquoPhotoelectrocatalyticoxidation of rose Bengal in aqueous solution using a TiTiO

2

mesh electroderdquo Environmental Science and Technology vol 34no 20 pp 4401ndash4406 2000

[66] P F Lee X Zhang D D Sun J Du and J O LeckieldquoSynthesis of bimodal porous structuredTiO

2microspherewith

high photocatalytic activity for water treatmentrdquo Colloids andSurfaces A vol 324 no 1ndash3 pp 202ndash207 2008

[67] J Xie D Jiang M Chen et al ldquoPreparation and characteriza-tion ofmonodisperse Ce-doped TiO

2microspheres with visible

light photocatalytic activityrdquo Colloids and Surfaces A vol 372no 1ndash3 pp 107ndash114 2010

[68] H Nemec C Kadlec F Kadlec et al ldquoResonant mag-netic response of TiO

2microspheres at terahertz frequenciesrdquo

Applied Physics Letters vol 100 no 6 Article ID 061117 pp 891ndash894 2012

[69] F Cesano D Pellerej D Scarano G Ricchiardi and AZecchina ldquoRadially organized pillars in TiO

2and in TiO

2C

microspheres synthesis characterization and photocatalytictestsrdquo Journal of Photochemistry and Photobilogy A vol 242 pp51ndash58 2012

[70] M Ye Y Yang Y Zhang T Zhang andW Shao ldquoHydrothermalsynthesis of hydrangea-like F-doped titania microspheres forthe photocatalytic degradation of carbamazepine underUVandvisible light irradiationrdquo Journal of Nanomaterials vol 2012Article ID 583417 8 pages 2012

[71] S Mozia P Brozek J Przepiorski B Tryba and A W Mora-wski ldquoImmobilized TiO

2for phenol degrdation in a pilot-

scale photocatalytic reactorrdquo Journal ofNanomaterials vol 2012Article ID 949764 10 pages 2012

[72] P Zhou J G Yu and Y X Wang ldquoThe new understandingon photocatalytic mechanism of visible-light response N-Scodoped anatase TiO

2by first-principlesrdquo Applied Catalysis B

vol 142-143 pp 45ndash53 2013[73] E S Elmolla and M Chaudhuri ldquoThe feasibility of using com-

bined TiO2photocatalysis-SBR process for antibiotic wastewa-

ter treatmentrdquo Desalination vol 272 no 1ndash3 pp 218ndash224 2011[74] Y Jin M Wu G Zhao and M Li ldquoPhotocatalysis-enhanced

electrosorption process for degradation of high-concentrationdye wastewater on TiO

2carbon aerogelrdquo Chemical Engineering

Journal vol 168 no 3 pp 1248ndash1255 2011[75] L Prieto-Rodriguez S Miralles-Cuevas I Oller A Aguera

G L Puma and S Malato ldquoTreatment of emerging contam-inants in wastewater treatment plants (WWTP) effluents bysolar photocatalysis using low TiO

2concentrationsrdquo Journal of

Hazardous Materials vol 211-212 pp 131ndash137 2012[76] M N Chong and B Jin ldquoPhotocatalytic treatment of high

concentration carbamazepine in synthetic hospital wastewaterrdquoJournal of Hazardous Materials vol 199-200 pp 135ndash142 2012

[77] M Chen andW Chu ldquoDegradation of antibiotic norfloxacin inaqueous solution by visible-light-mediated C-TiO

2photocatal-

ysisrdquo Journal of Hazardous Materials vol 219-220 pp 183ndash1892012

[78] D Nasuhoglu A Rodayan D Berk and V Yargeau ldquoRemovalof the antibiotic levofloxacin (LEVO) in water by ozonation andTiO2photocatalysisrdquo Chemical Engineering Journal vol 189-

190 pp 41ndash48 2012[79] R Nakano R Chand E Obuchi K Katoh and K Nakano

ldquoPerformance of TiO2photocatalyst supported on silica beads

for purification of wastewater after absorption of reflow exhaustgasrdquo Chemical Engineering Journal vol 176-177 pp 260ndash2642011

[80] N N Rao V Chaturvedi and G Li Puma ldquoNovel pebble bedphotocatalytic reactor for solar treatment of textile wastewaterrdquoChemical Engineering Journal vol 184 pp 90ndash97 2012

[81] I Catanzaro G Avellone G Marcı et al ldquoBiological effectsand photodegradation by TiO

2of terpenes present in industrial

wastewaterrdquo Journal of Hazardous Materials vol 185 no 2-3pp 591ndash597 2011

[82] R Qiu D Zhang Z Diao et al ldquoVisible light induced photocat-alytic reduction of Cr(VI) over polymer-sensitized TiO

2and its

synergism with phenol oxidationrdquo Water Research vol 46 no7 pp 2299ndash2306 2012

[83] W Zhang Y Li Y Su K Mao and Q Wang ldquoEffect of watercomposition on TiO

2photocatalytic removal of endocrine


disrupting compounds (EDCs) and estrogenic activity fromsecondary effluentrdquo Journal of Hazardous Materials vol 215-216 pp 252ndash258 2012

[84] Y Xu J Jia D Zhong and Y Wang ldquoDegradation of dyewastewater in a thin-film photoelectrocatalytic (PEC) reactorwith slant-placed TiO

2Ti anoderdquo Chemical Engineering Jour-

nal vol 150 no 2-3 pp 302ndash307 2009[85] Y Yao K Li S Chen J Jia YWang andHWang ldquoDecoloriza-

tion of Rhodamine B in a thin-film photoelectrocatalytic (PEC)reactor with slant-placed TiO

2nanotubes electroderdquo Chemical

Engineering Journal vol 187 pp 29ndash35 2012[86] M Boroski A C Rodrigues J C Garcia L C Sampaio J

Nozaki andN Hioka ldquoCombined electrocoagulation and TiO2

photoassisted treatment applied to wastewater effluents frompharmaceutical and cosmetic industriesrdquo Journal of HazardousMaterials vol 162 no 1 pp 448ndash454 2009

[87] H El Hajjouji F Barje E Pinelli et al ldquoPhotochemicalUVTiO

2treatment of olive mill wastewater (OMW)rdquo Biore-

source Technology vol 99 no 15 pp 7264ndash7269 2008[88] K Nakano E Obuchi S Takagi et al ldquoPhotocatalytic treat-

ment of water containing dinitrophenol and city water overTiO2SiO2rdquo Separation and Purification Technology vol 34 no

1ndash3 pp 67ndash72 2004[89] DWu H You R Zhang C Chen and D J Lee ldquoBallast waters

treatment using UVAg-TiO2+O3advanced oxidation process

with Escherichia coli and Vibrio alginolyticus as indicatormicroorganismsrdquo Chemical Engineering Journal vol 174 no 2-3 pp 714ndash718 2011

[90] C Han M Pelaez V Likodimos et al ldquoInnovative visible light-activated sulfur doped TiO

2films for water treatmentrdquo Applied

Catalysis B vol 107 no 1-2 pp 77ndash87 2011[91] R Bergamasco F V da Silva F S Arakawa et al ldquoDrinking

water treatment in a gravimetric flow system with TiO2coated

membranesrdquo Chemical Engineering Journal vol 174 no 1 pp102ndash109 2011

[92] T Ochiai K Nakata TMurakami et al ldquoDevelopment of solar-driven electrochemical and photocatalytic water treatmentsystem using a boron-doped diamond electrode and TiO

2

photocatalystrdquoWater Research vol 44 no 3 pp 904ndash910 2010[93] E Lopez Loveira P S Fiol A Senn G Curutchet R Candal

and M I Litter ldquoTiO2-photocatalytic treatment coupled with

biological systems for the elimination of benzalkonium chloridein waterrdquo Separation and Purification Technology vol 91 pp108ndash116 2012

[94] M Pelaez P Falaras A G Kontos et al ldquoA comparative studyon the removal of cylindrospermopsin and microcystins fromwater with NF-TiO

2-P25 composite films with visible and UV-

vis light photocatalytic activityrdquoApplied Catalysis B vol 121 pp30ndash39 2012

[95] M G Antoniou P A Nicolaou J A Shoemaker A A de laCruz and D D Dionysiou ldquoImpact of themorphological prop-erties of thin TiO

2photocatalytic films on the detoxification

of water contaminated with the cyanotoxin microcystin-LRrdquoApplied Catalysis B vol 91 no 1-2 pp 165ndash173 2009

[96] K Zhang K C Kemp andV Chandra ldquoHomogeneous anchor-ing of TiO

2nanoparticles on graphene sheets for waste water

treatmentrdquoMaterials Letters vol 81 pp 127ndash130 2012[97] L Rizzo ldquoInactivation and injury of total coliform bacteria after

primary disinfection of drinking water by TiO2photocatalysisrdquo

Journal ofHazardousMaterials vol 165 no 1ndash3 pp 48ndash51 2009[98] G E Romanos C P Athanasekou F K Katsaros et al

ldquoDouble-side active TiO2-modified nanofiltration membranes

in continuous flow photocatalytic reactors for effective waterpurificationrdquo Journal of Hazardous Materials vol 211-212 pp304ndash316 2012

[99] P A Pekakis N P Xekoukoulotakis and D MantzavinosldquoTreatment of textile dyehouse wastewater by TiO

2photocatal-

ysisrdquoWater Research vol 40 no 6 pp 1276ndash1286 2006[100] L Lin M N Xie Y M Liang Y Q He G Y S Chan

and T G Luan ldquoDegradation of cypermethrin malathionand dichlorovos in water and on tea leaves with O

3UVTiO

2

treatmentrdquo Food Control vol 28 no 2 pp 374ndash379 2012[101] J L Esbenshade J C Cardoso and M V B Zanoni ldquoRemoval

of sunscreen compounds from swimming pool water usingself-organized TiO

2nanotubular array electrodesrdquo Journal of

Photochemistry and Photobiology A vol 214 no 2-3 pp 257ndash263 2010

[102] N Ma X Quan Y Zhang S Chen and H Zhao ldquoIntegrationof separation and photocatalysis using an inorganic membranemodified with Si-doped TiO

2for water purificationrdquo Journal of

Membrane Science vol 335 no 1-2 pp 58ndash67 2009[103] D N Bui S Z Kang X Li and J Mu ldquoEffect of Si doping on

the photocatalytic activity and photoelectrochemical propertyof TiO

2nanoparticlesrdquo Catalysis Communications vol 13 no 1

pp 14ndash17 2011[104] J A Rengifo-Herrera and C Pulgarin ldquoPhotocatalytic activity

of N S co-doped and N-doped commercial anatase TiO2

powders towards phenol oxidation and E coli inactivationunder simulated solar light irradiationrdquo Solar Energy vol 84no 1 pp 37ndash43 2010

[105] H Sun H Liu J Ma X Wang B Wang and L Han ldquoPrepa-ration and characterization of sulfur-doped TiO

2Ti photoelec-

trodes and their photoelectrocatalytic performancerdquo Journal ofHazardous Materials vol 156 no 1ndash3 pp 552ndash559 2008

[106] D Zhao C Chen Y Wang et al ldquoEnhanced photocatalyticdegradation of dye pollutants under visible irradiation onAl(III)-modified TiO

2 structure interaction and interfacial

electron transferrdquo Environmental Science and Technology vol42 no 1 pp 308ndash314 2008

[107] S M Chang and W S Liu ldquoSurface doping is more beneficialthan bulk doping to the photocatalytic activity of vanadium-doped TiO

2rdquo Applied Catalysis B vol 101 no 3-4 pp 333ndash342

2011[108] Q Li C Zhang and J Li ldquoPhotocatalytic and microwave

absorbing properties of polypyrroleFe-doped TiO2composite

by in situ polymerization methodrdquo Journal of Alloys andCompounds vol 509 no 5 pp 1953ndash1957 2011

[109] R Xu J Li J Wang et al ldquoPhotocatalytic degradation oforganic dyes under solar light irradiation combined withEr3+YAlO

3Fe- and Co-doped TiO

2coated compositesrdquo Solar

Energy Materials and Solar Cells vol 94 no 6 pp 1157ndash11652010

[110] R F Chen C X Zhang J Deng and G Q Song ldquoPreparationand photocatalytic activity of Cu2+-doped TiO

2SiO2rdquo Interna-

tional Journal of Minerals Metallurgy and Materials vol 16 no2 pp 220ndash225 2009

[111] G G Liu X Z Zhang Y J Xu X S Niu L Q Zheng and X JDing ldquoEffect of ZnFe

2O4doping on the photocatalytic activity

of TiO2rdquo Chemosphere vol 55 no 9 pp 1287ndash1291 2004

[112] E Pipelzadeh A A Babaluo M Haghighi A Tavakoli MV Derakhshan and A K Behnami ldquoSilver doping on TiO

2

nanoparticles using a sacrificial acid and its photocatalytic per-formance undermediumpressuremercuryUV lamprdquoChemicalEngineering Journal vol 155 no 3 pp 660ndash665 2009


[113] X Lin F Rong D Fu and C Yuan ldquoEnhanced photocatalyticactivity of fluorine doped TiO

2by loaded with Ag for degra-

dation of organic pollutantsrdquo Powder Technology vol 219 pp173ndash178 2012

[114] L G Devi B Nagaraj and K E Rajashekhar ldquoSynergisticeffect of Ag deposition and nitrogen doping in TiO

2for the

degradation of phenol under solar irradiation in presence ofelectron acceptorrdquo Chemical Engineering Journal vol 181-182pp 259ndash266 2012

[115] X Li R Xiong and G Wei ldquoPreparation and photocatalyticactivity of nanoglued Sn-doped TiO

2rdquo Journal of Hazardous

Materials vol 164 no 2-3 pp 587ndash591 2009[116] Z M El-Bahy A A Ismail and R M Mohamed ldquoEnhance-

ment of titania by doping rare earth for photodegradation oforganic dye (Direct Blue)rdquo Journal of Hazardous Materials vol166 no 1 pp 138ndash143 2009

[117] A F Shojaie and M H Loghmani ldquoLa3+ and Zr4+ co-dopedanatase nano TiO

2by sol-microwave methodrdquo Chemical Engi-

neering Journal vol 157 no 1 pp 263ndash269 2010[118] C Fan P Xue and Y Sun ldquoPreparation of Nano-TiO

2doped

with cerium and its photocatalytic activityrdquo Journal of RareEarths vol 24 no 3 pp 309ndash313 2006

[119] L Yang P Liu X Li and S Li ldquoThe photo-catalytic activities ofneodymium and fluorine doped TiO

2nanoparticlesrdquo Ceramics

International vol 38 no 6 pp 4791ndash4796 2012[120] W Sangkhun L Laokiat V Tanboonchuy P Khamdahsag and

N Grisdanurak ldquoPhotocatalytic degradation of BTEX usingW-doped TiO

2immobilized on fiberglass cloth under visible lightrdquo

Superlattices and Microstructures vol 52 no 4 pp 632ndash6422012

[121] J Gong C YangW Pu and J Zhang ldquoLiquid phase depositionof tungsten doped TiO

2films for visible light photoelectro-

catalytic degradation of dodecyl-benzenesulfonaterdquo ChemicalEngineering Journal vol 167 no 1 pp 190ndash197 2011

[122] J Y Gong W H Pu C Z Yang and J D Zhang ldquoTungstenand nitrogen co-doped TiO

2electrode sensitized with Fe-

chlorophyllin for visible light photoelectrocatalysisrdquo ChemicalEngineering Journal vol 209 pp 94ndash101 2012

[123] Y Hu Y T Cao P X Wang et al ldquoA new perspective for effectof Bi on the photocatalytic activity of Bi-doped TiO

2rdquo Applied

Catalysis B vol 125 pp 294ndash303 2012

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


The TiO2photocatalysis has attracted great attention as

a promising water treatment technology due to quite a fewintrinsic advantages (i) Powerful ability to decompose thepollutants the organic pollutants can be decomposed andeven mineralized due to the dramatic powerful oxidationability of HO∙ Consequently this technique can be utilizedwidely in the cases once the advanced treatment of waterparticularly containing the recalcitrant organic compoundsis demanded (ii) Ambient operating conditions the processcan be realized under ambient operating conditions and thusis acknowledged as quite safe (iii) Low cost the sunlightcan be employed as the energy source and the oxidant isambient O

2 Meanwhile as the TiO


recycled the cost can be further cut off to run the process(iv) Environmentally friendliness TiO

2are chemically sta-

ble nearly non-toxic or relatively safe even in the harshconditions Thus the TiO

2photocatalysis is thought as an

environmentally-friendly approach even if recent concernsarise on its environmental risks as it is released into theenvironment [15]

Many reports available in recent years have evidencedthat numerous organic contaminants can be decomposed oreven mineralized [16] by photocatalysis and the mechanismgoverning the photocatalytic process on the TiO

2surface has

been disclosed in depth [17] Also efforts for preparationof photocatalytic materials [7ndash9 18ndash22] which include themodification of the commonly used TiO

2photocatalyst

and the design and preparation of new photocatalysts areintensively exerted These efforts inclusively have attemptsto accelerate the degradation process of target substrates byenhancing the separation efficiency of e-h pairs or to extendthe wavelength of excitation light from ultraviolet (UV)regime to visible regime

Along with the lab-scale investigation of simulated waste-water some pilot-scale tests have been performed [23 24]It could be understood that although the results served todemonstrate the technical feasibility for water purificationtheTiO

2photocatalysis is still facing a series of technical chal-

lenges for a large-scale installment in wastewater treatmentplants (WWTPs) It must be noted that challenges such asfabrication of active TiO

2photocatalyst rapid separation and

recycling of TiO2after use and optimization of the overall

process are determined by the fundamentals In particularthe connection between the fundamentals and the actualscenario should be well understood by both the scientistsand engineers as they are performing their own individualtasks

However the fundamentals which have been well estab-lished in the literature are far from being fully understoodor sophisticatedly applied in the actual application Also theoperating parameters optimized in laboratory often suffera mismatching with the actual scenario In view of theproblem this review highlights the connection between thetwo tasks which is expected to bring valuable informationfor the academia and industry Along with this highlight theeconomic consideration was covered which eventually leadsto a balance between the various variables working in theTiO2photocatalytic process

2 Water Purification of TiO2

Photocatalysis

With the increasing demand of the water sources novel tech-nologies which serve to purify the polluted water with pow-erful ability to decompose the pollutants in energy-saving andenvironmentally-benignmanners are becoming essential Aswell known conventional technologies such as filtrationcoagulation adsorption and precipitation principally serveto transfer the pollutants from one phase to another Thenewly emerging membrane separation technology [25] alsobelongs to such category Following them further steps mustbe taken to treat or dispose the transferred pollutants In thisregard chemical oxidation featured by their ability to destroythe molecular structures of pollutants is an indispensibleoption During the processes the toxicity of the involvedorganic moieties is likely to extenuate or aggravate and thusthe degree of pollutant degradation should be in controlMoreover the concept of wastewater will become out of datesince the so-called wastewater is just staying at a stage duringthe overall recycling of water as a resource The TiO

2-based

photocatalytic process harnessing the sunlight as an energysource and ambient O

2as an oxidant for water purification

is capable of exhaustively mineralizing the organic pollutantsand eliminating the toxicity Thus the TiO

2-based photo-

catalysis is accepted as an effective tool to purify the pollutedwater

As listed in Table 1 the water which is polluted by textilepesticides medicine and so forth can be treated by theTiO2-based photocatalysis It should be pointed out that the

TiO2-based photocatalysis is just employed as an individ-

ual method herein Actually however effluents dischargedfrom industrial processes commonly contain recalcitrant ornonbiodegradable pollutants and thus an integrated processcontaining biological and physicochemical units will bemoreapplicable Basically in the front of integrated process theinfluent can be subjected to a physical unit such as filtration orgrilling to separate the solids Following is a physicochemicalunit such as coagulation to remove the colloidal substancesSubsequently a biological unit including aerobic or inaero-bic oxidation serves to degrade most of the biodegradableorganic molecules and thus the water quality is usuallyimproved in terms of the indexes such as chemical oxygendemand (COD) and NH

3-N Finally a chemical means such

as the TiO2-based photocatalytical process is incorporated to

purify the water by finally removing the recalcitrant organicpollutants

From the technical point of view the TiO2-based pho-

tocatalytical process is applicable as one prerequisite issatisfied which is the transparency of water The transpa-rency ensures the transmission of the light The TiO

2-based

photocatalytical process can be installed as a pretreatmentunit or an advanced unit If the effluent is in need of furtherpurification to destroy the nondegradable organic pollutantsthe TiO

2-based photocatalytical process follows the bio-

logical unit Alternatively the TiO2-based photocatalytical

process is put in front of the biochemical unit to improvebiodegradability for a better biological oxidization of theorganic substrates




References



retention time[73]

TiO2carbonaerogel




and wavelength[74]




[75]



[76]






[78]






[80]



[81]




[83]




[84 85]


concentration[86]











[89]






TiO2microsphere





thickness[95]


Table 1 Continued


References



[96]


TiO2 P25Organic



[60]








[100]



VB

Self-combination

Ligh

t


level

O2

CB

HOO∙

HO∙

HO∙


O2

H2O2

H+

Eg

h+

h+

eminus

∙O2

minus


3 Fundamentals








2can be



2minus h) + (TiO

2minus e) (2a)

Redinterface1198961

lArrrArr

119896minus1



1198962

lArrrArr

119896minus2



2minus Red)surface

1198963

lArrrArr

119896minus3


(2d)






2band gap The


2(119899-type)




Photocatalysts

OH

-inde

x

100

60

40

80

20

0 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) P25

(2) Am

(3) A

(4) R

(5) ZnO

(6) SnO2

(7) SrTiO3

(8) BaTiO3

(10) V2O5

(13) CdS

(14) CuS

(15) BiOCl

(16) BiOI

(17) ZnS

(19) BiOBr

(20) CuO

(26) NiO

(11) Bi2WO4

(12) CeO2

(18) Cr2O3

(21) MnO2

(22) Fe2O3

(23) BiVO4

(24) ZrO2

(25) La2O3

(9) WO3 (27) Bi2O3








2is more efficient



[29]











2










2



2appears to be a




2










SiO3












Cu2+-dopedTiO2SiO2
























2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






References



retention time[73]

TiO2carbonaerogel




and wavelength[74]




[75]



[76]






[78]






[80]



[81]




[83]




[84 85]


concentration[86]











[89]






TiO2microsphere





thickness[95]


Table 1 Continued


References



[96]


TiO2 P25Organic



[60]








[100]



VB

Self-combination

Ligh

t


level

O2

CB

HOO∙

HO∙

HO∙


O2

H2O2

H+

Eg

h+

h+

eminus

∙O2

minus


3 Fundamentals








2can be



2minus h) + (TiO

2minus e) (2a)

Redinterface1198961

lArrrArr

119896minus1



1198962

lArrrArr

119896minus2



2minus Red)surface

1198963

lArrrArr

119896minus3


(2d)






2band gap The


2(119899-type)




Photocatalysts

OH

-inde

x

100

60

40

80

20

0 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) P25

(2) Am

(3) A

(4) R

(5) ZnO

(6) SnO2

(7) SrTiO3

(8) BaTiO3

(10) V2O5

(13) CdS

(14) CuS

(15) BiOCl

(16) BiOI

(17) ZnS

(19) BiOBr

(20) CuO

(26) NiO

(11) Bi2WO4

(12) CeO2

(18) Cr2O3

(21) MnO2

(22) Fe2O3

(23) BiVO4

(24) ZrO2

(25) La2O3

(9) WO3 (27) Bi2O3








2is more efficient



[29]











2










2



2appears to be a




2










SiO3












Cu2+-dopedTiO2SiO2
























2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




Table 1 Continued


References



[96]


TiO2 P25Organic



[60]








[100]



VB

Self-combination

Ligh

t


level

O2

CB

HOO∙

HO∙

HO∙


O2

H2O2

H+

Eg

h+

h+

eminus

∙O2

minus


3 Fundamentals








2can be



2minus h) + (TiO

2minus e) (2a)

Redinterface1198961

lArrrArr

119896minus1



1198962

lArrrArr

119896minus2



2minus Red)surface

1198963

lArrrArr

119896minus3


(2d)






2band gap The


2(119899-type)




Photocatalysts

OH

-inde

x

100

60

40

80

20

0 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) P25

(2) Am

(3) A

(4) R

(5) ZnO

(6) SnO2

(7) SrTiO3

(8) BaTiO3

(10) V2O5

(13) CdS

(14) CuS

(15) BiOCl

(16) BiOI

(17) ZnS

(19) BiOBr

(20) CuO

(26) NiO

(11) Bi2WO4

(12) CeO2

(18) Cr2O3

(21) MnO2

(22) Fe2O3

(23) BiVO4

(24) ZrO2

(25) La2O3

(9) WO3 (27) Bi2O3








2is more efficient



[29]











2










2



2appears to be a




2










SiO3












Cu2+-dopedTiO2SiO2
























2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





2can be



2minus h) + (TiO

2minus e) (2a)

Redinterface1198961

lArrrArr

119896minus1



1198962

lArrrArr

119896minus2



2minus Red)surface

1198963

lArrrArr

119896minus3


(2d)






2band gap The


2(119899-type)




Photocatalysts

OH

-inde

x

100

60

40

80

20

0 21 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

(1) P25

(2) Am

(3) A

(4) R

(5) ZnO

(6) SnO2

(7) SrTiO3

(8) BaTiO3

(10) V2O5

(13) CdS

(14) CuS

(15) BiOCl

(16) BiOI

(17) ZnS

(19) BiOBr

(20) CuO

(26) NiO

(11) Bi2WO4

(12) CeO2

(18) Cr2O3

(21) MnO2

(22) Fe2O3

(23) BiVO4

(24) ZrO2

(25) La2O3

(9) WO3 (27) Bi2O3








2is more efficient



[29]











2










2



2appears to be a




2










SiO3












Cu2+-dopedTiO2SiO2
























2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials






2










2



2appears to be a




2










SiO3












Cu2+-dopedTiO2SiO2
























2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









2O to form ∙OH









2O2




2


HO2

∙+∙OH 997888rarr H

2O +O

2(3)

H2O2+ 2h 997888rarr O

2+ 2H+ (4)


2-based







2O






2


2








2surface





119889119862

119889119905

= 119870119886119870119903

119862

1 + 119870119886119862

(5)








Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





Ln(1198620

119862

) = 119870ap119905 (6a)







Ln(1198620

119862

) + (1198620minus 119862) = 119896ap119905 (6b)


2 Obviously such






2photo-










2





2has an








2


















(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a)

(a)

(b)





2powders



2particles will







2powders are




2and the



2can be





2sol and



2microspheresThe




2microsphere
















2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





2photocalytic












Acknowledgments


References




2photocatalytic














2rdquo Journal of


















2photocata-

































2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials
























2and crys-



















2under low power






2-ZrO2nanoparticles


















2particles



2photocatalysts and




2O3anodesrdquo










2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials









2syn-










2rdquo










2nanoparti-




2



2microspherewith









2and in TiO

2C



















2photocatal-











2and its



























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials


























2


















2photocatal-



3UVTiO

2














2Ti photoelec-















2SiO2rdquo Interna-






2







2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








2for the









2doped















2rdquo Applied









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



review article -based photocatalytic process for purification of...

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