Sonocatalytic degradation of a textile dye over Gd-doped ZnO nanoparticles synthesized through sonochemical process

Article abstract of DOI:10.1016/j.ultsonch.2014.08.023

The present study was performed to sonochemically synthesize Gd_xZn_1-xO (x = 0-0.1) nanoparticles for sonocatalysis of Acid Orange 7 (AO7) in an aqueous medium. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM

Full text of DOI:10.1016/j.ultsonch.2014.08.023

Accepted Manuscript
Sonocatalytic degradation of a textile dye over Gd-doped ZnO nanoparticles
synthesized through sonochemical process
Alireza Khataee, Reza Darvishi Cheshmeh Soltani, Atefeh Karimi, Sang Woo
Joo
PII:
S1350-4177(14)00293-4
DOI:
http://dx.doi.org/10.1016/j.ultsonch.2014.08.023
ULTSON 2692
Reference:
To appear in:
Ultrasonics Sonochemistry
Received Date:
Revised Date:
Accepted Date:
9 July 2014
26 August 2014
27 August 2014
Please cite this article as: A. Khataee, R.D.C. Soltani, A. Karimi, S.W. Joo, Sonocatalytic degradation of a textile
dye over Gd-doped ZnO nanoparticles synthesized through sonochemical process, Ultrasonics Sonochemistry
(
2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.08.023
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Sonocatalytic degradation of a textile dye over Gd-doped ZnO nanoparticles
synthesized through sonochemical process
a,
b
a
c, **
Alireza Khataee *, Reza Darvishi Cheshmeh Soltani , Atefeh Karimi , Sang Woo Joo
a
Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied
Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
b
Department of Environmental Health Engineering, School of Health, Arak University of Medical
Sciences, Arak, Iran
c
School of Mechanical Engineering, Yeungnam University, Gyeongsan 712-749 South Korea
*
Corresponding author (communicator)
E–mail address: a_khataee@tabrizu.ac.ir (ar_khataee@yahoo.com)
Tel.: +98 411 3393165; Fax: +98 411 3340191
*
*
Corresponding author
E–mail address: swjoo@yu.ac.kr
Tel: +82 53 810 1456
1

Abstract
The present study was performed to sonochemically synthesize Gd Zn1-xO (x = 0–0.1)
x
nanoparticles for sonocatalysis of Acid Orange 7 (AO7) in an aqueous medium. The results of X-
ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared
spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) analysis confirmed proper
synthesis of Gd-doped sonocatalyst. 5% Gd-doped ZnO nanoparticles with band gap of 2.8 eV
exhibited the highest sonocatalytic decolorization efficiency of 90% at reaction time of 90 min.
The effects of initial dye concentration and sonocatalyst dosage on decolorization efficiency
were evaluated. In the presence of sodium sulfate, sodium carbonate and sodium chloride the
decolorization efficiency decreased from 90 to 78, 65 and 56%, respectively. Among various
enhancers, the addition of potassium periodate improved the decolorization efficiency from 90 to
1
00%. The highest decolorization efficiency was obtained at pH value of 6.34 (90%). The
decolorization efficiency decreased only 6 % after 4 repeated runs. Therefore, Gd-doped ZnO
nanoparticles can be used as a promising catalyst for degradation of organic pollutants with great
reusability potential.
Keywords: Sonocatalysis; Nanocatalyst; Acid Orange 7; Nano-sonocatalyst; Doped ZnO.
2

1
. Introduction
There are many industries discharging organic dyes such as textile, leather, plastic, paper, rubber,
dyestuff, pharmaceuticals, cosmetic and food processing [1-3]. The presence of organic dyes in
aqueous environments can cause detrimental effects on aquatic life and human health [1, 4]. Azo
dyes, which are characterized by the presence of one or more azo bonds (–N=N–) associated
with one or more aromatic systems, are identified as the carcinogenic substances [1, 2].
Additionally, the presence of azo dyes in aqueous environments can hinder light penetration and
subsequently photosynthesis [5]. Due to the adverse effects of discharged organic dyes on human
health, several treatment technologies have been developed for treating colored effluents,
including biological processes, adsorption, electrochemical process, membrane technology,
chemical oxidation and advanced oxidation processes (AOPs) [1, 4, 6-8]. In recent decade, the
application of sonochemical process by using ultrasound as an advanced oxidation process has
gained more attention for the purification of polluted effluents because of its high efficiency and
easy operation [9-11]. During sonochemical process, ultrasound waves cause rapid growth and
cavitational collapse of the bubbles within the solution, which produces extremely high
temperature and pressure in the bubble. Consequently, high temperature near the bubbles leads to
•
thermal dissociation of water, resulting in the generation of hydroxyl radicals (OH ) as one of the
most powerful oxidants for non-selective oxidation of target organic pollutanst [10-14].
Nevertheless, complete mineralization of the target pollutants via sonolysis alone requires large
amounts of energy and higher reaction time. To solve this problem, ultrasonic irradiation can be
performed in the presence of an appropriate semiconductor as sonocatalyst to accelerate the
reaction [2, 11, 15, 16]. Sonocatalytic degradation is a novel technique of treating contaminated
water [12]. Among various catalysts, the application of ZnO nanoparticles has attracted much
3

more attention due to its exceptional physical, chemical and optical properties such as high UV
absorption potential, wide band gap (3.37 eV) and large exciton binding energy (60 meV) [4, 17,
1
8]. But the application of pure ZnO nanoparticles as sonocatalyst contains some drawbacks.
The main limitation of the application of pure ZnO nanoparticles is the fast recombination rate of
the generated electron–hole pairs. Therefore, it is essential to reduce electron–hole
recombination rate for improvement of the catalytic activity. Recent experimental results have
shown that incorporation of suitable metal ions such as rare-earth metals (REMs) into the
structure of pure catalyst is a practical approach to enhance its catalytic activity [11, 18-21].
Therefore, in the present study, the application of gadolinium (Gd), as a silvery-white, malleable
and ductile rare-earth metal, was considered as doping element to improve catalytic activity of
ZnO nanomaterials for the sonocatalysis of Acid Orange 7 (AO7). To the best of our knowledge
and based on the literature review, there is no report on the use of Gd-doped ZnO nanomaterials
prepared from a simple sonochemical method as sonocatalyst for sonocatalytic degradation of a
textile dye. The efficiency of the process was evaluated under different operational parameters
and GC-MS analysis was also performed to identify the intermediates produced during
sonocatalysis of AO7.
2
. Materials and methods
2
.1. Chemicals
All chemicals, which were of analytical grade, were purchased from Merck, Germany, except for
Gd(NO ·6H O, powder and C OH (99%) which were purchased from Sigma Aldrich, USA
and Zn(CH COO) .2H O (99%) which was purchased from Carlo Erba Reagenti, India. Acid
3
)
3
2
2
H
5
3
2
2
4

Orange 7 as an azo dye was provided by Shimi Boyakhsaz Co., Iran. The specifications of AO7
are given in Table 1.
2
.2. Synthesis of ZnO and Gd-doped ZnO nanoparticles
ZnO and Gd-doped ZnO nanostructures were synthesized using a simple sonochemical method.
It has been demonstrated that the catalysts prepared by sonochemical method exhibit higher
photocatalytic activity in comparison with the catalysts prepared by the conventional methods
[
16]. In a typical procedure, different amount of Gd(NO
3
)
3
·6H O was added to the solution of
2
zinc acetate to achieve 1, 3 and 5% Gd. 1 M NaOH solution was added to the resulted mixture
until the pH reached 10. Then, the mixture was sonicated in a bath type sonicator (Ultra 8060,
England) with a frequency of 36 KHz for 3 h. The resulting white product was washed thrice
with absolute ethanol and distilled water, and dried at 80 °C for 12 h to achieve the crystalline
structure. To synthesize undoped ZnO nanoparticles the above procedure was carried out without
the addition of Gd (NO
3
)
3
·6H O.
2
2
.3. Sonocatalysis procedure
Sonocatalytic degradation of AO7 was evaluated over undoped and Gd-doped ZnO
nanoparticles. In a typical approach, 100 mL of AO7 solution with a known initial concentration
containing 100 mg nanocatalyst was irradiated with a frequency of 36 KHz and 150 W output
power at natural pH. The experiments were carried out in dark to remove the effect of photolysis.
The samples with volume of 4 mL were withdrawn from the solution at regular time interval of
1
5 min and residual AO7 concentration was measured using a UV–Vis spectrophotometer (WPA
lightwave S2000, England) at maximum wavelength of 507 nm.
5

2
.4. Analysis of nanocatalysts
The X-ray diffraction (XRD) patterns of undoped and Gd-doped ZnO nanoparticles were
recorded by using a Siemens X-ray diffractometer (D8 Advance, Bruker, Germany), Cu Ka
radiation (l = 1.54065 Å), an accelerating voltage of 40 kV, and an emission current of 30 mA.
To evaluate surface morphology of ZnO and Gd-doped ZnO samples, scanning electron
microscopy (SEM) was performed via TESCAN microscope (Model: MIRA3, Czech Republic).
Additionally, manual microstructure distance measurement software was applied to estimate size
distribution of as-prepared nanoparticles. To examine surficial functional groups and chemical
bonds on undoped and Gd-doped ZnO nanoparticles, a Tensor Fourier transform infrared (FT–
IR) spectroscope (Model: Tensor 27, Bruker, Germany) was used in the wavenumber ranging
-1
from 400 to 4000 cm . The analysis was performed on KBr pellets. Chemical states and
compositions of samples were characterized using X-ray photoelectron spectroscopy (XPS)
through a Thermo Scientific spectrometer (Model: K-ALPHA, UK). The sample was mixed
thoroughly in double distilled water under sonication for 15 min to reach a homogeneous
mixture, and then absorbance of the mixture was recorded between 200 and 700 nm using an
UV–Vis spectrophotometer (WPA lightwave S2000, England) to determine the band gap of
undoped and Gd-doped ZnO nanoparticles. To determine generated intermediates of AO7,
during sonocatalysis process an Agilent 6890 gas chromatograph with a 30m to 0.25 mm HP-
5
MS capillary column coupled with an Agilent 5973 mass spectrometer (Canada) was applied.
6

3
. Results and discussion
3
.1. Structural analysis of synthesized nanocatalysts
The results of SEM analysis are shown in Fig. 1. As shown, undoped ZnO nanoparticles (Fig. 1
(
(
a)) are quite irregular in size and shape in comparison with that of Gd-doped ZnO nanoparticles
Fig. 1 (b)). As can be observed, in the case of Fig. 1 (b) the surface roughness seems to be lesser
with lower aggregation of synthesized nanoparticles. Irregular size of undoped ZnO
nanoparticles may be attributed to the aggregation of ZnO nanoparticles and the growth of
irregular crystalline grains during their synthesis. It can be concluded that the incorporation of
Gd into crystalline structure of ZnO nanoparticles can restrain the aggregation phenomenon and
subsequently decreases the crystalline size and increases the shape uniformity of the
nanoparticles. Apart from that, sonochemical synthesis of the catalyst causes de-aggregation of
the catalyst particles [10]. It allows more active sites for the catalytic reaction. Fig. 2 shows the
size distribution of Gd-doped ZnO nanoparticles calculated by manual microstructure distance
measurement software. As depicted, the particle size distribution plot of Gd-doped ZnO
nanoparticles shows that most of the nanoparticles are ranged between 20 and 30 nm with
frequency of 52.17%. The frequency of the particle size range of <10, 10-20 and 30-40 nm are
2
.17, 41.30 and 4.34%, respectively.
Fig. 3 shows the X-ray diffraction patterns of undoped and 1% Gd-doped ZnO nanoparticles
prepared via sonochemical method. As depicted, the main dominant peaks of undoped ZnO are
at 2θ of 31.68, 34.32, 36.13, 47.48, 56.50, 62.80, 66.54, 67.88, 69.07, 73.00 and 77.34 °,
corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202)
planes of hexagonal wurtzite ZnO according to JCPDS Card number of 36-1451, respectively. In
addition, as shown in Fig. 3, no peaks corresponding to gadolinium oxides or other crystalline
7

material are found in the recorded patterns, which indicate that gadolinium ions have
successfully replaced in ZnO lattice. However, the peaks located at (100), (002), and (101)
planes shifted to lower angles in the case of Gd-doped ZnO in comparison with undoped ZnO.
This observation proves successful incorporation of Gd ions into ZnO lattice and expansion of
ZnO lattice through doping by gadolinium. This can be also explained by the fact that the ionic
3
+
2+
radius of Gd (0.938 Å) is larger than that of Zn (0.74 Å) [22, 23].To evaluate the effect of
doping agent on the crystalline size of prepared samples, Debye–Sherrer’s equation was used as
represented in Eq. (1):
D = 0.9λ / β Cos θ
(1)
where D, λ, β and θ are the average crystalline size (Å), the wavelength of the X-ray (Cu
Kα=1.54056 Å), the full width at half maximum (FWHM) intensity of the peak (rad) and the
diffraction angle, respectively [24]. Accordingly, the average crystallite size of the undoped and
Gd-doped ZnO nanoparticles were found to be about 16.04 and 11.15 nm, respectively. Fig. 4
exhibits the results of FT-IR analysis conducted on undoped and Gd-doped ZnO nanoparticles.
-1
The band located at 573 cm is associated with Zn–O vibration. The broad peaks at 3418–3450
-1
cm is attributed to the stretching vibration of O–H in the adsorbed water. As can be seen, the
intensity of Zn–O stretching peak has been reduced in the doped samples which may be related
2
+
3+
to the substation of Zn ions by the Gd dopant in the lattice of Gd-doped ZnO [23].
XPS is a surface sensitive method that probes the electrons ejected from the surface of the
prepared samples. XPS analysis was conducted on undoped and Gd-doped ZnO nanoparticles to
demonstrate incorporation of Gd into the ZnO lattice and to identify the oxidation state of Gd in
the ZnO lattice (Fig. 5). The XPS survey spectra of undoped ZnO and 5% Gd-doped ZnO
samples are presented in Fig. 5 ((a) and (d)).
8

The energy scale was calibrated with the C 1s peak of the carbon contamination at 285.97 eV.
Location of Zn 2p and O 1s peaks in the XPS spectra of ZnO and Gd-doped ZnO samples are
provided in Table 2. Figs. 5 (b) and (e) show binding energy for the Zn2p region of undoped and
Gd-doped ZnO. In this Figure, the Zn2p3/2 and Zn2p1/2 peaks are observed at about 1023 and
1
2
046 eV which can be associated with the Zn element in ZnO and confirms the oxidation state of
for Zn in the prepared sample. Figs. 5 (c) and (f) show the XPS core level spectra of O 1s of
undoped ZnO and 3% Gd-doped ZnO samples. The asymmetric peaks centred at 532.19 and
2
-
5
31.6 eV are ascribed to the O ions in the structure of undoped and Gd-doped ZnO samples.
The shoulder observed on the high energy side of the peaks of undoped and Gd-doped ZnO
samples is as a result of the presence of specific species such as carbonate, adsorbed O and H O.
2
2
As can be seen in Fig. 5 (g), the spectrum contains a peak centred at 140.63 eV, which is not
seen in the XPS spectrum of undoped ZnO sample. The peak located at 140.63 eV demonstrates
3
+
the presence of Gd ions in the ZnO lattice [25]. The difference between the spectra of undoped
and Gd-doped ZnO samples is attributed to the electron density changes around the Zn and O
atoms via incorporation of Gd into ZnO lattice. It causes the change in the location of Zn 2p and
O 1s peaks in the XPS spectrum of Gd-doped ZnO sample in comparison with undoped ZnO.
Overall, the shift in the location of Zn 2p and O 1s peaks demonstrates that Gd has been
successfully incorporated into ZnO structure.
3
.2. Comparative study
A comparative study for degradation of AO7 was carried out and the results are shown in Fig. 6.
In this set of experiments, initial AO7 concentration, sonocatalyst dosage, dopant content and
reaction time were constant at 5 mg/L, 1 g/L, 5% and 105 min, respectively. Fig. 6 exhibits that
9

sonolysis alone, sonocatalysis over undoped ZnO nanoparticles and sonocatalysis over Gd-doped
ZnO nanoparticles are responsible for the decolorization efficiency of 27.5, 46.5 and 91.5%,
respectively. Thus, sonocatalysis using Gd-doped ZnO nanoparticles produces the highest
decolorization efficiency. The following two viewpoints, including ‘‘hot spot” and
‘‘sonoluminescence” may be approved to explain the mechanism of sonocatalytic degradation of
AO7. As the first mechanism collapse of the cavitation bubbles in water medium produces ‘‘hot
5
6
spot” with high temperature of 10 °C or 10 °C. These hot spots can prompt pyrolysis of H O
2
•
•
molecules to form hydroxyl radicals (OH ) and hydrogen radicals (H ) as exhibited in Eqs. (2)
and (3) [26, 27].
•
•
H O + Ultrasonic irradiation → OH + H
(2)
(3)
2
•
•
OH + OH → H O
2
2
Secondly, the ultrasonic irradiation can result in the formation of the light with a wide
wavelength range, exciting the sonocatalyst to act as photocatalyst for the generation of high
•
amounts of OH on the surface of the catalyst (Eq. (4)) [28]. The electron-hole pairs can react
with electron donors and acceptors adsorbed on the semiconductor surface Eqs. (5)-(7) [29].
Generated radicals can then react with organic dye molecules and enhance the decolorization
efficiency.
+
−
Undoped and Gd − doped ZnO + Ultrasonic irradiation → ZnO / Gd − doped ZnO (h + e ) (4)
−
•−
2
e + O → O
(5)
2
+
•
+
h + H O → OH + H
(6)
(7)
2
+
−
•
h
+ OH → OH
10

As displayed in Fig. 6, Gd-doped ZnO nanoparticles produced more sonocatalytic activity in
comparison with undoped ZnO nanoparticles. The incorporation of dopant agent into the pure
catalyst can result in hindering the recombination rate of produced electron-hair pairs [11, 19].
3
.3. Effect of dopant content
The effect of dopant content on the sonocatalytic degradation of AO7 was studied by varying the
amount of dopant between 1 and 10%, while initial dye concentration, sonocatalyst dosage and
reaction time were constant at 5 mg/L, 1 g/L and 90 min, respectively (Fig. 7). Fig. 7 revealed
that increasing dopant content from 1 to 5% results in increasing the decolorization efficiency
(
%) from 53 to 90.9%, respectively, while further increase in the amount of dopant to 10%
caused a significant decrease in the decolorization efficiency (%) to 48%. Enhanced
sonocatalytic activity with increasing Gd content can be interpreted by two distinct mechanisms.
Firstly, the incorporation of Gd into ZnO lattice can change absorbance properties of the catalyst.
According to this mechanism, named as sonoluminescence mechanism, band gap of the
sonocatalyst plays a significant role in the sonocatalysis. For this, band gap of undoped and Gd-
doped ZnO nanoparticles containing different amounts of dopant agent was measured via Eq. (8)
[
30]:
2
(
Ahυ ) = K(hυ −E )
(8)
g
where hν is the photon energy (eV), A is the absorption coefficient, K is a constant and E
g
is the
2
band gap. To determine band gap energy, (Ahν) was plotted against hν (Fig. 8). The value of hν
(
eV) can be computed through Eq. (9):
1
240
hυ =
(9)
λ
11

2
By extrapolating the linear region in a plot of (Ahν) versus photon energy, the band gap of
undoped ZnO and Gd-doped ZnO nanoparticles can be determined (Table 3). As given in Table
3
, the values of band gap for undoped, 1%, 3% and 5% Gd-doped ZnO nanoparticles were
obtained to be 3.2, 3.0, 2.9 and 2.8 eV, respectively. Secondly, the presence of higher amounts of
Gd ions than the optimal value in ZnO structure results in lower electron/hole recombination rate
during the sonocatalysis of AO7 . Also, the penetration depth of light, which has been produced
because of ultrasonic irradiation, into ZnO can greatly exceed the space charge layer with
+
3
increasing Gd ion concentration. Therefore, the recombination of generated electron-hole pairs
becomes easier, which leads to a low sonocatalytic activity of Gd-ZnO for AO7 degradation. In
+
3
addition, appropriate loading of Gd ions is highly necessary for producing a significant
potential difference between the surface and the center of the particles in order to efficiently
separate the electron-hole pairs. The excess amount of dopant covering the surface of ZnO
nanoparticles leads to a decrease in the sonocatalytic activity of the catalyst due to an increase in
the number of electron-hole recombination centers [11, 31].
3
.4. Effect of sonocatalyst dosage
Fig. 9 shows the effect of sonocatalyst dosage on the sonocatalytic decolorization of AO7. As
shown, at sonocatalyst dosages of 0.25, 0.5, 0.75, 1.0 and 1.25 g/L, the sonocatalytic
decolorization of AO7 were found to be 32.0, 45.0, 62.0, 90.0 and 91.75% within a reaction time
of 90 min. This indicated the high amount of sonocatalyst dosage favored the decolorization of
AO7. The increase in sonocatalytic decolorization of AO7 with increasing catalyst dosage
resulted from increasing in the available surface area containing active reaction sites.
•
Consequently, it would generate more OH radicals for the degradation of AO7 [2]. Ultrasonic
12

irradiation is expected to diminish the blockage of active reaction sites of the catalyst and
aggregation of the synthesized catalyst particles in the solution [32]. Anyway, increasing the
sonocatalyst dosage from 1 to 1.25 g/L led to no increase in the decolorization efficiency (%).
This may be attributed to the fact that excessive amount of sonocatalyst in the solution can result
in the scattering of ultrasound and blocking the transmission of ultrasound waves and energy
near to the surface of the sonocatalyst, which decreases the rates of sonocatalytic degradation
•
reactions [32, 33]. In addition, excessive sonocatalyst dosage can cause: (i) increased OH
consumption at the interface of bubble and solution and (ii) accumulation of by-products in the
cavity bubbles for lower collapse temperatures leading to less powerful collapse [9].
3
.5. Effect of initial dye concentration
From an application viewpoint, it is important to study the dependence of sonocatalytic
decolorization of AO7 on its initial concentration. The effect of initial AO7 concentration on its
sonocatalytic decolorization efficiency (%) was studied by varying initial dye concentration in
the range between 5 and 15 mg/L. The values of sonocatalyst dosage, reaction time and the
amount of dopant were 1 g/L, 90 min and 5%, respectively. The results exhibited that the
decolorization efficiency (%) of AO7 was inversely proportional to its initial concentration. As
result, increasing initial dye concentration from 5 to 15 mg/L led to decreasing the sonocatalytic
decolorization (%) of AO7 from 90.9 to 44.0%, respectively (Fig. 10). This negative effect
maybe because of the following reasons (i) The number of dye molecules that adsorbed on the
surface of catalyst increases by increasing dye concentration. In addition, at a high dye
concentration, a significant amount of UV light resulting from ultrasonic irradiation, maybe
absorbed by the dye molecules rather than by the catalyst particles and that reduces the
13

•
efficiency of catalytic reaction. (ii) The generation of OH radicals on the surface of catalyst is
likely reduced since active sites on the surface of the catalyst are occupied by the dye ions. (iii)
With increasing dye concentration, the number of hydroxyl radicals required for decolorization
of dye molecules increases. However, the formation of hydroxyl radicals remains constant for a
given catalyst dosage, irradiation time and intensity.[2, 34]. The results clearly illustrated that the
sonocatalytic degradation process is promising at low concentration of pollutant [26].
3
.6. Effect of the presence of radical scavengers
Real wastewaters usually contain ionic species which can affect the sonocatalytic activity of
catalyst due to their scavenging properties. Hence, the study of the effect of ionic species,
especially anions, on the sonocatalytic degradation of target pollutants is important considering
commercial application of the technique. To assess the effect of the presence of radical
scavengers on the sonocatalytic decolorization (%) of AO7, sodium sulfate, sodium carbonate
and sodium chloride were chosen with the same initial concentration of 5 mg/L where initial dye
concentration, catalyst dosage, dopant content and reaction time were set to 5 mg/L, 1 g/L, 5%
and 90 min, respectively. As can be seen from Fig. 11, the presence of all studied radical
scavengers in the solution caused inhibiting effect on the sonocatalytic decolorization of AO7. At
the presence of sodium sulfate, sodium carbonate and sodium chloride, the decolorization
efficiency (%) of AO7 decreased from 90% to 78, 65 and 56%, respectively. This set of
experiments was performed at natural pH, which is below the point of zero charge of ZnO
nanoparticles (pHzpc=9.0) [4]. Therefore, the ZnO nanoparticles carry positive charge, which
electrostatic attraction will be happened between negatively charged anion and the surface of
sonocatalyst. Consequently, the reaction of generated holes with these undesirable adsorbed
14

•
anions results in the decrease in the generation of OH , causing low sonocatalytic decolorization
efficiency [35]. Among the studied radical scavengers, sodium chloride produced the greatest
inhibiting effect on the decolorization efficiency (%) of AO7. The chloride ion scavenges
•
generated OH as shown in Eqs. (10) – (13) [35, 36]:
−
+
•
Cl + h → Cl
(10)
(11)
(12)
(13)
−
•
•−
2
Cl + Cl → Cl
•
−
•−
OH + Cl → HOCl
•
−
+
•
HOCl + H → Cl + H O
2
However, low inhibiting effect of sulfate ions on the sonocatalytic degradation of AO7 can be
ascribed to the fact that produced sulfate radicals during deactivation of hydroxyl radicals are
also strong oxidants which can degrade organic dye to some extent as illustrated in Eqs. (14) –
(
16) [35]:
2
4
−
•
•−
4
−
•
SO + OH → SO + OH
(14)
(15)
(16)
•
−
2−
4
+
SO + H O → SO + OH + H
4
2
•
−
SO + Organic dye → By − products
4
Moreover, peroxydisulfate (ꢀ ꢂꢃꢁꢄ), which is an in-situ chemical oxidant with a standard
ꢁ
oxidation potential of 2.01V, can be produced as a result of the reaction between two sulfate ions
as represented in Eq. (18) [36, 37]:
•
−
•−
2−
SO4 + SO4 → S O8
(17)
2
Sulfate radicals react with water and organic molecules in the first step. Consequently, the
probability of reaction (18) significantly reduces and this reaction has a very small role. It should
be pointed out that the oxidation potential of the aforementioned oxidants (ꢀꢂꢅ•ꢄꢆandꢆꢀ ꢂ ꢁꢄꢆ)
ꢁ
ꢃ
15

•
produced by sulfate ions are lower than that of OH , leading to lower decolorization efficiency of
AO7 in comparison with the process without the addition of scavengers. The possible reactions
representing the scavenging effect of sodium carbonate are given in Eqs. (18)-(20) [23, 38].
2
−
•
•−
−
CO3 + OH → CO3 + OH
(18)
(19)
•
−
−
−
2
CO3 + H O → 2CO + HO + OH
2 2 2
−
•
−
OOH + OH → OH + OOH
(20)
•−
The oxidation potential of the generated radical anions (CO
3
) is less than that of the hydroxyl
radicals. In addition, the active sites on the surface of the sonocatalyst may be blocked by the
•
anions, deactivating the sonocatalysts toward the dye. The results demonstrated that OH plays a
significant role in the sonocatalytic decolorization of AO7. Similar results have been reported by
Shimizu et al. in their study on the sonocatalytic degradation of methylene blue dye [39].
3
.7. Effect of the presence of enhancers
The effect of the presence of peroxydisulfate, hydrogen peroxide and potassium periodate as
enhancers with initial concentration of 2 mM on the sonocatalytic degradation of AO7 was
evaluated and the results are illustrated in Fig. 12. For this, the amount of dopant, sonocatalyst
dosage, initial dye concentration and reaction time were constant at 5%, 1 g/L, 5 mg/L and 75
min, respectively. As illustrated, the presence of hydrogen peroxide, peroxydisulfate and
potassium periodate results in increasing the sonocatalytic decolorization (%) of AO7 from 75%
to 77.0, 83.0 and 96.0, respectively. In the case of the addition of hydrogen peroxide, the
•
increase in the sonocatalytic degradation of AO7 is mainly due to increased generation of OH
•
radicals. The presence of hydrogen peroxide increases the generation rate of OH by means of the
16

reduction of hydrogen peroxide at the conduction band and the self-decomposition of hydrogen
peroxide as a result of ultrasound irradiation, which is shown in Eq. (21) [2, 28].
•
•−
4
H + ultrasonic irradiation → 2SO
(21)
The addition of peroxydisulfate can also enhance the sonocatalytic decolorization efficiency
•
because of the production of sulfate radicals and subsequently the formation of OH as illustrated
in Eq. (16) and the equation as below [11]:
2
−
•−
S O8 + ultrasonic irradiation → 2SO4
(22)
2
The addition of potassium periodate ions produced the greatest favorable effect on the
sonocatalytic decolorization efficiency of AO7. As a reason, it can be pointed out that periodate
ions increase the efficiency of sonocatalysis by capturing generated electrons as represented in
Eq. (23) [40]:
−
4
−
+
−
IO + 8e + 8H → 4H O + I
(23)
2
3
.8. Effect of ultrasonic power
Ultrasonic power is an important instrumental parameter deciding the efficiency of the
sonocatalytic degradation of target pollutants because of its effect on the cavitational activity
[
38]. Fig. 13 exhibits the effect of ultrasound power on the sonocatalytic decolorization of AO7.
The values of initial AO7 concentration, sonocatalyst dosage and dopant content were constant at
5
1
9
mg/L, 1 g/L and 5% within a reaction time of 90 min, respectively. At ultrasonic powers of
50, 300 and 400 W/L, the sonocatalytic decolorization (%) of AO7 was reached to 75, 89 and
1% within a reaction time of 75 min, respectively, while further increase in reaction time to 90
min led to insignificant difference between the decolorization efficiencies (%) of AO7 under
different ultrasonic powers. Therefore, an ultrasonic power of 150 W/L was selected as optimal
17

value for performing the experiments. Increasing the decolorization efficiency with increasing
ultrasonic power is due to the increased collapsing bubbles. Hence, the overall cavitational
•
activity would be higher at higher ultrasonic power resulting in increased net production of OH
and consequently enhanced degradation of target pollutants [11, 38]. But, increasing frequency
of irradiation for getting better results, especially for chemical processing applications, is not a
good idea and there exists an optimum frequency of operation. Also, continuous operation with
high frequency irradiation leads to an erosion of the transducers surface. The power requirement
for inception of cavitation events in a high frequency operation is also high. Use of multiple
frequency operation can be considered as an efficient alternative to the drawbacks associated
with the single frequency operation especially when high cavitational intensities are required
[
41].
3
.9. Effect of initial pH
The pH of solution is an important parameter which plays a significant role in the sonocatalytic
oxidation of various target pollutants. Thus, in the present study, the effect of pH on the
sonocatalysis of AO7 was studied at acidic, natural and basic values. Fig. 14 displays that natural
pH is favored the sonocatalytic degradation of AO7. At initial pHs of 3, 6.34 and 10, the
decolorization efficiency (%) of AO7 were 29.6, 90.0 and 42.5%, respectively. Similar trend was
observed and reported by Tang et al. in their study on the sonocatalytic degradation of
Rhodamine B in aqueous solution [42]. The solution chemistry of Acid Orange 7 (L) can be
described by the following equilibriums:
2
−
+
−
L + H ← K → HL
(24)
(25)
1
−
+
HL + H ← K → H L
2
2
18

Based on the calculated values for the equilibrium constants (pK =11.4 (for the naphthalene–OH
1
group) and pK
2
= 1 (for SO
3
–H group)) [43], existence of only negatively charged dye species in
-
the whole examined pH solution range of 3–10 is stated with a predominant role of HL species.
Due to amphoteric behavior of most semiconductor oxides, the pH of the reaction has significant
effect on the surface properties of semiconductor such as surface charge of catalyst and the size
of the aggregations. The Point of Zero Charge (PZC) of ZnO is about 9±0.3 [10]. The surface of
Gd-doped ZnO nanoparticles are positively charged at pH values lower than point of zero
charge. Thus, AO7 molecules, as an anionic dye with negative charge, can be adsorbed on the
surface of catalyst at pH values lower than 9.0 due to attraction force. Beside this, the decrease of
sonocatalytic decolorization efficiency in acidic solution compared with the neutral conditions
may be due to the addition of HCl to the solution for the pH adjustment, leading to an increase in
−
the amount of Cl anions in the solution which is a powerful radical scavenger. Moreover, the
low decolorization efficiency at pH=3 may be related to the corrosion of the catalyst in acidic
solution. In alkaline solution (pH > pH pzc), there is an electrostatic repulsion between negatively
-
charged surface of sonocatalyst and OH ions. This prevents the formation of hydroxyl radicals
and consequent reducing of decolorization efficiency.
3
.10. Reusability test
The consecutive application of the sonocatalyst as well as maintenance of its sonocatalytic
activity is of critical concern for long-term use of the sonocatalyst from an application point of
view. Therefore, the reusability test for 5% Gd-doped ZnO nanoparticles was conducted within
four repeated cycles with an initial AO7 concentration of 5 mg/L, photocatalyst dosage of 1 g/L
and ultrasonic power of 150 W/L during a reaction time of 90 min. The results revealed a
19

negligible drop in the sonocatalytic decolorization efficiency (%) of AO7 within four
experimental runs. At first, second, third and fourth experimental runs, the decolorization
efficiency (%) were obtained to be 100, 92, 90 and 94%, respectively. These results indicated
that 5% Gd-doped ZnO nanoparticles can be an efficient and durable sonocatalyst for treating
colored effluents.
3
.11. AO7 degradation by-products
To identify AO7 degradation by-products, samples subjected to different reaction times were
analyzed using a GC/MS. The results of GC-MS analysis are given in Table 4. Accordingly, nine
compounds were successfully detected, namely 1,3-bis (3-phenoxyphenoxy) benzene, acetic
acid, 2-Propanoic acid, BETA.-THUJONE, Bicyclo[2.2.1]heptan-2-one, 2-methyl-3-phenyl-
Propanal,
trimethyl[5-methyl-2-(1-methylethyl)phenoxy],
1-methyl-4-(1-methylethenyl)
Cyclohexene and Ethyl benzene [44, 45].
4
. Conclusions
In this study, ZnO nanoparticles were doped by means of gadolinium through a simple
sonochemical method and their sonocatalytic activity was assessed for the degradation of a
textile dye. XRD analysis showed crystalized nature of ZnO with pure wurtzite phase. Low
+
3
aggregation of sonocatalyst was observed via SEM images as a result of doping by Gd ions.
The incorporation of Gd ions into the ZnO lattice was confirmed by the results of XPS analysis.
The enhanced decolorization efficiency was observed in the presence of 5% Gd-doped ZnO
nanoparticles compared with undoped ZnO and sonolysis alone. The optical absorption spectra
of synthesized samples showed the lowest band gap for 5% Gd-doped ZnO nanoparticles. High
20

sonocatalyst dosage and low initial dye concentration were favored the sonocatalytic
decolorization. Addition of radical scavengers reduced the sonocatalytic decolorization, while
the presence of process enhancer, especially potassium periodate, accelerated the decolorization
efficiency. Increasing ultrasonic power did not significantly improve the efficiency of
sonocatalysis. Overall, the application of Gd-doped ZnO nanoparticles can be a promising and
efficient approach for the sonocatalysis of colored effluents with high reusability potential.
Acknowledgements
The authors thank the University of Tabriz (Iran) and Arak University of Medical Sciences (Iran)
for all of the support provided. This work is funded by the Grant 2011-0014246 of the National
Research Foundation of Korea.
21

References
1] R. Darvishi Cheshmeh Soltani, A.R. Khataee, M. Safari, S.W. Joo, Preparation of bio-
[
silica/chitosan nanocomposite for adsorption of a textile dye in aqueous solutions, International
Biodeterioration & Biodegradation, 85 (2013) 383-391.
[
2] N. Ertugay, F.N. Acar, The degradation of Direct Blue 71 by sono, photo and
sonophotocatalytic oxidation in the presence of ZnO nanocatalyst, Applied Surface Science,
(
[
2014). DOI: 10.1016/j.apsusc.2014.01.178.
3] A. Khataee, Y. Hanifehpour, M. Safarpour, M. Hosseini, S. Joo, Synthesis and
Characterization of Er
Science of Advanced Materials, 5 (2013) 1074-1082.
4] R. Darvishi Cheshmeh Soltani, A. Rezaee, A. Khataee, Combination of Carbon Black–
x
Zn1–x Se Nanoparticles: A Novel Visible Light Responsive Photocatalyst,
[
ZnO/UV Process with an Electrochemical Process Equipped with a Carbon Black–PTFE-Coated
Gas-Diffusion Cathode for Removal of a Textile Dye, Industrial & Engineering Chemistry
Research, 52 (2013) 14133-14142.
[
5] A. Hassani, R.D.C. Soltani, S. Karaca, A. Khataee, Preparation of montmorillonite–alginate
nanobiocomposite for adsorption of a textile dye in aqueous phase: Isotherm, kinetic and
experimental design approaches, Journal of Industrial and Engineering Chemistry, (2014). DOI:
1
0.1016/j.jiec.2014.05.034.
[
6] R. Darvishi Cheshmeh Soltani, A. Rezaee, A.R. Khataee, M. Safari, Photocatalytic process
by immobilized carbon black/ZnO nanocomposite for dye removal from aqueous medium:
Optimization by response surface methodology, Journal of Industrial and Engineering
Chemistry, 20 (2014) 1861-1868.
22

[
7] A. Khataee, A. Khataee, M. Fathinia, Y. Hanifehpour, S.W. Joo, Kinetics and Mechanism of
Enhanced Photocatalytic Activity under Visible Light Using Synthesized PrxCd1–xSe
Nanoparticles, Industrial & Engineering Chemistry Research, 52 (2013) 13357-13369.
[
8] A. R. Khataee, S. Fathinia, M. Fathinia, Y. Hanifehpour, S. W. Joo, B. Soltani, Synthesis,
Characterization and Photocatalytic Properties of Nanostructured Sm-doped CdSe, Current
Nanoscience, 9 (2013) 780-786.
[
9] Z. Eren, N.H. Ince, Sonolytic and sonocatalytic degradation of azo dyes by low and high
frequency ultrasound, Journal of Hazardous Materials, 177 (2010) 1019-1024.
10] N. Ghows, M.H. Entezari, Exceptional catalytic efficiency in mineralization of the reactive
[
textile azo dye (RB5) by a combination of ultrasound and core–shell nanoparticles (CdS/TiO
2
),
Journal of Hazardous Materials, 195 (2011) 132-138.
[
11] A. Khataee, A. Karimi, S. Arefi-Oskoui, R. Darvishi Cheshmeh Soltani, Y. Hanifehpour, B.
Soltani, S.W. Joo, Sonochemical synthesis of Pr-doped ZnO nanoparticles for sonocatalytic
degradation of Acid Red 17, Ultrasonics Sonochemistry, (2014).
[
12] Z.-D. Meng, W.-C. Oh, Sonocatalytic degradation and catalytic activities for MB solution of
Fe treated fullerene/TiO composite with different ultrasonic intensity, Ultrasonics
Sonochemistry, 18 (2011) 757-764.
13] K.P. Mishra, P.R. Gogate, Intensification of degradation of aqueous solutions of rhodamine
2
[
B using sonochemical reactors at operating capacity of 7 L, Journal of environmental
management, 92 (2011) 1972-1977.
[
14] A.B. Pandit, P.R. Gogate, S. Mujumdar, Ultrasonic degradation of 2: 4: 6 trichlorophenol in
presence of TiO catalyst, Ultrasonics Sonochemistry, 8 (2001) 227-231.
2
23

[
15] Y.L. Pang, A.Z. Abdullah, Effect of carbon and nitrogen co-doping on characteristics and
sonocatalytic activity of TiO nanotubes catalyst for degradation of Rhodamine B in water,
Chemical Engineering Journal, 214 (2013) 129-138.
16] S. Shirsath, D. Pinjari, P. Gogate, S. Sonawane, A. Pandit, Ultrasound assisted synthesis of
doped TiO nano-particles: Characterization and comparison of effectiveness for photocatalytic
oxidation of dyestuff effluent, Ultrasonics sonochemistry, 20 (2013) 277-286.
17] R. Darvishi Cheshmeh Soltani, A. Rezaee, M. Safari, A. Khataee, B. Karimi, Photocatalytic
2
[
2
[
degradation of formaldehyde in aqueous solution using ZnO nanoparticles immobilized on glass
plates, Desalination and Water Treatment, (2013) 1-8.
3
+
[
18] S. Kumar, P.D. Sahare, Gd incorporated ZnO nanoparticles: A versatile material,
Materials Research Bulletin, 51 (2014) 217-223.
19] K. Rekha, M. Nirmala, M.G. Nair, A. Anukaliani, Structural, optical, photocatalytic and
[
antibacterial activity of zinc oxide and manganese doped zinc oxide nanoparticles, Physica B:
Condensed Matter, 405 (2010) 3180-3185.
[
20] J. Karamdel, C.F. Dee, B.Y. Majlis, Characterization and aging effect study of nitrogen-
doped ZnO nanofilm, Applied Surface Science, 256 (2010) 6164-6167.
21] J.-H. Sun, S.-Y. Dong, J.-L. Feng, X.-J. Yin, X.-C. Zhao, Enhanced sunlight photocatalytic
[
performance of Sn-doped ZnO for Methylene Blue degradation, Journal of Molecular Catalysis
A: Chemical, 335 (2011) 145-150.
[
22] O. Oprea, O. Vasile, G. Voicu, L. Craciun, E. Andronescu, Photoluminescence, magnetic
3
+
properties and photocatalytic activity of Gd doped ZnO nanoparticles, Digest J. Nanomater.
Biostruc, 7 (2012) 1757-1766.
24

[
23] A. Khataee, R. Darvishi Cheshmeh Soltani, Y. Hanifehpour, M. Safarpour, H. Gholipour
Ranjbar, S.W. Joo, Synthesis and Characterization of Dysprosium-Doped ZnO Nanoparticles for
Photocatalysis of a Textile Dye under Visible Light Irradiation, Industrial & Engineering
Chemistry Research, 53 (2014) 1924-1932.
[
(
[
24] A.L. Patterson, The Scherrer Formula for X-Ray Particle Size Determination, Phys. Rev., 56
1939) 978-982.
25] Y.-S. Lin, Y. Hung, J.-K. Su, R. Lee, C. Chang, M.-L. Lin, C.-Y. Mou, Gadolinium (III)-
incorporated nanosized mesoporous silica as potential magnetic resonance imaging contrast
agents, The Journal of Physical Chemistry B, 108 (2004) 15608-15611.
[
26] J. Wang, Z. Jiang, L. Zhang, P. Kang, Y. Xie, Y. Lv, R. Xu, X. Zhang, Sonocatalytic
degradation of some dyestuffs and comparison of catalytic activities of nano-sized TiO , nano-
2
sized ZnO and composite TiO2/ZnO powders under ultrasonic irradiation, Ultrasonics
Sonochemistry, 16 (2009) 225-231.
[
27] K. Zhang, W.-C. Oh, Kinetic study of the visible light-induced sonophotocatalytic
degradation of MB solution in the presence of Fe/TiO
2
-MWCNT catalyst, Bulletin Korean
Chemal Society, 31 (2010) 1589-1595.
[
28] Z. Frontistis, D. Mantzavinos, Sonodegradation of 17α-ethynylestradiol in environmentally
relevant matrices: Laboratory-scale kinetic studies, Ultrasonics Sonochemistry, 19 (2012) 77-84.
29] A.E. Morales, E.S. Mora, U. Pal, Use of diffuse reflectance spectroscopy for optical
characterization of un-supported nanostructures, Revista Mexicana de Fisica S, 53 (2007) 18.
30] S. Farhadi, K. Pourzare, S. Sadeghinejad, Synthesis and characterization of Co
nanoplates by simple thermolysis of the [Co (NH (C ·4H O complex, Polyhedron, 67
2014) 104-110.
[
[
3
O
4
3
)
6
]
2
2
O
4
)
3
2
(
25

[
31] M. Khatamian, A. Khandar, B. Divband, M. Haghighi, S. Ebrahimiasl, Heterogeneous
3
+
3+
3+
photocatalytic degradation of 4-nitrophenol in aqueous suspension by Ln (La , Nd or Sm )
doped ZnO nanoparticles, Journal of Molecular Catalysis A: Chemical, 365 (2012) 120-127.
3
+
[
32] Y.L. Pang, A.Z. Abdullah, Fe doped TiO
2
nanotubes for combined adsorption–
sonocatalytic degradation of real textile wastewater, Applied Catalysis B: Environmental, 129
(
[
2013) 473-481.
33] Y.L. Pang, A.Z. Abdullah, S. Bhatia, Review on sonochemical methods in the presence of
catalysts and chemical additives for treatment of organic pollutants in wastewater, Desalination,
77 (2011) 1-14.
34] A.G. Chakinala, P.R. Gogate, A.E. Burgess, D.H. Bremner, Intensification of hydroxyl
radical production in sonochemical reactors, Ultrasonics sonochemistry, 14 (2007) 509-514.
35] S.G. Anju, S. Yesodharan, E.P. Yesodharan, Zinc oxide mediated sonophotocatalytic
degradation of phenol in water, Chemical Engineering Journal, 189–190 (2012) 84-93.
36] R. Darvishi Cheshmeh Soltani, A. Rezaee, H. Godini, A.R. Khataee, A. Hasanbeiki,
2
[
[
[
Photoelectrochemical treatment of ammonium using seawater as a natural supporting electrolyte,
Chemistry and Ecology, 29 (2012) 72-85.
[
37] Y. Deng, C.M. Ezyske, Sulfate radical-advanced oxidation process (SR-AOP) for
simultaneous removal of refractory organic contaminants and ammonia in landfill leachate,
Water Research, 45 (2011) 6189-6194.
[
38] N. Golash, P.R. Gogate, Degradation of dichlorvos containing wastewaters using
sonochemical reactors, Ultrasonics Sonochemistry, 19 (2012) 1051-1060.
39] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Sonocatalytic degradation of methylene
blue with TiO pellets in water, Ultrasonics Sonochemistry, 14 (2007) 184-190.
[
2
26

[
40] B. Gözmen, Applications of response surface analysis to the photocatalytic mineralization
of acetaminophen over silver deposited Tio2 with periodate, Environmental Progress &
Sustainable Energy, 31 (2012) 296-305.
[
41] V.S. Sutkar, P.R. Gogate, Design aspects of sonochemical reactors: techniques for
understanding cavitational activity distribution and effect of operating parameters, Chemical
Engineering Journal, 155 (2009) 26-36.
[
42] S.K. Tang, T.T. Teng, A.F.M. Alkarkhi, Z. Li, Sonocatalytic Degradation of Rhodamine B
in Aqueous Solution in the Presence of TiO Coated Activated Carbon, APCBEE Procedia, 1
2
(
[
2012) 110-115.
43] J. Bandara, J. Mielczarski, J. Kiwi, 1. Molecular mechanism of surface recognition. Azo
dyes degradation on Fe, Ti, and Al oxides through metal sulfonate complexes, Langmuir, 15
(
[
1999) 7670-7679.
44] J. Wu, H. Zhang, J. Qiu, Degradation of Acid Orange 7 in aqueous solution by a novel
2
+
electro/Fe peroxydisulfate process, Journal of hazardous materials, 215 (2012) 138-145.
[
45] X. Zhong, S. Royer, H. Zhang, Q. Huang, L. Xiang, S. Valange, J. Barrault, Mesoporous
silica iron-doped as stable and efficient heterogeneous catalyst for the degradation of CI Acid
Orange 7 using sono–photo-Fenton process, Separation and Purification Technology, 80 (2011)
1
63-171.
27

Figure captions
Figure 1. SEM images of the (a) undoped ZnO and (b) 5% Gd-doped ZnO nanostructures.
Figure 2. Size distribution of Gd-doped ZnO nanostructures.
Figure 3. XRD pattern of undoped and Gd-doped ZnO nanostructures.
Figure 4. FT-IR spectra of undoped ZnO and Gd-doped ZnO nanoparticles.
Figure 5. (a) survey spectrum of undoped ZnO, (b) Zn2p spectrum in undoped ZnO, (c) O1s
spectrum in undoped ZnO, (d) survey spectrum of Gd-doped ZnO, (e ) Zn2p spectrum in Gd-
doped ZnO (f) O1s spectrum in Gd-doped ZnO and (g) Gd4d spectrum in Gd-doped ZnO
samples.
Figure 6. Comparison of the processes in the degradation of AO7. Initial dye concentration = 5
mg/L, catalyst dosage = 1 g/L, initial pH=6.34 and ultrasonic power= 150 W/L.
Figure 7. Comparison of different dopant percentage in the sonocatalytic degradation of AO7.
Initial dye concentration = 5 mg/L, catalyst dosage = 1 g/L, initial pH=6.34 and ultrasonic
power= 150 W/L.
2
Figure 8. (Ahv) –hv curves of the undoped and Gd-doped ZnO nanoparticles.
Figure 9. Effect of the catalyst dosage on the sonocatalytic degradation of AO7. Dopant
percentage = 5%, initial dye concentration = 5 mg/L, initial pH=6.34 and ultrasonic power= 150
W/L.
Figure 10. Varying sonocatalytic degradation efficiency versus initial dye concentration. Dopant
percentage = 5%, catalyst dosage = 1 g/L, initial pH=6.34 and ultrasonic power= 150 W/L.
Figure 11. Varying sonocatalytic degradation efficiency at the presence of different radical
scavengers. Radical scavenger concentration = 5 mg/L, dopant percentage = 5%, catalyst dosage
=
1 g/L, initial dye concentration = 5 mg/L, initial pH=6.34 and ultrasonic power= 150 W/L.
28

Figure 12. Effect of the presence of peroxydisulfate, hydrogen peroxide and potassium periodate
on the sonocatalytic degradation of AO7. Dopant percentage = 5%, catalyst dosage = 1 g/L and,
initial dye concentration = 5 mg/L, peroxydisulfate and hydrogen peroxide and potassium
periodate concentration = 2 mM, initial pH=6.34 and ultrasonic power= 150 W/L.
Figure 13. The effect of ultrasonic powers on the sonocatalytic degradation of AO7. Initial dye
concentration = 5 mg/L, dopant percentage = 5%, catalyst dosage = 1 g/L and initial pH=6.34.
Figure 14. The effect of pH on the sonocatalytic degradation of AO7. Dopant percentage = 5%,
catalyst dosage = 1 g/L, initial dye concentration = 5 mg/L and ultrasonic power= 150 W/L.
29