Enhanced mechanism of catalytic ozonation by ultrasound with orthogonal dual frequencies for the degradation of nitrobenzene in aqueous solution

Article abstract of DOI:10.1016/j.ultsonch.2009.07.001

The experiments have been performed with a semi-continuous batch reactor to investigate the degradation efficiency of nitrobenzene in aqueous solution by ultrasound with the different orthogonal dual frequencies catalytic ozonation. The introduction of ultrasound can enhance the degradation efficiency of nitrobenzene compared to the results obtained from the processes of ozonation alone and ultrasound alone. The degradation of nitrobenzene is found to be zero-order in the two systems of ultrasound alone, and the reactions follow the pseudo-first-order kinetic model in the processes of ozone alone and ozone/ultrasound. The investigation confirms that the degradation of nitrobenzene follows the mechanism of hydroxyl radical (^?OH) oxidation, and the enhancement function is even more pronounced in the presence of ultrasound with the greater difference between the orthogonal dual frequencies due to the obvious synergetic effect between ozone and ultrasound, which increases the utilization efficiency of ozone, and accelerates the initiation of ^?OH and the formation of H₂O₂, resulting in the rapid formation of an increasing diversity of byproducts and the advancement degree of mineralization of total organic carbon (TOC). The oxidative byproducts have been, respectively identified in the different processes selected, including o, p, m-nitrophenols, phenol, malonic acid, 4-nitrocatechol, nitrate ion, maleic acid, oxalic acid, hydroquinone, p-quinone, 1,2,3-trihydroxy-5-nitrobenzene and acetic acid.

Full text of DOI:10.1016/j.ultsonch.2009.07.001

Ultrasonics Sonochemistry 17 (2010) 84–91
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
Enhanced mechanism of catalytic ozonation by ultrasound with orthogonal
dual frequencies for the degradation of nitrobenzene in aqueous solution
Lei Zhao a,b,*, Jun Ma b,*, Xuedong Zhai a
a
School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, People’s Republic of China
National Engineering Research Center of Urban Water Resources, State Key Laboratory of Urban Water Resources and Environment, Harbin Institute of Technology,
b
Harbin 150090, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
The experiments have been performed with a semi-continuous batch reactor to investigate the degrada-
tion efficiency of nitrobenzene in aqueous solution by ultrasound with the different orthogonal dual fre-
quencies catalytic ozonation. The introduction of ultrasound can enhance the degradation efficiency of
nitrobenzene compared to the results obtained from the processes of ozonation alone and ultrasound
alone. The degradation of nitrobenzene is found to be zero-order in the two systems of ultrasound alone,
and the reactions follow the pseudo-first-order kinetic model in the processes of ozone alone and ozone/
ultrasound. The investigation confirms that the degradation of nitrobenzene follows the mechanism of
Received 20 February 2009
Received in revised form 2 July 2009
Accepted 8 July 2009
Available online 14 July 2009
Keywords:
Enhanced mechanism
Catalytic ozonation
Ultrasound
Nitrobenzene
Degradation
ꢀ
hydroxyl radical ( OH) oxidation, and the enhancement function is even more pronounced in the pres-
ence of ultrasound with the greater difference between the orthogonal dual frequencies due to the obvi-
ous synergetic effect between ozone and ultrasound, which increases the utilization efficiency of ozone,
ꢀ
and accelerates the initiation of OH and the formation of H
2 2
O , resulting in the rapid formation of an
increasing diversity of byproducts and the advancement degree of mineralization of total organic carbon
TOC). The oxidative byproducts have been, respectively identified in the different processes selected,
(
including o, p, m-nitrophenols, phenol, malonic acid, 4-nitrocatechol, nitrate ion, maleic acid, oxalic acid,
hydroquinone, p-quinone, 1,2,3-trihydroxy-5-nitrobenzene and acetic acid.
Ó 2009 Elsevier B.V. All rights reserved.
1
. Introduction
Nitrobenzene, as a carcinogenic pollutant [1], is one of the fast-
In recent years, ozonation combined with ultrasound has been
actively researched as a promising method of AOPs [8]. Comparing
with ozonation or ultrasound alone, it appears to be more effective
and uses less energy, and it has the advantages of being non-selec-
tive, leading to no secondary pollution, and being particularly
effective in removing persistent and biorefractory pollutants from
water [9]. Many researches have been reported on the removal of
contaminants in aqueous solution by the process of ozone/ultra-
sound, including methyl orange, phenol, p-nitrotoluene, aniline,
anthraquinone dye, C.I. reactive blue 19, chitosan and pentachloro-
phenol [10–12]. In addition, the experimental result indicates that
the presence of ultrasound can enhance the ozonolysis of nitroben-
zene [13]. However, there is no report on the degradation of trace
est-growing end-use synthetic products of benzene [2]. Its com-
mercial uses are reduction to aniline, solvent, metal polishes,
shoe-black, perfume, dye intermediates [3,4], plastics, explosives,
pharmaceuticals [5], pesticides [6] and a combustible propellant
[
7]. However, nitrobenzene resists to oxidation by conventional
chemical oxidation due to the strong electron-withdrawing prop-
erty of the nitro-group. Mineralization of nitrobenzene by microor-
ganisms is prevented due to the effects of toxic and mutagenic
deriving from nitrobenzene and its transformation on biological
systems, such as nitrosobenzene, hydroxylaminobenzene and ani-
line [1,3]. In order to afford the inexpensive and effective processes
for the water treatment, various biological treatment processes,
chemical reduction methods and advanced oxidation processes
ꢁ1
nitrobenzene (
lg L level) by ozone/ultrasound. In fact, the re-
moval of organic micro-pollutant in water is very important to
the practical operation in wastewater treatment and drinking
water supply with source water of the river and the lake.
(
AOPs) have been studied for the degradation of nitrobenzene in
aqueous solution.
In order to further increase the degradation efficiency of ozon-
ation and develop a convenient and clean operation for the water
treatment, the comparative experiments have been performed to
investigate the degradation efficiency of organic micro-pollutant
in the different processes, including ultrasound alone, ozone alone
and ozone/ultrasound. Nitrobenzene reacts slowly with molecular
*
Corresponding authors. School of Municipal and Environmental Engineering,
Harbin Institute of Technology, Harbin 150090, People’s Republic of China. Tel.: +86
4
8
51 82291644/86283010; fax: +86 451 82368074 (L. Zhao), tel.: +86 451
6282292/86283010; fax: +86 451 82368074 (J. Ma).
E-mail addresses: zhaolei999999@126.com (L. Zhao), majun@hit.edu.cn (J. Ma).
1
350-4177/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.07.001

L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
85
ꢁ
1 ꢁ1
8
ozone (0.09 ± 0.02 M
s
), while reacts quickly with hydroxyl
sound with the different orthogonal dual frequencies, namely the
combined ultrasonic fields AB and AC. Therefore, typical power
intensity and power density were same to AB and AC, which were
ꢀ
ꢁ1 ꢁ1
radical ( OH) (2.2 ꢂ 10 M
s ). Therefore, nitrobenzene, as a
ꢀ
special indicator of OH and a major environmental pollutant, is
chosen as a target organic compound due to its toxicity of the cen-
tral nervous system and its refractory nature to conventional
chemical oxidation. The originality of the present study is to reveal
the enhanced mechanism of combined process from the aspects of
the synergistic effect between ozone and ultrasound with the dif-
ferent orthogonal dual frequencies.
ꢁ2
ꢁ1
0.65 W cm and 64.1 W L , respectively.
Ozone was produced from pure oxygen (Harbin Gas Co. Ltd.,
China, 99.999%) through an XFZ-5 ozone generator (Qinghua Tongli
Co. Ltd., China) at a power setting of 40 W, and was subsequently
fed into the ultrasonic reactor to contact thoroughly with water
samples through a porous titanium plate at the bottom of the reac-
tor. The total applied ozone in this experiment was controlled at
ꢁ1
1
.0 mg L . Before the experimental operation, the reactor was
2
. Experimental
pre-ozonated for 4 min to satisfy any ozone demand in the reactor,
and then was washed several times with distilled water to exclude
any possible side effects. During the degradation experiment, the
synthetic solution (4 L) with the nitrobenzene concentration of
2
.1. Materials and reagents
ꢁ1
The synthetic solution was prepared by spiking 50 lg L nitro-
ꢁ1
5
0
l
g L was pumped into the reactor by a MP-20R magnetic
benzene (Beijing Chemical Factory, China, purified by distillation
pretreatment, 99.80%) in Milli-Q water (Millipore Q Biocel system).
Other chemicals used in the experiments were analytical grade re-
agents. A diluted sodium thiosulphate solution was used in the
experiments for quenching the reaction. Except for volumetric
flasks, the other glassware was muffled overnight at 673 K. The
volumetric flasks were washed by soaking them in chromic acid
and then rinsing with distilled water.
pump (Shanghai Xishan Pump Co. Ltd., China) and then circulated
ꢁ1
at a rate of 4 L min . A cooling system was kept outside the reac-
tor and cold water was pumped from a thermostatic bath in order
to control reaction temperature constantly at 298 K. Water sam-
ples were taken from the reactor at various reaction times to ana-
lyze the residual concentration of nitrobenzene. The oxidation
reaction was quenched by the addition of a small amount of so-
dium thiosulphate solution. In addition, compared to the scavenger
ꢁ
2ꢁ
ꢁ
ꢁ
effect of buffer solution (HCO , CO , H
2
PO , and H
2
PO ), the deg-
2
.2. Experimental procedure
3
3
4
2
ꢁ1
radation of trace initial nitrobenzene concentration 50 lg L led
to a slight reduction to 6.72–6.80 from initial pH 6.87 which scar-
cely affect the experimental results, and could be neglected. There-
fore, the experiments were carried out at initial pH 6.87 in this
study without adding any buffer solution to maintain the pH at a
constant value.
The experiments were carried out in a semi-continuous ultra-
sonic reactor, as shown in Fig. 1, which was made entirely of stain-
less steel with a breadth of 11.5 cm, a depth of 11.5 cm and a
height of 26.8 cm. The three flattened sides of the reactor were,
respectively mounted with three series of piezoelectric transducers
(
3 No. each), which were arranged in series and driven at 20, 28
and 40 kHz (A, B and C field), respectively. The emitting system
was connected to a frequency generator and a power supply. The
ultrasonic power input of every field was 100 W. The experiment
focused more attention on the enhancement function of ultra-
2.3. Analytical method
The concentration of ozone in the gas was measured by iodo-
metric titration method [14]. The concentration of residual ozone
Fig. 1. Schematic of experimental apparatus.

8
6
L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
in aqueous solution was measured by spectrophotometer using the
indigo method [15]. The concentration of H formed in the oxi-
1.00
0.0
2
R
= 0.998
2
O
2
(a)
dation system was determined by the photometric method [16].
The concentration of nitrobenzene was determined by a GC-14C
gas chromatograph (Shimadzu, Japan) according to the previous
method [17]. An electron paramagnetic resonance (EPR) experi-
0
0
0
.96
.92
.88
2
2
= 0.999 -0.5
R
= 0.999
R
ꢀ
ment was conducted for the determination of OH generated in
-1.0
the selected processes [17]. Mineralization of pollutant was moni-
tored for total organic carbon (TOC) removal, which was deter-
mined with a TOC analyzer (Analytic Jena Multi N/C 3100). A gas
chromatographic and mass spectrometric analyzes system (GC/
MS, 6890GC/5973MS, Agilent, USA) and ion chromatography (IC,
CDD-6A, Shimadzu Co., Japan) were used to identify the byprod-
ucts of the degradation of nitrobenzene [18].
(b)
2
2
R
= 0.992
R
= 0.997
0.84
-1.5
-2.0
AB
AC
Ozone alone
Ozone/AB
Ozone/AC
0
0
.80
.76
0
10 20 30 40 50 60
0
10 20 30 40 50 60
Reaction time (min)
3
. Results and discussion
Fig. 3. Kinetic plots for the degradation efficiency of nitrobenzene in the different
3.1. Degradation efficiency of nitrobenzene in the selected processes
processes (reaction conditions – temperature: 298 K; initial pH: 6.87; initial
ꢁ
1
ꢁ1
nitrobenzene concentration: 50
l
g L ; total applied ozone: 1.0 mg L ; C: the
instantaneous concentration of nitrobenzene;
nitrobenzene; (a) kinetic plots of the processes of AB and AC; (b) pseudo-first-
order plots of the processes of ozone alone, ozone/AB and ozone/AC).
C
0
: the initial concentration of
The degradation efficiency of nitrobenzene was investigated in
the selected processes, including AB, AC, ozone alone, ozone/AB
and ozone/AC. The results are illustrated in Fig. 2.
As shown in Fig. 2, after the 60 min treatment of a nitrobenzene
ꢁ
1
aqueous solution of 50
l
g L , the degradation efficiency of nitro-
tion reactions are the zero-order with respect to nitrobenzene in
benzene in the process of ozone alone is 32.5%, and ultrasonic
fields with the different orthogonal dual frequencies of AB and
AC only results in the degradation of 14.2% and 20.7%, respectively.
The combined processes of ozone/AB and ozone/AC can enhance
greatly the degradation efficiency of nitrobenzene. Comparing
the result of ozone/AB with the cumulative effect of ozone alone
and AB, an increment of approximately 11.9% of nitrobenzene deg-
radation is observed. Under the same experimental conditions,
ozone/AC system leads to about 83.8% nitrobenzene conversion,
indicating an increment of 30.6% compared to the cumulative ef-
fect of ozone alone and AC, suggesting that the combined processes
of ozone with ultrasound selected by the present study can pro-
duce the synergetic effect to improve the degradation of nitroben-
zene in aqueous solution, and the process of ozone/AC can obtain
the more obvious synergetic effect.
the processes of AB and AC. The plots of ln (C/C ) versus reaction
time for representative kinetic runs are presented in Fig. 3b, and
0
it is interesting to emphasize that the three reactions follow the
pseudo-first-order kinetic model in the processes of ozone alone,
ozone/AB and ozone/AC. These phenomena in Fig. 3 indicate that,
as an effectively assistant enhanced method, the introduction of
ultrasound with the different orthogonal dual frequencies does
not cause the conversion of kinetic order during the ozonation
for the degradation of nitrobenzene.
Generally, it is now widely believed that ozone reacts in aque-
ous solution on various organic and inorganic compounds, either
by direct, selective reactions of molecular ozone or through a rad-
ꢀ
ical type reaction involving OH [19,20]. Since the oxidizing poten-
ꢀ
ꢀ
tial of OH is much higher than that of molecular ozone, OH is a
less selective and more powerful oxidant than molecular ozone,
and it is one of the most reactive free radicals and one of the stron-
gest oxidants [21]. Furthermore, the reaction rate constant of nitro-
In addition, the results of kinetics investigation are shown in
Fig. 3. Based on normalized concentration (C/C
0
) versus reaction
ꢁ1 ꢁ1
time, the linearity of the plots in Fig. 3a depicts that the degrada-
benzene with ozone alone is only 0.09 ± 0.02 M
the rate constants for reaction of nitrobenzene with OH is
s
[22], while
ꢀ
8
ꢁ1 ꢁ1
ꢀ
2
.2 ꢂ 10 M
s
[23]. In fact, the initiation of OH also can be,
1
00
respectively observed in the processes of ultrasound alone and
ultrasound/ozone [24]. Therefore, for the experimental phenomena
in Fig. 2, it may be assumed that nitrobenzene is degraded mainly
AB
AC
80
60
40
20
0
Ozone alone
Ozone/AB
Ozone/AC
ꢀ
by OH oxidation in the selected processes.
ꢀ
3
.2. Initiation of OH in the selected processes
In order to further identify the reaction mechanism of the deg-
radation of nitrobenzene in the selected processes, the experi-
ꢀ
ments were carried out to determine the initiation of OH by
means of the spin trapping/EPR technique, which can detect unsta-
ble radicals by measuring the intensity of DMPO–OH adduct signal.
The experimental results are summarized in Fig. 4.
Firstly, Fig. 4a presents the profile of the DMPO–OH adduct sig-
nal in the process of ozone alone without addition of nitrobenzene.
Like the profile in Fig. 4a, the spectra of the DMPO–OH adduct sig-
nal in the other processes selected by the present experiment are
all composed of quartet lines having a peak height ratio of
0
10
20
30
40
50
60
Reaction time (min)
1
a
:2:2:1, and the parameters are hyperfine constants
N
a = 1.49 mT,
Fig. 2. Comparison of the degradation efficiency of nitrobenzene in the different
= 1.49 mT and g-value = 2.0055, which coincide with those of
processes (reaction conditions – temperature: 298 K; initial pH: 6.87; initial
H
ꢁ
1
ꢁ1
nitrobenzene concentration: 50
l
g L ; total applied ozone: 1.0 mg L ).
the DMPO–OH adduct as demonstrated previously [25]. These

L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
87
0
0
0
.075
.060
.045
4
3
2
1
000
200
400
600
1
1
500
000
AB
1
360
Without nitrobenzene
Ozone alone
AC
Ozone alone
Ozone/AB
Ozone/AC
500
0
-
500
1000
1500
(
a)
-
-
0.030
3450
3465
3480
3495
3510
Gauss
(
b)
0
0
.015
.000
800
Addition of nitrobenzene
Without nitrobenzene
0
0
10
20
30
40
50
60
AB
AC
Ozone alone Ozone/AB
Ozone/AC
Reaction time (min)
The experimental processes
Fig. 5. Evolution of the concentration of H
2
O
2
formation in the different processes
(
reaction conditions – temperature: 298 K; initial pH: 6.87; initial nitrobenzene
Fig. 4. Comparison of the relative intensity of DMPO–OH adduct signal in the
ꢁ
1
ꢁ1
concentration: 50 lg L ; total applied ozone: 1.0 mg L ).
different processes (reaction conditions – temperature: 298 K; initial pH: 6.87;
ꢁ
1
ꢁ1
initial nitrobenzene concentration: 50 lg L ; total applied ozone: 1.0 mg L ;
ꢁ
1
initial DMPO concentration 100 mmol L ; (a) the intensity of DMPO–OH adduct
signal in the process of ozone alone without addition of nitrobenzene; (b) the
relative intensity of DMPO–OH adduct signal in the different processes).
down until the concentration of H
of 0.0375 mg L at reaction time 60 min. Otherwise, the concen-
2
O
2
eventually reaches a plateau
ꢁ1
2 2
tration of H O formation in the processes of ozone/AB and
ꢁ
1
ꢀ
ozone/AC increases continuously to 0.0543 and 0.0672 mg L with
the increasing reaction time to 60 min, respectively.
experimental results indicate that the initiation of OH exists in the
every reaction system mentioned above.
Comparing the results in Fig. 5, it is seen that the process of
ozone alone, ozone/AB or ozone/AC can, respectively obtain a high-
er concentration of H O formation than that of AB or AC system at
2 2
the same reaction time point. As a consequence, the processes of
AB and AC present the positive effect during the beginning 30
and 40 min, respectively, and the negative effect after this time
point, resulting in an appearance of extremum. This may be attrib-
Secondly, Fig. 4b displays the relative intensity of DMPO–OH
adduct signal in the different selected processes. From the results
in Fig. 4b, it can be found that the relative intensity of DMPO–OH
adduct signal increases according to the order of AB, AC, ozone
alone, ozone/AB and ozone/AC, which is consistent with the evolu-
tion trend of degradation efficiency of nitrobenzene in these pro-
cesses, confirming directly the expectation mentioned above that
ꢀ
uted to the lower level of H
accumulated concentration of H
effect derived from the those species in the presence of reaction
system and the thermal decomposition of H at the ‘‘hot spots”
2
O
2
formation, and the decrease in
the degradation of nitrobenzene is mainly attributed to OH
2 2
O is mainly due to the scavenger
oxidation.
Otherwise, comparing the relative intensity of DMPO–OH ad-
duct signal in the every selected process, it is also observed that
the process without nitrobenzene always has a stronger relative
intensity than that with addition of nitrobenzene, suggesting that
the former can obtain a higher concentration of OH because there
is no nitrobenzene, an OH scavenger, to react competitively with
2 2
O
shell region formed during the ultrasonic irradiation of aqueous
solution [26]. However, a plateau situation is presented in the pro-
cess of ozone alone, meaning that the equilibrium is established
between the positive effect and the negative effect due to the rel-
atively higher concentration of H O formation compared to the
2 2
either case of ultrasound alone. Under the same experimental con-
ditions, the ozone/AB and ozone/AC systems is, respectively pro-
ꢀ
ꢀ
ꢀ
OH. Based on the reaction rate constant of nitrobenzene with
OH, this phenomenon is an auxiliary exemplification of OH oxida-
ꢀ
ꢀ
tion mechanism.
vided with the synergistic effect for the formation of H
overcome the negative effect leading to a persistent increase trend
of H formation within reaction time investigated, and the
2 2
O to
Moreover, as an intermediate species production, the initiation
of OH maybe influences the generation of other highly oxidative
ꢀ
2 2
O
species, for example, H
processes.
2 2
O also should be formed in the selected
increasing trend is even more pronounced in the process of
ozone/AC.
ꢀ
The initiation of OH, an active oxidative species, is the most
important characteristic of AOPs. A common objective of enhanced
process is to produce OH in sufficient quantity to improve the de-
2 2
3.3. Formation of H O in the selected processes
ꢀ
gree of mineralization of the target organic compound.
As another significant new intermediate species generated from
ozone decomposition, the evolution of accumulated concentration
of H formation was investigated in the processes of AB, AC,
O
2 2
3.4. Removal efficiency of TOC in the selected processes
ozone alone, ozone/AB and ozone/AC. The results are shown in
Fig. 5.
Due to the significance of mineralization of organic compounds,
the experiments were performed to detect the removal efficiency
of TOC in the selected processes. The results are illustrated in Fig. 6.
The results of Fig. 6 indicate that the two combined ozone/ultra-
sound processes are more effective than each single one to remove
TOC from aqueous solution containing nitrobenzene. The processes
of AB and AC result in the removal efficiency of 7.7% and 12.4%,
respectively. 37.6% of initial TOC is removed by ozone/AB
From Fig. 5, it can be found that certain concentrations of H
are formed in all the selected processes. At first, the concentration
of H in the processes of AB and AC reaches the maximum of
.0146 and 0.0225 mg L at reaction time 30 and 40 min, respec-
2 2
O
2 2
O
ꢁ1
0
ꢁ
1
tively, and then decreases to 0.0059 and 0.0143 mg L at 60 min
treatment. However, in the ozone alone system, H forms with
a rapid rate within the initial 40 min, and then the formation slows
2 2
O

8
8
L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
80
64
48
32
16
0
measured when the concentrations of p-nitrophenol and m-nitro-
phenol all reach the maximum value.
AB
As also can be observed from Fig. 7, there are three new byprod-
ucts to be measured in the process of ozone alone: maleic acid,
oxalic acid and hydroquinone. The possible contribution of oxida-
tive cleavage of a hydroxylated aromatic compound, derived from
AC
Ozone alone
Ozone/AB
Ozone/AC
ꢀ
the interaction of OH with nitrophenols, leads to the formation of
ꢀ
maleic acid and oxalic acid [31]. The addition of OH to p-nitrophe-
nol results in the formation of hydroquinone, and hydroquinone
also can be formed via a simultaneous cleavage of the nitro-group
during aromatic nuclear hydroxylation. Otherwise, oxidation of
ꢀ
phenol by OH also can result in hydroquinone formation [32].
Furthermore, it should be noted that 1,2,3-trihydroxy-5-nitro-
benzene and acetic acid, as the novel kinds of byproducts com-
pared to those of the process of ozone alone, appear in the
ozone/AB system. The additional formations of 1,2,3-trihydroxy-
5
-nitrobenzene and acetic acid are the results of aromatic nuclear
0
10
20
30
40
50
60
ꢀ
hydroxylation of 4-nitrocatechol and the further OH oxidation of
malonic acid, respectively [33,34].
Reaction time (min)
Fig. 6. Comparison of removal efficiency of TOC in the different processes (reaction
Notice from Fig. 7 that the degradation of nitrobenzene in the
process of ozone/AC leads to the appearance of other byproducts:
p-quinone and o-nitrophenol. The formation of p-quinone is de-
rived from the further degradation of hydroquinone through the
conditions – temperature: 298 K; initial pH: 6.87; initial nitrobenzene concentra-
ꢁ
1
ꢁ1
tion: 50 lg L ; total applied ozone: 1.0 mg L ).
abstraction of two hydrogens [32], and o-nitrophenol comes from
compared to 20.8% by ozone alone with the same total applied
ꢀ
ꢁ1
the OH attack of nitrobenzene at the
addition [27].
a-carbon via electrophilic
ozone of 1.0 mg L . Simultaneously, the improvement of removal
efficiency of TOC is even more pronounced in the process of ozone/
AC, approximately 67.8% of initial TOC is removed after 60 min
treatment.
Moreover, comparing the removal efficiency of TOC in Fig. 6
with the degradation efficiency of nitrobenzene in Fig. 2 by the
same process, it is found that the removal of TOC is always lower
than the disappearance of nitrobenzene, indicating that nitroben-
zene has been mineralized partly into carbon dioxide and water,
and the byproducts are formed via the degradation of initial com-
pound in the every selected processes.
Comparing the results of Fig. 7, it is obvious that the combined
process of ozone/AB or ozone/AC increases, respectively the con-
centrations and diversities of byproducts, and the appearance time
of byproducts also becomes shorter correspondingly, and the
improvement of them is even more pronounced in the ozone/AC
system. Also, it should be remembered that most byproducts
achieve nearly the complete conversion in process of ozone/AC.
Though byproducts are rapidly produced from the beginning peri-
od of the treatment, they can not be removed absolutely from
aqueous solution to yield other intermediates and/or carbon diox-
ide and water in the process of ozone/AB. Combining with the re-
sults of Figs. 4 and 6, the phenomenon of Fig. 7 suggests that a
ꢀ
3.5. Formation and evolution of byproducts in the selected processes
higher concentration of OH can lead to the rapid formation of an
increasing diversity of byproducts, and enhance the total removal
of byproducts and the degree of mineralization of target organic
compound.
Based on the analysis by GC/MS and IC, the formation and evo-
lution of byproducts were identified and compared in the selected
processes, and the results are shown in Fig. 7.
ꢀ
Combining with the determination results of initiation of OH in
Fig. 4, the formation and evolution of byproducts can be explained
reasonably in the every selected process. At first, the results in
Fig. 7 illustrate that in the process of AB, only four byproducts
are detected: p-nitrophenol, m-nitrophenol, phenol and malonic
3.6. Discussion on the enhanced mechanism
The chemistry of ozone in aqueous solution is complex. Molec-
ular ozone can oxidize water impurities via direct, selective reac-
acid. On the one hand, p-nitrophenol and m-nitrophenol can be
tions or can undergo decomposition via
a
chain reaction
ꢀ
ꢀ
formed if the attacks of OH occur, respectively at
c- or b-carbons
mechanism resulting in the generation of OH [20]. Firstly, ozone
is very unstable in aqueous solution due to its high active reso-
nance structures of molecule. The half-life time of ozone molecular
varies from a few seconds up to few minutes and depends on pH,
water temperature and concentration of organic and inorganic
compounds in aqueous solution [20]. Furthermore, the major sec-
ondary oxidant formed from ozone decomposition in aqueous
of the benzene ring of nitrobenzene via electrophilic addition [27].
ꢀ
On the other hand, OH also can attack the nitrobenzene molecule,
resulting in the occurrence of electron transfer and denitration, and
the formation of phenyl radicals [28]. Phenyl radicals can be trans-
ꢀ
formed into phenol through the further attack of OH [29]. Other-
wise, reactions of the intermediates mentioned above and
ꢀ
ꢀ
nitrobenzene with OH lead to the cleavage of benzene ring and
solution is the OH [35], the reaction rate of which with organic
6
9
ꢁ1 ꢁ1
the formation of oxygenated aliphatic compounds, which can be
degraded to carboxylic acids via alcohols, aldehydes or ketones
molecules is usually in the order of 10 –10 M
s
[36]. There-
fore, it is deduced from the results of Fig. 4 that in the ozone alone
ꢀ
(
from quinones), for example, malonic acid [29].
system, OH is initiated from the self-decomposition of ozone in
Secondly, as two new byproducts, nitrate ion and 4-nitrocate-
aqueous solution under this experimental conditions. However,
ozone alone and ultrasound alone have been shown to achieve a
very limited mineralization of organic compounds in micro-pollu-
tant removal in aqueous solution, as shown in Figs. 2 and 6. There-
fore, the combined process of ozone/ultrasound is selected in the
present study, and the emphasis is located on the mechanism
chol are found in the AC system. Nitrate ion is a result of the min-
eralization of organically bound nitrogen during the degradation of
nitrobenzene, and the formation of 4-nitrocatechol may be ex-
ꢀ
plained by the electrophilic attack of OH on the ortho position of
p-nitrophenol and m-nitrophenol [30]. In fact, nitrate ion can be

L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
89
8
6
4
2
AB
0
0
1
AC
8
6
4
2
0
2
1
Ozone alone
9
6
3
0
6
1
1
Ozone/AB
2
8
4
0
0
2
1
1
6
2
8
4
0
Ozone/AC
0
10
20
30
40
50
60
Reaction time (min)
Fig. 7. Formation and evolution of byproducts in the different processes (reaction conditions – temperature: 298 K; initial pH: 6.87; initial nitrobenzene concentration:
ꢁ
1
ꢁ1
5
0
l
g L ; total applied ozone: 1.0 mg L ; (s) p-nitrophenol; (d) m-nitrophenol; (4) phenol; (N) malonic acid; (q) nitrate ion; (w) 4-nitrocatechol; (h) maleic acid; (j)
oxalic acid; (5) hydroquinone; (.) 1,2,3-trihydroxy-5-nitrobenzene; (}) acetic acid; (ꢀ) p-quinone; (+) o-nitrophenol).
investigation of ultrasound with the different orthogonal dual fre-
quencies for the enhancement efficiency of ozonation.
rapid formation, growth and implosive collapse of gas vacuoles
generate short-lived (-ns), localized ‘‘hot spots” whose peak tem-
peratures and pressures have been measured at 5000 K and
1000 atm [24]. The combining irradiation of dual-frequency ultra-
sound can significantly increase the ultrasonic cavitation yield
compared with that coming from two individual ultrasonic irradi-
ations [37]. This enhancement effect can essentially be ascribed to
the following factors. When two sound waves encounter in solu-
tion, not only the original waves but also waves with the sum fre-
quency, difference frequency and multiple frequencies may be
produced in the focal acoustic field and the larger amplitudes are
created [38]. Furthermore, the simultaneous combined multi-fre-
quency ultrasound can disturb the sample solution more strongly
than the single-frequency ultrasound and may even break the
surface continuity of the sample solution, thus resulting in more
The present study illuminates that, due to the lower power
ꢁ
2
intensity and the lower power density (0.65 W cm
6
and
ꢁ1
4.1 W L ), the single ultrasonic field (A, B or C) scarcely causes
the degradation of nitrobenzene, and also no enhancement effi-
ciency of ozonation is presented at the lower total applied ozone
ꢁ1
of 1.0 mg L (not shown). However, ultrasound with the dual fre-
quencies catalytic ozonation can enhance the degradation effi-
ciency of nitrobenzene compared to the both case of ozone alone
and ultrasound alone under the same conditions (Fig. 2). This
may be mainly attributed to the difference in the nature of the
acoustic field generated, and the reactions are listed as follows.
On the one hand, it is generally believed that the chemical ef-
fects of ultrasound arise from acoustic cavitations of liquids: this

9
0
L. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 84–91
ꢀ
cavitation nuclei in the solution. This increased disturbance leads
to the enhancement of bubble–bubble and bubble–sound interac-
tions by Bjerkness forces. The simultaneous combined ultrasonic
irradiation causes more efficient mass transfer, leading to an in-
creased cavitation activity [39]. On the other hand, this combined
dual-frequency ultrasound also can produce the interference pat-
tern of superposition waves to disturb the standing wave pattern
leading to the higher energy efficiencies [40].
Except for the reasons mentioned above, it is especially impor-
tant that the orthogonal dual-frequency can significantly increase
the inertial cavitation activity, leading to the improvement of cav-
itation yield compared to the algebraic sum of the individual ultra-
sound irradiations [39,41]. As a result, the synergistic effect is
presented obviously between ozone and ultrasound with the
orthogonal dual frequencies, and the greater the difference be-
tween the orthogonal dual frequencies, the stronger enhancement
efficiency is obtained in the present study. Furthermore, the exper-
imental results (not shown) demonstrate that the utilization effi-
ciency of ozone is 52.4%, 74.6% and 97.5%, respectively, in the
processes of ozone alone, ozone/AB and ozone/AC, namely the
introduction of ultrasound can increase the utilization efficiency
of ozone. In other words, the synergistic effect appears to transfer
ozone into aqueous solution more efficiently and also accelerate
ever, under relatively higher initiation efficiency of OH, the overall
disappearance of target organic compound can follow first-order
kinetics because the reaction rate is not limited by the concentra-
tion of OH initiated in the processes of ozone alone, ozone/AB and
ꢀ
ozone/AC [45].
Therefore, the enhancement degradation of nitrobenzene in the
present combined process is derived from the synergetic effect be-
tween ozone and ultrasound, resulting in the acceleration initia-
ꢀ
tion of OH which determines the improvement removal of
target organic compound.
4
. Conclusions
Ultrasound with the different orthogonal dual frequencies has
been used to enhance ozonation for the degradation efficiency of
nitrobenzene in aqueous solution. The experimental results indi-
cate the process of ultrasound alone is lower than ozone alone
for the degradation of nitrobenzene, but ozone/ultrasound combi-
nation is found more efficient than the additive effects of single
operations. The kinetics in the two processes of ultrasound alone
exhibit the zero-order with respect to nitrobenzene, while the deg-
radation reactions all follow the pseudo-first-order kinetic model
in the three processes of ozone alone and ozone/ultrasound. The
the initiation of novel oxidative intermediate species from the
ꢀ
ꢀ
degradation of nitrobenzene is mainly attributed to the OH oxida-
decomposition of ozone, such as OH and H
2
O
2
, which produces
tion in the every selected process.
a higher removal efficiency of TOC, resulting in a higher degree
of mineralization (Figs. 4–6).
Comparing to the ozone alone and ultrasound alone systems,
the combination process of ozone/ultrasound can increase the ini-
ꢀ
Based on the initiation of OH detected, it is revealed that the
ꢀ
tiation of OH, the formation of H
2
O
2
and the removal efficiency of
degradation of nitrobenzene occurs via several significant steps
in the present selected processes, including the hydroxylation of
aromatic ring, the mineralization of organic nitrogen, the transfor-
mation of aromatics to aliphatics by destruction of ring structures,
the oxidation of the aliphatic chains, and the formation of organic
acids (Fig. 7). This phenomenon is consistent with the primary
objective of AOP, which can entail complete mineralization, imply-
ing that the final products of degradation reactions are carbon
dioxide, short-chain organic acids, and inorganic ions, typically less
toxic and amenable to biodegradation [24].
Otherwise, the introduction of ultrasound accelerates the deg-
radation rate of nitrobenzene, and the reactions of the ozone/AB
and ozone/AC systems all follow the pseudo-first-order kinetic
model, the same to that of ozone alone (Fig. 3b), meaning that
ultrasound is a effectively subordinate enhanced approach to ozon-
ation, a dominating oxidation technique. Or else, when ozone alone
is the subordinate enhanced approach, the degradation efficiency
of nitrobenzene in the process of ultrasound alone will be higher
than that of ozone alone, and the reaction kinetics of ozone/ultra-
sound may be same to that of ultrasound alone. This phenomenon
is reasonable. On the one hand, the degradation byproducts will
contribute to the degradation of target organic compound until
TOC. The phenomena are derived from the obvious synergetic ef-
fect between ozone and ultrasound with the different orthogonal
dual frequencies, and the greater the difference between the
orthogonal dual frequencies, the stronger enhancement efficiency
is obtained.
The byproducts analysis illustrates the degradation of nitroben-
zene occurs via the hydroxylation of aromatic ring, the mineraliza-
tion of organic nitrogen, the transformation of aromatics to
aliphatics by destruction of ring structures, the oxidation of the ali-
phatic chains, and the formation of organic acids. Observed
byproducts include the following: o, p, m-nitrophenols, phenol,
4
-nitrocatechol, hydroquinone, p-quinone, 1,2,3-trihydroxy-5-
nitrobenzene, maleic acid, malonic acid, oxalic acid, acetic acid,
and nitrate ion.
Acknowledgements
Authors gratefully acknowledge the China Postdoctoral Science
Foundation (Grant No. 20080440130), the Scheme of 863 High
Technology Research and Development Program of China (Grant
No. 2006AA06Z306) and the National Natural Science Foundation
of China under the Scheme of National Creative Research Groups
(Grant No. 50821002) for the financial support of this study.
2
the carbon species are oxidized completely to CO [13]. It is ob-
served that the ultrasonic degradation of nitrobenzene or p-nitro-
phenol follows, respectively the first-order kinetics [13,42,43],
while the formation of 4-nitrocatechol from p-nitrophenol follows
zero-order kinetics during the course of ultrasonic irradiation [42].
As another important byproduct, the sonolysis of phenol in aque-
ous solution is zero-order [44]. Therefore, when degradation
byproducts with zero-order kinetics contribute dominantly to the
removal, the degradation of target organic compound unquestion-
ably follows zero-order kinetics. Strictly speaking, the ultrasonic
reaction does not follow intrinsically a single kinetics order, which
is dependent on the species and the concentrations of reactants
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