Photocatalytic and sonolytic oxidation of acid orange 7 in aqueous solution

Photocatalytic and sonolytic oxidation of acid

orange 7 in aqueous solution

Theodora Velegraki a, Ioannis Poulios b, Magdalini Charalabaki a,Nicolas Kalogerakis a, Petros Samaras c, Dionissios Mantzavinos a,*

aDepartment of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greeceb Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, GR-54006 Thessaloniki, GreececDepartment of Pollution Control Technologies, Technological Educational Institute of West Macedonia, GR-50100 Kozani, Greece

Received 15 April 2005; received in revised form 29 June 2005; accepted 4 July 2005

Available online 26 August 2005

Abstract

The oxidation of 50 mg/L azodye acid orange 7 (AO7) in water was investigated by means of photocatalysis in the presence of various

semiconducting catalysts and ultrasound irradiation. The UVA-induced photocatalytic degradation over TiO2 anatase suspensions was found

to increase with increasing catalyst concentration, decreasing solution pH, as well as in the presence of dissolved oxygen. The presence of 1,4-

benzoquinone, sodium azide or sodium chloride in the reaction mixture decreased degradation due to the scavenging of radicals and other

reactive moieties. Interestingly, addition of hydrogen peroxide at various concentrations from 21 to 1050 mg/L also inhibited degradation. The

sonochemical degradation of AO7 was found to increase with increasing frequency and decreasing temperature. Under similar treatment

conditions, ultrasound irradiation resulted in higher conversion than photocatalysis; moreover, the sonochemically irradiated solution

consistently contained low aliphatic intermediates, while the photocatalytically treated solution mainly consisted of aromatic intermediates as

confirmed by GC–MS analysis. The acute toxicity to marine bacteria V. fischeri decreased following oxidation with either process.

Furthermore, deep sonochemical treatment slightly improved the aerobic biodegradability as assessed by shake flask tests.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Acid orange 7; Intermediates; Photocatalysis; Toxicity; TiO2; Ultrasound; Wastewater

www.elsevier.com/locate/apcatb

Applied Catalysis B: Environmental 62 (2006) 159–168

1. Introduction

Textile manufacturing involves several processes which

generate large quantities of wastewaters. These effluents are

highly variable in composition with relatively low BOD and

high COD contents and are typically characterized by: (i)

strong color due to residual dyes, (ii) recalcitrance due to the

presence of compounds such as dyes, surfactants and sizing

agents and (iii) high salinity, high temperature and variable

pH [1]. Given the complex and bioresistant character of

textile effluents, their effective treatment usually requires a

combination of various physical, chemical and biological

technologies.

* Corresponding author. Tel.: +30 28210 37797; fax: +30 28210 37846.

E-mail address: [email protected] (D. Mantzavinos).

0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2005.07.007

Several recent studies deal with the treatment of model

solutions containing various commercial dyes with empha-

sis on azodyes since they are extensively used in dyeing

processes. These molecules are chemically stable and hardly

biodegradable aerobically. Although they are easily reduced

under anaerobic conditions, they produce potentially more

hazardous aromatic amines [1]. Special attention has been

paid on the oxidative degradation of acid orange 7 (AO7), a

representative mono-azodye, by means of various advanced

oxidation processes (AOPs) [2–6]. Heterogeneous photo-

catalytic oxidation has been extensively studied for the

treatment of textile effluents. The TiO2-mediated degrada-

tion of AO7 (as well as of other azodyes) by means of near-

ultraviolet (UVA), visible and solar irradiation has been

reported in several publications, as well as in a recent review

article [7]. Most of these publications, deal with the kinetics

T. Velegraki et al. / Applied Catalysis B: Environmental 62 (2006) 159–168160

of AO7 decolorization and mineralization as a function of

operating conditions, while relatively little is known about

reaction by-products and pathways as well as the effect

of photocatalytic treatment on subsequent toxicity and

biodegradability [7]. TiO2 photocatalysis is an emerging

treatment technology with key advantages including the

lack of mass transfer limitations, operation at ambient

conditions and the possible use of solar irradiation, while the

catalyst itself is inexpensive, readily available, non-toxic

and chemically stable. The process can easily decolorize

and reduce considerably the organic load of dyehouse and

related effluents.

A relatively innovative AOP based on the use of low to

medium frequency (typically in the range 20–1000 kHz)

and high energy ultrasound to catalyze the destruction of

organic pollutants in waters and wastewaters has been

recently given a lot of attention [8]. Sonochemical

treatment typically operates at ambient conditions and

does not require the addition of extra chemicals or

catalysts; unlike other AOPs that are exclusively based on

hydroxyl radical-induced reactions, ultrasonic irradiation

may selectively degrade less hydrophilic compounds

through thermal degradation reactions. Various investiga-

tions report successful removal of a wide range of organic

pollutants from aqueous solutions [9], including the

treatment of textile dyes [10]. Nonetheless, unlike the

large amount of information concerning AO7 photocata-

lytic degradation available in the literature, information on

AO7 sonochemical degradation is scarce with only two

publications dealing with the medium frequency (e.g. at

400–520 kHz) ultrasound irradiation of the dye [11,12].

The aim of this work was to investigate the oxidation of

AO7 by means of heterogeneous photocatalysis and low

frequency ultrasound irradiation regarding the effect of

various operating conditions (type and concentration of

catalyst, solution pH, water matrix for photocatalytic runs

and reaction temperature, ultrasound frequency and

periodicity of irradiation for sonochemical runs) on

conversion. Reaction intermediates were identified and

the effect of chemical pre-oxidation on acute ecotoxicity

and aerobic biodegradability was evaluated.

2. Experimental and analytical

2.1. Chemicals

Acid orange 7 (C16H11N2NaO4S) was purchased

from Fluka and used without further purification. 1,4-

Benzoquinone (>98% purity) and potassium peroxodi-

sulfate (>99% purity), hydrogen peroxide (as a 35% v/v

solution) and sodium chloride (>99.5% purity), and

sodium azide (>99% purity) all used as matrix compo-

nents during photocatalytic experiments were supplied by

Fluka, Merck and Aldrich, respectively. Acetonitrile used

in HPLC analysis was Suprasolv quality and purchased

from Merck, while ammonium acetate (>98% purity) was

purchased from Fluka. Deionized water used for sample

preparation was prepared on a water purification system

(EASYpureRF) supplied by Barnstead/Thermolyne

(USA).

Five photocatalystswere employed in this study, namely:

(a) TiO2 P-25 (Ti-D) supplied by Degussa Huels with

an anatase:rutile composition ratio of 3.6:1 and a surface

area of 58 m2/g, (b) TiO2 anatase (Ti-A) supplied by

Tronox-McGee with a surface area of 12 m2/g, (c) TiO2

(anatase) supplied by Aldrich (Ti-C), (d) ZnO supplied by

Fluka with a surface area of 10 m2/g, and (e) CdS supplied

by Aldrich.

2.2. Photocatalytic degradation experiments

Photocatalytic experiments were conducted in a 500 ml

Pyrex vessel in the center of which was placed a glass

cylindrical tube (17 � 3.5 cm) housing a 9 W UVA lamp

(Radium Ralutec, 9 W/78, 350–400 nm). The vessel was

covered with aluminum foil and immersed in a water bath,

which was connected to a temperature control unit (Polystat,

cc2 model, Huber), thus maintaining a constant liquid-phase

temperature of 25 8C. In a typical run, 200 ml of an AO7

aqueous solution at an initial concentration of 50 mg/L were

prepared daily, loaded in the vessel and slurried with the

appropriate concentration of photocatalyst. The reactor

contents were magnetically stirred, while (unless otherwise

stated) CO2 free air was continuously sparged in the liquid.

In most cases, experiments were performed at ambient pH

(this was 5.5 for the TiO2/AO7 system and 7.5 for the ZnO/

AO7 system) and left uncontrolled during the reaction. In

those cases where runs were carried out at basic or acidic

conditions, the initial pH was adjusted adding the

appropriate amount of NaOH or HC1 as needed. Samples

periodically drawn from the vessel were filtered through

0.45 mm disposable filters to remove catalyst particles and

then analyzed with respect to AO7 and color conversion, as

well as determination of reaction by-products and sample

ecotoxicity.

2.3. Sonochemical degradation experiments

An Ultrason 250 (LabPlant, UK) ultrasound generator

connected to a titanium-made horn operating in pulse or

continuous mode at a fixed frequency of 80 kHz and a

variable electric power output up to 150 W was used for

most sonication experiments. For those experiments carried

out at a frequency of 24 kHz, a UP 400S (Dr Hielscher

GmbH, Germany) horn-type generator capable of operating

at a variable electric power output up to 450 W was used

instead. Reactions were carried out in a 250 ml cylindrical

glass reaction vessel, which was closed during ultrasonic

irradiation. The vessel was immersed in a water bath, which

was connected to a temperature control unit (Polystat cc2

model, Huber). In all cases, 200 ml of AO7 aqueous solution

T. Velegraki et al. / Applied Catalysis B: Environmental 62 (2006) 159–168 161

Fig. 1. AO7 conversion (closed symbols) and color removal (open symbols)

during photocatalytic degradation with various semiconductors. (*,*) Ti-

A; (&,&) Ti-D; (~,~) Ti-C; (^,^) ZnO; (x) AO7 conversion with Ti-A

in dark. Catalyst concentration: 100 mg/L. Initial solution pH 5.5 for TiO2

and 7.5 for ZnO.

at an initial concentration of 50 mg/L were prepared daily

and subjected to ultrasonic irradiation, while samples

periodically drawn from the vessel were analyzed according

to the procedures described below.

2.4. High performance liquid chromatography (HPLC)

A fully computer controlled HPLC system (manufac-

tured by Shimadzu) comprising a two solvent delivery

pump, diode array and fluorescence detectors and an

autosampler was used to follow AO7 concentration-time

profiles. AO7 was separated on a Alltech Inertsil C8 5 mm,

250 mm � 4.6 mm column using 70:30 aqueous solution of

ammonium acetate (20 mM):acetonitrile as an isocratic

mobile phase at 1 ml/min and ambient temperature. The

injection volume was 20 ml and detection was achieved with

the diode array detector set at 485 nm. Quantification was

based on the chromatograms taken using the Shimadzu

Class-VP chromatography data handling software. The

linearity between absorbance and concentration was tested

using external standards at various AO7 concentrations

between 1 and 50 mg/L and the response was found to be

linear (with a correlation coefficient r2 = 0.9973) over the

whole range of concentrations under consideration. The

inter-day repeatability of the method was checked for

analyzing seven AO7 samples and was found to be excellent

with a relative standard deviation of 0.6%. Blank samples

were run between two consecutive HPLC runs to ensure that

no residuals from the previous run were carried over to the

next run.

2.5. Color

Changes in sample color were followed measuring

absorbance at 485 nm on a Helios Unicam UV–vis

spectrophotometer.

2.6. Solid-phase microextraction coupled with gas

chromatography–mass spectrometry (SPME-GC–MS)

GC–MS analysis was employed to identify reaction by-

products from photocatalytic and sonochemical degrada-

tion. Prior to GC–MS analysis; the samples were pre-

concentrated using a solid-phase microextraction (SPME)

method, a selective tool for the trace analysis of organic

compounds in water samples, according to the procedures

described in detail elsewhere [13]. A Shimadzu GC-17A

(Version 3) QP-5050A GC–MS system equipped with a

30 m � 0.25 mm, 0.25 mm HP-5MS capillary column

(Agilent Technologies) was used for all analyses. The

injector was operating at 270 8C in the splitless mode with

the split closed for 5 min. Helium (>99.999% purity) was

used as the carrier gas at a flow-rate of 1.4 ml/min. The

column oven was initially set at 50 8C for 5 min, then

programmed to 160 at a 10 8C/min rate, where it was held

for 2 min and finally to 310 8C at a 5 8C/min rate, where it

was held for 10 min. The interface temperature was set at

320 8C and the detector voltage at 1.4 kV. The ionization

mode was electron impact (70 eV) and data was

collected in the full scan mode (m/z 50–400). For a

compound to be considered as a likely reaction inter-

mediate with a certain degree of confidence, its mass

spectrum should match that of the GC–MS mass spectrum

library by at least 90%.

2.7. The Microtox acute toxicity test

The luminescent marine bacteria Vibrio fischeri was used

to assess the acute ecotoxicity of AO7 samples prior to and

after photocatalytic and sonochemical treatment. The

inhibition of V. fischeri exposed to AO7 samples for

15 min was measured using a Microtox 500 Analyzer (SDI,

USA) according to the Microtox 82% screening test

(Microbics Corporation, MicrotoxManual, 1992, AToxicity

Testing Handbook, vol. 1–5, Carlsbad, CA, USA). Each

sample was run in triplicate and the results were corrected to

account for sample color.

2.8. Shake flask experiments

Shake flasks experiments were performed to assess the

aerobic biodegradability of AO7 prior to and following

sonochemical treatment. Flasks containing either original or

sonicated AO7 solutions and 0.5 ml of activated sludge

taken from the municipal wastewater treatment plant of

Chania, Greece were shaken at 200 rpm and ambient

temperature. These runs were repeated in the presence of

sodium azide that inhibits biological activity to test whether

possible substrate removal could be due to adsorption onto

biomass.


Fig. 2. AO7 conversion after 240 min of photocatalytic treatment with Ti-A

at various concentrations. Initial solution pH 5.5.


during photocatalytic degradation with Ti-A and various initial pH values.

(*, *) pH 2; (&, &) pH 9. Catalyst concentration: 100 mg/L.

3. Results and discussion

3.1. Photocatalytic degradation experiments

3.1.1. Screening of catalysts

Screening experiments were conducted to assess the

relative efficiency of various semiconductors, namely Ti-A,

Ti-C, Ti-D, ZnO and CdS, in terms of AO7 degradation and

decolorization based on a common catalyst loading. Fig. 1

shows that substantial conversion could be achieved with all

three TiO2 photocatalysts used with Ti-A being superior

compared to the other two yielding about 66 and 88% color

and AO7 removal, respectively, after 240 min of irradiation;

the respective values with Ti-D were 60 and 73% even

though the catalyst surface area was about five times greater

than that with Ti-A. Although Ti-D is generally considered

as highly active commercial catalyst for the photodegrada-

tion of various pollutants, there are several cases where pure

anatase (such as Ti-A or Ti-C) appears to have better

photocatalytic properties than Degussa P-25 [14]. Ti-D owes

its high photoreactivity to a slow electron/hole recombina-

tion rate, whereas the high photoreactivity of the anatase

form is due to a fast interfacial electron transfer rate [14,15].

We can hypothesize that in our case, where Ti-A is more

efficient than Ti-D, the photocatalytic oxidation of AO7 is

limited more from the electron transfer reaction than from

the electron/hole recombination. For the run with ZnO, the

corresponding removal values were 10 and 20%, while CdS

led to neither decolorization nor degradation possibly due to

the lower oxidative power of the photogenerated holes, as

well as to its photo-corrosion (data for CdS is not shown for

clarity). To test whether AO7 removal was due to

photocatalytic degradation rather than adsorption onto the

catalyst surface, blank runs were performed in the dark;

representative data for Ti-A is also given in Fig. 1 showing

that substrate adsorption was negligible after 240 min, while

similar results were obtained for the other catalysts. As can

also be seen, the extent of decolorization was always lower

than AO7 conversion since, other than residual AO7,

reaction by-products that contribute to sample absorbance

may be more stable than AO7 to degradation. This is in good

agreement with the results of Comparelli et al. who studied

the photocatalytic degradation of azodyes methyl orange

and methyl red over TiO2 [16] and ZnO [17] and reported

that dye degradation was generally faster than decoloriza-

tion. Similar results were also reported by Augugliaro et al.

[18] who studied the solar photocatalytic degradation of

methyl orange and AO7 over TiO2. In view of the results of

Fig. 1, all subsequent photocatalytic runs were carried out

with Ti-A.

3.1.2. Effect of catalyst concentration

TiO2 loading in slurry photocatalytic processes is an

important factor that can influence strongly dye degradation.

The effect of catalyst loading on conversion was studied in

the range 0–350 mg/L and the extent of AO7 degradation

after 240 min of reaction is shown in Fig. 2. As can be seen,

decomposition increases with increasing catalyst concentra-

tion up to about 100 mg/L after which dye conversion

remains practically unchanged. The catalyst concentration

abovewhich conversion levels off depends on several factors

(e.g. reactor geometry, operating conditions, wavelength and

intensity of light source) and corresponds to the point where

all catalyst particles, i.e. all the surface exposed, are fully

illuminated [7,19–21]. At higher-concentrations, a screening

effect of excess particles occurs, thus masking part of the

photosensitive surface and consequently hindering light

penetration [21]; this usually results in conversion reaching a

plateau, while at excessive catalyst concentrations conver-

sion may also decrease due to increased light reflectance

onto the catalyst surface. In this study, the optimum

concentration at which all subsequent experiments were

conducted was about 100 mg/L.

3.1.3. Effect of solution pH

Fig. 3 shows AO7 conversion and color removal as a

function of irradiation time during photocatalytic degradation


Fig. 4. Effect of air sparging and hydrogen peroxide addition on photo-

catalytic degradation. (*) & Run 1: with air sparging; (&) & Run 2:

without air sparging; (^) & Run 3: without air sparging in the presence of

H2O2; (~) & Run 4: with air sparging in the presence of H2O2; (*) & Run

5: UV/H2O2 without Ti-A (impulse addition of H2O2); (&) & Run 6: UV/

H2O2 without Ti-A (stepwise addition of H2O2). Catalyst concentration:

100 mg/L. H2O2 concentration: 200 mg/L.

under acidic and alkaline conditions. As can be seen from

Figs. 1 and 3, degradation strongly depends on the solution pH

and is substantially hindered at alkaline conditions. For

instance, the extent of AO7 conversion after 240 min at pH

values of 2, 5.5 and9were 80, 88 and 44%, respectively,while

the respective values for decolorization were 57, 66 and 29%.

Blank runs were also performed in the dark showing no AO7

adsorption after 240 min. The effect of pH on degradation is a

complex issue closely related to the pathways of dye

degradation, the amphoteric behavior of TiO2, as well as the

loweringof theoxidativepowerof thephotogeneratedholesby

increasing the pH value.

Irradiation of an aqueous TiO2 suspension with light

energy greater than the band gap energy of the semi-

conductor (hv > Eg = 3.2 eV) yields valence band holes and

conduction band electrons; the former can react with water

and the hydroxide ion (i.e. under alkaline conditions) to

generate hydroxyl radicals which are strong oxidizing

agents as well as oxidize directly the dye [22,23]. On the

other hand, the photogenerated electrons react with adsorbed

molecular oxygen on the Ti(III) sites, reducing it to

superoxide radical anion which, in turn, reacts with protons

to form peroxide radicals. Furthermore, the electrons may

directly degrade the dye through the reductive cleavage of

the azo bond; finally, the dye may also undergo degradation

through its reaction with hydroxyl and peroxide radicals.

The aforementioned reaction sequence is described expli-

citly elsewhere [7]. At pH values greater than about 6.5 (this

value corresponds to the point of zero charge for TiO2 [24])

the catalyst surface becomes negatively charged, thus

preventing the negatively charged dye as well as the

hydroxide anion from adsorbing onto the surface; this would

explain the reduced degree of degradation recorded at

alkaline conditions. Conversely, acidic conditions would

favor the reductive cleavage of AO7 [7] as well as the

electrostatic attraction between the positively charged

surface and the dye both of which would result in increased

degradation.

3.1.4. Effect of air sparging and addition of hydrogen

peroxide

In further experiments, the effect of air sparging and

addition of hydrogen peroxide on AO7 photocatalytic

degradation was studied and the results are shown in Fig. 4.

As expected, the presence of oxygen enhances degradation

since it reacts with conduction band electrons (avoiding e�/

h+ recombination, a major cause of low TiO2 photocatalytic

quantum yield) to form superoxide radical anions that

eventually yield reactive peroxide radicals. For example, the

extent of AO7 conversion and color removal after 240 min

with air sparging was about 10 and 20% greater than that

without sparging.

Interestingly, addition of 200 mg/L hydrogen peroxide

with or without air sparging had a detrimental effect on

conversion; this adverse effect was more pronounced when

hydrogen peroxide addition was combined with air sparging

rather than in the absence of oxygen. In general, hydrogen

peroxide is expected to promote degradation since it may

react with conduction band electrons and the superoxide

radical anion to yield hydroxyl radicals and anions as

follows [25,26]:

H2O2 þ e� ! OH� þOH� (1)

H2O2 þO2�� ! OH� þOH� þ O2 (2)

Nonetheless, it is well documented that, depending on the

reaction conditions and system in question, there is an

optimum H2O2 concentration, above which H2O2 acts as

electron and radical scavenger, thus leading to reduced

degradation [7,25]. In light of this, we studied the effect of

H2O2 concentration in the range 0–1050 mg/L and the

results are shown in Fig. 5. At the conditions under

consideration, degradation is impeded for the whole range of

H2O2 concentrations studied. To assess the oxidizing ability

of combined H2O2 and light irradiation, two runs were

performed in the absence of Ti-A and the results are also

shown in Fig. 4. Fifty millilitres of hydrogen peroxide


Fig. 5. Effect of H2O2 concentration on AO7 conversion (black bars) and

color removal (hatched bars) after 240 min of photocatalytic treatment with

Ti-A. Catalyst concentration: 100 mg/L.

solution were added in the reaction mixture either

instantaneously (i.e. at t = 0) or stepwise at equal time

intervals (i.e. every 30 min); in both cases, no decolonization

occurred, while AO7 conversion was always less than 20%.

An additional blank run in the dark was also performed

showing no degradation in the presence of H2O2. The

rationale behind the stepwise addition of oxidant has to do

with the fact that reactions involving H2O2 are very fast and

the instantaneous addition of relatively large oxidant

amounts may cause radical scavenging, thus suppressing

reaction rates as well as wasting the precious oxidant [27].

3.1.5. Effect of water matrix

In a final set of photocatalytic degradation experiments,

the effect of water matrix on AO7 conversion was studied.

Runs were carried out in the presence of 1,4-benzoquinone,

sodium azide and sodium chloride and the results are shown

in Fig. 6. In all cases, an inhibition of AO7 degradation was

recorded which is due to the quenching behavior of the

Fig. 6. Effect of water matrix on AO7 conversion after 240 min of photo-

catalytic treatment with Ti-A. Run 1: AO7 only; Run 2: 5.5 mM 1,4-

benzoquinone; Run 3: 4.5 mM NaN3: Run 4: 4.5 mM NaCl; Run 5: 86 mM

NaCl. Catalyst concentration: 100 mg/L.

matrix compounds. 1,4-Benzoquinone may scavenge the

supertixide radical anions, while sodium azide is a scavenger

of both oxygen and hydroxyl radicals [28]. These results are

in line with those reported by Stylidi et al. [29] who studied

the TiO2 visible light photocatalytic degradation of 100 mg/

L AO7 aqueous solutions and found that complete inhibition

of AO7 degradation occurred in the presence of 4.3 mM of

1,4-benzoquinone. Addition of 4.3 mM sodium azide

retarded considerably (but not quenched fully) the degrada-

tion of 15 mg/L AO7 solutions.

Experiments conducted at different NaCl concentrations

showed that degradation decreases with increasing NaCl

concentration; for instance, the extent of AO7 removal after

240 min of reaction at 86 mM NaCl concentration (this

corresponds to about 5 g/L) was as low as about 20%. This is

an important consideration bearing in mind that textile

dyehouse effluents often contain salt at concentrations as

high as 100 g/L [1]. The detrimental effect of NaCl on

degradation can be explained as a result of the reaction of

photogenerated holes and hydroxyl radicals with chloride

ions to form chloride radicals; although the latter are also

capable of oxidizing organic compounds, their oxidation

potential is considerably lower than that of hydroxyl radicals

[30].

3.2. Sonochemical degradation experiments


function of time during sonochemical degradation at

80 kHz, 150 W and various temperatures. Ultrasound was

operated in pulse mode, i.e. 5 s on and 5 s off and the

timescale corresponds to the actual irradiation time (e.g.

overall run duration was twice as much as the irradiation

time). As can be seen, about 90 and 70% AO7 and color

removal was, respectively, achieved afier 120 min of

reaction at 25 8C, while degradation substantially decreasedwith increasing temperature and was nearly quenched at


during pulsed (5 s on �5 s off) sonochemical degradation at 80 kHz and

various temperatures. (*, *) 25 8C; (&, &) 60 8C; (~, ~) without

temperature control. Electric power: 150 W.


60 8C. It should be pointed out that for the run performed

without temperature control, the temperature gradually

increased throughout the course of the reaction due to

insufficient dissipation of heat. For instance, temperature

increased from ambient at time zero to 45 8C after 15 min, to

51 8C after 60 min and then remained constant up to

120 min of sonication. The detrimental impact of tempera-

ture on degradation can be explained as follows: the

maximum temperature (Tmax) obtained during the bubble

collapse is given as follows:

Tmax ¼ To

�P

Po

�ðg � 1Þ (3)

where To is the liquid bulk temperature, Po is the vapor

pressure of the solution, P is the liquid pressure during the

collapse and g is the specific heat ratio (i.e. the ratio of

constant pressure to constant volume heat capacities).

Although increased temperatures are likely to facilitate

bubble formation due to an increase of the equilibrium

vapor pressure, this beneficial effect is compensated by

the fact that bubbles contain more vapor which cushions

bubble implosion and consequently reduces the maximum

temperature that can be achieved upon bubble collapse. In

addition to this, increased temperatures are likely to favor

degassing of the liquid phase, thus reducing the number of

gas nuclei available for bubble formation [9]. It should be

pointed out that the effect of temperature on sonochemical

degradation rates is a relatively complex issue closely

related to the properties and reaction conditions of each

specific system in question; therefore, it is not surprising that

several investigators have reported contradictory findings

regarding the temperature effect [8,9]. In certain reaction

systems for instance, the net effect of an increase in bulk

temperature is an increase in degradation rates. This occurs

up to the point at which the cushioning effect of the vapor


during continuous sonochemical degradation at various frequencies. (*,

*) 24 kHz; (~, ~) 80 kHz. Electric power: 150 W.

begins to dominate the system and further increases in liquid

temperature result in reduced reaction rates.


function of time during continuous sonochemical degrada-

tion at 24 and 80 kHz, 25 8C and 150 W. As can be seen,

nearly complete and about 85%AO7 and color removal was,

respectively, achieved after 240 min at 80 kHz; conversely,

no conversion was practically achieved for the experiment

carried out at 24 kHz. This behavior is not unexpected since

higher frequencies are usually needed to degrade hydro-

philic and non-volatile organics, while lower frequencies are

more suitable for hydrophobic and volatile compounds [9].

From Figs. 7 and 8, it is evident that, pulsed irradiation is

more efficient than continuous under similar operating

conditions and this may be associated with the more

effective utilization of hydroxyl radical achieved under

pulsed conditions [11]. Moreover, pulsed operation facil-

itates temperature control because there is more time

available for heat dissipation.

In aqueous phase sonolysis, there are three potential sites

for sonochemical activity, namely: (i) the gaseous region of

the cavitation bubble where volatile and hydrophobic species

are easily degraded through pyrolytic reactions as well as

reactions involving the participation of hydroxyl radicalswith

the latter being formed through water sonolysis:

H2O ! H� þ OH� (4)

(ii) the bubble–liquid interfacewhere hydroxyl radicals are

localized and, therefore, radical reactions predominate

although pyrolytic reactions may also, to a lesser extent,

occur and (iii) the liquid bulk where secondary sonochemical

activity may take place mainly due to free radicals that have

escaped from the interface and migrated to the liquid bulk.

It should be pointed out that hydroxyl radicals can

recombine yielding hydrogen peroxide which may, in turn,

react with hydrogen to regenerate hydroxyl radicals:

OH� þ OH� ! H2O2 (5)

H2O2 þH� ! H2O þ OH� (6)

Given that AO7 is a non-volatile and highly soluble ionic

compound, hydroxyl radical-mediated reactions occurring

primarily in the liquid bulk as well as at the bubble interface

are likely to be the dominant degradation pathway.

3.3. Determination of deep oxidation by-products

To identify reaction by-products accompanying the deep

oxidation of AO7, samples subjected to 240 min of

photocatalytic or sonochemical degradation were analyzed

by means of SPME-GC–MS, a method suitable for the trace

analysis of relatively hydrophobic compounds. It should be

noticed that we were primarily interested in identifying

the very deep and consequently most stable oxidation

by-products of AO7 photocatalysis and sonolysis. This

was so because such end-products, even at minute


concentrations, are likely to determine eventually the

biodegradability and ecotoxicity of the treated solution.

For the samples treated by photocatalysis, the extent of AO7

conversion was 73% with Ti-D and 88% with either Ti-A or

Ti-C; for the sonochemically irradiated sample, the

respective conversion was nearly 100%. All identified by-

products are shown in Fig. 9. As can be seen, photocatalytic

degradation by-products can be classified into three groups:

(i) naphthalene-like compounds such as 2-naphthol which is

a primary degradation intermediate accompanying AO7

Fig. 9. By-products of deep AO7 photocatalytlc and sono

cleavage in the vicinity of the azo bond. Its formation has

also been corroborated in several previous studies concern-

ing AO7 photocatalytic degradation [7], (ii) aromatic

intermediates such as benzyl alcohol, benzaldehyde, 2-

methyl phenol, toluene and benzophenone and (iii) ring-

cleavage compounds such as 2-ethyl-l-hexanol and

t-butylamine. It should be pointed out that none of the

previous studies dealing with AO7 UVA-induced photo-

catalytic degradation gives information on reaction by-

products; such information is only available for visible- and

chemical degradation identified by SPME-GC–MS.


Fig. 10. Acute ecotoxicity to V. fischeri of samples prior to and after photo-

catalytic or sonochemical degradation. Run 1: original AO7; Run 2: UV/Ti-A

(200 mg/L); Run 3: UV/Ti-A (250 mg/L); Run 4: UV/Ti-A (100 mg/L)

combined with H2O2; Run 5: 80 kHz ultrasound. All samples were run in

triplicate. Inset values show AO7 degradation achieved in the respective run.

Table 1

Combined sonochemical and biological degradation of AO7. Duration of

shake flask tests: 13 days

Treatment efficiency Combined

treatment

Biological

treatment only

AO7 conversion during

sonochemical step (%)

91 –

AO7 conversion during

biological step (%)

14.4 0

Overall AO7 conversion (%) 92.7 0

Color removal during sonochemical

step (%)

71 –

Color removal during biological

step (%)

29.6 0

Overall color removal (%) 80 0

solar light-induced photocatalytic experiments [7]. For

instance, Stylidi et al. [28,29] proposed a reaction pathway

for the visible and solar AO7 degradation involving the early

formation of 2-naphthol and other naphthalene-like com-

pounds whose further oxidation yielded phthalic derivatives;

the latter underwent further degradation to form aromatic

intermediates further oxidation of which resulted in

aromatic ring cleavage. With the exception of 2-napthol,

other reaction intermediates proposed by Stylidi et al.

[28,29] and other investigators [7] were not detected in our

study and this might have been due to the different

mechanisms involved in UVA, visible and solar degradation

pathways as well as to the different experimental conditions

and analytical techniques employed in various studies.

It should be pointed out that, other than the compounds

successfully detected by SPME-GC–MS, several other

chromatographic peaks were also found but could not be

positively identified (e.g. the match factor of the mass

spectrum was below 90%). It is interesting to note that

distribution of by-products appears to depend on the type of

photocatalyst used since, although identical treatment

conditions were employed, some intermediates were not

common for all three catalysts; this is more pronounced

comparing the intermediates from Ti-A and Ti-C runs for

which the same AO7 conversion of 88% was achieved.

The effect of catalyst properties on selectivity has been

recently demonstrated by Comparelli et al. [16] who studied

the by-products formed during the photocatalytic degrada-

tion of methyl orange and methyl red over various TiO2

powders.

After 240 min of ultrasound irradiation, the reaction

mixture nearly exclusively consists of aliphatic compounds

presumably due to the deep oxidation of AO7 and its ring

intermediates; this implies that, at the conditions in question,

sonochemical treatment is more effective than photocata-

lysis in terms of AO7, color and by-products removal. It is

presumed that the differences in the identified by-products of

photocatalysis and ultrasound may also, to some extent, be

due to the different degradation mechanisms involved.

3.4. Effect of treatment on ecotoxicity and

biodegradability

The effect of AO7 treatment on acute ecotoxicity was

assessed using the marine bacteria V. fischeri and the results

are shown in Fig. 10. It should be noted that photocatalytic

runs were carried out at different experimental conditions

(i.e. Ti-A concentrations, treatment times, addition of

H2O2), thus leading to various degrees of AO7 degradation

(shown as inset values in Fig. 10). With the exception of the

photocatalytic run with H2O2, residual AO7 and degradation

by-products appear to be less toxic than the original,

untreated sample. In the case of H2O2, the increased toxicity

is believed to be due to the presence of unreacted hydrogen

peroxide (a well known disinfectant) which is toxic to

microorganisms. Furthermore, mild sonochemical treatment

yielding only about 15% AO7 reduction was capable of

decreasing toxicity from about 55 to 45%.

Information regarding the effect of sonochemical

treatment of azodyes on subsequent biodegradability is

scarce. Tezcanli-Guyer and Ince [12] who studied the

sonochemical degradation of AO7 at 520 kHz reported that,

although treatment for 60 min led to about 70% decoloriza-

tion, aerobic biodegradation (as assessed by changes in the

BOD5/TOC ratio) was not possible. For the purpose of the

present investigations, AO7 was subjected to pulsed

ultrasonic irradiation for 120 min at 80 kHz, 150 W and

25 8C and the resulting solution was then inoculated with

0.5 ml of activated sludge to initiate biodegradation. In

parallel, a solution containing 50 mg/L of AO7 was also

inoculated to assess the aerobic biodegradability of

untreated AO7. As can be seen in Table 1, AO7 was not

degradable aerobically after 13 days of biodegradation.

However, coupling ultrasonic irradiation with biological

degradation led to slightly increased overall treatment

efficiencies. The results of Fig. 10 and Table 1 suggest that

chemical oxidation may be successfully employed as a pre-

treatment stage to convert initially, bioresistant and toxic

molecules to more biodegradable intermediates.


4. Conclusions

The conclusions drawn from this study can be

summarized as follows:

(1) T
iO2-mediated photocatalysis is capable of oxidizing
acid orange 7 in synthetic aqueous solutions. The extent

of conversion depends on the operating conditions

employed such as type and concentration of catalyst,

solution pH, dissolved oxygen and the water matrix,

while degradation proceeds through a series of aromatic

and ring-cleavage intermediates.

(2) L
ow frequency, high power ultrasound can promote the
deep oxidation of the azodye to lower ring-cleavage

intermediates. The extent of conversion is a function of

the liquid bulk temperature, ultrasound frequency and

the periodicity of ultrasound operation.

(3) A
dvanced oxidation reduces the acute ecotoxicity to
marine bacteria V. fischeri and improves the aerobic

biotreatability. In this context, chemical oxidation could

be used as a pre-treatment step to convert initially

bioresistant compounds to more readily biodegradable

ones followed by biological treatment.

Acknowledgments

The authors wish to thank V. Tsiridis for his involvement

with toxicity analysis. Financial support for T. Velegraki was

provided by the Hellenic Ministry of National Education &

Religious Affairs under the ‘‘PYTHAGORAS’’ program.

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Photocatalytic and sonolytic oxidation of acid orange 7 in aqueous solution

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