Development of an empirical kinetic model for sonocatalytic process using neodymium doped zinc oxide nanoparticles

Article abstract of DOI:10.1016/j.ultsonch.2015.09.004

The degradation of Acid Blue 92 (AB92) solution was investigated using a sonocatalytic process with pure and neodymium (Nd)-doped ZnO nanoparticles. The nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The 1% Nd-doped ZnO nanoparticles demonstrated the highest sonocatalytic activity for the treatment of AB92 (10 mg/L) with a degradation efficiency (DE%) of 86.20% compared to pure ZnO (62.92%) and sonication (45.73%) after 150 min. The results reveal that the sonocatalytic degradation followed pseudo-first order kinetics. An empirical kinetic model was developed using nonlinear regression analysis to estimate the pseudo-first-order rate constant (k_app) as a function of the operational parameters, including the initial dye concentration (5-25 mg/L), doped-catalyst dosage (0.25-1 g/L), ultrasonic power (150-400 W), and dopant content (1-6% mol). The results from the kinetic model were consistent with the experimental results (R² = 0.990). Moreover, DE% increases with addition of potassium periodate, peroxydisulfate, and hydrogen peroxide as radical enhancers by generating more free radicals. However, the addition of chloride, carbonate, sulfate, and t-butanol as radical scavengers declines DE%. Suitable reusability of the doped sonocatalyst was proven for several consecutive runs. Some of the produced intermediates were also detected by GC-MS analysis. The phytotoxicity test using Lemna minor (L. minor) plant confirmed the considerable toxicity removal of the AB92 solution after treatment process.

Full text of DOI:10.1016/j.ultsonch.2015.09.004

Ultrasonics Sonochemistry 29 (2016) 146–155
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Development of an empirical kinetic model for sonocatalytic process
using neodymium doped zinc oxide nanoparticles
Alireza Khataee a, , Behrouz Vahid , Shabnam Saadi , Sang Woo Joo
⇑
,1
b
a
c,
⇑
a
Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran
Department of Chemical Engineering, Tabriz Branch, Islamic Azad University, 51579-44533 Tabriz, Iran
School of Mechanical Engineering, Yeungnam University, 712-749 Gyeongsan, South Korea
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2015
Received in revised form 2 September 2015
Accepted 3 September 2015
Available online 14 September 2015
The degradation of Acid Blue 92 (AB92) solution was investigated using a sonocatalytic process with pure
and neodymium (Nd)-doped ZnO nanoparticles. The nanoparticles were characterized by X-ray diffraction
(XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The 1%
Nd-doped ZnO nanoparticles demonstrated the highest sonocatalytic activity for the treatment of AB92
(
(
10 mg/L) with a degradation efficiency (DE%) of 86.20% compared to pure ZnO (62.92%) and sonication
45.73%) after 150 min. The results reveal that the sonocatalytic degradation followed pseudo-first order
Keywords:
kinetics. An empirical kinetic model was developed using nonlinear regression analysis to estimate the
pseudo-first-order rate constant (kapp) as a function of the operational parameters, including the initial
dye concentration (5–25 mg/L), doped-catalyst dosage (0.25–1 g/L), ultrasonic power (150–400 W), and
dopant content (1–6% mol). The results from the kinetic model were consistent with the experimental
Kinetic modeling
Non-linear regression
ZnO nanoparticles
Sonocatalyst
2
Degradation
results (R = 0.990). Moreover, DE% increases with addition of potassium periodate, peroxydisulfate, and
hydrogen peroxide as radical enhancers by generating more free radicals. However, the addition of
chloride, carbonate, sulfate, and t-butanol as radical scavengers declines DE%. Suitable reusability of the
doped sonocatalyst was proven for several consecutive runs. Some of the produced intermediates were
also detected by GC–MS analysis. The phytotoxicity test using Lemna minor (L. minor) plant confirmed
the considerable toxicity removal of the AB92 solution after treatment process.
Ó 2015 Published by Elsevier B.V.
1
. Introduction
The application of ultrasonic processes in advanced oxidation
(a hot-spot approach). This collapse generates high temperature
and pressure in the collapsing cavities and in the liquid immedi-
ately surrounding them (the interface area) [5,6]. Consequently,
thermal cleavage of water occurs as the liquid and dissolved
oxygen molecules produce reactive oxygen species (ROS),
processes (AOPs) has been proposed recently for the treatment of
various hazardous organic compounds, including halogenated
hydrocarbons, pesticides components, and dyes in aqueous
mediums [1–3]. Considerable amounts of dyes (up to 20%) can be
lost in effluents during dyeing processes in the textile industry,
which results in serious pollution of surface and ground waters
that are resistant to biological treatment [4]. Sonication with a
simple mechanism is an inexpensive solution to overcome this
problem that can be used in ambient conditions.
Å
ꢀ
Å
Å
Å
including O
2
, O, OOH, and in particular OH radicals. The hydroxyl
Å
radical ( OH) as a strong oxidizing agent, reacts with pollutants
effectively and unselectively to degrade them without generating
secondary waste. [7,8].
However, the degradation rate of organic pollutants by ultra-
sonic processes is low, and complete mineralization rarely occurs
[9,10]. To overcome these drawbacks, the ultrasonic method can
2
+
The origin of the sonochemical effects is cavitation, which
involves the generation, growth, and implosion of micro-bubbles
within the liquid medium acting as localized micro-reactors
be combined with Fe , H
2
O
2
, Fenton reagent, and semiconductor
Å
oxides to increase the production of OH radicals [11,12]. For
example, using appropriate catalysts (heterogeneous catalysis) like
semiconductors (heterogeneous catalysis) can accelerate the
sonochemical reactions by a synergistic effect of ultrasound and
the solid semiconductor in the sonocatalytic process in an environ-
mentally friendly coupled technique [8,9].
⇑
Corresponding authors.
E-mail addresses: a_khataee@tabrizu.ac.ir, ar_khataee@yahoo.com (A. Khataee),
swjoo@yu.ac.kr (S.W. Joo).
1
Communicator.
http://dx.doi.org/10.1016/j.ultsonch.2015.09.004
1
350-4177/Ó 2015 Published by Elsevier B.V.

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
147
Among the diverse catalysts, zinc oxide (ZnO) nanoparticles
2.3. Characterization of nanocatalysts
have attracted particular attention for properties such as a wide
band gap (3.37 eV), high UV absorption potential, and high exciton
binding energy (60 meV) [13]. Doping with rare-earth metals
XRD of the undoped and Nd-doped ZnO was conducted using a
Siemens X-ray diffractometer (D8 Advance, Bruker, Germany) with
(
REMs) is a practical way to increase the catalytic activity of ZnO
a
Cu K radiation (l = 1.54065 Å), an accelerating voltage of 40 kV,
by preventing fast recombination of the electron–hole pairs in
the doped semiconductor [10,14]. The advantages of sonochemical
methods for nanostructured material synthesis are the simple
experimental conditions, quick procedure, and low cost compared
to conventional methods [15].
Using intrinsic elementary reactions as a basis for a kinetic
model is one of the best methods for model development because
it involves exact understanding of the process reactions and the
influences of the operational parameters [16–18]. However, this
kind of modeling is sometimes difficult due to the diversity of reac-
tions, particularly for coupled processes [19,20]. Empirical kinetic
modeling of a process via non-linear regression analysis provides
a nonlinear model that can be used for complicated combined
and an emission current of 30 mA. The surface morphology of the
synthesized catalysts was investigated by SEM (TESCAN, MIRA3,
Czech Republic), and a thermo scientific spectrometer (K-ALPHA,
UK) was used for XPS measurements. The band gap of the nanocat-
alysts was measured by preparing a sample solution in distilled
water and sonicating it for 20 min to obtain a homogeneous mix-
ture. Microstructure distance-measurement software (Microstruc-
ture Measurement version 1.0) was applied to determine the size
distribution of the as-prepared nanoparticles. The solution absor-
bance was recorded between 200 and 800 nm using an UV–Vis
spectrophotometer (WPA lightwave S2000, England). Some of the
intermediates during the sonocatalytic degradation of AB92 were
detected by an Agilent 6890 gas chromatograph with a 30-m to
0.25-mm HP-5MS capillary column coupled with an Agilent 5973
mass spectrometer (Canada).
2 2
AOPs, including photoelectro-Fenton and US/UV/H O processes
[
19,21]. The validity of empirical modeling has been evaluated by
comparing experimental and theoretically calculated data [22].
In this study, a simple sonochemical method was used to syn-
thesize pure and Nd-doped ZnO nanoparticles. X-ray diffraction
2.4. Sonocatalytic degradation of the dye
(
XRD) and scanning electron microscopy (SEM) were used to
Sonocatalytic degradation experiments of AB92 were done for
50 min each using an ultrasonic bath (Ultra 8060, England) with
characterize the prepared sonocatalysts. X-ray photoelectron
spectroscopy (XPS) was used to confirm the presence of Nd in
the catalyst structure. AB92 was treated as a model organic azo
dye to determine the sonocatalytic activity of pure and Nd-doped
ZnO nanoparticles, and the results were compared to those of
sonolysis alone. To the best of our knowledge, sonocatalysis using
Nd-doped ZnO nanoparticles for the degradation of AB92 has not
been studied previously. Non-linear regression analysis was used
to develop an empirical kinetic model considering the effect of
the main operational parameters on kapp, including initial dye
concentration, doped-catalyst dosage, ultrasonic power, and
dopant content. The effects of various process enhancers and
radical scavengers on the dye degradation efficiency (DE%) were
studied. Finally, some of the intermediates generated during the
sonocatalytic process were detected by GC–MS technique.
1
a frequency of 36 kHz at the natural pH of the dye in the presence
of various ZnO nanoparticles. A certain amount of nanocatalyst was
added to 100 mL of AB92 solution with a specified concentration,
and the prepared solution was sonicated in the dark to eliminate
the photocatalysis effect. A typical sample of the ultrasonically
treated solution was withdrawn at distinct process times, and
the AB92 solution absorbance was measured using the UV–Vis
spectrophotometer at its maximum absorbance wavelength
(kmax = 571 nm).
3. Results and discussion
3.1. Characterization of undoped ZnO and Nd-doped ZnO
Fig. 1 illustrates the X-ray diffraction patterns of pure and 1%
Nd-doped ZnO nanoparticles. The main peaks were identified for
undoped ZnO at 2 h of 31.92, 34.6, 36.48, 47.68, 56.72, 63, 66.08,
68, 68.28, 71.64, and 75.96, which are related to the (100),
2
. Materials and methods
2.1. Chemicals
(
002), (101), (102), (110), (103), (200), (112), (201), (004),
All chemicals were analytical grade and used without further
and (202) planes of hexagonal wurtzite ZnO, respectively (JCPDS
Card 36-1451) [23]. Identical peaks for the doped ZnO were seen
without any diffraction peaks from neodymium oxides or other
impurities. The diffraction values of the (100), (002), and (101)
planes revealed a shift to lower angles in the case of Nd-doped
ZnO compared to pure ZnO, indicating appropriate doping of Nd
ions into the ZnO lattice. This shift can be explained by the expan-
sion of the ZnO lattice through doping by neodymium due to the
purification. ZnCl
OH, 99%) and neodymium chloride (NdCl
purchased from Sigma Aldrich, USA. Acid Blue 92 (molecular
formula = C26 Na max = 571 nm, = 695.58 g/mol,
2
was provided by Merck, Germany. Ethanol
(
C
2
H
5
3
.6H O) were
2
H
16
N
3
3
O
S
10 3
,
k
M
w
color index number = 13,390), as an anionic monoazo dye, was
obtained from Shimi Boyakhsaz Company (Iran).
3
+
2+
larger ionic radius of Nd (0.983 Å) in comparison with Zn
(0.74 Å).
2
.2. Undoped ZnO and Nd-doped ZnO nanoparticles synthesis
procedure
Fig. 2a and b demonstrate SEM images of the undoped ZnO and
% Nd-doped ZnO nanoparticles, respectively. Fig. 2a shows that
1
Undoped ZnO and Nd-doped ZnO nanoparticles were sono-
chemically synthesized as follows: (1) a molar pre-specified
amount of NdCl O was added to an aqueous solution of zinc
chloride, (2) NaOH solution (1 M) was added dropwise to the
prepared solution to set the pH to 10, (3) the obtained solution
was sonicated for 3 h by an ultrasonic bath (Ultra 8060, England)
with a frequency of 36 kHz, and (4) the resulting white crystalline
product was washed with double distilled water and ethanol and
dried at 80 °C for 12 h.
the undoped ZnO nanoparticles are irregular in shape and size in
comparison with the Nd-doped ZnO nanoparticles, which is related
to the growth of irregular crystalline grains during the synthesis
and their aggregation. Fig. 2b shows that the incorporation of Nd
into the crystal structure of ZnO can decrease the aggregation phe-
nomenon and hence reduce the size of nanoparticles and improve
the shape uniformity of them. The presence of the Nd dopant
decreases the crystal size of Nd-doped ZnO sample, which was
mainly attributed to the generation of Nd–O–Zn on the surface of
3
ꢁ6H
2

1
48
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
degradation of AB92 (10 mg/L) was compared under sonication
in the presence and absence of ZnO nanoparticles at identical
operational conditions in 150 min of treatment (Fig. 5). The results
revealed that DE% was 45.73%, 62.9%, and 86.20% for sonolysis,
sonocatalysis with undoped ZnO, and sonocatalysis with
Nd-doped ZnO nanoparticles, respectively. Combining semicon-
ductor catalysts with sonolysis enhances the performance of the
ultrasound process by formation of electron–hole pairs after
excitation of the electrons from the valence to conduction band.
Then, interface oxidation of the adsorbed hydroxyl anions and
water molecules occurs on the catalyst surface as a result of the
Å
holes, which generates OH radicals. Interaction of the
conduction-band electrons with adsorbed oxygen molecules pro-
Å
ꢀ
Å
duces O
Eqs. (2)–(6)) [32,33].
2
and OOH radicals, which can oxidize the contaminants
(
Fig. 1. XRD patterns of undoped and 1% Nd-doped ZnO nanoparticles.
the catalyst preventing the growth of crystal grains [24]. Moreover,
sonochemical synthesis of the catalyst generally results in
de-aggregation of the synthesized nanoparticles and hence good
spherical shape of the doped nanoparticles observed owing to their
lower crystalline size. Also, the de-aggregation of the particles
provides more active sites for the reaction [25]. Fig. 2c shows that
the major size distribution of the 1% Nd-doped particles was in the
range of 40–60 nm with a frequency of 67.5%.
Fig. 3 illustrates the XPS spectra of the 1% Nd-doped ZnO
nanoparticles, and it is clear that there are Zn, O, C, and Nd peaks
on the surface of the catalyst. The corresponding energies of these
elements are given in Table 1. The O1s, C1s, Zn2p3/2, and Zn2p1/2
binding energies are close to 532.08, 286.08, 1023.08, and
1
046.08 eV, respectively, which corresponds to the wurtzite
structure of the hexagonal zinc (II) oxide [26]. It should be stated
that the other binding energies which are observed at 11.08,
9
2.08, 141.08 and 1198.08 eV are attributed to Zn3d, Zn3p, Zn3s
and Zn2s, respectively. Also, the specified area in the Fig. 3 is
ascribed to Auger Zn LMM peaks [27–29].
Fig. 3 also depicts the spectra of the Nd 3d region. The 5/2 and
3
/2 spin–orbit double components of the Nd 3d core level
photoemission are located at 1004.98 and 984.08 eV, respectively,
and are in good agreement with the Nd3d3/2 and Nd3d5/2 peaks. This
shows that the Nd ions exist in a trivalent state based on literature
data for Nd ions in Nd
The band gap of the ZnO semiconductor was calculated using
Eq. (1) [31]:
2 3
O [30].
2
ðAh
m
Þ ¼ Kðh
m
ꢀ E
g
Þ
ð1Þ
where h
constant, and E
by extrapolating the linear region in the diagram of (Ah
the photon energy (Fig. 4). The band gap values of 3.20, 2.60,
m
is the energy of a photon (eV), A is the absorption, K is a
g
is the band gap. The band gap can be determined
2
m
) against
2
2
.80, 2.85 and 2.90 eV were obtained for the undoped ZnO, 1%,
%, 4% and 6% Nd-doped ZnO nanoparticles, respectively
3.2. Comparison of sonolysis and sonocatalysis processes
AB92 adsorption on the ZnO catalysts (approximately 7% after
0 min) can be neglected for removal from the solution. The
Fig. 2. SEM images of (a) undoped ZnO, (b) 1% Nd-doped ZnO, and (c) 1% Nd-doped
ZnO particle size distribution.
6

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
149
water-soluble compound, so its presence in the gaseous phase of
a cavity is not likely [39]. As a consequence, the main routes for
AB92 degradation are chemical oxidation by ROS, especially hydro-
xyl radicals in the interfacial area of the gas and bulk solution and
on the surface of the sonocatalyst [40].
3.3. Kinetic of sonocatalytic degradation using Nd-doped ZnO and
modeling of operational parameters
The operational conditions of the sonocatalytic process for
empirical kinetic modeling are summarized in Table 2. According
to the pseudo-first-order kinetic assumption from the slope of ln
0
(C /C) against time (t), kapp was estimated for each experiment
2
with high correlation coefficients (R ), which were greater than
.99, thus confirming the suggested kinetic. The non-linear relation
0
of kapp with each of the operational parameters can be separately
modeled by an empirical power law equation for the initial dye
concentration, doped-catalyst dosage, ultrasonic power, and
dopant content (Eq. (7)) [41]:
Fig. 3. XPS spectra of 1% Nd-doped ZnO nanoparticles.
b
k
app
¼
aðoperational parameterÞ
ð7Þ
Undoped ZnO=Nd ꢀ doped ZnOþÞÞÞ
!
The model constants
regression analysis of the experimental results, and the results
are shown in Figs. 6–9.
a
and b were calculated by non-linear
Undoped ZnO=Nd ꢀ doped ZnOðh þ eꢀÞ
þ
ð2Þ
ð3Þ
þ
Å
þ
kapp and thus DE% were increased by increasing the catalyst
h
h
þ H
2
O ! 2 OH þ H
dosage (Fig. 6) and ultrasound power (Fig. 7) during the 150-min
process time. The effect of catalyst dosage is attributed to the
greater available surface area with active reaction sites, which
increases the production of hydroxyl radicals for the AB92 degra-
dation [42]. However, after an optimum value of 1 g/L, DE%
decreases due to nanoparticle aggregation and ultrasound scatter-
ing, which decreases the active sites and blocks the transmission of
ultrasound waves, respectively [13,43].
þ
þ OHꢀ ! OH
Å
ð4Þ
ð5Þ
ð6Þ
ꢀ
Å
ꢀ
e
þ O
2
! O
2
þ
Å
ꢀ
Å
H
þ O ! OOH
2
More holes can also be produced by an oxygen escape mecha-
The ultrasound power is an important instrumental parameter
that affects the cavitational activity [44]. The improvement of DE
with the ultrasonic power is due to the greater formation of
nism where some surface oxygen atoms escape from the crystal
lattice as a result of strong shock waves generated by cavitation
bubbles during their collapse [34]. Furthermore, catalyst crevices
containing entrapped gas act as a cavitation nucleus and lower
the tensile strength of the solid–liquid interface compared to the
bulk liquid, which generates more active cavitation bubbles. A
sonoluminescence phenomenon also occurs, and the small activity
of the semiconductor as a photocatalyst cannot be eliminated [35].
Rare earth ion doping into the pure ZnO lattice also creates
impurity energy levels in the band gap and produces traps for
the generated charge carriers. This accelerates the interfacial
charge transfer and inhibits the recombination of electron–hole
pairs [36,37]. Finally, the ultrasonic waves not only increase the
active surface area of the sonocatalyst by de-aggregation but also
enhance the mass transfer of various species from the solution to
the semiconductor surface [38].
%
Å
collapsing bubbles. This enhances the formation of OH radicals
and increases the degradation of target contaminants [10,44].
Furthermore, at elevated ultrasound power, the turbulence of the
solution increased, which increased the mass transfer rate of the
dye, the generated intermediates, and the reactive radicals
between the solution and the catalyst surface. The available active
sites on the surface of the catalyst are also increased by increased
de-aggregation [45,46].
kapp and DE% were decreased by increasing the initial concen-
tration of AB92 (Fig. 8) and the dopant content (Fig. 9) during
the 150-min process time. At identical operational conditions,
The rest of the experiments were carried out using sonocataly-
sis with the Nd-doped ZnO catalyst. AB92 is a non-volatile and
Table 1
Location of Zn, C 1s, O 1s and Nd 3d peaks in the XPS spectrum of Nd-doped ZnO.
Nd-doped ZnO
Peak
Binding energy (eV)
Zn2s
1198.08
1046.08
1023.08
141.08
92.08
Zn2p1/2
Zn2p3/2
Zn3s
Zn3p
Zn3d
11.08
O
1s
532.08
984.08
1004.98
286.08
Nd3d5/2
Nd3d3/2
C
1s
Fig. 4. (Ahv)2 ꢀ h
v
curves of the undoped and 1%Nd-doped ZnO nanoparticles.

1
50
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
Fig. 6. The effect of catalyst dosage on kapp; Initial dye concentration = 10 mg/L,
dopant content = 1%, and ultrasonic power = 150 W.
Fig. 5. Comparison of different processes in the degradation of AB92. Initial dye
concentration = 10 mg/L, catalyst dosage = 1 g/L and ultrasonic power = 150 W.
the number of generated hydroxyl radicals is constant, and these
radicals have to react with more dye and degradation intermedi-
ates [10]. Hence, the sonocatalytic process is more promising at a
lower concentration of contaminants [47].
The enhanced sonocatalytic activity by incorporation of Nd can
be explained by two mechanisms. Firstly, the presence of Nd in the
ZnO lattice can alter the absorbance properties of the catalyst.
Based on this mechanism, the sonocatalyst band gap has a signifi-
cant role in the sonocatalysis [31,48]. As mentioned in Section 3.1,
the band gap of Nd-doped ZnO nanoparticles decreases compared
to pure ZnO because of the formation of a shallow level inside the
band gap. As a result, the transition from the ground to the excited
state is easier with the narrower band gaps.
3
+
Moreover, Nd acts as an electron scavenger and prevents
electron–hole pairs recombination [49,50]. However, appropriate
3
+
loading of Nd ions is important for generating a significant
potential difference between the surface and the center of the
nanoparticles to separate the electron–hole pairs effectively. This
loading was chosen as 1% of Nd. Excess dopant covers the surface
of ZnO nanoparticles and decreases the sonocatalytic activity
by increasing the space charge layer in the ZnO lattice and
consequently accelerating the electron–hole pair recombination
rate [10,24,51].
Fig. 7. The effect of ultrasonic power on
dosage = 1 g/L and initial dye concentration = 10 mg/L.
app
k . Dopant content = 1%, catalyst
3.4. Development and evaluation of empirical kinetic model
As mentioned, the degradation of AB92 obeys pseudo-first-
order kinetics. kapp is a function of the operational parameters as
follows (Eq. (8)):
a
b
k
app ¼ k0
½Doped ꢀ catalystꢂ ðUS powerÞ
ð8Þ
c
d
½
AB92ꢂ ðDopant contentÞ
Non-linear analysis regression was used to estimate a, b, c, and
0
d. Then, k could be calculated for each run using known values of
kapp, the initial AB92 concentration, the amount of doped catalyst,
ultrasound power, and dopant content applied in each experiment.
The mean value of k was obtained as 0.0026.
Fig. 8. The effect of AB92 initial concentration on kapp. Dopant content = 1%, catalyst
dosage = 1 g/L and ultrasonic power = 150 W.
0
Table 2
The operational conditions of sonocatalytic process.
By substituting the estimated values into Eq. (8), Eq. (9) was
obtained as the empirical kinetic model for prediction of kapp in
different experimental conditions:
Parameter
Range
Catalyst dosage (g/L)
0.25–1
150–400
5–25
Ultrasonic power (W)
Initial AB92 concentration (mg/L)
Dopant content (%)
0:59
0:5104
½
Doped ꢀ catalystꢂ ðUS powerÞ
kapp ¼ 0:0026
ð9Þ
0:409
0:177
1–6
½AB92ꢂ
ðDopant percentageÞ

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
151
Fig. 9. The effect of dopant content on kapp. Initial dye concentration = 10 mg/L,
catalyst dosage = 1 g/L and ultrasonic power = 150 W.
Fig. 10. Comparison between experimental and calculated pseudo-first order rate
constants (min ) for degradation of AB92 in the sonocatalytic process at different
ꢀ1
operational condition. For experimental details refer to Table 3.
The experimental and calculated kapp for degradation of AB92
(Table 3) were compared to evaluate the obtained empirical kinetic
model in Fig. 10, which shows a good agreement of the model and
2
the experimental results (R = 0.990).
3.5. Effect of enhancers
The effect on DE% was measured for enhancers such as hydro-
gen peroxide, peroxydisulfate, and potassium periodate with initial
concentrations of 0.15 mM. The results are plotted in Fig. 11. The
enhancers increase the AB92 sonocatalytic degradation from
8
6.20% to 90.08%, 92.50%, and 94.65%, respectively. For hydrogen
peroxide, the enhanced degradation of the dye is mainly due to
Å
the excess production of OH radicals by the ultrasound
(
Eq. (10)). Hydrogen peroxide also promotes the formation rate
Å
of OH radicals by reduction in the conduction band (Eq. (11))
[
42,52].
Å
H
2
2
O
2
þ ultrasonic irradiation ! 2 OH
ð10Þ
ð11Þ
Fig. 11. Effect of enhancers on the sonocatalytic degradation of AB92. Dopant
content = 1%, initial dye concentration = 10 mg/L, enhancer concentra-
tion = 0.15 mmol/L, catalyst dosage = 1 g/L and ultrasonic power = 150 W.
þ eꢀ ! OH þ OH
Å
ꢀ
H
O
2
The addition of peroxydisulfate can also enhance DE% due to the
production of sulfate radicals, which react with water to form extra
OH radicals (Eqs. (12)–(15)) [1]:
Åꢀ
2ꢀ
Å
þ
Å
SO þ RHðdyeÞ ! SO4 þ RðIntermediatesÞ þ H
ð14Þ
ð15Þ
4
2
ꢀ
S
2
ð12Þ
ð13Þ
Åꢀ
Å
2ꢀ
SO þ RðIntermediatesÞ ! SO þ CO þ NO
2
2
4
4
Åꢀ
O ! SO 24 ꢀ þ OH þ H
Å
þ
þ Other inorganics
SO þ H
2
4
Table 3
Apparent pseudo-first-order rate constants of the sonocatalytic process at different operational conditions.
appꢁexp (min 1
ꢀ
)
kappꢁcal (min
ꢀ1
)
[
AB92]
0
(mg/L)
[doped-catalyst] (g/L)
US power (W)
Dopant content (%)
k
1
1
1
1
5
1
2
2
1
1
1
1
1
1
1
1
0
0
0
0
0.25
0.5
0.75
1
1
1
1
1
1
1
1
1
1
1
1
1
150
150
150
150
150
150
150
150
300
400
150
150
150
150
150
150
1
1
1
1
1
1
1
1
1
1
2
4
6
1
1
1
0.0057
0.009
0.0057
0.0086
0.0109
0.0129
0.0171
0.0109
0.0097
0.0089
0.0184
0.0213
0.0114
0.0101
0.0094
0.0129
0.0129
0.0129
0.0111
0.0129
0.0162
0.0112
0.0096
0.0082
0.0178
0.0215
0.0116
0.0104
0.0093
0.0129
0.0128
0.0127
5
0
5
0
0
0
0
0
0
0
0

1
52
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
3.7. Reusability of the sonocatalyst
Consecutive application of the sonocatalyst while maintaining
its activity is critical for practical use. It has been proven that cat-
alyst doping with an appropriate dopant enhances its reusability
[
49]. Therefore, a reusability test for the 1% Nd-doped ZnO
nanocatalyst was performed for the AB92 (10 mg/L) treatment
Fig. 13). The data demonstrated negligible drops in DE% within
(
four repeated cycles to 86.20%, 86.01%, 85.81%, and 84.67%, respec-
tively. These data revealed that the 1% Nd-doped ZnO nanoparti-
cles are durable and effective for the remediation of colored
wastewaters.
3.8. AB92 degradation intermediates
The intermediates generated during the sonocatalytic degrada-
tion process were analyzed by GC–MS and identified by matching
their spectra with commercial standards recorded in the Mass
library (Wiley 7n) to find match factors above 90% [55]. Molecular
structure and retention time of four detected components after
Fig. 12. Differing sonocatalytic degradation efficiency at the presence of various
radical scavengers. Dopant content = 1%, catalyst dosage = 1 g/L, initial dye
concentration = 10 mg/L, scavenger concentration = 100 mg/L and ultrasonic
power = 150 W.
6
min of degradation are shown in Table 4. Generation of these
intermediates during the decolorization of AB92 confirmed active
degradation of this pollutant through the sonocatalytic degrada-
tion process.
Potassium periodate addition produces the most favorable
effect on DE% among the enhancers due to electron capture from
the conduction band (Eq. (16)) [53]:
IO þ 8eꢀ þ 8Hþ ! Iꢀ þ 4H
ꢀ
3.9. Phytotoxicity assessment
4
2
O
ð16Þ
Phytotoxicity is defined as the compound capacity to cause
damage to an aquatic plant. In this research, the phytotoxicity of
solutions containing 10 mg/L of AB92 and 1 g/L of 1% Nd-doped
ZnO sample was investigated according to our previous work
[56]. For the phytotoxicity experiment, twenty fully-grown fronds
of Lemna minor (L. minor) were carried to a 200 mL beaker contain-
ing 100 mL of tested solution and 2 mL of growth medium. The
tested solutions were selected as water, AB92 solution and treated
AB92 solution after the separation of the doped nanoparticles,
respectively. Relative frond number was calculated through Eq.
(23):
3
.6. Effect of radical scavengers
The effect of various anions and organic compounds on the
sonocatalytic process is significant from a practical point of view
because of their possible presence in real wastewater and their role
as radical scavengers. Hence, sodium sulfate, sodium carbonate,
sodium chloride, and t-butanol were selected with an initial con-
centration of 100 mg/L to evaluate the influence on DE%. Fig. 12
shows an inhibiting effect on the sonocatalytic degradation of
AB92 for all of the radical scavengers, confirming the free radical
attack mechanism for the treatment process. The possible reac-
tions that can occur in the presence of the radical scavengers are
as follows (Eqs. (17)–(22)) [44,54]:
ðfrond N
7
ꢀ frond N
0
Þ
Relative frond numberðRFNÞ ¼
ð23Þ
frond N
0
SO4 þ OH ! SO þ OHꢀ
2
ꢀ
Å
Åꢀ
0 7
where N and N are the number of fronds on days 0 and 7, respec-
tively. The RFN was calculated as 0.7, 0.3 and 0.6 for the above men-
tioned tested solutions, respectively. RFN was smaller in the AB92
ð17Þ
4
CO3 þ OH ! CO þ OHꢀ
2
ꢀ
Å
Åꢀ
ð18Þ
ð19Þ
ð20Þ
ð21Þ
3
Å
Cl þ Clꢀ ! Cl
Å
ꢀ
2
ꢀ
Å
Å
ꢀ
Cl þ OH ! Cl þ OH
Å
Å
ðCH
3
Þ COH þ OH ! H
2
O þ CH
2
CðCH Þ2OH
3
3
C
4
H
9
OH þ 2hþ ! C
4
H
8
O þ 2Hþ
ð22Þ
Experiments were performed at pH 6 (the natural pH of dye),
which is below the zero charge point of the ZnO nanoparticles
pHzcp = 9.0) [14]. The positively charged ZnO nanoparticles attract
(
the negatively charged anions electrostatically, and as a conse-
quence, the reaction of generated holes with the adsorbed anions
Å
reduces the OH radical formation. The active sites of the sonocat-
alyst occupied by these anions deactivate the sonocatalyst surface
for the dye and the adsorption of degradation intermediates, which
react with adsorbed hydroxyl radicals and result in lower DE% [32].
Å
The data suggest that OH radicals have an essential function in the
Fig. 13. Reusability of the 1% Nd-doped ZnO nanoparticles within 4 consecutive
experimental runs. Dopant content = 1%, catalyst dosage = 1 g/L, initial dye con-
centration = 10 mg/L, reaction time = 150 min and ultrasonic power = 150 W.
sonocatalytic process. Similar data have been reported for the
sonocatalytic degradation of methylene blue dye [7].

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
153
Table 4
Identified intermediates during sonocatalytic degradation of Acid Blue 92.
No.
1
Structure
t
R
(min)
Main fragments (m/z)
Compound
OH
O
31.424
149.00 (100.00%), 57.10 (44.43%), 167.00 (38.82%), 55.00 (35.49%), 207.00 (33.46%)
1,2-Benzenedicarboxylic acid
OH
O
2
3
4
O
2.285
4.681
2.684
75.00 (100.00%), 117.00 (58.55%), 73.00 (12.09%), 76.00 (7.62%), 118.00 (5.80%)
75.00 (100.00%), 116.00 (75.43%), 73.00 (13.26%), 76.00 (7.56%), 117.00 (7.48%)
59.00 (100.00%), 60.00 (2.55%), 57.10 (0.5%), 55.10 (0.41%), 71.10 (0.36%)
Acetic acid
OH
CH
3
O
Acetamide
CH
3
3
NH
2
O
Methylformamide
CH
NH
Fig. 14. Macroscopic images of L. minor (a) control sample, (b) treated AB92 solution and (c) AB92 solution.
solution compared to the control and treated samples (Fig. 14),
which indicates that the toxicity of the AB92 solution was more
than other samples. After treatment of the AB92 solution, RFN
increases from 0.3 to 0.6, which shows the toxicity reduces owing
to the effectively degradation of the dye. However, it is yet less than
the control sample, which can be attributed to the incomplete dye
degradation (DE% = 86.20%) after 150 min of treatment.
aqueous solution. XRD patterns demonstrated that the particles
were crystalized in a hexagonal wurtzite lattice. SEM images
revealed that the incorporation of Nd into the ZnO crystal structure
prevents particle aggregation. XPS spectra confirmed the presence
of Nd in the doped samples. Based on the obtained results, DE% for
the sonocatalysis with doped catalysts was higher than sonocatal-
ysis with undoped catalyst and sonolysis alone. Furthermore, DE%
was influenced by the dopant content, and 1% Nd-doped ZnO
nanoparticles were selected as the most effective catalyst in the
other experiments. DE% was noticeably affected by experimental
parameters like the catalyst dosage, initial AB92 concentration,
dopant content, and ultrasound power. The addition of enhancers
4
. Conclusion
Sonochemically prepared Nd-doped ZnO nanoparticles were
applied as a sonocatalyst for the degradation of AB92 azo dye in

1
54
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 146–155
and radical scavengers accelerated and reduced DE%, respectively,
indicating that free radicals are the predominant controlling
mechanism for sonochemical degradation. Also, the sonocatalyst
demonstrated high durability after four consecutive experiments,
and some of the degradation intermediates were recognized by
GC–MS analysis. Finally, by applying a nonlinear regression analy-
sis, an empirical kinetic model was developed for calculating kapp
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