Degradation of orange-G by advanced oxidation processes

Article abstract of DOI:10.1016/j.ultsonch.2009.10.008

The degradation and mineralization of orange-G (OG) in aqueous solutions by means of ultrasound irradiation at a frequency of 213 kHz and its combination with a heterogeneous photocatalyst (TiO₂) were investigated. The effects of various operational parameters such as, the concentration of the dye and solution pH on the degradation efficiency were studied. The degradation of the dye followed first-order like kinetics under the conditions examined. The sonolytic degradation of OG was relatively higher at pH 5.8 than that at pH 12. However, an alkaline pH was favoured for the photocatalytic degradation of OG using TiO₂. Total organic carbon (TOC) measurements were also carried out in order to evaluate the mineralization efficiency of OG using sonolysis, photocatalysis and sonophotocatalysis. The hybrid technique of sonophotocatalytic degradation was compared with the individual techniques of photocatalysis and sonolysis. A simple additive effect was observed during the sonophotocatalytic oxidation of OG using TiO₂ indicating that the combined treatment offers no synergistic enhancement. TOC results also support the additive effect in the dual treatment process.

Full text of DOI:10.1016/j.ultsonch.2009.10.008

Ultrasonics Sonochemistry 17 (2010) 338–343
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Degradation of orange-G by advanced oxidation processes
*
Jagannathan Madhavan, Franz Grieser, Muthupandian Ashokkumar
School of Chemistry, University of Melbourne, VIC 3010, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 August 2009
Received in revised form 8 October 2009
Accepted 15 October 2009
Available online 21 October 2009
The degradation and mineralization of orange-G (OG) in aqueous solutions by means of ultrasound irra-
diation at a frequency of 213 kHz and its combination with a heterogeneous photocatalyst (TiO2) were
investigated. The effects of various operational parameters such as, the concentration of the dye and solu-
tion pH on the degradation efficiency were studied. The degradation of the dye followed first-order like
kinetics under the conditions examined. The sonolytic degradation of OG was relatively higher at pH 5.8
than that at pH 12. However, an alkaline pH was favoured for the photocatalytic degradation of OG using
TiO2. Total organic carbon (TOC) measurements were also carried out in order to evaluate the minerali-
zation efficiency of OG using sonolysis, photocatalysis and sonophotocatalysis. The hybrid technique of
sonophotocatalytic degradation was compared with the individual techniques of photocatalysis and son-
olysis. A simple additive effect was observed during the sonophotocatalytic oxidation of OG using TiO2
indicating that the combined treatment offers no synergistic enhancement. TOC results also support
the additive effect in the dual treatment process.
Keywords:
Sonolysis
Photocatalysis
Sonophotocatalysis
Orange-G
Advanced oxidation processes
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
posed by many scientists and engineers. Different advanced oxida-
tion processes (AOPs) include, electrochemical oxidation processes,
Textile dyes constitute one of the largest groups of organic com-
pounds that represent an increasing environmental concern with
reference to water pollution. Of the dyes available in the market,
approximately 50–70% is azo dyes followed by the anthraquinone
dyes. Azo dyes can be divided into monoazo, diazo and triazo clas-
ses depending on the presence of one or more azo groups (–N@N–).
Azo dyes belong to the largest class of dyes used in textile and
paper industries. They are very stable to ultraviolet and visible
light irradiation. Moreover, they are resistant to aerobic degrada-
tion [1] and can be reduced to potentially carcinogenic aromatic
amines under anaerobic conditions or in vivo [2,3]. About 1–20%
of the total synthetic dyes in the world is lost during the dyeing
process and is released in the textile effluents [4,5]. The release
of this contaminated water into the environment is a considerable
source of non-aesthetic pollution and eutrophication which can
produce dangerous by-products through oxidation, hydrolysis, or
other chemical reactions taking place in the wastewater itself
[6,7]. The various methods through which the degradation and
mineralization of such dye effluents can be achieved have there-
fore received increasing attention. Chlorination and ozonation
have been used for the removal of certain dyes but they work at
slow rates, as well as have high operating costs [8,9]. Therefore,
in order to overcome the drawbacks of some of the conventional
treatment methods, advanced oxidation processes have been pro-
sonolysis, supercritical water oxidation and homogeneous and het-
erogeneous photochemical processes [10–12]. Particularly, sono-
chemical degradation of certain organic pollutants and azo dyes
were widely studied by Hoffmann and coworkers [13–15]. Sono-
chemical degradation often fails to achieve the complete mineral-
ization. A few studies have focused on combined AOPs [16–18].
Ragaini et al. [16] observed a synergetic enhancement in the deg-
radation rate for the sonophotocatalytic degradation of 2-chloro-
phenol using a 20 kHz ultrasound. Peller et al. [17] carried out
combined high frequency (660 kHz) sonolysis/photocatalysis and
achieved synergistic degradation of 2,4-dichlorophenol without
making any toxic intermediates even at very low catalyst loadings.
Selli [18] reported a synergy in combining sonolysis and photoca-
talysis for the degradation of an azo dye, acid orange 8.
Recently, Collings et al. [19] studied the rapid degradation of
polychlorinated biphenyls (PCB’s) and certain organochloride pes-
ticides present in the soil using ultrasound (20 kHz, 160 W). It was
reported that 1 min of sonication was sufficient to achieve 90%
destruction of the PCB’s and 99% destruction was attained after
7 min. Such a rapid decomposition of the pollutants was believed
to be due to the solid particles in slurry acting as foci for the nucle-
ation and collapse of micro bubbles. These quite remarkable obser-
vations have prompted us to study the effect of solid particles on
the sonolytic degradation of a textile dye, orange-G (OG).
The photocatalytic degradation of OG has been reported in sev-
eral studies [20,21]. Nagaveni et al. [22] reported the degradation
of OG using combustion-synthesized nano-TiO2 under solar light,
* Corresponding author. Tel.: +61 3 83447090.
E-mail address: masho@unimelb.edu.au (M. Ashokkumar).
1350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.10.008

J. Madhavan et al. / Ultrasonics Sonochemistry 17 (2010) 338–343
339
and found that the initial degradation rates were 1.6 times higher
than that using Degussa P25 TiO2. Sun et al. [23] studied the deg-
radation of OG on nitrogen doped TiO2 and reported that the doped
TiO2 nanocatalysts demonstrated higher activity than the commer-
cial Degussa P25 under visible light.
In the present study, sonolysis, heterogeneous photocatalysis
and a combination of both (sonophotocatalysis) have been chosen
for the degradation of OG, with the aim of investigating effect of
solid particles in a multi-AOP to find the best possible conditions
for the degradation of the dye.
phase consisted of 50/50 (acetonitrile/water). The flow rate of the
solvent was 0.03 mL/min and the capillary voltage was set at
3.5 kV. The degradation intermediates were identified by compar-
ing the chromatograms to those of known compounds using mass
spectrometry and HPLC. The concentration of OG was determined
by a high-performance liquid chromatograph, Shimadzu LC-10
AT VP system with a Shimadzu SPD-10 AVP UV–vis absorbance
detector with an Altech reversed phase column (C18, 150 mm ꢀ
4.6 mm inner diameter, 5 lm beads); a mobile phase of acetoni-
trile/water = 90/10; flow rate of 1.0 mL/min; detection wavelength
of 480 nm. For intermediate separation, an acetate buffer (0.1 M
ammonium acetate in acetic acid) was used as the mobile phase
with a flow rate of 1 mL/min and the detection wavelength of
280 nm.
TOC was determined using a total organic carbon analyser,
which utilizes oxidative combustion followed by infrared detec-
tion. The instrument used was a TOC-VCSH (Shimadzu) pro-
2. Materials and methods
2.1. Experimental conditions
Sonolysis experiments were performed at an ultrasound fre-
quency of 213 kHz in a continuous wave mode. The ultrasound
unit used was ELAC LVG-60 RF generator coupled with ELAC Al-
lied signal transducer with a plate diameter of 54.5 mm [12].
The power output on the RF generator for all experiments was
35% which corresponds to a calorimetric power of about 20 W.
All reagents used were of AR grade and were used without fur-
ther purification. Aqueous solutions of OG (Sigma–Aldrich) were
prepared using Milli-Q water. All experiments were performed
at natural solution pH, which was left uncontrolled during the
reaction. For those runs carried out at alkaline or acidic condi-
tions, the initial pH was adjusted by adding appropriate amounts
of NaOH or H2SO4. Experiments were carried out under atmo-
spheric conditions and at room temperature. The temperature
was maintained (20 2 °C) by circulating water through a jacket
surrounding the cell. The volume of the solutions sonicated was
250 mL. Photoirradiation was carried out by means of a Xe-arc
lamp (450 W, Oriel).
All sonolytic, photocatalytic and sonophotocatalytic experi-
ments were performed in the same experimental assembly. A typ-
ical experimental procedure adopted for the heterogeneous
photocatalytic degradation of dyes is described below. By dissolv-
ing appropriate amounts of the dye in 250 mL of Milli-Q water
within the reaction cell, the desired concentrations of the dye were
prepared. A known amount of the photocatalyst, viz., Degussa P25
TiO2 (particle size 20 nm, surface area 55 m2/g) was then added to
the dye solution. The sonication at higher frequencies is not ex-
pected to affect the particle size since the physical effects gener-
ated are relatively weaker. Prior to irradiation, the aqueous
suspension was mixed continuously in the dark for 45 min to en-
sure adsorption equilibrium of OG on the catalyst surface was
reached. Continuous stirring in this case is essential for maintain-
ing the photocatalyst in the illuminated part of the suspension.
During irradiation, 3 mL aliquots were withdrawn at appropriate
time intervals and the photocatalyst was removed immediately
grammed by TOC-control
V Software. The instrument was
calibrated before each use with standardized TOC solutions of
1000 mg/L. The absorption spectrum of OG was recorded with a
‘‘Cary Varian 50 Bio” UV–vis spectrophotometer at 25 °C. The spec-
trophotometer was coupled to a computer, which measured the
absorption spectrum using Scan 2 software. The samples were
placed in a sample compartment made of quartz and were scanned
in the wavelength range of 200–600 nm. The surface tension of the
aqueous solutions was measured using a McVan Analite surface
tension meter (Model 2141) with a glass Wilhelmy plate. All mea-
surements were done at room temperature.
3. Results and discussion
The degradation of OG was performed under different experi-
mental conditions that include, (i) photoirradiation alone (UV-
‘‘control”), (ii) ultrasound alone (US), (iii) photoirradiation in the
presence of TiO2 (UV + TiO2) and (iv) a combination of photocatal-
ysis and ultrasound (US + UV + TiO2) and the results are shown in
Fig. 1. The preliminary experiments revealed no significant degra-
dation of OG occurred either in the absence of US or UV. How-
ever, under the presence of US with 35% amplitude at the
frequency of 213 kHz, about 43% degradation of OG occurred
within 75 min.
ꢀ
It is known that sonolysis of aqueous solutions produces OH
created by the implosion of cavitation bubbles (Reaction (1)) [24].
H2O ! ꢀH þ ꢀOH
ð1Þ
1.2
1
by filtration through a 0.45
lm syringe filter (Pall Corporation).
0.8
0.6
2.2. Analytical determinations
The solute concentration was monitored over time using elec-
trospray mass spectrometry (ESMS). The mass spectrometer used
was a Micromass QUATTRO 11 coupled to a Hewlett Packard series
1100 degasser. The instrument was calibrated using the automatic
tuning procedure with respective to the parent compound as the
standard. The mass range scanned was m/z 50–500 and several
spectra were obtained across each chromatographic peak. All mass
spectra were the averages of 3–4 spectra obtained across the top of
each chromatographic peak with background noise subtracted. The
analysis was carried out in positive electrospray ionization mode at
cone voltages of 30 V, 50 V and 80 V, respectively. The mobile
blank
US
0.4
0.2
0
UV+TiO2
US+UV+TiO2
0
10
20
30
40
50
60
70
80
Time, min
Fig. 1. The degradation of orange-G (0.09 mM) using various advanced oxidation
processes (TiO2 (1 g/L) was used for photocatalytic and sonophotocatalytic runs).

340
J. Madhavan et al. / Ultrasonics Sonochemistry 17 (2010) 338–343
There are three potential sites for sonochemical reactions in
(a) 1.4
aqueous solutions, 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 par-
ticipation of hydroxyl radicals (ii) the bubble–liquid interface
where hydroxyl radicals are localized and therefore, radical reac-
tions may predominate although pyrolytic reactions occur to a les-
ser extent, and (iii) the liquid bulk where sonochemical activity
may take place mainly due to free radicals that have escaped from
the interface and migrated into the bulk liquid. We speculate that
the degradation of OG takes place mainly in the bubble–liquid
interface and further discussion on this issue is provided later.
It is observed in Fig. 1 that photoirradiation in the presence of
TiO2 leads to 59% degradation in 75 min. Bandgap excitation of
the TiO2 leads to the formation of electrons in the conduction band
and holes in the valence band [25]. As a consequence of such pho-
toinduced charge separation on the semiconductor surface, elec-
tron transfer reactions occur at the ꢁ semiconductor–water
1.2
0.01 mM
0.03 mM
0.05 mM
0.07 mM
0.09 mM
0.12 mM
1
0.8
0.6
0.4
0.2
0
0
10
20
30
40
50
60
70
80
Sonication time, min
1
(b)
0.01 mM
0.03 mM
0.05 mM
0.07 mM
0.09 mM
0.12 mM
0.8
0.6
0.4
0.2
0
ꢀ
interface. The superoxide radical anion O2 is formed by interac-
tion of conduction band electrons with adsorbed oxygen mole-
cules, while the oxidation of adsorbed organic substrates
simultaneously occurs either via their direct interaction with va-
lence band holes or via valence band hole oxidation of adsorbed
water or hydroxyl anions.
However, when both the US and UV are combined (sonophotoc-
atalysis), a significant enhancement in the degradation (85% in
75 min) of OG was observed, suggesting the involvement of hydro-
xyl radicals formed by both bandgap excitation of the photocata-
lyst as well as by the water sonolysis route.
OG has a pKa value of 11.5 hence it is in its –OH form below its
pKa and –Oꢁ form above its pKa. It was also found that the UV–vis
spectrum of OG above and below its pKa changed significantly.
Therefore, it was anticipated that different degradation rates
would be observed at different pH values above and below its
pKa. Hence, the sonolytic, photocatalytic and sonophotocatalytic
degradation of OG at pH values well below (5.8) and above (12)
its pKa were examined.
0
10
20
30
40
50
60
70
Sonication time, min
Fig. 2. (a) Effect of orange-G concentration on the sonolytic degradation kinetics at
pH 5.8. (b) Effect of orange-G concentration on the sonolytic degradation kinetics at
pH 12.
8
7
pH 5.8
pH 12
6
3.1. Sonolytic degradation of orange-G
5
4
3
2
1
0
To study the effect of pH on the sonolytic degradation of OG,
experiments were carried out with OG at various initial concentra-
tions and the result is shown in Fig. 2a and b. The sonochemical
degradation of OG followed first-order like kinetics, which was
confirmed from the plot of ln C0/C vs. time. Note that this cannot
be strictly considered as a first-order reaction since the rate con-
stant changes with the initial concentration of OG. We have ob-
served similar behavior in our earlier work on the sonochemical
degradation of benzoic acid [26]. This anomaly was explained
based on the changes in bubble temperatures affecting the amount
of OH radicals generated during cavitation and detailed discussion
on this issue is provided in our previous report [26]. The only rea-
son we use this kinetic data is to compare the rate of degradation
under various experimental conditions. As can be seen in Fig. 2, the
concentration of OG decreased with an increase in the sonication
time at both pH values. In Fig. 2a, it is shown that the extent of deg-
radation was significantly affected by the initial concentration of
OG. For example, with an initial concentration of 0.12 mM at pH
5.8, about 33% of OG was degraded after 75 min, whereas about
54% degradation was achieved at 0.05 mM concentration. The ini-
tial reaction rate ([OG]i ꢀ corresponding slopes from ln plots) was
found to increase with increasing initial concentration of OG.
The effect of pH on the sonochemical degradation rate of OG is
shown in Fig. 3. Singla et al. [26] studied the sonolytic degradation
of benzoic acid at both acidic and alkaline pH and reported that
with an increase in the concentration of benzoic acid in the bulk
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
[OG], mM
Fig. 3. Effect of orange-G concentration on the sonolytic degradation rate at pH 5.8
and 12.
solution, the bubble/solution interfacial concentration was also in-
creased at low pH. This occurs because the neutral form of benzoic
acid is more hydrophobic than the ionized form and therefore
more surface active. The consequence of this effect is that the reac-
tion between OH radicals (generated within the bubbles) and OG is
enhanced thereby leading to an increase in the degradation rate.
In present study, the rate of OG degradation was relatively
slow at higher solution pH than at lower pH. Since the OG degrada-
tion rate showed a similar trend with pH to that observed for
benzoic acid, the same explanation holds for OG. Surface tension

J. Madhavan et al. / Ultrasonics Sonochemistry 17 (2010) 338–343
341
measurements (data not shown) indicate that OG is slightly more
surface active in acidic pH consistent with the slightly higher
sonolytic degradation at pH 5.8 compared with that at pH 12.
The presence of particles under an ultrasound field (called sonoca-
talysis) may enhance degradation by providing additional nuclei
for bubble generation. Therefore, in order to monitor the effect of
suspended solids (TiO2) on cavitation, the following experiment
was conducted under the experimental conditions of (OG) =
0.09 mM and TiO2 amount of 1 mg/mL, and the results obtained
are shown in Fig. 4. From Fig. 4, it can be deduced that within
experimental error, there is no significant enhancement in the deg-
radation rate by the addition of TiO2 during sonolysis, and that the
sonocatalytic degradation follows the degradation trend of sono-
lytic degradation at both acidic and alkaline pH.
In order to explore ways to enhance the OG degradation rate
experiments combining photocatalysis and sonolysis were carried
out [31,32]. For any hybrid technology, it is obviously expected
that the efficiency of the hybrid system (AB) should be higher than
the sum of the efficiencies of the two single technologies (A and B),
comprising the hybrid technology, that is, AB > A + B.
3.2. Photocatalytic degradation of orange-G
The pH of the solution is an important parameter for the photo-
catalytic degradation since it controls the surface charge properties
of the photocatalyst and the size of aggregates that it forms. The
influence of pH on the photocatalyzed degradation of OG at pH
5.8 and 12.0 was investigated with constant dye concentration
(0.09 mM) and photocatalyst amount (1 mg/mL). Fig. 4 shows that
the photocatalytic degradation rate at pH 5.8 is comparatively low-
er than that at pH 12. The rate of photocatalytic degradation of OG
by Degussa P25 TiO2 was observed to be 17.1 ꢀ 10ꢁ7 M minꢁ1 at
pH 12 which is 1.3 times higher than that of at pH 5.8
(12.6 ꢀ 10ꢁ7 M minꢁ1). The higher photocatalytic activity at alka-
line condition seems to be a characteristic feature for many photo-
catalytic systems and similar results have been obtained for the
photodegradation of methyl orange [27] and phenoxyacetic acid
[28]. Hung et al. [29] also monitored the effect of pH on the photo-
catalytic degradation of OG and detected that the OG degradation
was high when the pH was greater than 9 and slowest under neu-
tral conditions.
R
ðUSþUVþTiO2
Þ
Synergy index ¼
ð2Þ
RUS þ R
ðUVþTiO2
Þ
where RUS, R
and R are sonolytic, photocatalytic
ðUSþUVþTiO2Þ
ðUVþTiO2
Þ
and sonophotocatalytic degradation rates.
The sonophotocatalytic degradation experiments at pH 5.8 and
12 were carried out using Degusa P25 and the data is shown in
Fig. 4. As can be seen that, at pH 5.8, OG underwent relatively
slow degradation under ultrasound irradiation in the presence
of titanium dioxide (US + TiO2) while, under photocatalysis
(UV + TiO2) the degradation occurred at a higher rate. A further
significant increase in the reaction rate was observed when the
illuminated suspensions were simultaneously sonicated, i.e., un-
der sonophotocatalysis (UV + US + TiO2), where the degradation
rate was higher than that of the individual sonolysis or photoca-
talysis rates. For example, the degradation rates at pH 5.8 with
sonolysis, sonocatalysis and photocatalysis are, RUS = 6.2 ꢀ 10ꢁ7
The effect of pH on the photocatalytic degradation rate can be
explained by considering the surface charge of TiO2 (zero point
charge (pHZPC) ꢂ 6.5) and its relation to the acid dissociation con-
stants of sulfonic acid in OG. Since the pKa value of the sulfonic acid
group is low, it will exist in an anionic form at the pH studied. As
the TiO2 surface is positively charged below its pHZPC and nega-
tively charged above its pHZPC, it can be expected that the titanium
dioxide particles would repel the dye at pH 12 and attract the same
at pH 5.8. Therefore, based on a simple adsorption density basis
higher photocatalytic degradation should occur at acidic pH than
at alkaline pH. Contrary to this explanation, in the present investi-
gation, it was observed that the rate reached a maximum at alka-
line pH. The enhancement in the degradation rate at pH 12 may
M minꢁ1, R
¼ 6:1 ꢀ 10ꢁ7 M minꢁ1 and R
¼ 12:6 ꢀ
ðUSþTiO2
Þ
ðUVþTiO2Þ
10ꢁ7 M minꢁ1, respectively. A soꢁn1ophotocatalytic degradation rate
of R
¼ 22:3ꢀ 10ꢁ7 M min was observed, giving a synergy
ꢀ
be due to formation of OH from the excess OHꢁ anions adsorbed
ðUVþTiO2
Þ
index of 1.1 0.2. Considering that this value is only slightly
greater than 1, and within experimental error of the measure-
ments, it suggests that there is an additive effect in combining
sonolysis and photocatalysis, rather than a synergistic effect.
Within experimental error, the sonophotocatalytic degradation
rate at pH 12 is almost equal to the photocatalytic degradation rate
(Fig. 4), giving a synergy index of 0.7 0.2. Hence, we propose that
on the catalyst surface. Moreover, at sufficiently higher pH values,
the formation of oxidizing species such as the superoxide radical
anion (O2ꢀꢁ) could also be responsible for the rate enhancement
[30].
3.3. Sonophotocatalytic degradation of OG
Suspended solids may also influence the level of cavitation gen-
erated in solution and hence the sonochemical degradation rate.
1.2
1
25
0.8
0.6
0.4
0.2
0
pH 5.8
20
pH 12
15
10
5
0
1
2
3
4
5
Time (h)
0
US
US+TiO2
UV+TiO2
US+UV+TiO2
Fig. 5. Change in TOC as a function of time for different degradation processes at pH
5.8 and 12. (ꢁ) Sonolysis at pH 5.8; (d) photocatalysis at pH 5.8; (N) sonophot-
ocatalysis at pH 5.8; (h) sonolysis at pH 12; (s) photocatalysis at pH 12 and (4)
sonophotocatalysis at pH 12.
Fig. 4. Comparison of degradation rates for different processes for the degradation of
orange-G (0.09 mM) in the presence of TiO2 (1 mg/mL) with and without ultrasound.

342
J. Madhavan et al. / Ultrasonics Sonochemistry 17 (2010) 338–343
35
30
25
20
15
10
5
clusions were made: (1) the TOC removal rate was much less than
that of the decolourization process under the same experimental
conditions. (2) Mineralization becomes important after the initial
degradation of OG. (3) Sonophotocatalysis achieves higher TOC re-
moval than the individual processes such as sonolysis and photo-
catalysis. (4) Similar to degradation, an additive effect was
observed with TOC removal rate. (5) The TOC decrease at pH 5.8
is more favourable than pH 12 which is evident from 82% TOC
reduction after 4 h of sonophotocatalytic reaction at pH 5.8
whereas at pH 12 about 60% TOC reduction was observed under
the same experimental conditions.
The above trend may be due to the formation of more stable
intermediates in the process of decolourization and also the differ-
ence between the photocatalytic mechanisms of decolourization
and TOC decrease. The decolouring mechanism of OG might pro-
ceed predominantly through the destruction of the azo group by
oxidation via reaction with positive holes and by reduction from
conduction band electrons in a manner similar to that of decolou-
rization of other azo dyes in photocatalytic processes, while TOC
was decreased only by the oxidation of the organic material [26].
0 min
1 hour
2 hour
I
OG
Phenol Aniline
Unidentified
products
0
0
100
200
300
400
500
-5
Time, s
Fig. 6. HPLC chromatogram obtained as a function of sonication time for orange-G
(0.09 mM) at pH 5.8.
3.5. Possible degradation pathway
sonophotocatalytic degradation at pH 5.8 is more efficient than at
pH 12, due to the sonolytic effect rather than photocatalysis.
Since the sonolysis, photocatalysis and sonophotocatalytic deg-
radation showed almost the same degradation products, the degra-
dation products formed during sonolysis were analyzed in the
present investigation.
3.4. Mineralization studies
The mechanism of the degradation of the dye by sonolysis can
be expected to be very complex due to the non-selective nature
of attack of OH radicals on OG. Bokare et al. [33] reported the
degradation of OG using Fe–Ni bimetallic nanoparticles and pro-
posed the reaction mechanism involving the formation of aniline
and naphthol amine. Kanazawa and Onami [34] proposed a mech-
anism for the degradation of OG by sodium hypochlorite. Based
on their studies, it was evident that certain reaction products are
A rather straightforward way of measuring oxidation progress
of an organic compound is the determination of carbon content
of the oxidation product mixture. This can be obtained by monitor-
ing the total organic carbon (TOC) content of the treated solutions.
The TOC content directly relates to the extent of water pollution.
Based on our TOC studies of the OG degradation using sonolysis,
photocatalysis and sonophotocatalysis (Fig. 5) the following con-
NH2
OH
NH OH
O
S O
+
OH
O
S
Aniline
II
HO
O
m/z = 335
I
OH
N
7-hydroxy-8-(hydroxyamino)naphthalene-1,3-disulfonic acid
O
S O N
OH
OH
O
?
S
HO
O
Orange-G
m/z-408
III
Phenol
OH
O N
N
?
O
S
OH
OH
O
S
HO
O
OH
IV
OH
5,7-dihydroxy-8-[(E)-phenyldiazenyl]naphthalene-1,3-disulfonic acid
OH
O N
N
O
S
OH
?
O
S
HO
m/z = 456
O
OH
V
8-[(E)-(2,4-dihydroxyphenyl)diazenyl]-5,7-dihydroxynaphthalene-1,3-disulfonic acid
Scheme 1. A schematic representation showing the initial degradation pathway of orange-G during sonolysis.

J. Madhavan et al. / Ultrasonics Sonochemistry 17 (2010) 338–343
343
subsequently converted to other products, step by step. Similarly,
different reaction intermediates due to the different reaction pos-
sibilities, like hydrogen abstraction, hydroxyl radical addition
etc., would occur in the present study.
(2) Even though the sonophotocatalysis process is shown to be
more efficient than the individual processes for the degrada-
tion of OG, only an additive effect of combining sonolysis
and photocatalysis was observed rather than a synergy
effect. TOC studies also support the above conclusion.
(3) Aniline, phenol and aromatic hydroxyl amine are formed as
the intermediates during the sonolysis process.
In order to identify reaction by-products accompanying OG deg-
radation, samples collected at various time intervals were analyzed
by means of HPLC and ESMS. The results indicate that sonolysis de-
grades the azo group of OG and results in its irreversible transfor-
mation into a number of products including aromatic amines.
HPLC chromatogram obtained for the sonication of OG at differ-
ent time intervals is given in Fig. 6. As seen in Fig. 6, the peak due to
OG (Rt = 300–380 s) decreased with increased sonication time
while the peaks for the products correspondingly increased. The
major product of sonication is 7-hydroxy-8-(hydroxyamino)naph-
thalene-1,3-disulfonic acid (I) (Rt = 100–130 s) was found as a pro-
tonated form in the mass spectrum. Formation of aniline and
phenol were confirmed by comparing the retention time of pure
aniline and phenol standards with the retention time of the reac-
tion sample at 170–200 s and 140–170 s, respectively (Scheme 1).
By comparing the product formation yields, it was observed that
the quantity of aniline (II) formation was very low and this leads to
the conclusion that it undergoes further degradation into other
products. Since sonolysis mainly involves the attack of OH radicals
on the aromatic ring, it can be expected that the first intermediates
formed would be OH-added products of OG. Apart from aniline,
other products detected were also OH-added products such as 5,7-
dihydroxy-8-[(E)-phenyldiazenyl]naphthalene-1,3-disulfonic acid
(IV, m/z 424) and 8-[(E)-(2,4-dihydroxyphenyl)diazenyl]-5,7-dihy-
droxynaphthalene-1,3-disulfonic acid (V, m/z 456). The above ESMS
findings together with the results of HPLC support the notion that
during the sonolytic oxidation of dye molecule two processes occur
(a) hydroxylation of the aromatic ring and, (b) reductive cleavage of
the azo bond that lead to the formation of aromatic primary amines.
From all our observations, it can be concluded that partially
water soluble materials such as OG do not degrade synergistically
using a combined AOP process. Also, enhanced degradation by
operating ultrasound in the presence of particles, for these types
of pollutants is not observed under our operating conditions.
Acknowledgements
The authors thank DIISR, Australia and DST, New Delhi for the
financial support of an India–Australian strategic research fund
(INT/AUS/P-1/07 dated 19 September 2007).
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(1) Degradation of OG depends on the initial dye concentration
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