Heterogeneous sono-Fenton-like process using nanostructured pyrite prepared by Ar glow discharge plasma for treatment of a textile dye

Article abstract of DOI:10.1016/j.ultsonch.2015.09.012

The plasma-treated pyrite (PTP) nanostructures were prepared from natural pyrite (NP) utilizing argon plasma due to its sputtering and cleaning effects resulting in more active surface area. The NP and PTP were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM) methods. The performance of the PTP was greater than NP for treatment of Reactive Red 84 (RR84) by the heterogeneous sono-Fenton process. The optimum amounts of main operational parameters were obtained as PTP of 4 g/L, initial dye concentration of 10 mg/L, pH of 5, and ultrasonic power of 300 W after 120 min of reaction time. Also, the effects of enhancers, and inorganic salts and t-butanol as hydroxyl radical scavengers on the degradation efficiency were investigated. Gas chromatography-mass spectroscopy analysis (GC-MS) was applied for detection of some degradation intermediates. Environmentally friendly plasma modification of the NP, in situ production of H₂O₂ and ^OH radicals, low leached iron concentration and repeated reusability at the milder pH are the significant benefits of the PTP utilization.

Full text of DOI:10.1016/j.ultsonch.2015.09.012

Ultrasonics Sonochemistry 29 (2016) 213–225
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Heterogeneous sono-Fenton-like process using nanostructured pyrite
prepared by Ar glow discharge plasma for treatment of a textile dye
a
b
Alireza Khataee a, , Peyman Gholami , Behrouz Vahid
⇑
a Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, 51666-16471 Tabriz, Iran
b Department of Chemical Engineering, Tabriz Branch, Islamic Azad University, 51579-44533 Tabriz, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The plasma-treated pyrite (PTP) nanostructures were prepared from natural pyrite (NP) utilizing argon
plasma due to its sputtering and cleaning effects resulting in more active surface area. The NP and PTP
were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brun
auer–Emmett–Teller (BET) and scanning electron microscopy (SEM) methods. The performance of the
PTP was greater than NP for treatment of Reactive Red 84 (RR84) by the heterogeneous sono-Fenton pro-
cess. The optimum amounts of main operational parameters were obtained as PTP of 4 g/L, initial dye
concentration of 10 mg/L, pH of 5, and ultrasonic power of 300 W after 120 min of reaction time. Also,
the effects of enhancers, and inorganic salts and t-butanol as hydroxyl radical scavengers on the degra-
dation efficiency were investigated. Gas chromatography–mass spectroscopy analysis (GC–MS) was
applied for detection of some degradation intermediates. Environmentally friendly plasma modification
Received 24 August 2015
Received in revised form 17 September
2015
Accepted 18 September 2015
Available online 30 September 2015
Keywords:
Nanostructures
Plasma-treated pyrite
Heterogeneous sono-Fenton
Ar glow discharge plasma
Å
of the NP, in situ production of H2O2 and OH radicals, low leached iron concentration and repeated
reusability at the milder pH are the significant benefits of the PTP utilization.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
remediation methods like AOPs, which can not only degrade but
also mineralize different contaminants without producing sec-
Increases in the variety of compounds generated by human
activities have led to a considerable volume of various industrially
polluted wastewaters. Conventional wastewater treatments com-
prising of biological, physical and chemical processes are not effec-
tive for the removal of all pollutants from contaminated effluents.
In recent years, considerable efforts have been devoted to develop-
ing effective, simple and cheap treatment processes to remove pol-
lutants from contaminated aqueous media. Advanced oxidation
processes (AOPs) have attracted special attention as promising
alternatives to the conventional processes due to the high effi-
ciency for the treatment of wastewater [1,2]. Among AOPs, the
Fenton process is an effective and simple treatment method that
is extensively used in the degradation of various chemicals from
contaminated water sources [3].
ondary waste [5]. Among AOPs, the Fenton and sonication pro-
cesses are simple and efficient methods which are applied for the
mineralization of various contaminants from polluted water
sources [3]. Hydroxyl radicals (ÅOH) as the most powerful oxidizing
agent in AOPs, can be generated by the Fenton reaction involving
ferrous iron (Eq. (1)) or from water dissociation under ultrasonic
irradiation (Eq. (2)) through cavitation phenomenon [6]. After
directing ultrasonic waves into a liquid, the cavitation leads to for-
mation, growth, and finally collapse of microbubbles producing
high localized temperatures and pressures (hot spot approach)
[7]. However, ultrasonic process consumes more time and energy
compared to other methods due to the low degradation rate; hence
it can be combined with other processes like Fenton process to
enhance their efficiency for the wastewater treatment [8].
Textile industry wastewater has large amounts of organic dyes,
which are resistant to the biological methods. Moreover, other
physical and chemical processes like adsorption and coagulation
merely transfer contaminants to a secondary phase requires more
treatment [4]. Hence, it is significant to find effective wastewater
Fe2þ þ H2O2 ! Fe3þ þ ꢀOH þ OHꢁ
H2OþÞÞÞ ! ꢀOH þ Hꢀ
ð1Þ
ð2Þ
On the other hand, homogeneous Fenton process can be just
performed in acidic condition (pH 3) preventing the iron precipita-
tion; it also needs to risky storage and transportation of hydrogen
peroxide (H2O2). Moreover, recycling of homogeneous catalyst as
⇑
Corresponding author.
E-mail addresses: a_khataee@tabrizu.ac.ir, ar_khataee@yahoo.com (A. Khataee).
http://dx.doi.org/10.1016/j.ultsonch.2015.09.012
1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

214
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
well as its separation from the treated wastewater confines the
application of Fenton process. Application of heterogeneous Fenton
process with no need for catalyst separation and low leached iron
is the practical solution to overcome these obstacles [2]. Heteroge-
neous Fenton catalysts including pyrite [9], magnetite [10], and
goethite [11] are used in this process in which superficial solid
Fe ions catalyze the production of ÅOH. The pyrite (FeS2) is the most
abundant metal sulfide in nature and nontoxic. It has potential for
utilization in the heterogeneous Fenton reaction. The usage of syn-
thesized pyrite has been studied in several wastewater treatment
procedures, such as the Fenton and adsorption processes [12,13].
It should be mentioned that the heterogeneous Fenton process
has some limitations compared to the homogeneous one including
high mass transfer resistance and few active reaction sites. The
effective methods to solve these drawbacks are applying nanos-
tructured particles and ultrasonic irradiation [14].
Plasma is ionized gas, which composes of negative and positive
ions, electrons and neutral species, which is noticed as the forth
state of matter, which is an environmentally-friendly method to
produce various nanostructures for various applications [15]. For
instance, non-thermal plasma techniques such as glow discharge,
radio frequency, and silent discharge have been used for modifica-
tion of different catalysts surfaces and enhancement of their effi-
ciencies [16,17]. For example, the plasma treatment alters the
activity and surface structure of natural clinoptilolite and synthe-
sized zeolites [16]. The catalytic activity and stability of the Pd/
HZSM-5 catalyst improve after plasma treatment [18]. The selec-
tivity for hydrogenation of acetylene increases after using the H2,
Ar and O2 atmosphere plasma for modified Pd/TiO2 catalyst [19].
Glow discharge plasma treated magnetite using oxygen and argon
was applied for treatment of an oxazine dye by catalytic ozonation
[20].
reactor, which comprised of a Pyrex tube, which was sealed with
two aluminum bonnets on its both sides. High-voltage direct cur-
rent (DC) supplied by a DC power source was connected by the alu-
minum bonnets. Argon gas was pumped within the reactor to bring
the pressure to 40 Pa, applying rotary and turbo-molecular pumps.
A schematic diagram of the plasma set-up is illustrated in Fig. 1.
2.3. Characterization of pyrite nanostructures
An X-ray diffractometer (XRD, D-5000, Siemens, Germany) was
applied for identification of the pyrite phase before and after
plasma treatment. FT-IR spectra of the NP and PTP were recorded
on a Bruker Tensor 27 FT-IR spectrophotometer (Germany) using
KBr pellets. The size and morphology of the NP and PTP samples
were investigated using a SEM equipped with an EDX microanaly-
sis (MIRA3 FEG-SEM Tescan, Czech). Microstructure Distance Mea-
surement software (Nahamin Pardazan Asia Co., Iran) was applied
to determine the size distribution of the structures developed on
the surface of the PTP by using of SEM micrographs. The surface
area and pore volume of the both samples were measured by nitro-
gen adsorption/desorption at 77 K using a Gmini series instrument
(Nitrometrics, Japan).
The point of zero charge (pHPZC) of the PTP is a pH value in
which the sample does not alter the solution pH. pHPZC was deter-
mined based on a procedure explained by Mustafa et al. [21].
Accordingly, 0.2 g of the PTP were added to nine Erlenmeyer flasks
containing 40 mL of NaNO3 solution (0.1 N), individually. The pH of
the prepared suspensions was adjusted to the range of 2–10 by
adding of HNO3 and NaOH solutions using a pH meter (Metrohm,
Switzerland). The suspensions were agitated in a shaker incubator
(Fannavaran ISH55LD, Iran) at a rate of 175 rpm for 48 h at ambi-
ent temperature. Then, the suspension pH was determined as well
The aim of this research is to prepare nanostructured pyrite
from natural pyrite with the Ar glow discharge plasma for treat-
ment of RR84 synthetic solutions by the heterogeneous sono-
Fenton-like process (US/PTP). To the best of our knowledge there
is no report for modification of the NP by plasma and also its usage
in combination of ultrasonic for treatment of a model textile dye.
The characterizations of treated NP were carried out by XRD, FT-
IR, BET and SEM methods. The effect of operational parameters
including the PTP dosage, initial RR84 concentration, solution pH,
ultrasonic power, presence of enhancers, t-butanol, and inorganic
salts were studied on the degradation efficiency in a series of batch
experiments. Degradation intermediates of RR84 were recognized
by the GC–MS.
as the difference between the initial and final pH (DpH).
2.4. Heterogeneous sono-Fenton-like process
Degradation experiments were performed in a 250 mL Erlen-
meyer, which was placed in an ultrasonic bath (EP S3, 40 kHz,
300 W, Sonica, Italy). The distance between the bottom of the reac-
tion vessel and the sonication source was adjusted at 1.0 cm,
where the surface of the solution had the maximum turbulence.
In general, 100 mL of the RR84 solution with distinct concentration
and certain dosage of the PTP were used in all of the experiments.
Then, the solution pH was adjusted by adding H2SO4 (0.1 M) and
NaOH (0.1 M) and measured by the pH meter to the known values.
During the degradation process, the 3 mL of sample was with-
drawn with pipette at 15 min reaction intervals. Then the sus-
pended particles were separated completely from the treated
solution with a centrifugal separator. The absorbance of the dye
solution was measured at the maximum wavelength of the dye
(kmax = 495 nm) using an UV–Vis spectrophotometer (Lightwave
S2000, England). An atomic absorption spectroscopy (AAS) appara-
tus (Novaa 400, Analytikjena, Germany) was used for measure-
ment of the concentration of dissolved iron in the solution. GC–
MS analysis was carried out applying a method described in our
previous work for identification of the generated degradation
intermediate of RR84 during the process [6].
2. Experimental procedure
2.1. Materials
Natural pyrite was obtained from Morvarid iron mine (Zanjan,
Iran). The mono azo dye, Reactive Red 84 was provided from
Ciba-Geigy Ltd (Switzerland), which was used as a textile dye for
wool and silk. The properties of RR84 are presented in Table 1.
Hydrochloric acid (37%), sodium hydroxide (99%) and hydrogen
peroxide (30%) were purchased from Merck (Germany). Ethanol
was supplied from Jahan Alcohol Teb Co. (Arak, Iran). Distilled
water was used throughout the experiments.
3. Results and discussion
2.2. Preparation of pyrite nanostructures
3.1. Characterization of the PTP
Argon glow discharge plasma was utilized to produce pyrite
nanostructures from NP. It was crushed by rod and ball milling
(Kian Madan Pars Co, Tehran, Iran) to obtain micro-grained pyrite
particles. Then, 2 g of the pyrite particles were placed in a plasma
Fig. 2 demonstrates XRD patterns of the NP and PTP samples. In
these spectra, peaks were seen at 2h values of 28. 18, 33.20, 37.22,
40.88, 47.56, 56.38, 59.10, 62.02, and 64.56°, which were attributed

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
215
Table 1
Characteristic of RR84.
Name
Chemical structure
Molecular formula
C26H19BrN4Na2O9S3
Color index number
13429
kmax (nm)
Mw (g/mol)
753.53
Reactive Red 84
495
Fig. 1. Schematic diagram of the glow discharge plasma system used in this study.
Fig. 2. XRD patterns of (a) NP and (b) PTP samples.
to the characteristic (111), (200), (210), (211), (220), (311),
(222), (023), and (321) planes of pyrite (JCPDS card 42-1340),
respectively. Comparison between these two spectra and with the
standard pyrite pattern confirms that the crystal structure of pyrite,
which does not alter by the Ar plasma treatment [21]. Moreover, It
seems that the XRD peaks at 2h of 27.28, 38.14, 39.72 and 48.88°,
relate to the (110), (211), (011) and (200) reflection planes of bis-
muth [22]. This implies that the presence of bismuth in low quanti-
ties in pyrite samples.
FT-IR spectra of the NP and PTP samples are plotted in Fig. 3 In
both curves, the peaks of the Fe–S stretching vibration mode (540,
615.2, and 781.1 cmꢁ1), the Fe–O–OH vibration (1020.3 cmꢁ1), the

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A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
Fig. 3. FT-IR spectra of (a) NP and (b) PTP samples.
Fig. 4. SEM micrographs of spectra of (a and b) NP and (c and d) PTP samples.

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
217
Fig. 5. Particle size distribution of pyrite nanostructures.
SEM images of the NP and PTP are shown in Fig. 4 with different
magnifications. The SEM image of NP (Fig. 4a and b) shows a bulky
structure. However, Fig. 4c and d indicate the presence of fine
nanostructures in PTP sample. As can be seen in Fig. 5, the average
width of these structures is 30–50 nm. Based on the results from
the XRD and FT-IR analyses, the Ar plasma treatment does not con-
vert the pyrite to the other material. Comparison of the SEM
images for the NP and PTP shows the development of nanostruc-
tures on the PTP surface by the plasma treatment. These nanos-
tructures increase the surface area and hence improve the
performance of the PTP.
EDX spectra of the NP and PTP samples are shown in
Fig. 6a and b, which prove that the main elements such as Fe and
S still exist in both of the structures. The weight percent (wt%) of
Fe and S elements were found to increase after plasma treatment,
which implied some impurities of NP were removed from the pyr-
ite surface [26]. By comparison of curve a (NP) with curve b (PTP)
in the FT-IR spectra, the intensity of peaks for the PTP sample were
found to be increased. This can also confirm the obtained results
from the EDX analysis.
The BET isotherm model was used to analyze the data obtained
from the N2 adsorption/desorption isotherm of NP and PTP. After
plasma treatment, the estimated BET surface area and pore volume
of the NP were found to be 6.486 m2/g and 1.424 cm3/g, which
increased to 9.652 m2/g and 2.199 cm3/g, respectively. These
enhancements also confirm the development of pyrite
nanostructures.
Fig. 7 illustrates the
initial pH of the solution was 6.3,
D
pH change versus the initial pH. When the
pH was calculated zero, indicat-
D
ing that the net charge of the PTP was zero at this pH. Moreover,
when the solution pH was below pHPZC, the PTP surface charge
was positive, while it was negatively charged with pH above pHPZC
[27].
3.2. Comparison of different treatment processes for removal of RR84
The performance of ultrasonic alone and heterogeneous Fenton-
like processes using the NP and PTP in the absence and presence of
ultrasonic irradiation for the removal of RR84 in aqueous solution
were compared to study the effect of Ar plasma treatment and son-
ication on the NP performance. As can be observed from Fig. 8,
after 120 min of treatment, 93.7% degradation occurs applying
Fig. 6. EDX spectra of (a) NP and (b) PTP samples.
Fe–SO4 vibration (1623.9 cmꢁ1), and the O–H vibration
(3450.5 cmꢁ1
) were found [23,24]. The peaks at 2924 and
2853 cmꢁ1 are related to the asymmetric and symmetric C–H
bonds, respectively [25].

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A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
Fig. 7. Plot for determination of PTP pHpzc
.
Fig. 8. Comparison of the removal efficiency of RR84 with different processes; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, ultrasonic power = 300 W, and pH = 5. The inset shows
mentioned processes follow pseudo-first order kinetic.
Table 2
the US/PTP, which was selected to carry out the rest of experiments
Effect of the different processes on the apparent pseudo-first order constants of
as the most efficient process.
degradation of RR84.
Nanostructured pyrite is formed under the plasma treatment,
No.
Process
kapp (minꢁ1
)
Correlation coefficient (R2)
which increases the surface area of the NP. Furthermore, the
plasma removes some impurities from the surface of the NP, which
increased the presence of iron on its surface [28]. Hence, more iron
is available for the heterogeneous Fenton-like process to produce
in-situ H2O2 and ÅOH radicals by following reactions on the surface
of the PTP [29]:
1
2
3
4
5
US
NP
PTP
US/NP
US/PTP
0.0022
0.0040
0.0085
0.0096
0.0205
0.989
0.998
0.985
0.991
0.994
Fe2þ þ O2 ! Fe3þ þ Oꢀ2ꢁ
ð3Þ
ð4Þ
ð5Þ
surf
surf
Fe2þ þ Oꢀ2ꢁ þ 2Hþ ! Fe3þ þ H2O2
The effect of ultrasonic irradiation can be explained by dissoci-
ation of water molecules (Eq. (2)) and regeneration of Fe2+ from Fe3
surf
surf
+
Fe3þ þ H2O ! Fe2þ þ ꢀOH þ Hþ
(Eq. (6)), which catalyzes Fenton reaction (Eq. (1)) to produce
surf
surf
Å
extra OH radicals [14]:
Then, Fenton reaction can be occurred on the surface of the PTP
and in the aqueous solution by Fe2+ ions which are existed in pyrite
structure and dissolved in the solution, respectively (Eq. (1)) [9].
ÞÞÞ
Fe3þ þ H2O2 ! Fe2þ þ HO2ꢀ þ Hþ
ð6Þ
surf
surf

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
219
Fig. 9. The effect of suspension pH on the degradation efficiency of RR84 by US/PTP process; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, and ultrasonic power = 300 W. The inset plot
was depicted based on pseudo-first order kinetic assumption for degradation of RR84.
In addition, turbulence of the solution and solid–liquid mass
transfer coefficient increase under ultrasonic waves [30]. The
number of active sites on the catalyst’s surface also enhance owing
to deaggregation of the particles in the so; moreover, the number
of cavitation microbubbles can enhance at low tensile strength of
the solid–liquid interface and more cavitation nuclei in the crevices
of the solid [7].
3.3. Effect of the main operational parameters on the sono-Fenton-like
process
The effect of main operational parameters including pH, PTP
dosage, initial RR84, and ultrasonic power on the degradation of
the organic pollutant was studied. pH is one of the most effective
parameters in the Fenton process. Accordingly, the effect of this
parameter on the degradation of RR84 was investigated, and the
obtained results are shown in Fig. 9. At low pHs, high degradation
efficiencies are achieved, and degradation efficiency (%) declines
with increasing of the solution pH. It can be attributed to the oxi-
dation potential of ÅOH/H2O redox pair, which decreases from 2.59
to 1.65 V vs. standard hydrogen electrode (SHE) by increasing the
solution pH from 0 to 14 [32]. This reveals that in acidic mediums,
The inset of Fig. 8 reveled that all of the applied processes obey
pseudo-first order kinetic; from the slope of straight lines in the
inset plot, the apparent pseudo-first order rate constants (kapp
)
for the degradation of RR84 were estimated and presented in
Table 2. High correlation coefficients (R2), which was more than
0.98 verified the proposed kinetic.
Considerable synergistic effect of the ultrasonic irradiation and
Fenton-like process using the PTP for degradation of RR84 can be
expressed in the terms of the apparent pseudo-first-order rate con-
stants through Eq. (7) [31] as 0.48:
Å
the oxidation potential of the OH radicals for the degradation of
RR84 molecules is more than natural or alkaline mediums. More-
over, based on the pHpzc analysis (Fig. 7), at pH values lower than
pHpzc, which is 6.3, the PTP surface charge is positive. It enhances
the adsorption of anionic dye molecules on its surface, and thus
kUS=PTP ꢁ ðkUS þ kPTP
Þ
Synergy ¼
ð7Þ
kUS=PTP
Fig. 10. Dissolved iron concentration in solution phase after 120 min; [PTP] = 4 g/L.

220
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
Fig. 11. The effect of PTP dosage on the degradation efficiency of RR84 by US/PTP process; [RR84]0 = 10 mg/L, ultrasonic power = 300 W, and pH = 5. The inset plot was
depicted based on pseudo-first order kinetic assumption for degradation of RR84.
more RR84 molecules are degraded by the adsorbed reactive radi-
cals [33,34]. Finally, to measure the total concentration of dis-
solved iron ions, which released into the solution at various pHs,
AAS was used and the data are plotted in Fig. 10. The dissolved iron
concentration increases with decreasing of the pH. Hence, high
degradation is observed when the pH was adjusted in the range
of 2– 5, which were approximately the same. Thus, the pH 5 is
selected as the optimized value. It should be mentioned that, the
total leaking iron concentration is lower than 0.8 mg/L after
heterogeneous sono-Fenton-like process in the presence of PTP at
different pH values (Fig. 10). At this level of dissolved iron concen-
tration, the generation of hydroxyl radicals is mainly owing to the
heterogeneous sono-Fenton-like and the homogeneous Fenton
reaction in the bulk solution contributed little to the degradation
of RR84 [35].
pens due to the high active sites for the reactive radical formation
and adsorption [36]. Afterwards, the degradation efficiency
remains almost constant owing to the scavenging effect of Fe2+
on the hydroxyl radicals (Eq. (8)), [37,38].
ꢀOH þ Fe2þ ! OHꢁ þ Fe3þ
ð8Þ
Also, screening of the ultrasonic waves by the more amount of
solid particles inhibits the solution from receiving the same value
of dissipated energy [39]. Since the highest degradation efficiency
of RR84 is observed with 4 g/L of PTP and after that remains
approximately constant after 120 min of treatment, hence, this
dosage is selected as the desired amount for the rest of the
experiments.
By increasing of the RR84 concentration from 10 to 40 mg/L, the
degradation efficiency decreases from 93.7% to 30.5% in 120 min
(Fig. 12). At the identical operational conditions, the available sur-
face area of the PTP to adsorb dye molecules are constant, and also
the same number of hydroxyl radicals were generated, accordingly,
Fig. 11 demonstrates the degradation of RR84 as a function of
the PTP dosage in the solution, which increases from 23.8% to
93.7% after 120 min over the PTP dosage range of 0–4 g/L. This hap-
Fig. 12. The effect of dye concentrations on the decolorization efficiency of RR84 by US/PTP process; [PTP] = 4 g/L, ultrasonic power = 300 W, and pH = 5. The inset plot was
depicted based on pseudo-first order kinetic assumption for degradation of RR84.

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
221
Fig. 13. The effect of ultrasonic power on the decolorization efficiency of RR84 by US/PTP process; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, and pH = 5. The inset plot was depicted
based on pseudo-first order kinetic assumption for degradation of RR84.
Table 3
Effect of the operational parameters on the apparent pseudo-first order constant of
degradation for sono-Fenton-like process.
3.4. Effect of hydroxyl radical scavengers
Chloride, sulfate and carbonate are regular inorganic anions in
textile effluents and may influence the treatment efficiency, which
is called the salting-out effect. Fig. 14 indicates that the presence of
these chemicals in the dye solutions decline the RR84 degradation
efficiency. These negative ions are adsorbed on the positively
charged surface of PTP at pH 5 and occupied active sites that con-
tributed to the US/PTP process; this reduces the amount of ÅOH pro-
duction [43].
Operational parameters and
amounts
kapp
Correlation coefficient
(minꢁ1
)
(R2)
pH
2
3
5
7
0.0264
0.0223
0.0216
0.0074
0.0029
0.985
0.978
0.969
0.993
0.986
9
PTP dosage (g/L)
The effect of NaCl on the RR84 degradation was higher than that
of Na2CO3 and Na2SO4. This can be attributed to the scavenging of
ÅOH by chloride and the formation of reactive radicals with low
0
0.5
1
2
3
0.0023
0.0032
0.0042
0.0081
0.0153
0.0198
0.0211
0.994
0.992
0.004
0.966
0.986
0.992
0.989
Å
oxidation potential rather than OH by the Eqs. (9)–(11) [6,41]:
Å
Furthermore, the generated Cl radicals have a great tendency to
4
5
Å
react with H2O2, which declines OH formation (Eq. (12)) [44].
Clꢁ þ Hþ ! ꢀClOHꢁ
ð9Þ
ð10Þ
ð11Þ
ð12Þ
RR84 concentration (mg/L)
10
20
30
40
0.0205
0.0071
0.0047
0.0033
0.994
0.998
0.997
0.988
ꢀClOHꢁ þ Hþ ! H2O þ ꢀCl
Clꢁ þ ꢀCl ! ꢀClꢁ2
Ultrasonic power
0
100
200
300
0.0085
0.0102
0.0145
0.0205
0.985
0.991
0.976
0.994
ꢀCl þ H2O2 ! Hþ þ Clꢁ þ HO2ꢀ
Moreover, carbonate ions may react with ÅOH and lead to
the generation of COÅ3ꢁ radicals with low oxidation potential
(Eq. (13)). This decreases the RR84 degradation efficiency [45].
the number of RR84 molecules and their degradation intermedi-
ates that can react with hydroxyl radicals is thus constant even
with increasing of the initial dye concentration [38,40,41].
The degradation efficiency of 71.2, 84.4, and 93.7% was achieved
under ultrasonic power of 100, 200 and 300 W, respectively
(Fig. 13) indicating the positive effect of ultrasound intensity on
the dye degradation. With increasing of the ultrasonic power, more
energy is dissipated to the system, which enhances the number of
cavitation bubbles and consequently reactive radicals as explained
in Section 3.2.1 [42].
CO32ꢁ þ ꢀOH ! CO3ꢀꢁ þ OHꢁ
Sulfate ions can also react with OH (Eq. (14)), however, the
formed SOÅ4ꢁ radicals have oxidation potential of 2.6 V [46,47].
Hence, the scavenging effect of sulfate was declined by the SO4Åꢁ
radicals formation.
ð13Þ
Å
SO42ꢁ þ ꢀOH ! SO4ꢀꢁ þ OHꢁ
Finally, t-butanol as an organic ÅOH radical scavengers was
added to the RR84 solution and the degradation efficiency also
decreased owing to the following reaction (Eq. (15)) [48]:
ð14Þ
In all of the experiments, the degradation rate also obeys the
pseudo-first order kinetics and the apparent pseudo-first-order
rate constants were determined from the inset plots of Figs. 9
and 11–13 and presented in Table 3.
ꢀOH þ ðCH3Þ3COH ! H2O þ ꢀCH2CðCH3Þ2OH
ð15Þ

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A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
Fig. 14. Effect of scavengers on the degradation of RR84 by US/PTP process; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, ultrasonic power = 300 W, pH = 5, and [Scavenger]0 = 10 mg/L.
Fig. 15. Effect of the presence of hydrogen peroxide and peroxydisulfate on the degradation of RR84 by US/PTP process; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, ultrasonic
power = 300 W, pH = 5, and [Enhancer]0 = 10 mg/L.
As a consequence, the radical mechanism for the degradation of
RR84 with the main role of ÅOH radicals was confirmed, which is in
consistent with other studies [49].
3.6. RR84 degradation intermediates
Identified compounds of RR84 degradation by US/PTP process
were presented in Table 4, when the match factor of mass spec-
trum was above 90%. Comparison of the recognized compounds
and the dye molecule indicates the conversion of RR84 to the small
aromatic and aliphatic compounds, which can occur by cleavage of
C–C, C–N, C–S or N@N bonds [52]. However, identification of all
intermediates is not possible due to their slight accumulation
and limitation related to the GC–Mass method [53].
3.5. Effect of enhancers
Fig. 15 shows the effect of adding enhancers on the degradation
efficiency of RR84. The enhanced degradation efficiency in the
Å
presence of H2O2 is due to the increased formation of OH by the
Fenton reaction (Eq. (1)) [50].
The peroxydisulfate anion (S2O28ꢁ) effect is also studied on the
RR84 degradation. This oxidant can be converted to sulfate radicals
(SOÅ4ꢁ) in the presence of PTP as a heterogeneous catalyst (Eq. (16)),
which is more powerful oxidant (2.6 V) than peroxydisulfate
(2.01 V) [51]:
3.7. Stability of PTP in successive US/PTP process
One of the most significant properties of a catalyst from practi-
cal point of view is its stability in the successive applications.
Hence, the PTP was used in five repeated sono-Fenton-like
Fe2þ þ S2O2ꢁ ! Fe3þ þ SO4ꢀꢁ þ SO42ꢁ
ð16Þ
surf
8
surf

A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
223
Fig. 16. Degradation efficiencies for five successive cycles; [RR84]0 = 10 mg/L, [PTP] = 4 g/L, ultrasonic power = 300 W, and pH = 5.
Table 4
Identified by-products during degradation of RR84 by sono-Fenton-like process.
No.
1
Compound name
Structure
Retention time (min)
9.424
Main fragments
b-Naphthylamine
71, 115, 116, 143
2
3
Nitrobenzene
p-Cresol
24.559
18.625
77, 51, 130
39, 51, 77, 107
4
5
1,2-Benzenedicarboxylic acid
Hydroquinone
9.464
104, 76, 50
13.109
45, 73, 112, 239
6
7
Butanamide
5.106
7.680
27, 41, 44, 59
43, 53, 73, 147
Butenedioic acid
processes, and its function and reusability were determined. After
each degradation experiment, the PTP particles were separated
from the solution, washed, dried, and applied for the next experi-
ment. The RR84 degradation efficiency for these five successive
processes is demonstrated in Fig. 16. The results reveal that the
degradation efficiency was still high after several usages. More-
over, the data of released iron measurements (Fig. 10) shows that
at pH 5, the dissolved iron concentration was about 0.45 mg/L. In

224
A. Khataee et al. / Ultrasonics Sonochemistry 29 (2016) 213–225
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a
novel Fe-impregnated nanocatalyst for the
4. Conclusions
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Å
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The authors thank the University of Tabriz for the support
provided.
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