Agustina Journal of Photo Chemistry and Photo Biology C Photo Chemistry Reviews 6 264 2005

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Journal of Photochemistry and Photobiology C: Photochemistry Reviews

6 (2005) 264273

Review

A review of synergistic effect of photocatalysisand ozonation on wastewater treatment

T.E. Agustina , H.M. Ang, V.K. Vareek

Department of Chemical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia

Received 18 August 2005; received in revised form 9 November 2005; accepted 28 December 2005

Abstract

For the treatment of wastewater that contain recalcitrant organic compounds, such as organo-halogens, organic pesticides, surfactants, andcolouring matters, wastewater engineers are now required to develop advanced treatment processes. A promising way to perform the mineralization

of this type of substance is the application of an advanced oxidation process (AOP).

Photocatalytic oxidation and ozonation appear to be the most popular treatment technologies compared with other advanced oxidation processes

(AOPs) as shown by the large amount of information available in the literature. The principal mechanism of AOPs function is the generation of

highly reactive free radicals. Consequently, combination of two or more AOPs expectedly enhances free radical generation, which eventually leads

to higher oxidation rates.

The use of combine photocatalysis and ozonation is an attractive route because of the enhancement of the performance for both agents by means

of the hydroxyl radical generation, a powerful oxidant agent that can oxidize completely the organic matter present in the aqueous system. The

scope of this paper is to review recently published work in the field of integrated photocatalysis and ozonation on wastewater treatment.

In this review the chemical effects of various variables on the rate of degradation of different pollutants are discussed. The mechanism and

kinetics has also been reported.

It can be concluded that photocatalytic oxidation in the presence of ozone is a process that is qualitatively and quantitatively different from the

well-known photocatalytic oxidation with oxygen and the ozonation without photocatalyst. The reason for the higher oxidation rate is probably a

photocatalytic induced decay of ozone, initiated by the combination of titanium dioxide and UV-A radiation. 2006 Elsevier B.V. All rights reserved.

Keywords: Synergistic effect; Photocatalysis ozonation; Wastewater treatment

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

2. Photocatalysis and ozonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267

3. Mechanism and kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268

4. Intermediates detected . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

5. Effect of ozone dosage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270

6. Effect of pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

7. Effect of crystal composition of the photocatalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

8. Economics aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

Corresponding author. Tel.: +61 8 9266 3874; fax: +61 8 9266 2681.

E-mail address: [email protected] (T.E. Agustina).

1389-5567/$20.00 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.jphotochemrev.2005.12.003


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T.E. Agustina et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 6 (2005) 264273 265

T.E. Agustina was born in Indonesia, in 1972. She

received a bachelor degree in chemical engineering

from the Department of Chemical Engineering at Sri-

wijaya University (South Sumatera, Indonesia) in 1996

anda master degree in chemical engineering from Gad-

jah Mada University (Yogyakarta, Indonesia) in 2001.

She joined the Faculty of Sriwijaya University as a

Lecturer in 2000. She is currently studying her PhD

at Curtin University of Technology, Western Australia,under the direction of Ass. Prof. Ming Ang. During

this time she has been supported by a Technological

and Professional Skills Development Sector Project (TPSDP) scholarship. Her

research interest is in the area of photocatalytic on the wastewater treatment.

H.M. Ang is an Associate Professor in Chemical Engi-

neering and the Deputy Head of Department at Curtin

University of Technology. He is a fellow of both Engi-

neers Australia as well as the Institution of Chemical

Engineers, United Kingdom. Dr. Ang is also on the

Board of the Institution of Chemical Engineers in Aus-

tralia. He has supervised numerous PhD and Master

students over the years and his research areas are in

industrial crystallisation and environmental engineer-

ing with special emphasis on wastewater treatment and

contaminated soil remediation.

V.K. Vareek is currently a Senior Lecturer in the

Department of Chemical Engineering at Curtin Uni-

versity of Technology, Perth, Western Australia. Dr.

Pareek completed his PhD in the area of photocatalysis

from University of New South Wales. He is a Chemical

Engineer and has received a masters degree from IIT

Delhiand a BE(Hons) from theUniversityof Rajasthan

(India). His expertise is in the area of computational

fluid dynamics (CFD) and reactor modelling. He is a

member of IChemE.

1. Introduction

For the treatment of wastewater that contain trace recal-

citrant organic compounds, such as organo-halogens, organic

pesticides, surfactants, and colouring matters, environmental

engineers are now required to develop more advanced treatment

processes. In general, a combination of several methods gives

high treatment efficiency compared with individual treatment.

Forexample, a certain organic compound canhardly be degraded

by ozonation or photolysis alone and the treated wastewater

may be more dangerous as a result of ozonation [14] but a

combination of several treatment methods, such as O3/VUV

(ozone/vacuumultraviolet),O3/H2O2/UV(ozone/hydrogenper-oxide/ultraviolet), and UV/H2O2 (ultraviolet/hydrogen perox-

ide), improves the removal of pollutants from the wastewater

[57].

On the other hand, photocatalysis has a great potential for the

removal of organic pollutants from wastewater [8] although it is

still not in practical use because of its low oxidation rate. There-

fore,a combinationof photocatalysistogether with ozone, which

is a strong oxidizer, is reasonable for the treatment of difficult

to degrade organic compounds since the organic compounds are

then expected to decompose more quickly and thoroughly in the

presence of ozone, to carbon dioxide, water, or inorganic ions.

Sanchez et al. [9] combined these two methods for the removal

Fig. 1. Degradation of various organic compound (textile effluent in 1 h, aniline

in 2 h and2,4-dichlorophenoxyaceticacid in6 h) using photocatalysis, ozonation,

and combined photocatalysis ozonation ([15,9,25]).

of aniline from water and found that the decomposition rate of

aniline is larger than when individually treated by either of the

two methods. Wang et al. in 2002 studied the decomposition of

formic acid in an aqueous solution using photocatalysis, ozona-

tion, and their combination. As a result, the decomposition rate

of formic acid by the combination of photocatalysis and ozona-

tion was found to be higher than the sum of the decomposition

rates when formic acid was individually decomposed by a com-

bination of the two methods, which indicates the presence of a

synergistic effect of photocatalysis and ozonation [10].

The scope of this paper is to review recently publishedwork in

the field of integrated photocatalysis and ozonation on wastewa-

ter treatment. The summary of compounds degraded by various

researchers using photocatalytic ozonation including informa-

tionaboutthe UV source used, intermediates detected and results

is presented in Table 1. All results using different organic efflu-ents always showed that the application of photocatalytic ozona-

tion gave the minimum total degradation time and produced

the highest removal of the organic effluent (see Figs. 1 and 2).

In particular, for the textile effluent, the decolourization was

almost complete with the combined method. The advantages of

photocatalysis giving decrease in TOC, and the advantages of

ozonation giving no increase of high intermediate concentra-

tions, were combined here.

Fig. 2. Degradation of nitrogen containing organic compounds by photocataly-

sis, ozonation and combined photocatalysis and ozonation [30].


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266 T.E. Agustina et al. / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 6 (2005) 264273

Table 1

Summary of compounds degraded by various researchers using photocatalytic ozonation including information about the UV used, intermediates detected and results

and comments

Compound

degraded/source of

effluent

Source of light Intermediates detected Results and comments Parameter monitored References

Acid compounds

Acetic acid Six 6 W medium pressureHg vapor lamp

Formic acid,formaldehyde

The acetic acic degradation ratewas seven times higher in the

combined systems than that using

photocatalysis alone

DOC and acetic aciddegradation

[58]

Formic acid 6 W black light blue

fluorescent lamp

Not analysed The kinetic constant for the

combination methods is more

than the sum of both the kinetics

constans for photocatalysis and

ozonation taken separately

Decomposition of

formic acid

[10]

Formic acid Six 6 W medium pressure

Hg vapor lamp

None The degradation rate of formic

acid using combined method was

nearly five times higher than

those using photocatalysis alone

DOC and formic acid

degradation

[58]

Propionic acid Six 6W medium pressure

Hg vapor lamp

Acetic acid, formic acid,

acetaldehyde,

formaldehyde

The degradation rate of propionic

acid using photocatalysis

ozonation system was almost 10times higher than that in the

photocatalysis system

DOC and propionic

acid degradation

[58]

Monochloroacetic acid

and pyridine

UV lamp Not analysed When using the combined

method, the degradation rate of

monochloroacetic acid and

pyridine led to 4 and 24 times

higher degradation rate,

respectively

Degradation of

monochloroacetic

acid and pyridine

[32]

2,4-

Dichlorophenoxyacetic

acid

20W black light 2,4-Dichlorophenol

chlorohydroquinone

1,2,4-trichlorobenzene

Combined method for 4 h

resulted in 2/3 of TOC removal

leading to about the same level

for photocatalysis alone after 20 h

TOC removal [44]

Sulfosalicylic acid 14W 254nm lamp

(waters)

Not analysed The degradation efficiency at all

pH values has the same order:

O3/VO/TiO2 > O3/TiO2/UV> O3/UV

COD removal rate [51]

Humic acids 125 W black white

fluorescent lamp

Not analysed The degradation of humic acids

in a sequential system including

preozonation and photocatalysis

were found to be efficient in

organic matter and colour

removal

Organic matter

reduction and colour

reduction

[38]

Nitrogen compounds

Aniline 125 W medium pressure

Hg lamp

Not analysed The degradation rate of aniline by

the combination of photocatalysis

and ozonation is larger than when

individually treated

TOC removal [9]

Free cyanide ions 6W black light lamp Cyanate, nitrate and

carbonate ions

The degradation time of cyanide

is almost complete in 90, 60, and

18 min run time when using

photocatalysis, ozonation, and

photocatalysis ozonation,

respectively

Degradation of

cyanide

[37]

Nitrogen containing

organic compound

450 W xenon short arc

lamp

Ammonia, nitrite nitrate,

ethylamine

A considerable increase in the

degradation efficiency of the

N-compounds is reached by a

combination of photocatalysis

and ozonation

TOC removal [57]

Alcohols

Phenol, p-chlorophenol

(CP), p-nitrophenol

(NP)

700 W high pressure Hg

lamp

Hydroquinone,

resorcinol, catechol,

oxalic acid fumaric acid,

glyoxylic acid, phenol

Photoctalytic ozonation led to

lower degradation times for COD

and TOC removal. The

percentage of mineralization was

nearly 100, 90 and 75% in

phenol, CP and NP

COD and TOC

removal degradation

of catechol

[40]


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Table 1 (Continued)

Compound

degraded/source of

effluent

Source of light Intermediates detected Results and comments Parameter monitored References

Catechol 15 W low pressure UV

lamp

Not analysed The combined photocatalysis

ozonation process (TiO2/UV/O3)

considerably increased TOC

removal rate compared toTiO2/UV, O3, and O3/UV process

TOC removal [43]

Other organic compounds

Textile effluent 125W high pressure Hg

lamp

Not analysed The decolorization was almost

complete and the highest TOC

reduction was achieved when

using photocatalysis ozonation

mehods

TOC removal

decolorization

[29]

Pesticides 6 W black light lamp Not analysed Photocatalytic ozonation process

led to rapid decrease of the

concentration of the

biorecalcitrant pesticides and to a

strong TOC reduction

TOC reduction [36]

4-Chlorobenzaldehyde 10 W low pressure Hg

lamp

Not analysed The highest efficiency for the

substrate degradation was

obtained by simultaneous

ozonation and photocatalysis

Degradation of

substrate

[41]

Acetic acid,

monochloro acetic acid

and dimethyl-2,2,2-

trichloro-1-

hydroxyethylphospate

(DEP)

6 W low pressure Hg lamp Formic acid, acetic acic,

glycoxylic acid, glycolic

acid

The most rapid degradation of the

intermediate compounds by the

combined method explains the

greatest TOC elimination rate

produced by this combination

Degradation of acetic

acid monochloro

acetic acid, DEP

[60]

Dibutyl phthalate 15W low pressure UV

lamp

Not analysed The mineralization rate constant

of dibutyl phthalate with

TiO2/UV/O3 is 1.21.8 times

higher than that of O3/UV with

the same ozone concentration

TOC removal [42]

Oxalate ion 700 W medium pressure

Hg lamp

Not analysed The highest decrease of oxalate

concentration takes place in the

photocatalysis ozonation system

Degradation of

oxalate concentration

[39]

Post harvest fungal

spoilage of kiwi fruit

6 W UV lamp Not analysed TiO2/UV/O3 process

synergystically degraded organic

compounds and inhibited

conidial germination when

compare to a single treatment of

O3 or TiO2/UV

Fungicide

decomposition

[61]

Organic pollutants in

raw water (phthalate

esters (PAEs) and

persistent organic

pollutants (POPs))

15 W low pressure UV

lamp

Not analysed TiO2/UV/O3-BAC (biological

activated carbon) was more

efficient than UV/O3-BAC in

DOC removal and its synergetic

effect occurred more significantly

DOC removal PAEs

and POPs removal

[62]

2. Photocatalysis and ozonation

According to their recent paper, Serpone and Emiline [11]

defined that photocatalysis, in its most simplistic description,

is the acceleration of a photoreaction by action of a catalyst.

An earlier IUPAC document defined it as a catalytic reaction

involving light absorption by a catalyst or a substrate.

Photocatalytic degradation has proven to be a promising

technology for degrading various organic compounds includ-

ing refractory chlorinated aromatics [1216] and more than

1700 references have been collected on this discipline [17].

Compared with other conventional chemical oxidation methods,

photocatalysis may be more effective because semiconductors

are inexpensive and capable of mineralizing various refractorycompounds [18].

Due to its capability of generating OH radicals, which is a

powerful oxidant species, photocatalysis can be considered as

an advanced oxidation process (AOP). However, photochem-

ical AOPs are light induced reactions, mainly oxidations that

rely on the generation of hydroxyl radicals by the combination

with added oxidants or semiconductors. The degradation effi-

ciency of photochemical AOPs is greatly enhanced using either

homogeneous or heterogeneous photocatalysis [19]. Heteroge-

neous processes employ semiconductor slurries (e.g. TiO2/UV,

ZnO/UV) for catalysis, whereas homogeneous photochemistry

(e.g. H2

O2/UV, Fe3+/UV) is used in a single-phase system.


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Fig.3. Conductionand valencebands andelectronhole pairgeneration in semi-

conductor.

Photocatalytic reactions occur when charge separation are

induced in large bandgap semiconductor by excitation with ultra

bandgap radiations [20]. In this way, the absorption of light

by the photocatalyst greater than its bandgap energy excites

an electron from the valence band of the irradiated particle to

its conduction band, producing a positively charged hole in the

valence band and an electron in the conduction band [21] as

shown in Fig. 3. The hole in the valence band may react with

water absorbed at the surface to form hydroxyl radicals andon the other hand, the conduction band electron can reduce

absorbed oxygen to form peroxide radical anion that can fur-

ther disproportionate to form OH through various pathways

[22]. During the photocatalytic process, other oxygen contain-

ing radicals are also formed including superoxide radical anion

and the hydroperoxide radical [23]. In addition, the band elec-

tron may also react directly with the contaminant via reductive

processes [24]. It is not clear under which experimental condi-

tions, one reaction pathway is more important than the other.

It is generally accepted that the substrate adsorption in the sur-

face of semiconductors plays an important role in photocatalytic

oxidation [25].Titaniumdioxide (TiO2) is a semiconductor that absorbs light

at < 385 nm with the corresponding promotion of an electron

from the valence band to the conduction band. This excitation

process leaves behind a positively charged vacancy called a hole

(h+). The hole by itself is either a very powerful oxidizing agent

or it may generate hydroxyl radicals in presence of water and

molecular oxygen [26,27]. Unlike other semiconductors, TiO2photocatalysts can function using sunlight as the energy source.

Titanium dioxide is extensively used as photocatalyst due to

its high chemical stability, optical and electronic properties

[28].

Photocatalysis is a potential new method capable of elimi-

nating relatively recalcitrant organic compounds [26,29]. Thismethodology is based on the production of electronhole pairs

by illumination with light of suitable energy, of a semicon-

ductor powder such as TiO2 dispersed in an aqueous medium.

These charges migrate to the particle surface and react with

adsorbed species of suitable redox potential. In aerated media,

adsorbed molecular oxygen accepts photogenerated electrons,

while water molecules can react with photogenerated holes to

produce hydroxyl radicals.

However, although photocatalysis has shown to be adequate

for the destruction of a wide variety of compounds, in some

cases the complete mineralization is slowly attained and the

efficiency of the processes, in terms of energy consumption,

Fig. 4. Laboratory-scale set-up for photocatalysis ozonation.

is only advantageous for very dilute effluents [26]. To overcome

this difficulty some additives such as H2O2, Fe2+, Fe3+, S2O82,Ag+, etc., with different chemical roles have been added to the

photocatalytic systems [9].

Furthermore, ozone can also be used for degradation of

organic pollutants in water. Ozone is a powerful oxidant (electro-

chemical oxidation potential of 2.07 V versus 2.8 V for hydroxyl

radical), which is generally produced by an electric discharge

method in the presence of air or oxygen. Two reactions of ozone

with dissolved organic substance can be distinguished in water:

a highly selective attack of molecular ozone takes place on the

organic molecules at low pH, whereas free radicals from ozone

decomposition can also react non-selectively with the organic

compounds [30]. Additional free hydroxyl radicals can be pro-duced in the aqueous media from ozone by pH modification

or can be introduced by combining ozone either with hydrogen

peroxide or UV-irradiation from a high pressure mercury lamp

[31].

On the other hand, the use of ozone for the destruction of

organics in water is also a well-known water treatment tech-

nique. Unlike photocatalysis, ozonation, due to its capability

for selective destruction of recalcitrant organics, is used as a

pre-treatment step before ordinary biological techniques, thus

being more efficient for highly contaminated wastewater. The

simultaneous application of ozonation and photocatalysis has

the capabilityfor efficienttreatment of organically contaminated

waters over a wide range of concentrations [9]. A schematic dia-

gram of a laboratory-scale set-up for photocatalysis ozonation

is illustrated in Fig. 4.

3. Mechanism and kinetics

Photocatalytic oxidation and ozonation appear to be the

most promising pre-treatment technologies compared with other

advanced oxidation processes (AOPs) as shown by the large

amount of information available in the literature. The principal

mechanism of AOPs function is the generation of highly reactive

free radicals.


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Consequently, a combination of two or more AOPs enhances

free radical generation, which eventually leads to higher oxida-

tion rates.

Sanchez et al. [9] in their investigation, found that although

the strategy of ozonation pre-treatment followed by photo-

catalysis would be a satisfactory route for aniline degradation,

the simultaneous ozonation and photocatalysis methodology

resulted in much higher total organic carbon (TOC) removal

which, in spite of more energy and material demanding, could

be preferred from an application point of view (see Table 1).

In fact, besides the direct ozonation of the intermediate com-

pounds, in the presence of TiO2 under illumination, ozone can

generate OH radicals through the formation of an ozonide rad-

ical (O3) in the adsorption layer:

TiO2+h e+ h+ (1)

O3+ e O3

(2)

The generated O3 species rapidly reacts with H+ in the

solution to give HO3

radical, which evolves to give O2 andOH as shown in Eqs. (3) and (4) below:

O3+H+ HO3

(3)

HO3 O2+OH

(4)

In photocatalysis, the hydroxyl radical is generally consid-

ered to be mainly responsible for the organic attack. It must be

considered that the OH radical can react with O3

OH + O3 O2 +HO2 (5)

in competition with aniline and the corresponding organic inter-

mediates. Further, the O2 species participates in a closed loop

reaction scheme, which leads to a continuous consumption of

ozone. In the absence of O3, dissolved O2 itself can accept TiO2conduction band electron and generate O2

O2+ e O2

(6)

Which in turn can be protonated to form HO2:

O2+H+ HO2

(7)

In contrast with HO3 (see Eq. (3)), this species cannot give

OH radicals in a single step and an alternative reaction pathway

has been proposed to account for OH radicals generation from

HO2:

2HO2 H2O2+O2 (8)

H2O2 +O2 OH + HO+O2 (9)

This mechanism requires a total of three electrons for the

generation of a single OH species, which is a less favoured

situation if compared with the one electron needed through the

O3 reaction pathway [9].

To get more information about the photocatalytic ozonation

processes, Kopf et al. [32] measured the ozone decomposition

by illumination in aqueous TiO2 suspension in the absence of

organic compounds. The solutions (pH 3) were saturated with

ozone. After stopping the gas supply illumination was started.

It is obvious that the decomposition is accelerated by the appli-

cation of titanium dioxide and appropriate light. The reaction

depends on light intensity. If the light intensity is decreased,

the ozone decomposition is also decreased. These results show

that a real photocatalytic reaction occurs in the suspension, not

the radiation alone but the combination of radiation and pho-

tocatalyst initiates a decomposition reaction of ozone. Another

investigation by Kopf et al. [32] indicated that oxygen should

have an influence on the photocatalytic ozonation. Besides other

reaction paths, such as direct ozone attack (Eq. (29)), or direct

electron transfer from TiO2 to theozonemolecule(Eqs.(1)(4)),

a further possible reaction path is proposed [32].

Charge separation:

TiO2+h h++ e (10)

Charge transfer of positive charge:

h++H2O OH+ H+ (11)

Electron transfer and further reactions:

e+O2 O2 (12)

O2+H+ HO2

(13)

O3+O2 O3

+O2 (14)

O3+H+ HO3

(15)

HO3 OH + O2 (16)

Oxidation of the organic compound R-H:

OH + R-H R + H2O (17)

or

OH + R R-OH (18)

In reactions (13)(16) the ozone decomposition according to

the mechanism of Weiss [33] with the extension of Buhler et al.

is described [34].

The difference between photocatalytic ozone decomposition

and ozone decomposition in aqueous solution is the initiation

of the reaction. The starting radical is formed photochemically

by an electron transfer from titanium dioxide to oxygen and

not, like in the Weiss-mechanism, by the reaction of OH ion

with ozone. In both cases primarily O2 is formed. O2 reactswith ozone forming the ozonide ion (14). After protonation OH-

radicals areformed ((15) and (16)). TheformationofOHradicals

in acidic solutions via direct electron transfer from the catalyst

to the ozone molecule or via O2 explains the degradation

of organic compounds by photocatalytic ozonation which react

very slowly with the ozone molecule or with HO2, one of the

primary oxidising species in photocatalytic processes [32].

Furthermore, Wang et al. [10] indicated in their investiga-

tion that a synergistic effect occurred when the photocatalytic

ozonation treatments are carried out simultaneously. Two possi-

ble mechanisms are considered for the synergistic effect. Firstly,

it is considered that in the photocatalytic reactor, the dissolved


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ozone was easy to get electrons produced on the surface of tita-

nium dioxide according to the mechanism:

O3(ads) + e O3(ads)

(19)

The recombination of electrons and positive holes could be

interferred by the reaction between ozone and electrons on the

surface of titanium dioxide. Consequently, a larger number ofradicals are produced, thereby accelerating the photocatalytic

reaction. Secondly, it is considered that a larger number of

hydrogen peroxide and hydroxyl radicals were produced from

the dissolved ozone as a result of UV-irradiation [35] In the

ozonation under UV-irradiation, the hydroxyl radical is pro-

duced according to the following mechanism [10]:

O3 +H2OhH2O2 +O2 (20)

H2O2h2OH (21)

H2O2 HO2+H+ (22)

O3+HO2

O3

+HO2

(23)

HO2 O2

+H+ (24)

O3+O2 O3

+O2 (25)

O3+H+ HO3

(26)

Recently, Farre et al. [36] suggested thatfor the photocatalytic

ozonation system, the reaction also proceeded by radical attack

to organic molecule. However, the OH radical is produced by (i)

reaction of absorbed H2O molecule with photogenerated holes

at the illuminated TiO2 particle:

TiO2+h h++ e (27)

H2O + h+ OH + H+ (28)

and (ii) by reaction of adsorbed O3 and photogenerated electrons

at the TiO2 particles, namely reaction (1)(4) above by Sanchez

et al. [9] The presence of dissolved ozone in the irradiated TiO2aqueous suspension increases the OH radical production and

decreases the electronhole recombination, increasing the effi-

ciency of the photocatalytic process.

To date, a kinetic model based on LangmuirHinshelwood

(LH) equations adequately describes the reactivity results and

provides the values of kinetic constant and equilibrium adsorp-

tion constant in the degradation of organic compound using

photocatalyc ozonation methods [9,10,3739].Farre et al. found that the degradation processes follow zero

order kinetics when photocatalytic ozonation is applied [36].

On the other hand, Beltran et al. [40] discovered that phenol

removalobeyed first orderkinetics. Other researchers, Balcioglu

etal. [41] reported in their investigation whichalsoemployed the

combined method, the decomposition modesfollow pseudo-first

order kinetics. And Li et al. [42] described in their experimen-

tation on photocatalysis ozonation process, the mineralization

of dibuthyl phthalate follows pseudo-zero-order kinetics depen-

dence on ozone concentration. The photocatalytic ozonation of

catechol also followed pseudo-zero-order-kinetics, as reported

by Li et al. [43].

4. Intermediates detected

Intermediates are formed during the process of degradation.

Table 1 shows that most of the researchers did not analyse for

intermediates and focused mainly on the degradation rate of

organics. Some of them analysed TOC with only a few men-

tioning the intermediates formed.

Regarding the intermediates, Muller et al. [44] got quite

different results in the photocatalysis, ozonolysis and the

combination of both methods in the treatment of 2,4-

dichlorophenoxyacetic acid (2,4-D). They could identify 2,4-

dichlorophenol (2,4-DCP), chlorohydroquinone (CHQ) and

1,2,4-trichlorobenzene (THB). While CHQ and THB are minor

intermediates, 2-4-DCP is the main and long living intermediate

in all three methods. In photocatalysis, the 2,4-DCP disappeared

after a total running time of about 40h. In ozonolysis, 2,4-DCP

formed onlyin negligible amounts. In the combination approach,

the maximum concentration of 2,4-DCP was only about 10%

of the maximal level in photocatalysis and it vanished within

1012h, which is in about the same amount of time with thedegradation of 2,4-dichlorophenoxyacetic acids.

Beltran et al. [40] reported the intermediates found in

photocatalytic ozonation of phenols. Intermediate compounds

formed can be classified into three categories. These categories

are polyphenols (mainly hidroquinone), unsaturated carboxylic

acids (mainly fumaric acid) and saturated carboxylic acids

(mainly oxalic acid). Oxalic acid was the main end product

that remained in water, especially during the oxidation of p-

nitrophenol. Also, nitrogen as nitrate from p-nitrophenol and

chloride from p-chlorophenol were detected in solution.

Other researchers did not analyse the intermediates without

giving any particular reasons. However, the researchers used theadvantage of ozonolysis, which results in no build up of high

intermediates concentration.

5. Effect of ozone dosage

Li et al. [42] accounted the effect of ozone dosage in their

research on photocatalytic ozonation of dibutyl phthalate over

TiO2 film. The addition of ozone considerably improved TOC

removal compared to photocatalysis and ozonolysis near UV

(UV/O3). After 30 min, TOC removal rate was over 75% when

ozone dosage was over 25 mg/h, while only 73% using UV/O3when ozone dosage was 50 mg/h. These results showed the

synergistic effect of UV/O3, TiO2 /UV and TiO2/O3 [4547].When the semiconductor is in contact with water, the surface

of TiO2 is readily hydroxylated. Under near-UV illumination,

electronhole pairs are formed in the semiconductor, the oxida-

tion potential of hydroxylated TiO2 must lie above the position

of the semiconductor valence band, and the oxidation of surface-

bound OH andH2O byTiO2 valence band holes to form OH is

thermodynamically possible and expected [48]. Thus the min-

eralization of dibuthyl phthalate was accelerated.

Effect of ozone dosage was also examined by Li et al. [48]

in their study on removing organic pollutants in secondary

effluents using photocatalysis ozonation and subsequent biolog-

ical activated carbon. They found that DOC removal efficiency


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increased with ozone dosage. The total DOC removal efficiency

increased from about 52 to 59% in the process when ozone

dosage increased from 3 to 9 mg/L. However, DOC removed

per unit ozone consumption decreased with ozone dosage. It

was 2.11 mg DOC/mg O3 at 3 mg/L ozone dosage, while it

decreased to 0.95 mg DOC/mg O3 when ozone dosage increased

to 9 mg/L. At three ozone dosages (39 mg/L), the amount of

ozone consumption is 2.38, 4.39, and 6.01 mg/L, respectively.

And it indicated that low ozone dosage was much more econom-

ically efficient.

Kerc et al. [38] investigated the effect of ozone dosage of

between 1.47 and 7.35 mg/L on the degradation of humic acids

by photocatalysis and ozonation. They found that for higher

ozone application rates the maximum reduction achieved was

80% for organic matter degradation and 90% for colour reduc-

tion. A slight decrease in pH was observed with increasing ozone

dosage rates. The pH decrease may be attributed to the forma-

tion of carboxylic groups with more acidic characteristic. Li et

al. [43] also studied the effect of ozone dosage on photocat-

alytic ozonation of catechol. The concentration of ozone rangedbetween 0 and 50 mg/h. The increase of TOC removal by apply-

ing TiO2/O3 /UV was evident compared with UV/O3. These

results showed the synergistic effect of the UV/O3, TiO2/UV

and TiO2/O3 [4547]. The TOC removal rate was over 94.5%

after 30 min when the flowrate of ozone was over 37.5 mg/h.

6. Effect of pH

Ozone has many desirable oxidizing properties for use in

wastewater treatment. Unlike other oxidants, there are two

mechanisms through which ozone can degrade organic pollu-

tants, namely (i) direct electrophilic attack (Eq. (29)) and (ii)indirect attack through the formation of hydroxyl radicals (Eqs.

(30)(31)) [30,49]. Direct attack by molecularozone(commonly

known as ozonolysis) occurs at acidic or neutral conditions and

is a selective reaction resulting in the formation of carboxylic

acids as end products that cannot be oxidized further by molec-

ular ozone:

O3+R Rox (29)

where R is the organic solutes and Rox is the oxidised organic

products. Compounds susceptible to ozonolysis are those con-

taining C C double bonds, specific functional groups (e.g. OH,

CH3,OCH3) andatomscarryingnegativecharge (N,P, O, S) [49]At high pH value, ozone decomposes to non-selective hydroxyl

radicals, which, in turn, attack the organic pollutants:

O3+OH OH + (O2

HO2

) (30)

OH + R Rox (31)

where Rox is the oxidised organic products for radical-type

reactions.

Therefore, the pH of the effluent is a major factor determin-

ing the efficiency of ozonation since it can alter degradation

pathways as well as kinetics. Ozonation is usually coupled with

another oxidant such as hydrogen peroxide or UV-irradiation to

enhance the formation of hydroxyl radicals in aqueous phase

[50].

Many researches [51,52] found that increasing pH will

accelerate ozone decomposition to generate OH radical, which

destroys the organic compounds more effectively than ozone

does.

For the photocatalytic process, generally pH has a varied

effect on theprocess andit is expectedthat increase or decreasein

the pH should affect the rate of degradation as adsorption varies

with pH. In the case of substances which are weakly acidic, the

photocatalytic degradation increases at lower pH because of an

increase in the adsorption. Some substances undergo hydrolysis

at alkaline pH, which is one of the reasons for the increase in the

photocatalytic degradation at alkaline pH values [53] At alka-

line pH values, the concentration of OH radicals is relatively

higher and this may also be another reason for the increase in

the photocatalytic degradation rate. It can be concluded that the

effect of pH is negligible so pH adjustment is not required in the

photocatalytic degradation.

Forthe photocatalysisand ozonolysis, Muller et al. [44] foundthat neither the preference for low pH values of the photocatal-

ysis nor that for high pH values for the ozonolysis could be

better than the degradation rate obtained by simultaneous appli-

cation of both methods at neutral pH values. The kinetic results

showedthat degradation at pH 7 using thecombination approach

is more than 1.5 times faster than the best result of ozonolysis at

pH 11, and more than three times faster than the fastest degra-

dation applying photocatalysis using the unbuffered set-up (pH

3.43.5).

Tong et al. [51] reported that the pH values for the degrada-

tion of sulfosalicylic acid solution by photocatalysis ozonation

decreased firstly within a period of time and increased subse-quently with the mineralization of intermediates. They found

that the pH value in the system increased an initial pH of 2.22.8

for ozonation of 45 min.

7. Effect of crystal composition of the photocatalyst

Taking into account the impact of different possible photo-

catalyst on the: (1) chemical activity, (2) stability under different

operating conditions, (3) availability and handiness in many

physical forms, (4) cost, and (5) lack of toxicity, the most widely

used photocatalyst is titanium dioxide [54].

In this review, TiO2 photocatalyst is used in all experiments.

TiO2 has three crystal structures, namely rutile, anatase andbrukite. Only rutile and anatase are stable enough and can be

used as a photocatalyst. It was shown that photocatalytical prop-

erties of different TiO2 materials might differ considerably, with

rutiles exhibiting the lowest photoactivity [55]. Anatase has bet-

ter activity than rutile TiO2. The combination of anatase and

rutile decrease the recombination of photogenerated electrons

and holes and hence it shows more activity than anatase, as in

the case of Degussa P-25.

Thevenet et al. compared the efficiency of P-25 Degussa

and PC500-Millennium titanium dioxide photocatalysts. P-25

Degussa is made of two allotropic phases of the titanium diox-

ide; 80% of it is crystallized into the anatase phase and 20%


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into the rutile phase. Its specific area is 50 m2 g1 with average

particle size of 30 nm. The PC500-Millennium photocatalyst is

made of only one crystallographic phase, anatase, with a specific

areaof300m2 g1 which is much higherthan P25but with lower

particle size of 510 nm. The mixed anatase/rutile P-25 Degussa

is reported as the most efficient in their experimental condition.

Crystallographic phase contact between anatase and rutile inP-25 Degussa, as well as the crystallite size, was suggested to

explain its efficiency [56]. Therefore Degussa P-25 is the most

suitable photocatalyst. All the investigations reported here used

Degussa P-25 TiO2 for their study [9,29,32,3640,57,58].

The photocatalyst derives its activity from the fact that when

photons of certain wavelength strike its surface, electrons are

promoted from the valence band and transferred to the conduc-

tance band. This leaves positive holes in the valence band, which

react with the hydroxylated surface to produce OH radicals. In

Degussa P-25 the conduction band electron of the anatase part

jumps to the less positive rutile part, reducing the rate of recom-

bination of electrons and positive holes in the anatase part [59].

Hernandez-Alonso et al. [37] tested two photocatalysts withknown activity for cyanide photodegradation using photocatal-

ysis ozonation method. Two commercial TiO2 samples were

used, that is Degussa P-25 (805 anatase, 20% rutile) and BDH

TiO2 (100% anatase). The synergetic effect of ozone and TiO2under irradiation is present in both TiO2 samples; however, it

is more pertinent with the BDH specimen than with the P-25.

The enhancement of reactivity in the presence of ozone with

respect to oxygen is related to the ability of ozone of trap-

ping electrons and, therefore, it also depends on the availability

of electrons in the photocatalyst. The used photocatalyst dif-

fer in their crystal composition: BDH specimen is pure anatase

while P-25 is mixture of anatase and rutile. The higher BDHphotoactivity is probably related to the lower band gap energy

of the anatase sample that facilitates the electron trapping by

ozone.

8. Economics aspects

The economic aspects of the different methods must be con-

sidered in light of the energy consumption for all of them.

Photocatalytic ozonation needs additional electrical energy for

the ozone generator. Therefore energy consumption of the three

processes was compared according to consumption of electrical

power (kW h) of the laboratory device under the experimentalconditions.

Kopf et al. [32] calculated the energy consumption for the

UV-lampand ozonegeneratorin theirinvestigation on the photo-

catalytic ozonation of monochloroacetic acid and pyridine. They

found that the energy consumption of the UV lamp and ozone

generator was 0.046 and 0.018 kW h in 60 min, respectively.

The energy consumption for DOC (dissolved organic carbon)

decrease was compared for three methods, i.e. TiO2/O2 (pho-

tocatalysis), H2O/O3 (ozonation) and TiO2/O3 (photocatalytic

ozonation) and is shown in Table 2. The photocatalytic ozona-

tion methodresulted in much lowerspecific energy consumption

than the other two alternatives.

Table 2

Photocatalytic ozonation of monochloroacetic acid; specific energy consump-

tion (kW h/g DOC-reduction)

TiO2/O2 H2O/O3 TiO2/O3

Monochloroacetic acid 19 110 5.4

Pyridine 120 73 11

Reprinted from Ref. [32] with permission from Elsevier.

9. Conclusion

It was shown that the photocatalytic oxidation in the presence

of ozone is a process, which is qualitatively and quantitatively

different from either the well-known photocatalytic oxidation

with oxygen or ozonation without photocatalyst.

Five possible mechanisms are considered for the synergis-

tic effect: (i) direct electron transfer from TiO2 to the ozone

molecule which generate OH radicals through the formation of

an ozonide radical (Eqs. (1)(4)), (ii) electron transfer from TiO2to the oxygen molecule to form O2 radical, then O2 radical

reacts with ozone to give OH radicals through the formation

of an ozonide radical (Eqs. (10)(16)), (iii) a recombination of

electrons and positive holes interfered by the reaction between

ozone and electrons on the surface of titanium oxide, so a large

number of radicals produced, thereby accelerating the photocat-

alytic reaction (Eq. (19)), (iv) production of hydrogen peroxide

and OH radicals increased by UV-irradiation of ozone (Eqs.

(20)(26)), and (v) OH radicals produced by the reaction of

absorbed H2O molecule with photogenerated holes at the illu-

minated TiO2 particle (Eqs. (27) and (28)).

The main problem of AOPs lies in the high cost of reagents

such as ozone,hydrogen peroxide or energy source such as ultra-violet light. For the photocatalytic ozonation methods theenergy

demand of the O3/TiO2/UV could be considerably decreased by

the use of solar-irradiation and in situ electrochemical O3 gen-

eration. However, the use of solar radiation as an energy source

can reduce costs.

The synergistic effect summarized in Table 1 showed the

advantages of the application of photocatalytic ozonation

method. Photocatalytic ozonation could be an effective alter-

native way for oxidation and elimination of recalcitrant organic

compounds under specific conditions for the treatment of indus-

trial wastewater.

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