photocatalytic applications of

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Photocatalytic Applications of Micro- andNano-TiO2 in Environmental EngineeringSoonchul Kwon a , Maohong Fan b , Adrienne T. Cooper c & HongqunYang da School of Civil and Environmental Engineering, Georgia Institute ofTechnology, Atlanta, Georgia, USAb School of Materials Science and Engineering, Georgia Institute ofTechnology, Atlanta, Georgia, USAc Department of Civil and Environmental Engineering, TempleUniversity, Philadelphia, Pennsylvania, USAd Department of Chemical and Materials Engineering, University ofAlberta, Edmonton, Alberta, Canada

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To cite this article: Soonchul Kwon, Maohong Fan, Adrienne T. Cooper & Hongqun Yang (2008):Photocatalytic Applications of Micro- and Nano-TiO2 in Environmental Engineering, Critical Reviews inEnvironmental Science and Technology, 38:3, 197-226

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Critical Reviews in Environmental Science and Technology, 38:197–226, 2008Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643380701628933

Photocatalytic Applications of Micro- andNano-TiO2 in Environmental Engineering

SOONCHUL KWON,1 MAOHONG FAN,2 ADRIENNE T. COOPER,3

and HONGQUN YANG4

1School of Civil and Environmental Engineering and 2School of Materials Science andEngineering, Georgia Institute of Technology, Atlanta, Georgia, USA

3Department of Civil and Environmental Engineering, Temple University, Philadelphia,Pennsylvania, USA, and 4Department of Chemical and Materials Engineering, University of

Alberta, Edmonton, Alberta, Canada

The photocatalytic activity of micro- and nano-titanium dioxide(TiO2) has been utilized to significantly improve the degradationefficiencies of various contaminants in both water treatment andair pollution control. This article is a review of the literature cov-ering current research on environmental applications of micro-and nano-TiO2. The mechanisms of contaminant degradation ofnanoparticle TiO2 are reviewed, and its special properties are com-pared to micro-sized TiO2 in air purification and water treatment.

1. INTRODUCTION

Because of their promise for a wide range of applications in catalysis, sensors,and molecular electronics, much attention has been paid recently to researchon micro and nano materials.1,2 Micro and nano materials evidence unusualchemical, mechanical, optical, electrical, and magnetic properties comparedto traditional materials.3 Since considerable research has shown that manyapplications of micro- and nanomaterial properties depend on their specificsurface area, it is clear that a decrease in particle size will lead to improvedand expanded application.4 For example, nanocatalysts having small particlesize, high surface area, and a high density of surface coordination unsaturatedsites offer improved catalytic performance over microscale catalysts. They are

Address correspondence to Mahong Fan, School of Materials Science and Engineering,Georgia Institute of Technology, Atlanta, GA 30332, USA. E-mail: [email protected]

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especially attractive candidates in applications where the degradation ratesof contaminants are of major concern.5−12

The surface effects of micro and nanoparticles are extremely important.13

For spherical nanoparticles with a diameter of 3 nm, about 50% of theatoms or ions are on the surface, allowing both the possibility of manip-ulating bulk properties by surface effects and near-stoichiometric chemicalreactions.7 Due to their extreme sensitivity, the structural failure of nanoma-terials frequently occurs on the material surface. Typically, optimization ofthe nanoparticle structure is conducted under the condition that the particlesare larger than 5 nm.14

The potential benefits of photocatalysis have been reported in a largenumber of studies published in recent decades. Heterogeneous photocataly-sis has been applied in water treatment and air pollution control. A photocat-alyst can facilitate the breakdown and removal of a variety of environmentalpollutants at room temperature by oxidation, using either sunlight or artificiallight as an energy source. In the photo-oxidative removal of potentially toxicorganic or inorganic compounds present in the environment, primary atten-tion has been given to the role of titanium dioxide (TiO2) over compoundssuch as ZnO, CdS, and WO3. This attention is due to its high photocatalyticactivity, biological and chemical inertness and stability, resistance to pho-tocorrosion, low cost, nontoxicity, and favorable band-gap energy.11,15−24

Indeed, laboratory studies have demonstrated that TiO2 is the most suitablephotocatalyst of all these compounds for widespread environmental treat-ment and other applications. These applications include the destruction ofmicroorganisms such as bacteria25−27 and viruses28 the inactivation of can-cer cel1s,29,30 odor control,31,32 the conversion of NOx,33−44 the removal ofmercury,45−53 the conversion of SO2,40,54−63 and the decomposition of oilspills.64−71 Furthermore, a TiO2 photocatalyst that exhibits high activity forthe oxidation of volatile organic compounds (VOCs) under ultraviolet (UV)radiation offers an economically and technically practical means to clean airand water. This article provides an overview of the underlying principlesgoverning environmental applications of TiO2 as a photocatalyst.

2. MECHANISM OF THE TIO2-BASED PHOTOCATALYTICDEGRADATION PROCESS

The primary applications of TiO2 fall in the areas of pigments, adsorbents,and catalytic supports. The sol–gel route is regarded as a good method forsynthesizing ultrafine metallic oxides,73,74 and therefore has been widely usedto prepare TiO2 particles. There are three different phases of TiO2: anatase,rutile, and brookite.81 Although all are expressed with the same chemicalformula (i.e., TiO2), their crystal structures are different. The anatase phase

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Photocatalytic Applications of Micro- and Nano-TiO2 199

has a tetragonal structure and, in terms of thermodynamics, is a metastablephase.81 An anatase-to-rutile phase transformation occurs during heat treat-ment at about 900 K, and has an energy band gap of 3.2 eV.81 The rutilephase has a tetragonal structure that is stable at high temperatures and hasan energy band gap of 3.0 eV.81 The brookite phase has an orthorhombicstructure but, as it has many defects in its crystal structure,81 has not been usedas a photocatalyst. Because the band gap energy of the rutile and anatasephases is slightly greater than that of brookite, they both absorb primar-ily ultraviolet rays. However, the rutile type can absorb some visible rays.Among the three types of TiO2, the anatase form appears to be the mostphotoactive11,15,16,22,24,50,52,72,77,82−84 and the most practical for widespreadenvironmental applications such as water purification, wastewater treatment,and air pollution control, although anatase mixed with rutile has been foundto be highly photoactive as well.81

Figure 1 illustrates the mechanism of the photocatalytic degradation pro-cess. When TiO2 photocatalyst is irradiated with energy equal to or greaterthan the band gap (E ), it is thought to undergo the mechanism described byEqs. (1)–(5):14,85−87

TiO2 + hν → h+vb + e−

cb (1)

H2O + h+vb → ·OH + H+ (2)

h+vb+Pollutant(ads) → Pollutant+ (3)

OH + Pollutant(sol) → CO2 + H2O (4)

O2 + e−cb → O−

2 (5)

FIGURE 1. Primary mechanism of photocatalytic reaction. Path 1: formation of charge carriesby a photon. Path 2: charge carrier recombination to liberate heat. Path 3: production ofsuperoxide by a conduction band electron. Path 4: production of hydroxyl radical (·OH).14

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With the absorption of photons by TiO2, electrons (e−cb) and positive holes

(h+vb) are produced; electrons move to the conduction band (CB) to create

positive holes in the valence band (VB), which disperse on the surface ofTiO2. These charges either can recombine to produce heat or can be usedto reduce or oxidize species in solution at the TiO2 surface. The positiveholes react with H2O [Eq. (2)] or the pollutants adsorbed at the TiO2 surfaceas shown in Eq. (3). Also, two highly reactive substances are formed, asindicated in Eqs. (2) and (5): hydroxyl radicals (·OH) and a super-oxideanion (O−

2 ).88 A possible second way of contaminant degradation occursat the liquid/solid interface by means of the hydroxyl radical [Eq. (2)],87

which can initiate the degradation of the adsorbed chemical species by oneor more forms of electron transfer reactions at room temperature with aUV light source [Eq. (4)]. Photocatalytic reactions provide not only photo-excited holes and electrons, but also oxidatively and reductively generatedactive oxygen species.88 As an example, the photocatalytic degradation ofethanol (CH3CH2OH) on TiO2 is described next:14

TiO2 + hν → h+vb + e−

cb (6)

H2O + h+vb → OH + H+ (7)

CH3CH2OH + ·OH → CH3C · HOH + H2O (8)

CH3C · HOH + O2 → CH3CH(OH)OO· (9)

CH3CH(OH)OO · +CH3CH2OH → CH3CH(OH)OOH + CH3·HOH (10)

CH3CH(OH)OOH → CH3CHO + H2O2 (11)

Ethanol reacts with O2 on TiO2 surfaces, and an organoperoxyl radical(CH3CH(OH)OO·) is subsequently formed as an intermediate step. Finally,acetaldehyde (CH3CHO) and hydrogen peroxide (H2O2) are produced.

3. PHOTOCATALYTIC DEGRADATION REACTORS

As shown in Figure 2, various types of photocatalytic reactors have beendesigned for water treatment. Well-mixed heterogeneous batch reactors havebeen employed in laboratory experiments, for the most part. In most cases(Figure 2c), UV sources from outside of the reactor illuminate on one side andthe photons from UV lamps (usually mercury lamps) are dispersed toward thepollutant inside the reactor (Figure 2d). The reactor designs for photocatalyticdegradation with micro and nano-TiO2 particles are similar.19,24,70,78,89

3.1. Photoreactors for Water Treatment Systems

Photoreactors were employed in water treatment processes to per-form photocatalytic reactions over nano-TiO2 powder in liquid

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FIGURE 2. Photocatalytic laboratory reactor configurations: (a) jacketed batch photoreactorfor water degradation with center UV source;24 (b) circulating flow photoreactor with UV lampat water phase;11 (c) fluidized-bed continuous-flow gas reactor illuminated from outside UVsource;89 (d) continuous-flow gas photoreactor with center UV lamp.78

suspensions.19,50,70,72,75,80,90 Inside the reactor, the mixture of the pho-tocatalytic reaction was maintained in suspension by means of a magneticstirrer. Synthesized nano-TiO2 particles are known for their dispersion inwater.77 The dispersion of nano-TiO2 was irradiated with a UV lamp as theUV source, placed in the center of the reactor, as shown in Figure 2, a andb. Cooling water (Figure 2a) or continuous gas (Figure 2b) was circulatedthrough the inner sleeve to control the temperature in the reactor. Li et al.19

used pure oxygen or ozone containing oxygen bubbled continuously into

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the solution to maintain a constant temperature, whereas Szabo-Bardos etal.11 have used continuous air or argon gas. Also, water can sometimes beused to maintain a constant temperature.

The photoreactor illustrated in Figure 2a has an internal light sourcecontained in a cylinder of quartz glass; therefore, degradation of crude oilcould be initiated with illumination in the wavelength range of λ = 240–375nm without direct contact with the light source.24 Toluene and asphaltenewere solubilized by 8 mM of anionic surfactant (sodium dodecyl sulfate, SDS)in water and their photocatalytic degradation was performed using H2O2 in0.1% (w/v) aqueous suspension of TiO2.24 The F2 crude oil was emulsifiedinto water under intensive cooling at 25 ± 0.5◦C with addition of H2O2 during10 h of irradiation.24

The photochemical reactor shown in Figure 2b uses continuously fedair, at a flow rate of 40 dm3/h, to provide circulation within the reactor. As inthe previously described reactor, the light source (40W, λmax = 350 nm) doesnot come in contact with the water. The light source is placed at the centerof the reactor, and a glass tube between the internal (quartz) and external(glass) walls separates the reactor of 3 dm3 into two parts.11 The outlet gasflows out at the top of the reactor and a septum is located for sampling fromthe bottom.

Initial TiO2 photocatalytic reactor design research for water treatmentevaluated slurry reactors. In numerous photocatalytic water treatment studies,both micro- and nano-TiO2 particles have demonstrated excellent degrada-tion capabilities,11,15,17−−21,23−25,30,31,33,34,38,45,46,51−53,67,74,77,78,80,84,88,89,91−99

sorption of contaminants,35,54,55,57−60,63,83 and load-carrying capacity.1,3,100

However, the slurry reactor design requires additional steps, such asfiltration and/or centrifugation to separate the photocatalyst particlesfrom the water. The addition of these unit processes adds significantlyto the process complexity and cost.86 Alternative configurations provideeither fluidized11,89,96,98,99 or fixed-bed reactors.22,24,51,52,70,72,85,87,101 Oneof the most practical applications of photocatalysis is the use of fixed-bed reactor configurations with immobilized particles or semiconductorceramic membranes.86 Fixed-bed reactors for water or gas-type treat-ment processes continuously use the photocatalyst, eliminate the needfor postprocess filtration and particle recovery, and facilitate catalystregeneration.

3.2. Photoreactors for Gas-Phase Treatment System

Contrary to the dispersion of micro and nano-TiO2 powder in water treat-ment processes, in the photocatalytic reaction of gas treatment systems theuniform distribution of free-flowing micro- and nano-TiO2 inside the reac-tor plays a key role. The distributed photocatalyst illuminated by UV light

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contacts the reactant pollutant as it flows continuously through the photore-actor. Research for gas-phase photocatalytic reactions has been carried outprimarily in glass continuous-bed reactors, utilizing a UV lamp placed eitheroutside or inside the reactor, as shown in Figures 2, c and d, respectively.78,89

A UV source from outside of the reactor, particularly if it is only from oneside, may not illuminate uniformly to the central location of the packed TiO2.Thus, a UV lamp located in the middle of the reactor affords better photo-catalyst performance.11,78,89

The fluidized-bed reactor shown schematically in Figure 2c is a flat-wall,parallel-plate reactor formed from 7042 Pyrex glass.89 As light is transmittedinto this photoreactor from outside the reactor, coarse glass frit sealed in thereactor feed tube maintains uniform gas distribution and provides support ofthe catalyst bed. A single 4-W fluorescent UV source (GE F4T5-BLB) alignedparallel to the flat reactor wall uniformly illuminates the fluidized catalyst.Inlet and outlet gases were sampled manually with a Carle valve and sub-sequently analyzed for the concentration profile using gas chromatography(GC) with flame ionization detection (FID).89

The photoreactor represented in Figure 2d consists of a UV lamp, pro-ducing 2.2 W (around λmax =365 nm), surrounded by several layers of TiO2

immobilized on fiberglass mesh. The layers of catalyst screen are optimizedto completely absorb/diffract the light and induce turbulent air flow.78 Massflow meters were used to insure a constant flow (6–24 L/h) of air contain-ing a given concentration of the pollutant studied and, in some cases, ofozone.78

4. ORGANIC POLLUTANT DEGRADATION OVER TIO2

Volatile organic compounds (VOCs) are emitted into the atmosphere, soil,and water from a variety of anthropogenic activities, such as transportationand discharges from residences, businesses, and industrial facilities. Theycause environmental degradation of soil and groundwater,24,66 and the con-tribute to air pollution.15,83 The two main criteria for photocatalyst suitabilityfor the degradation of volatile organic compounds are (1) that the redoxpotential of the H2O/·OH (OH− = ·OH + e−; Eo = −2.8 V) couple fallswithin the appropriate band gap of the catalyst and (2) that the photocata-lyst remains stable over long periods.86 Photochemical reaction propertiesof non-nano-sized TiO2 is the same as that of nano-sized TiO2, but thedegradation capability of nano-TiO2 is much greater because of the largespecific surface area and high density of surface coordination.4 Recent stud-ies have revealed the significant photocatalytic degradation capability ofnano-TiO2 for aromatic compounds such as benzene, toluene, phenol, andcatechol.19,73,77,78,102

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4.1. Decontamination of Benzene

Benzene is an aromatic hydrocarbon resulting from burning of coal, gasoline,and other fuels. Benzene is also used in the manufacture of plastics, deter-gents, pesticides, and other chemical processes.73 Fu et al.73 used a glassfixed-bed reactor with a UV lamp in the middle and 19 g of 350-nm TiO2

to investigate gas-phase mineralization of benzene. Utilizing benzene in airat a concentration of 491 ppmv, experiments were carried out in the rangeof 70–140◦C.73 The concentrations of benzene and carbon dioxide were an-alyzed by using the GC/FID and GC/TCD, respectively. Figure 3 shows theresults of these experiments as a function of temperature. Line 1 shows thecontrol, catalyst with no illumination, while line 2 illustrates the conversionof benzene with UV illumination. With and without illumination, the mineral-ization of benzene improved with increasing temperature. At 140◦C, conver-sion efficiency attained its maximum of 30% for the thermocatalytic process(line 1), while it reached 90% for photocatalytic reaction under UV irradia-tion (line 2). This result indicates that TiO2 photocatalysis is effective for themineralization of benzene. Because the initiation of photocatalytic energyis dominated by photons, photocatalytic reactions are generally minimallyaffected by increases in temperature;103 however, this study was affected byeven small variations in low temperature (70–140◦C). This is likely due to thehigh specific surface area and porosity of nanosized TiO2, which enhancesphotocatalytic activity and improves initiation of the photoreaction at lowertemperatures.

FIGURE 3. The decontamination of 491 ± 2% ppm benzene balanced with N2 at a concen-tration of (1) thermocatalytic control; (2) photocatalytic reaction with (350 nm) nano-TiO2

catalyst (line 2); mass of catalyst 19 g;.73

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4.2. Degradation of BTX

Benzene, toluene, and xylene (BTX) are recognized as harmful VOCs be-cause of their adverse effects on the human nervous system. Exposure to BTXoccurs via contact with gasoline, kerosene, heating oil, paints, and lacquers,either via inhalation or skin contact, and through drinking contaminated wellwater.104

Pichat et al.78 demonstrated the photocatalytic effect of TiO2 on BTXdegradation using the photoreactor previously described in section 3.2 andshown in Figure 2d. In the Pichat experiments a 9-W UV lamp is surroundedby layers of fixed TiO2 mesh that maintain turbulent air flow inside the reac-tor and completely absorb the radiant flux emitted by the UV lamp. Outsideair was introduced into the reactor before each experiment for 15 min. Thedetails of the experimental procedure are the same as those presented insection 3.2. The results of photocatalytic BTX degradation in this study areshown in Figure 4. This result clearly indicates that the concentrations ofbenzene, toluene, and xylene each declined within 1 h by photocatalytic re-action, with the removal efficiencies of toluene and xylene over 60% and 50%,respectively. However, although the conversion of benzene was almost 70%within 1 h, removal efficiency decreased to 25% at 15 ppb concentration inthis study. They found that in order to enhance photoreaction performance,contact time between contaminants and the TiO2 surface area should besustained by limiting the air flow rate (e.g., to 50 L/h). By comparison, aconversion efficiency of benzene greater than 90% was shown in the study

FIGURE 4. The degradation of BTX [benzene (x), toluene (♦), o-xylene (�), and both m- andp-xylene (�)] by nano-TiO2-coated fiber glass mesh (50 g/m2) in the prototype air purifier.78

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of Fu et al.73 They used a higher initial benzene concentration (490 ppm)than in the study of Pichat et al.

Several researchers have investigated the use of ozone to en-hance photocatalytic degradation while preventing the generation ofintermediates.19,72,78,86,94,96 Ozone is superior to oxygen in this regard asozone is a more powerful oxidant (E o = 2.07 V) when compared to oxygen(E o = 0.82 V), and the electron transfer from TiO2 to ozone is faster than theelectron transfer from TiO2 to O2. Additionally, ozone has a strong electron-withdrawing tendency that leads to a negative charge density and facilitatesreactions with the free radical (E o = 2.80 V).19,78

4.3. Degradation of Phenol and Catechol

Phenol, used in a number of products such as nylon and other syntheticfibers, is a common contaminant in industrial wastewater. Exposure via skincontact to high levels of phenol can cause liver damage, diarrhea, and discol-oration of the urine.19,77 Phenol poses a substantial challenge, because of itshigh stability and solubility in water. The decontamination of phenol usingseveral different removal techniques has been studied.

Among various degradation methods, Nagaveni et al.77 investigatedthe photocatalytic degradation of phenol and catechol by combustion-synthesized nano-TiO2 with both artificial UV and solar irradiation. More-over, to improve the degradation efficiency H2O2 was added into reactorand compared to combustion-synthesized nano-TiO2 without H2O2. Solu-tion combustion is a fast and simple one-step method applied to an aque-ous redox mixture containing stoichiometric amounts of metal salts andrapidly heated water-soluble fuel.77 The artificial light reactor used a medium-pressure mercury vapor lamp (125 W, λmax = 365 nm) in a jacketed quartztube77 in order to keep the constant light property. Cold water was cir-culated continuously through the inner sleeve of the reactor to removeheat from the solution. The samples were centrifuged and filtered through0.45-µm Millipore membrane filters to remove the catalyst particles prior toanalysis.77

While the use of TiO2 without a UV source had little or no effect on phe-nol degradation, significant degradation was realized through the applicationof a photocatalyst in combination with UV. The experimental results in Figure5 show the photocatalytic activity for phenol degradation over combustion-synthesized and commercial TiO2 (Degussa P-25) using both solar and artifi-cial UV illumination. Phenol degradation over combustion-synthesized TiO2

is more effective than that over commercial TiO2 for both solar and UV il-lumination (0.1 g of 90% over combustion-synthesized TiO2 and 35% overcommercial TiO2 under solar irradiation at 200 min of exposure). This likelyis due to the special characteristics of combustion-synthesized TiO2 (6–8 nm);

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FIGURE 5. Phenol degradation with 6–8 nm of combustion-synthesized and 30 nm of com-mercial nano-TiO2 photocatalyst at 0.5 mmol/L initial concentration of phenol.77

the small particle size results in a higher surface area, an anatase crystal struc-ture, and higher surface density of the hydroxyl group than Degussa P-25,which has a particle size in the 30 nm range. It has been reported that thecrystallinity of TiO2 affects phenol degradation.105

The use of hydrogen peroxide also enhances photocatalyticefficiency.24,77 Pernyeszi et al.24 described the effects of H2O2 concentra-tion for the photocatalytic process. For example, compared to photocatalyticreaction without added H2O2, the addition of 0.1 M and 1 M H2O2 improvesthe phenol photocatalytic degradation by 14% and 30%, respectively. Photo-catalysis with artificial UV irradiation results in a more effective degradationin comparison to solar irradiated photocatalysis under the same intensities.

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FIGURE 6. Degradation of phenol with of combustion-synthesized nano-TiO2 photocatalyst(6–8 nm) as a function of initial phenol concentration.77

For example, at 120 min of exposure time, the completion of phenol de-composition by combustion-synthesized TiO2 under artificial UV and solarillumination was 80% and 70%, respectively. Figure 6 shows that degradationrates depend on the phenol concentration, and that the rate of decrease inthe phenol concentration is slower as the initial concentrations increase, thusindicating that lower concentrations of phenol are much easier to degrade.

Catechol, used as a developing agent for fur dyes and photography, isrecognized as a chemical hazard in wastewater.106−108 The U.S. Departmentof Health and Human Services (HHS) has determined that catechol causesdepression of the central nervous system and skin disorders similar to thosecaused by phenol,107 thus making its oxidation a significant objective ofwastewater treatment systems. Li et al.19 investigated the photocatalytic oxi-dation and the addition of ozonation with photocatalysis of catechol over car-bon black-modified nano-TiO2 (CB-nano-TiO2) supported by an aluminumsheet.19 CB-nano-TiO2 thin films, formed by a sol–gel method, prevent sep-aration between the solid and liquid phases in wastewater treatment. Thespace vacated by nano-sized carbon black (d = 18 nm) increases the poros-ity of nanoparticle TiO2 film and the fraction of rutile, improving photocat-alytic activity, both of which aid in light absorption.19,95 Kang et al.109 alsoindicated that the carbon-black-incorporated TiO2 has a more uniform poresize than that of regular TiO2.

CB-nano-TiO2 thin films therefore yield photocatalytic activity 1.5 timesgreater than unmodified nano-TiO2 thin films for the degradation of catechol.Figure 7 shows that the efficiency of photocatalytic catechol degradation

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FIGURE 7. Catechol degradation at various ozone concentrations in the nano-TiO2 (18nm)/UV/O3 process.19

improved with the addition of ozone. At an ozone concentration of 50 mg/hand 5 min of irradiation, catechol degradation was 90%. Efficiencies de-creased as ozone rates decreased and without the presence of ozone cat-echol removal was only 10% after five minutes of irradiation. These resultsare consistent with those found for phenol, wherein the presence of ozoneimproves photocatalytic efficacy.

5. NOx REMOVAL BY TiO2

Another recently targeted source of environmental pollution is NOx, pro-duced by the combustion and exhaust gases of motor vehicles. Of the various

FIGURE 8. Comparison of oxidative methods for catechol degradation.19

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methods for removing NOx, photocatalytic reaction has received consider-able focus, due to its high removal efficiency and ease of use. There are twotypes of photocatalytic processes for removing NOx, namely, photocatalyticoxidation and the decomposition of NO. Both plants and microorganismsuse products of photocatalytic oxidation such as nitrite and nitrate ion. Pho-tocatalytic oxidation occurs in the presence of both water and oxygen toform nitrate, whereas photocatalytic decomposition occurs in the absenceof water and oxygen.33,34,92 Hashimoto et al.33 demonstrated that the pho-tocatalytic oxidation of NO with UV irradiation can effectively remove NOx.The experiment was carried out in a fixed continuous-bed reactor at 1 atmusing 120 mg of nano-TiO2 combined with zeolite (A- and Y-form zeolite:TiO2-AZ and -YZ), which was used as a photocatalyst under uniform UV il-lumination inside reactor. The concentrations of nitrogen oxide and nitrogendioxide were obtained using a chemiluminescence NOx analyzer (Yanako;ELC-88AO). Nitrogen and nitrous oxide were analyzed by GC/TCD (heliumcontaining 20% of O2 and 10 ppm of NO).33 The addition of zeolite improvesthe adsorption of NO and thus photocatalytic oxidation. The adsorption ofNO initiates rapidly and adsorption equilibrium is reached quickly. In Figure9, the efficiency of NOx oxidation is defined by the integration of the dif-ference between the inlet concentration of NO and outlet concentration ofNOx.33 The solid line shows the outlet concentration of NOx and the opencircle shows the outlet concentration of NO, with the difference betweenthe two indicating the outlet concentration of NO2. The results show almost100% oxidation efficiency. However, because an oxidation product (HNO3)was absorbed on the photocatalyst surface area and NO was desorbed until

FIGURE 9. Photocatalytic oxidation of NO and NO2 over nano-TiO2 (11 nm) photocatalyst astime variation: (A) Hycom TiO2; (B) TiO2-AZ composite (1:1); (C) TiO2-YZ composite (1:1);solid line: outlet concentration of NOx (NOx = NO + NO2); open circle: outlet concentrationof NO; mass of nano-TiO2 120 mg; inlet NO concentration: 10 ppm balanced with air; pressure1 atm; flow rate 110 ml/min; and T 300◦C.33

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the point of equilibrium, oxidation efficiency decreased gradually with theincrease in illumination time. As shown in Figure 9, b and c, because the des-orbed concomitants were absorbed on the zeolite, recovery in the NO con-centration over nano-TiO2 with AZ and YZ was lower than over nano-TiO2

alone after reaching equilibrium.33 In addition, for oxidation over nano-TiO2

alone (Figure 9a), the outlet concentration of NO2 as an oxidation productincreased gradually when illumination time increased. By contrast, due tothe absorption of NO2 on the zeolite surface area, smaller concentrationsof NO2 resulted from the oxidation over nano-TiO2 with AZ (Figure 9b)and YZ (Figure 9c). Therefore, these results suggest that the addition of ze-olite to titanium oxide enhances the photocatalytic oxidation of NO usingnano-TiO2.

6. REMOVAL OF HG WITH TiO2

Toxic metal ions such as Hg(II), Ag(II), and Cd(II) are recognized as non-degradable compounds having long life spans, concentrations of which mayreach toxic levels in the food chain.85 Yet increasing recognition of the toxi-city of these metal ions has done little to slow demand for them in the faceof accelerating global industrialization. Mercury, for example, is widely usedin batteries, relays, metallurgy, catalysts, paints, and pesticides, among otherapplications.85 However, because of its cumulative toxic effects, mercury inwater and wastewater significantly imperils water-based ecosystems.50 Vari-ous physical and chemical removal methods have therefore been developedto degrade this toxic metal in water and wastewater, including precipitation,activated carbon adsorption, ion exchange, reverse osmosis, and membraneseparation.50,85 However, these processes themselves produce by-productsin the forms of hazardous and/or toxic compounds that are themselves inneed of disposal.

Among other methods, TiO2 has been used as a photocatalyst to re-move mercury from aqueous solutions.47,50−52,85 In particular, Skubal etal.50 investigated the photocatalytic degradation of mercury(II) ions in waterover arginine-modified TiO2 nanoparticles, both with and without UV light.Anatase TiO2 colloids (40–60 A in diameter) were formed from the hydrolysisof titanium tetrachloride (TiCl4).50 Analysis of the synthesized colloids wasconducted using the method of Thompson,110 which is a spectrophotometricmethod that measures peroxotitanium(IV) from the dissolution of the colloidin sulfuric acid. Either the colloid remained unmodified or its surface wasmodified with l-arginine (ARG, C6H14N4O2). Arginine was used as a modifierbecause it effectively binds to both the TiO2 surface, through the carboxylfunctional group, and to mercury ions in solution, through the amino func-tional groups.50 Solutions of arginine-modified TiO2 [ARG (1.67 × 10−3 M),TiO2 (5.00 × 10−3 M)] were allowed to reach equilibrium condition for 1 d

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212 S. Kwon et al.

under an anoxic atmosphere (argon-purged, AGA 99.995%). Mercuric chlo-ride was injected under anoxic condition into the modified TiO2 solution anddiluted to 500 ml in a quartz flask with continuous mixing. After a 10-minequilibration period, the sample either remained in darkness or was illumi-nated by an eight-light Rayonet photochemical chamber reactor (RMR model600) (λ max = 253.7 nm).50 Four milliliters of samples were withdrawn andfiltered anoxically using pressure filters with Amicon YM100 membranes.50

Precipitate on the filter was examined for the presence of nonsoluble elemen-tal mercury using a saturated solution of potassium iodide per the methodof Tennakone et al.111 Filtered sample was analyzed using cold vapor atomicabsorption spectroscopy (Buck Scientific 200 A).50

In this study, the efficiency of mercury(II) degradation over non-modified nano-TiO2 was less than 10%. However, because of the effectivebinding capacity between mercury ions in solution through the amino func-tional group and the TiO2 surface through the carboxyl functional group,degradation was nearly 100% (99.98%) in the UV light reaction for mercuryover TiO2 nanoparticles modified with arginine.50,112 Figure 10 indicates thatthe dark reaction continued to absorb mercury for about 1 h until the sys-tem reached equilibrium. This equilibrium point initially represented 60%of mercury removal. By contrast, removal efficiency was almost 95% within1 h in the UV light reaction, and removal from the solution was virtuallycomplete (99.98%) within 2 h. These results indicate that the modifier, argi-nine, significantly improved the removal efficiency of mercury, facilitatedcharge transfer from the TiO2 surface to the sorbed mercury, and preventedcharge recombination.50 Skubal et al.50 have also studied the effects of addingmethanol to enhance the removal rate of mercury over a shorter length ofUV illumination, achieving total degradation within 32 min.

FIGURE 10. Photocatalytic removal of mercury by of arginine-modified nano-TiO2 (45 nm);[TiO2] = 5.0 × 10−3 M , [ARG] = 1.67 × 10−3M , [Hg] = 7.5 × 10−4 M (150 ppm). Errorbars are derived from the standard deviation of duplicate reactions.50

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7. DEGRADATION OF OIL SPILLS OVER TiO2

Due to its high efficiency and low costs, photocatalytic degradation overnano-TiO2 has attracted the attention of scientists as an emerging technologyto deal with the accidental discharge of crude oil into waterways. Ziolli et al.70

have investigated the ability of photocatalytic detoxification under nano-TiO2

with UV illumination to destroy seawater-soluble crude oil hydrocarbons.Brazilian crude oil samples were obtained from the Campos Basin on the

continental shelf of Rio de Janeiro State, Brazil. Seawater (salinity 33‰, andpH 8.4) used in all experiments was collected at Sao Sebastiao. The experi-ment was conducted in an aqueous suspension of nano-TiO2 photocatalyst(30 nm) at a concentration of 0.1% w/v irradiated with UV by high-pressuremercury lamp (HPL-N, 125W, λmax = 366 nm) in a Pyrex batch reactor (80mm × 210 mm) over 7 d of exposure.70 The suspension was stirred and kepthomogeneous in the reactor; cooling water was circulated via a quartz innersleeve to avoid a temperature increase.

The toxicity of the water-soluble fraction (WSF) was examined us-ing a Microtox kit with marine luminescent bacteria (V. fischeri) as a testorganism.70,113 The results are expressed as toxicity units (TU), where TU >

1 means that the compound shows acute toxicity.Figure 11 shows that photocatalytic degradation of crude oil using nano-

TiO2 and UV irradiation increased gradually and approached a nearly 90% re-moval efficiency, whereas there was no significant removal using only either

FIGURE 11. Photocatalytic decomposition of seawater crude oil by 30 nm nano-TiO2 pho-tocatalyst; WSF of Brazilian type A crude oil in seawater (�) WSF-A only (control); � WSF-Awith TiO2 in the dark; (�) WSF-A during exposure to UV–visible irradiation without TiO2; (•)WSF-A with TiO2 and irradiation.70

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FIGURE 12. Toxicity results of photocatalytic decomposition of A and B type seawater crude-oil with heterogeneous photocatalysis (�, 30-nm nano-TiO2 with UV irradiation) and photol-ysis (•, UV irradiation).70

nano-TiO2 or UV illumination. The degradation of oil over nano-TiO2 withUV irradiation was greatest between the first and second days of UV ex-posure, suggesting that it takes more than 1 d of UV illumination in thesuspension to substantially degrade spilled oil by photocatalysis.

However, transient toxic compounds have been detected during pho-tocatalytic degradation. Jardim et al.93 demonstrated that the decompositionof four aromatic chlorinated compounds over TiO2 photocatalyst with UVirradiation could generate intermediates more highly toxic than the originalcompound. In the case of removal using only either nano-TiO2 or UV sources,the degradation ratio (C /C0) in Figure 11 shows intermediate concentrationsof oil higher than initial concentrations due to the generation of toxic com-pounds. Figure 12 shows toxicity, indicating the degree of toxicity, resultsafter up to 5 d of irradiation both with and without the nano-photocatalyst.The toxicity value was 1.8 TU—the same as before irradiation. For photo-catalysis with nano-TiO2, however, acute toxicity increased significantly (3.5TU) after 1 d of irradiation, then decreased to a value of zero after 3 d. Thissuggests that both toxic compounds and intermediates were either totallydegraded or converted to non-toxic compounds during photocatalysis. Be-cause of this intermediate behavior, then, special attention is needed whenapplying this technique to the degradation of spilled oil.

8. COD DETERMINATION BY NANO-TiO2

Chemical oxygen demand (COD) and total organic carbon (TOC) aremajor indices in environmental monitoring.114 COD has been frequentlyused as a benchmark to reveal the degree of contamination of or-ganic compounds.97,114 Recently, Ai et al. demonstrated COD monitoring

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by photocatalytic oxidation using a nano-TiO2/K2Cr2O7 system. K2Cr2O7

can facilitate the acceptance of photo-excited conduction band electronsand improve the efficiency of the photocatalytic degradation of organiccompounds.72,91 The monitoring principle of COD lies in determiningchanges in Cr(III) concentrations after the photocatalytic oxidation of or-ganic matter,72 then correlating the COD values obtained with those realizedusing conventional methods (e.g., dichromate).

The photocatalytic experiments were performed in a water-jacketed,mixed batch reactor using aqueous suspensions of TiO2 powder. UV irradia-tion was provided by an 11-W lamp (Shanghai Jinguang Lamps Factory) witha maximum wavelength, λmax, of 253.7 nm in the center of the reactor.72 After10 min of UV irradiation, samples were centrifuged and filtered to removeTiO2 particles and the absorbance of Cr(III) was examined calorimetrically at610 nm with a UNICO 2100 spectrophotometer.72 Wastewater samples wereobtained from Shanghai.

The experiment was carried out in order to optimize operating condi-tions such as pH, catalyst dosage, temperature, and oxygen concentration.In optimizing pH values, it was found that the absorbance of Cr(III) wasreduced considerably within pH 2, but, due to the effects of pH on the dis-tribution of Cr(VI) species in aqueous solution, declined only minimally withan increase in pH at a fixed COD value of 100 mg/ml.72 Therefore, pH 0.5was determined to be the optimum pH condition for COD determination.

The effects of nano-TiO2 dosage are shown in Figure 13, and indicatethat the oxidation rate of organic compounds increases significantly with anincrease in nano-TiO2 dosages up to 4 g/L, with no considerable increase

FIGURE 13. Effect of nano-TiO2 photocatalyst (30 nm) dosage on the absorbance of Cr(III).Conditions: CCr(VI) 0.01 mg/L; pH 0.5; C6H12O6 100 mg/L;and T 80◦C.72

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216 S. Kwon et al.

at higher dosages of nano-TiO2. This is because higher nano-TiO2 dosageslower the adsorption of UV light into the catalyst, preventing photocatalyticoxidation over nano-TiO2 with UV sources.72 Therefore, 4 g/L of nano-TiO2

dosage was selected as the optimum catalyst dosage.All indications are that the absorbance of Cr(III) generally improves with

increased temperature, which, because it enhances the collision frequencybetween nano-TiO2 and the substrates, also increases the oxidation rate ,but, due to the evaporation, the absorbance of Cr(III) lessened at relativelyhigher temperatures.72 Hence, for this study 80◦C was selected as the opti-mum temperature.

The effects of oxygen were studied by streaming air balanced with N2

into the COD solution at pH 0.5. It was determined that oxygen concentrationhad no effect on the absorbance of Cr(III).

To summarize, optimum conditions in this experiment were 4 g/L ofnano-TiO2 at 80◦C and a pH of 0.5. In addition, a comparison of COD valuesderived using this method with values taken from actual wastewater samplesusing a conventional method (dichromate) showed little difference (±10%)between the two.

9. TOC REMOVAL USING NANO-TiO2

Total organic carbon (TOC) is used to determine the degree of organic con-tamination in wastewater. Li et al.19 have investigated photocatalytic oxida-tion and ozonation of TOC over carbon-black-modified nano-TiO2 (CB-nano-TiO2) supported by an aluminum sheet.19 The experiment was carried outusing the same methods described for catechol degradation in section 4.3.The photocatalytic oxidation of TOC over CB-nano-TiO2 shows 1.5 timesmore activity than that over TiO2 thin films.19 Compared to other oxida-tion methods, Figure 14 shows that the highest TOC decomposition ratewas obtained using TiO2/UV/O3. The degradation (C/C0) rate of TOC in-creased gradually compared to that of catechol, which increased sharply(Figure 8). TOC was almost completely degraded (i.e., 95%) after 1 h usingthe TiO2/UV/O3 process. However, without ozone, the efficiency of TOC re-moval in a TiO2/UV process was only 20% after 1 h. Accordingly, the rate ofTOC removal increased with increases in ozone concentration. These resultsindicate that ozonation enhances the photocatalytic degradation of TOC. Asshown in Figure 15, TOC was degraded by more than 55% at an ozoneflow rate of 12.5 mg/h, and was totally mineralized after 30 min at 50 mg/h,indicating that more hydroxyl radicals were generated by the TiO2/UV/O3

process.19 Therefore, the photocatalytic activity of CB-TiO2/UV/O3 is sig-nificant in the degradation of TOC compared with ozonation alone andUV/O3.

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TABLE 1. TOC Removal Kinetics of the UV/O3 and TiO2/UV/O3 Processes With Various FlowRates of Ozone21

UV/O3 TiO2/UV/O3

O3 (mg/h) Equations of kinetics R2 Equations of kinetics R2

12.5 –dTOC/dt = 0.0099 .9949 –dTOC/dt = 0.0164 .991425.0 –dTOC/dt = 0.0159 .9951 –dTOC/dt = 0.0287 .994837.5 –dTOC/dt = 0.0225 .9877 –dTOC/dt = 0.0330 .989650.0 –dTOC/dt = 0.0260 .9910 –dTOC/dt = 0.0343 .9867

FIGURE 14. Comparison of TOC degradation by different oxidation methods; TiO2 methodsemployed 18-nm nano-TiO2 photocatalyst.19

FIGURE 15. Effect of ozone concentration on degradation of TOC removal by 18-nm nano-TiO2 photocatalyst in the UV/O3 process.19

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218 S. Kwon et al.

Table 1 provides the kinetic study of semi-batch experiments with vari-ous ozone flow rate ozone.19 It revealed that catechol concentrations did nothave a significant effect on the kinetics of UV/O3 and TiO2/UV/O3 processes;these results demonstrated that the zero-order kinetics of TOC removal pro-cess by UV/O3 or TiO2/UV/O3 as well as the reaction process of ozone mayplay a key role for the complete TOC removal.

10. CONCLUSION

This review focuses on the application of nanoparticulated TiO2 for air pu-rification, water degradation, and hazardous waste remediation.

Generally, various types of catalysts in water treatment and air pollu-tion control show high removal efficiency of only certain contaminants. Bycontrast, compared to regular TiO2 and other catalysts, the photocatalyticactivities of nanoparticle TiO2, either alone or modified with other chemi-cals, provide significant efficiency and are environmentally benign during thedegradation or reduction of various pollutants (both organic and inorganiccompounds) in water and air treatment systems, indicating a promising ma-terial for use in environmental engineering applications. As indicated earlier,heterogeneous photocatalytic degradation over nano-TiO2 with UV irradi-ation provides high efficiency, due to its special properties, i.e., (1) smallparticle size with high surface area, (2) ease of contact with pollutants, and(3) small absorbent particles that prevent the agglomeration of gas-phasecontaminants. Thus, TiO2 should be treated as an emerging technology at-tractive in terms of photoreaction efficiency, ease of usage, and the potentialfor economically efficient contaminant degradation. In addition, for the re-moval of chemical compounds such as mercury, phenol, and catechol, thephotocatalytic activity of nano-TiO2 modified with other chemicals revealsmuch better performance than that of nano-TiO2 alone.

For the degradation of some components such as crude oil, the pho-tocatalytic removal efficiency of some pollutants by nano-TiO2 alone wasstill not high, and the concentration of contaminant was even returned toinitial concentration due to its desorption. Because of the slow reaction ofelectron transfer from TiO2 to O2, which is broadly used as an oxidant, otherstronger oxidants are needed to react with compounds. Moreover, the reac-tion process of TiO2 can also produces a waste that is in need of disposal; themethod of by-product treatment should also be considered at the same time[i.e., during ethanol degradation, acetaldehyde (CH3CHO), which is toxic,an irritant, and a probable carcinogen,115 can be produced]. Therefore, fur-ther research is needed to explore the effectiveness of this promising pho-tocatalyst, either by itself or modified with other chemicals, or the develop-ment of a reactor system to overcome its disadvantages in the degradation ofcontaminants.

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