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Intensification of sonochemical decolorization of anthraquinonic dye Acid Blue
25 using carbon tetrachloride
Houria Ghodbane, Oualid Hamdaoui
*
Department of Process Engineering, Faculty of Engineering, University of Annaba, P.O. Box 12, 23000 Annaba, Algeria
article info
Article history:
Received 30 September 2008
Received in revised form 9 December 2008
Accepted 9 December 2008
Available online 24 December 2008
Keywords:
High frequency ultrasound
Decolorization
Acid Blue 25
Anthraquinonic dye
Carbon tetrachloride
abstract
In this work, the influence of CCl
4
on the sonochemical decolorization of anthraquinonic dye Acid Blue 25
(AB25) in aqueous medium was investigated using high frequency ultrasound (1700 kHz). This fre-
quency, reputed ineffective, was tested in order to introduce the ultrasound waves with high frequency
in the field of degradation or removal of dyes from wastewater, due to its limited use in this field, and to
increase the application of high frequency ultrasound wave in the field of environmental protection. The
effects of various parameters such as the concentration of CCl
4
, frequency (22.5 and 1700 kHz), solution
pH, temperature and tert-butyl alcohol adding on the decolorization rate of AB25 was studied. The
obtained results clearly demonstrated the significant intensification of AB25 decolorization in the pres-
ence of CCl
4
. The enhancement effect of CCl
4
increased by decreasing temperature and by increasing
the CCl
4
concentration. The pH has a significant influence on the bleaching of dye both in the absence
and presence of CCl
4
. The three investigated dosimeter methods (KI oxidation, Fricke reaction and
H
2
O
2
production) well corroborate the improvement of the sonochemical effects in the presence of
CCl
4
. The best sonochemical decolorization rate of AB25 in aqueous solution both in the absence and
presence of CCl
4
is observed to occur at 1700 kHz compared to 22.5 kHz. The sonochemical oxidation
of CCl
4
generates oxidizing species in the liquid phase that are highly beneficial for oxidation of hydro-
philic and non-volatile pollutant, such as dyes, because they are less susceptible to free radical attack
due to lower stability of the generated free radicals.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Synthetic dyes are an important class of recalcitrant organic
compounds and are often found in the environment as a result of
their wide industrial use. Dyes are used in many industries such
as food, paper, plastics, cosmetics and textile in order to color their
products. Among various industries, textile industry ranks first in
usage of dyes where the main sources of wastewater generated
originate from the washing and bleaching of natural fibers and
from the dyeing and finishing steps. There are more than 100,000
different commercial dyes and pigments exist with over 7 10
5
tones of dyestuff produced annually [1,2]. It is estimated that
10–15% of the overall production of dyes is released into the envi-
ronment, mainly via wastewater [3].
The discharge of very small amounts of dyes (less than 1 ppm
for some dyes) is aestheticaly displeasing, impedes light penetra-
tion, affects gas solubility damaging the quality of the receiving
streams and may be toxic to treatment processes, to food chain
organisms and to aquatic life. Azo, anthraquinone and indigo are
the major chromophores found in commercial dyes. Decolorization
of these dyes by physical or chemical methods is financially and of-
ten also methodologically demanding, time-consuming and mostly
not very effective. Recently, there has been increasing interest in
the application of advanced oxidation processes (AOPs) as attrac-
tive alternative treatments for the degradation of dyes in
wastewater.
Among the existing AOPs, sonochemical oxidation has received
considerable attention because of its particular efficacy toward
volatile and/or hydrophobic compounds [4,5]. The use of ultra-
sound in degradation of dyes and other pollutants has developed
in recent decades [6–11]. Sonochemical techniques involve the
use of ultrasonic waves to produce an oxidative environment via
cavitation bubbles generated during the rarefaction period of
sound waves. The formation, growth, and collapse of cavitation
bubbles, leading to high local temperatures and pressures, are con-
sidered the main mechanism through which chemical reactions oc-
cur in sonochemistry. In a homogeneous aqueous system, three
different reaction sites have been postulated: (i) the gaseous inte-
riors of collapsing cavities where both temperature and pressure
are extremely high (up to and above 5000 K and 1000 atm, respec-
tively), resulting in dissociation of chemical compounds including
water [4,5,12]; (ii) the interfacial liquid region between cavitation
1350-4177/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2008.12.005
* Corresponding author. Tel.: +213 771 598 509.
E-mail addresses: ohamdaoui@yahoo.fr, oualid.hamdaoui@univ-annaba.org
(O. Hamdaoui).
Ultrasonics Sonochemistry 16 (2009) 455–461
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultsonch
bubbles and the bulk solution where high temperature (ca. 1000–
2000 K) and high temperature gradients exist [4,5]; (iii) the bulk
solution region at ambient temperature where small amounts of
OH radicals diffusing from the interface may contribute to oxida-
tion and organic contaminant destruction reactions. Therefore,
OH and
OOH radicals can be generated from water and oxygen
dissociation through the reactions (1)–(5) [4,5].
OH and
OOH rad-
icals combine to produce hydrogen peroxide that is released in the
medium (reactions (6) and (7)).
H
2
O ! H
þ
OH ð1Þ
O
2
! 2O ð2Þ
H
þ O
2
!
OOH ð3Þ
O þ H
2
O ! 2
OH ð4Þ
H
þ O
2
!
OH þ O ð5Þ
2
OH ! H
2
O
2
ð6Þ
2
OOH ! H
2
O
2
þ O
2
ð7Þ
Polar organic compounds are degraded by ultrasonic irradiation
to a much lower extent than volatile and hydrophobic substrates
[13]. It was reported that in the view point of practical application,
the present ultrasonic degradation rates need to be increased by at
least 10–100 times in order to be energy efficient, especially for the
non-volatile compounds, such as dyes [14]. Many studies have
demonstrated that the addition of chemicals, such as salt
[15–18], chloroalkanes [19] and carbon tetrachloride [20–30],in
the presence of ultrasonic waves increase the decomposition effi-
ciency and reduce the time required for removing the pollutants.
However, the use of CCl
4
for intensification of oxidation reactions
has been investigated using low frequency ultrasonic reactors.
Additionally, no work, to our knowledge, exists on the effect of
CCl
4
on the degradation of dyes using high frequency (1700 kHz)
ultrasonic waves. This frequency, reputed ineffective, was tested
in order to introduce the ultrasound waves with high frequency
in the field of degradation or removal of dyes from wastewater,
due to its limited use in this field, and to increase the application
of high frequency ultrasound wave in the field of environmental
protection.
In the present work, the effect of carbon tetrachloride adding on
the ultrasonic degradation in aqueous solution of the anthraqui-
nonic dye Acid Blue 25 (AB25) was investigated. Anthraquinonic
dyes represent the second most important class of commercial
dyes after azo-compounds and are mainly used for dyeing wool,
polyamide and leather. To optimize the kinetic of the degradation
process and to clarify the bleaching mechanism, several parame-
ters such as the concentration of CCl
4
, frequency, pH, temperature
and tert-butyl alcohol addition were investigated. Additionally, the
sonochemical efficiency in the presence of carbon tetrachloride
was evaluated using standard methods such as Fricke reaction, KI
oxidation and H
2
O
2
production.
2. Experimental
2.1. Reagents
Acid Blue 25 (abbreviation: AB25; C.I. number: 62055; chemical
class: anthraquinone; molecular formula: C
20
H
13
N
2
NaO
5
S) was
used as a model solute. Acid Blue 25 [1-amino-9,10-dihydro-
9,10-dioxo-4-(phenylamino)-2-anthracenesulfonic acid, monoso-
dium salt] (dye content 45%, molecular weight: 416.39 g mol
1
)
was purchased from Aldrich and was used as received. The molec-
ular structure of Acid Blue 25 (C.I. 62055) is shown in Fig. 1.
High purity carbon tetrachloride (99.9%) was purchased from
Sigma–Aldrich. Adequate amount of CCl
4
was added to dye solu-
tion in order to obtain the desired concentration and the mixture
was stirred vigorously using a magnetic stirring bar.
All reagents used in the present study were purchased among
the products of high purity (analytical grade).
Aqueous solutions of AB25 were prepared by dissolving the re-
quired amount in distilled water.
2.2. Reactor
Experiments were conducted in a 215 mL cylindrical water-
jacketed glass reactor (Fig. 2). Ultrasonic waves (1700 kHz) were
emitted from the bottom of the reactor through a piezo-electric
disc (diameter, 2 cm). Acoustic power dissipated in the reactor
(14 W) was measured using standard calorimetric method [31].
2.3. Procedure
Carbon tetrachloride is a compound with low solubility in water
(solubility 1200 mg L
1
at 25 °C [32]) and high vapor pressure
(114 mm Hg at 25 °C [32]). Consequently, an appropriate experi-
O
O
HN
NH
2
ONa
S
O
O
Fig. 1. Chemical structure of Acid blue 25 (AB25).
Coolant inlet
Transducer
Coolant outlet
Sampling port
Thermometer
XX °C
Geyser
Fig. 2. Scheme of the sonochemical reactor used for AB25 decolorization.
456 H. Ghodbane, O. Hamdaoui / Ultrasonics Sonochemistry 16 (2009) 455–461
mental procedure has been developed to ensure homogeneous
solution and to minimize evaporative losses.
Various solutions containing carbon tetrachloride was prepared
by adding the required amount of this agent and stirring overnight
using a magnetic stirring bar.
Ultrasonic decolorization of AB25 was carried out under iso-
thermal conditions using a constant solution volume of 100 mL.
Aqueous samples were taken from the solution and the concentra-
tions were analyzed. The concentrations of AB25 in the solution
before and after sonochemical bleaching were determined using
a UV–visible spectrophotometer (Jenway 6405) at 602 nm.
Sonochemical experiments involving oxidizing species genera-
tion were performed in the batch mode by sonicating 100 mL of
aqueous solution (distilled water, KI solution or Fricke solution)
in the absence and presence of CCl
4
using the same reactor used
for dye decolorization. The temperature was monitored continu-
ously and maintained at 20 °C.
Potassium iodide solution (0.1 M) was sonicated in the absence
and presence of CCl
4
. The absorbance of triiodide (I
3
) at 352 nm
(the molar absorptivity
e
= 26,000 L mol
1
cm
1
) was measured
with a UV–visible spectrophotometer.
Fricke solution was prepared by dissolving FeSO
4
(NH
4
)
2-
SO
4
6H
2
O (10
3
M), H
2
SO
4
(0.4 M) and NaCl (10
3
M) in water.
The obtained solution was sonicated in the absence and presence
of CCl
4
. The absorbance of Fe
3+
at 304 nm (the molar absorptivity
e
= 2197 L mol
1
cm
1
) was measured by using a UV–visible
spectrophotometer.
Hydrogen peroxide concentrations obtained in the absence and
presence of CCl
4
were determined using the iodometric method
[33]. The iodide ion (I
) reacts with H
2
O
2
to form the triiodide
ion (I
3
) that absorbs strongly at 352 nm (
e
= 26,000 L mol
1
cm
1
). Sample aliquots taken from the reactor were added in the
quartz cuvette of the spectrophotometer containing potassium io-
dide (0.1 M) and ammonium heptamolybdate (0.01 M). The mixed
solutions were allowed to stand for 5 min before absorbance was
measured.
All experiments were conducted in triplicate and the mean val-
ues were reported.
3. Results and discussion
3.1. Effect of CCl
4
adding on AB25 decolorization
Sonochemical decolorization of AB25 (50 mg L
1
) without and
with the addition of various concentrations of CCl
4
was shown in
Fig. 3. It was observed that the bleaching of AB25 solutions was sig-
nificantly improved by the addition of CCl
4
. Increasing of CCl
4
con-
centration will result in increase of ultrasonic decolorization. This
enhancement is due to the degradation of CCl
4
by pyrolytic cleav-
age in cavitation bubbles, which conducts to the release of oxidiz-
ing agents that can react with AB25 molecules. The sonolytic
degradation of CCl
4
has been studied by several research groups
[34–37]. The overall reaction mechanism can be written as [34–
37].
CCl
4
!
CCl
3
þ
Cl ð8Þ
CCl
4
!: CCl
2
þ Cl
2
ð9Þ
CCl
3
!: CCl
2
þ
Cl ð10Þ
CCl
3
þ
CCl
3
! CCl
4
þ : CCl
2
ð11Þ
CCl
3
þ
CCl
3
! C
2
Cl
6
ð12Þ
: CCl
2
þ : CCl
2
! C
2
Cl
4
ð13Þ
Cl þ
Cl ! Cl
2
ð14Þ
Cl
2
þ H
2
O ! HClO þ HCl ð15Þ
Pyrolysis reactions of CCl
4
and H
2
O (reaction (1)) proceed in the
hot cavitation bubbles. The carbon–chloride bond in CCl
4
was pref-
erentially broken at high temperature to produce large amount of
Cl radicals compared to the hydrogen–oxygen bond in H
2
O due to
the fact that the carbon–chloride bond strength in CCl
4
is 73
kcal mol
1
and the hydrogen–oxygen bond energy in H
2
Ois
119 kcal mol
1
[36]. The formation of
Cl radicals will lead to a ser-
ies of recombination reactions conducting to the formation of addi-
tional active species, such as HClO, Cl
2
and chlorine-containing
radicals (
Cl,
CCl
3
and :CCl
2
), having strong oxidizing property,
which will markedly accelerate the decolorization of AB25 in aque-
ous solution.
In this study, the pH of AB25 solution (50 mg L
1
) dropped from
the initial value of 5.7 to a final of 2.8 after 40 min of ultrasonic
irradiation in the presence of 399 mg L
1
of CCl
4
. This was partially
attributed to the formation of HClO and HCl during the bleaching
process. The same phenomenon in the initial pH decrease during
sonolytic decolorization experiment was observed for all the used
CCl
4
concentrations. Additionally, the drop in solution pH is signif-
icant as the CCl
4
concentration increased due to the enhancement
of HClO and HCl production.
During this study, different runs were compared using the ini-
tial decolorization rate (mg L
1
min
1
), rather than the pseudo-
first-order kinetic constant. For an initial dye concentration of
50 mg L
1
, the initial rate of decolorization increased significantly
from 0.1467 without CCl
4
to 15.609 and 17.325 mg L
1
min
1
in
the presence of 399 and 798 mg L
1
of CCl
4
, respectively. That is,
the addition of 798 mg L
1
of CCl
4
results in a 118-fold increase
of the rate of AB25 bleaching. The decolorization rate in the pres-
ence of 399 mg L
1
of CC1
4
was 106 times greater than that calcu-
lated in the absence of CCl
4
.
CCl
4
concentrations ranging from 100 to 798 mg L
1
were used
because the objective of this work is to study the effect of CCl
4
add-
ing on the sonochemical decolorization of AB25 using 1700 kHz
ultrasonic irradiation which is reputed as ineffective. Further stud-
ies are needed to optimize the concentration of CCl
4
and in order to
prevent residual amount of CCl
4
in the final discharge effluent
stream. Additionally, the losses of CCl
4
by evaporation will be in-
creased with the increase of its concentration, which is also against
the use of an over-high concentration.
In the present work, the decolorization rate for an initial AB25
concentration of 50 mg L
1
was enhanced 18 times by the addition
0
0.2
0.4
0.6
0.8
1
010203040506070
Time (min)
C/C
0
Without 100 mg/L 160 mg/L
239 mg/L 399 mg/L 798 mg/L
Fig. 3. Sonochemical decolorization of AB25 (C
0
= 50 mg/L) without and with the
addition of different concentrations of CCl
4
(conditions: volume: 100 mL; natural
pH (5.7); temperature: 20 ± 1 °C; frequency: 1700 kHz; power: 14 W).
H. Ghodbane, O. Hamdaoui / Ultrasonics Sonochemistry 16 (2009) 455–461
457
of 100 mg L
1
of CCl
4
, 36 times in the presence of 160 mg L
1
of
CCl
4
and 48 times by the addition of 239 mg L
1
of CCl
4
. This
finding is similar to that made in previous work on methyl orange
decolorization using horn type 20 kHz sonicator [25], which re-
ported that the rate of sonochemical decomposition of methyl or-
ange was enhanced more than 100 times by adding CCl
4
into the
dye solution. Okitsu et al. [23] have indicated that the decomposi-
tion ratio became 4.8 times larger by the addition of 100 ppm of
CCl
4
and 8.9 times larger by the addition of 150 ppm of CCl
4
and
11 times larger by the addition of 200 ppm of CCl
4
and 14 times
larger by the addition of 250 ppm of CCl
4
, respectively. The low
enhancement effect in the work of Okitsu et al. [23] may be attrib-
uted to different geometric and experimental conditions. Using a
high intensity ultrasonic irradiation (frequency: 200 kHz, calori-
metric power: 80 W), Okitsu et al. [23] reported that the decompo-
sition ratio became 41 times larger by the addition of 100 ppm of
CCl
4
and 100 times larger by the addition of 150 ppm of CCl
4
,
respectively. A direct comparison of literature data obtained using
CCl
4
for the enhancement of sonochemical decomposition is not
possible since experimental conditions are not the same.
3.2. Effect of CCl
4
on KI oxidation, Fricke reaction and H
2
O
2
production
The effect of different CCl
4
concentrations raging from 100 to
798 mg L
1
on the oxidation of 0.1 M KI solution is shown in
Fig. 4 in terms of the amount of I
3
formed. The rate of I
3
pro-
duced without addition of CCl
4
is 1.04
l
M min
1
, whereas those
in the presence of 100, 160, 239, 399 and 798 mg L
1
of CCl
4
are
respectively 12.96, 18.42, 27.68, 35.76 and 38.64
l
M min
1
. The
rates of I
3
produced in the presence of CCl
4
are approximately
12.4 to 37.1 times larger when compared with that formed in the
absence of CCl
4
. This behavior is analogous to that made in previ-
ous works on the production of I
3
in the presence of CCl
4
[23]. Re-
cently, Zhou et al. [29] have investigated the ultrasonic oxidation of
iodide in the presence of CCl
4
. They indicated that the ultrasonic
oxidation of iodide was found to be significantly promoted by
the addition of CCl
4
. Sonochemical reactions taking place in the
presence of CCl
4
and KI have been detailed by Rajan et al. [38].
Fig. 5 shows the results of Fricke dosimeter obtained in the ab-
sence and presence of 399 mg L
1
of CCl
4
. The production of Fe
3+
was significantly enhanced with the addition of CCl
4
. The rate of
Fe
3+
formed is 2.64 and 50.15
l
M min
1
in the absence and pres-
ence of CCl
4
respectively. In the presence of CCl
4
, the rate of Fe
3+
formed was calculated for the first 7 min (linear portion of the
curve). The formation rate of Fe
3+
becomes 19 times larger by the
addition of 399 mg L
1
of CCl
4
. After 30 min of sonication,
the yields of Fe
3+
in Fricke reaction are 83.7 and 744.2
l
M in the
absence and presence of 399 mg L
1
of CCl
4
, respectively.
The concentration of H
2
O
2
generated in water during sonication
was measured in the absence and presence of 399 mg L
1
of CCl
4
by using the potassium iodide method [33]. In this method, the tri-
iodide concentration is equivalent to the H
2
O
2
concentration. Fig. 6
shows the concentration of triiodide ion as a function of sonication
time. More triiodide ion was formed in the presence of CCl
4
than in
its absence. However, the increase of triiodide concentration may
be a combined result of the increase of both the formation of
H
2
O
2
and other oxidizing species produced during degradation of
CCl
4
. The same finding has been reported by Wang et al. [25].
The rate of triiodide formation in the absence of CCl
4
was
0.7789
l
M min
1
and in the presence of 399 mg L
1
of CCl
4
was
26.9810
l
M min
1
. It means that the triiodide concentration be-
came 35 times larger by the addition of 399 mg L
1
of CCl
4
. Using
the p-hydroxyphenyl acetic acid dimerization method, Zheng
et al. [27] have indicated that more hydrogen peroxide was pro-
duced during the early stages in the presence of 150
l
M of CCl
4
,
but at later times sonication with and without CCl
4
showed similar
hydrogen peroxide concentrations. These results confirmed that
the enhancement of triiodide concentration is due to a combined
contribution of H
2
O
2
and oxidizing species formed during CCl
4
sonolysis.
The three investigated methods (KI oxidation, Fricke reaction
and H
2
O
2
production) well corroborate the improvement of the
sonochemical effects in the presence of CCl
4
. Additional oxidizing
0
20
40
60
80
100
0 5 10 15 20 25 30
Time (min)
Triiodide concentration (µM)
Without
100 mg/L
160 mg/L
239 mg/L
399 mg/L
798 mg/L
Fig. 4. Iodide dosimeter in the absence and presence of different concentrations of
CCl
4
(conditions: volume: 100 mL; KI concentration: 0.1 M; natural pH; tempera-
ture: 20 ± 1 °C; frequency: 1700 kHz; power: 14 W).
0
100
200
300
400
500
600
700
800
0 5 10 15 20 25 30 35
Time (min)
Fe
3+
concentration (µM)
Without
399 mg/L
Fig. 5. Fricke dosimeter in the absence and presence of 399 mg L
1
of CCl
4
(conditions: volume: 100 mL; temperature: 20 ± 1 °C; frequency: 1700 kHz; power:
14 W).
0
50
100
150
200
250
300
350
400
450
500
0 5 10 15 20 25 30 35
Time (min)
Triiodide concentration (µM)
Without
399 mg/L
Fig. 6. Amount of triiodide ions formed in the absence and presence of 399 mg L
1
of CCl
4
using the iodometric method (conditions: volume: 100 mL; temperature:
20 ± 1 °C; frequency: 1700 kHz; power: 14 W).
458 H. Ghodbane, O. Hamdaoui / Ultrasonics Sonochemistry 16 (2009) 455–461
species in the liquid phase produced during CCl
4
sonolysis are
highly beneficial for oxidation of non-volatile solute species which
are less susceptible to free radical attack.
3.3. Comparison of AB25 decolorization at two frequencies: 22.5 and
1700 kHz
Most of the previous works carried out using CCl
4
for intensifi-
cation of oxidation reactions have been investigated using low fre-
quency ultrasonic reactors [19–22,24–27]. The decolorization of
50 mg L
1
AB25 solution in the absence and presence of 399 mg
L
1
of CCl
4
was investigated at two different frequencies: 22.5
and 1700 kHz. The 22.5 kHz ultrasonic irradiation was carried out
with a commercial supply Microson XL 2000 equipped with a tita-
nium horn (6 mm diameter) mounted at the top of the cylindrical
glass vessel.
Frequency is an important parameter in sonochemical oxida-
tion. It can affect cavitation in several ways: by modifying bubble
number, bubble size, cavitation threshold and the temperatures
reached during the collapse. The sonochemical decolorization of
AB25 both in the absence and presence of CCl
4
is shown in Fig. 7.
From this figure, it is clearly shown that the best decolorization
is obtained at 1700 kHz both without and with the addition of
CCl
4
. For both frequencies (22.5 and 1700 kHz), a significant
enhancement of the bleaching of AB25 solutions was obtained in
the presence of 399 mg L
1
of CCl
4
. At 22.5 kHz, the initial decolor-
ization rate increased significantly from 0.0418 mg L
1
min
1
in
the absence of CCl
4
to 3.6555 mg L
1
min
1
in the presence of
399 mg L
1
of CCl
4
. Thus, the addition of CCl
4
(399 mg L
1
) results
in a 87-fold increase of the initial decolorization rate. The best
sonochemical decolorization rate of AB25 in aqueous solution is
observed to occur at 1700 kHz. This is because the number of
acoustic cycles and the number of cavitation collapses increases
at high frequency leading to the increase of CCl
4
degradation and
therefore to the formation of more oxidizing species, which en-
hance AB25 decolorization. This result is consistent with the work
of Pétrier and Francony [37] focusing on elimination of CCl
4
that
reports a better degradation yield at high frequency (800 kHz).
3.4. Effect of pH
The influence of solution pH on the sonochemical decoloriza-
tion of 50 mg L
1
AB25 aqueous solution was investigated in the
presence of 399 mg L
1
of CCl
4
. Fig. 8 illustrates the initial decolor-
ization rate as a function of initial pH. The decolorization rates
were nearly unchangeable (15.614 and 15.609 mg L
1
min
1
)in
the pH range 3.2–5.7 and decrease in the pH range 7–10.5. The ini-
tial decolorization rates were decreased significantly at pH values
of 1 (11.964 mg L
1
min
1
) and 11.8 (3.2358 mg L
1
min
1
).
The absorbance data of dye solution determined spectrophoto-
metrically indicated that change of the initial pH of dye solution
has no effect on the k
max
of AB25 (pH 1–11.8). Based on this obser-
vation and in order to explain the obtained results in the presence
of CCl
4
, the influence of pH on the sonochemical decolorization of
AB25 in the absence of CCl
4
was shown in Fig. 9. It was found that
the rate of AB25 decolorization in the absence of CCl
4
was strongly
pH dependent. The initial bleaching rates in acidic solutions (1–3)
are higher, especially at pH 1, and decrease from pH 1 to 5, and
there is almost no change in the pH range of 5–8. Higher decolor-
ization rates are observed in basic media (9.3–11.8). AB25 is a non-
volatile compound and the region of decolorization would be at the
exterior of the cavitation bubbles. It is possible that the change in
the solution pH results in the change of hydrophobic property of
the dye, which affects the ultrasonic decolorization. The accelera-
tion of decolorization in acidic conditions is probably associated
with the effect of protonation of negatively charged SO
3
group in
acidic medium and, obviously, the hydrophobic character of the
resulting molecule enhances its reactivity under ultrasound treat-
ment because AB25 is accumulated in the interface of the cavita-
tion bubbles. Moreover, in acidic conditions (pH 1–3), the
recombination of
OH radicals (reaction (6)) is less effective before
react against AB25 concentrated in the interface. In the pH range
0
0.2
0.4
0.6
0.8
1
0 5 10 15 20 25 30 35 40 45
Time (min)
C/C
0
1700 kHz (without)
1700 kHz (399 mg/L)
22.5 kHz (without)
22.5 kHz (399 mg/L)
Fig. 7. Sonochemical decolorization of AB25 (C
0
=50mgL
1
) in the absence and
presence of 399 mg L
1
of CCl
4
at two frequencies: 22.5 and 1700 kHz (conditions:
volume: 100 mL; natural pH (5.7); temperature: 20 ± 1 °C; power: 14 W).
0
2
4
6
8
10
12
14
16
1 3.2 5.7 7 9.3 10.5 11.8
pH
Initial decolorization rate (mg/L min)
Fig. 8. Initial decolorization rate at various pHs in the presence of 399 mg L
1
of
CCl
4
(conditions: volume: 100 mL; initial dye concentration: 50 mg L
1
; tempera-
ture: 20 ± 1 °C; frequency: 1700 kHz; power: 14 W).
0
0.05
0.1
0.15
0.2
0.25
0.3
1 3 5 7 8 9.3 10.6 11.8
pH
Initial decolorization rate (mg/L min)
Fig. 9. Initial decolorization rate at various pHs in the absence of CCl
4
(conditions:
volume: 100 mL; temperature: 20 ± 1 °C; initial dye concentration: 50 mg L
1
;
frequency: 1700 kHz; power: 14 W).
H. Ghodbane, O. Hamdaoui / Ultrasonics Sonochemistry 16 (2009) 455–461
459
5–8, AB25 reaches the ionized state, and its hydrophilicity and sol-
ubility are enhanced, and thus the decolorization is carried out in
the bulk of the solution where there is a lower concentration of
OH
because only about 10% of the
OH generated in the bubble can dif-
fuse into the bulk solution [14]. Additionally, the bleaching rate de-
crease at pH 5 and 8 due to the fact that a higher number of
OH
species recombine to H
2
O
2
. The enhancement of decolorization
rate at basic conditions may be caused by the change of hydropho-
bic property of the dye. The same results were obtained for the
decolorization of reactive brilliant red by 20 kHz ultrasonic irradi-
ation [39].
In the presence of CCl
4
, the initial decolorization rates in the pH
range 3.2–9 are higher than that obtained at pH 1. At pH 1, AB25 is
accumulated in the interfacial region of the cavitating bubbles and
decreases the degradation of CCl
4
, which reduces the amount of
the oxidizing agents produced by the oxidation of CCl
4
and there-
fore the AB25 decolorization. At pH 3.2–9.3, the hydrophilicity of
AB25 is improved and the bleaching is carried out in the bulk solu-
tion. Another reason may be that at acidic pH, the chlorine is pres-
ent in the solution in the form of hypochlorous acid, which has a
higher oxidation potential (1.49 V) than hypochlorite (0.94 V)
[40–42]. The lower decolorization rates in basic conditions, espe-
cially at pH 11.8, seem to be due to the enhancement of AB25
hydrophobicity as well as to the decreased production of chlo-
rine/hypochlorite at higher pH conditions, because of the forma-
tion of chlorate or perchlorate [40–42], which react more slowly
with dye molecules. The hypochlorite is prevalent in alkaline con-
dition [40–42]. It seems that hypochlorous acid (HClO) is the key
reactive intermediate in the sonochemical decolorization of AB25
in the presence of CCl
4
.
3.5. Effect of temperature
The effects of aqueous temperature on the sonochemical decol-
orization rate were investigated in the presence of 399 mg L
1
of
CCl
4
. The initial rates of AB25 decolorization obtained for sonolysis
of a 50 mg L
1
solution at temperatures of 20, 30, 40 and 50 °C are
15.609, 14.087, 13.979 and 11.348 mg L
1
min
1
, respectively. The
rate of AB25 sonochemical decolorization in the presence of CCl
4
decreases with increasing temperature between 20 and 50 °C. This
is due to the increased solvent vapor pressure inside the bubble;
increasing solvent vapor pressure attenuates the efficacy of cavita-
tional collapse, the maximum temperature reached during such
collapse, and, consequently, the rates of cavitational reactions.
3.6. Effect of tert-butyl alcohol
The influence of tert-butyl alcohol on the sonochemical decolor-
ization of dye in the presence of 399 mg L
1
of CCl
4
was investi-
gated (Fig. 10). In our experiments, varying concentrations of
tert-butyl alcohol (399, 798 and 1570 mg L
1
) were added to
50 mg L
1
AB25 solutions. The obtained results indicated that the
addition of 399, 798 and 1570 mg L
1
of tert-butyl alcohol de-
creased the initial decolorization rate from 15.609 mg L
1
min
1
in the absence of tert-butanol to 6.5236, 1.7794 and 1.2742
mg L
1
min
1
, respectively. The degradation was effectively
quenched, but not completely, by the addition of tert-butyl alcohol.
Because tert-butyl alchol molecules pass in the cavitation bubbles,
they are able to scavenge oxidizing species in the bubble and re-
duce significantly the decolorization rate of dye. Another factor
that also affects the rate of AB25 bleaching is the formation of vol-
atile products from the tert-butyl alcohol degradation that accu-
mulate inside the bubble and react with oxidizing species,
conducting to the deceleration of the decolorization of dye. Addi-
tionally, Pétrier and Francony [37] have demonstrated that addi-
tion of an excess of 1-butanol (10-fold CCl
4
initial concentration)
does not affect rates of carbon tetrachloride destruction. It seems
that tert-butyl alcohol and its degradation products inhibited the
bleaching of AB25 by scavenging active species, such as HClO, Cl
2
and chlorine-containing radicals (
Cl,
CCl
3
and :CCl
2
), having strong
oxidizing properties which will markedly inhibit the decolorization
of AB25 in aqueous solution. The increase of the tert-butyl alcohol
concentration leads to the decrease of the decolorization rate. The
bleaching rate with respect to that obtained in the absence of tert-
butyl alcohol was decreased 58% in the presence of 399 mg L
1
of
tert-butanol, 89% in the presence of 798 mg L
1
of tert-butanol
and 92% in the presence of 1570 mg L
1
of tert-butanol. The ob-
served differences can be due to the increase of the accumulation
of tert-butyl alcohol and its degradation products in the cavitation
bubbles with increasing alcohol concentration, conducting to more
scavenging effect. Regardless of the absence and presence of tert-
butyl alcohol, the pH of the reaction matrix decreased from the ini-
tial value of 5.7 to a final of 2.9 after 30 min of ultrasonic irradia-
tion, which was attributed to the formation of HClO and HCl.
From these results, we can infer that AB25 sonochemical decolor-
ization in the presence of CCl
4
is mainly due to the reaction with
chlorine-containing radicals and other oxidizing species such as
HClO and Cl
2
.
4. Conclusion
The sonochemical decolorization of AB25 in aqueous phase
using 1700 kHz ultrasound was significantly enhanced and im-
proved in the presence of CCl
4
. The effects of some operational
parameters on the bleaching of AB25 were discussed and found
that the decolorization rate is strongly dependent on the concen-
tration of CCl
4
, the temperature and the solution pH. It seems that
hypochlorous acid (HClO) is the key reactive intermediate in the
sonochemical decolorization of AB25 in the presence of CCl
4
. The
used sonochemistry dosimeters well corroborate the enhancement
of the sonochemical effects in the presence of CCl
4
. The best sono-
chemical decolorization rate of AB25 in aqueous solution is
observed to occur at 1700 kHz compared to 22.5 kHz. The degrada-
tion of dye in the presence of CCl
4
was effectively quenched, but
not completely, by the addition of tert-butyl alcohol.
High frequency ultrasonic waves (1700 kHz) used in the pres-
ence of CCl
4
was shown to be of interest for the treatment of
wastewaters contaminated with anthraquinonic dyes. The sono-
chemical process in the presence of CCl
4
may be utilized for the
decomposition of hydrophilic and non-volatile pollutants in water
because they are less susceptible to free radical attack due to lower
stability of the generated free radicals.
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30 35 40
Time (min)
C/C
0
Without tert-butanol
399 mg/L tert-butanol
798 mg/L tert-butanol
1570 mg/L tert-butanol
Fig. 10. Effect of tert-butyl alcohol on the sonochemical decolorization of AB25
(C
0
=50mgL
1
) in the presence of 399 mg L
1
of CCl
4
(conditions: volume: 100 mL;
natural pH (5.7); temperature: 20 ± 1 °C; power: 14 W).
460 H. Ghodbane, O. Hamdaoui / Ultrasonics Sonochemistry 16 (2009) 455–461
This work has discussed the sonolytic degradation of only one
acid anthraquinone dye (Acid Blue 25) but this method could be
used for the degradation of other acid anthraquinone dyes such
as Acid Blue 40, 41, 51, 53, 129 and 230 that have very similar
chemical structures with Acid Blue 25.
Further studies are needed to optimize the concentration of
CCl
4
and in order to prevent residual amount of CCl
4
in the final
discharge effluent stream.
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