Draft
Facile approach for synthesis of stable, efficient and
recyclable ZnO through pulsed sonication and its application for degradation of recalcitrant azo dyes in wastewater
Journal: Canadian Journal of Chemistry
Manuscript ID cjc-2017-0696.R1
Manuscript Type: Article
Date Submitted by the Author: 26-Mar-2018
Complete List of Authors: Gunasekaran, Kumaravel; Indian Institute of Science Education and
Research Bhopal, Chemical Engineering Ramesh, Saranya; University of Dublin Trinity College, Department of Physics
Keyword: pulsed ultrasound, advanced oxidation process, water treatment, decolourization, azo dye
Is the invited manuscript for consideration in a Special
Issue?: N/A
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Facile approach for synthesis of stable, efficient and recyclable ZnO through pulsed
sonication and its application for degradation of recalcitrant azo dyes in wastewater
G Kumaravel Dinesh*
Department of Chemical Engineering
Indian Institute of Science Education and Research Bhopal
Bhopal (M.P.)
India
Rameshkumar Saranya
Polymeric Materials & NanoComposites (PMNC)
Department of Physics
Trinity College Dublin
University of Dublin
Dublin, Ireland
*Corresponding author
Email: [email protected], [email protected] (G Kumaravel Dinesh)
Tel: +91 755 669 2571
Fax: +91 755 669 2392
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Highlights
•Synthesis of ZnO using ultrasound-assisted sol–gel and hydrothermal method.
•ZnO synthesized via ultrasound has large specific surface area and high pore volume.
•ZnO is better for adsorption and sonocatalysis.
•Simultaneous application of sonolysis and photolysis gives highest decolorization.
•97% decolorization in 20 minutes and 87% COD removal in 17 minutes were achieved.
Abstract
In the present study, the ultrasound in pulsed mode used as a part of advanced oxidation
method. The influence of pulsed ultrasound mode for the preparation of zinc oxide wurtzite
nanoparticle were synthesized. The catalysts synthesized were analysed using SEM, TEM,
EDAX, BET surface area, XRD and DRS to study about their morphological and structural
characterizations. The ZnO nanoparticles exhibited highly hexagonal structure from pulsed
sonication synthesis route. The efficiency of the decolourization of RR4 were studied under
different operation parameters like dye concentration, initial solution pH, antioxidants like
H2O2 concentration and catalyst loading. The hybrid combined process of pulsed sonolysis,
pH (4.0), H2O2 (17.64 mmol) and Catalyst (0.35 g/L)) achieved 97% degradation and 87.5%
COD removal in about 20 minutes of reaction time. The cyclic degradation studies of RR4
removal with 0.35 g/L of ZnO showed the reusability of catalyst upto 5th removal cycle with
negligible loss in the catalytic performance. GC–MS study, used for the detection of the RR4
intermediates revealed the oxidation/reduction reaction by the reactive radicals proceeded via
the reductive cleavage of the azo bonds. The studied process based on the pulsed ultrasound
is found to be effective for the degradation of RR4 dye.
Keywords: pulsed ultrasound, azo dye, advanced oxidation process, reactive red 4, water
treatment, decolourization.
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1. Introduction
In the recent decades the disposal of wastewater from the different types of industries
like pesticides, cosmetics, paper, textiles and leather has been increasing day by day. Among
all textile and dying industry are the major contributor of wastewater. The accumulation of
toxic compounds in the water resources have been entailed as unsolved puzzle for the
environment and more hazardous to human health. In fact, approximately 200 litres of water
required to produce l kg of textiles [1]. About 20% of the dyes and dye products enter into the
water resources through the discharge of textile industries. The azo dyes constitute the major
dye used in textile and dying industries exceeding over 60% of the dye used [2]. The
intermediates of the dye compounds have adverse effects as deplete the dissolved oxygen in
aquatic system and endanger to flora and fauna. Hence it is more essential to treat with an
efficient method to transform all these dyes and intermediates into toxic free compounds [3].
Moreover, the anaerobic and incomplete degradation of dyes generates aromatic amines and
other recalcitrant secondary pollutants which are more toxic and carcinogenic [4]. Hence,
there is a need for an efficient oxidation method to treat the dyes and its by-products before
being disposed into water resources. For decades advanced oxidation processes have proved
to be an efficient and powerful tool to eliminate these dyes from wastewater [5].
Environmental sonochemistry is an example for advanced oxidation process which is
widely followed treatment methods for the complete destruction of recalcitrant pollutants in
the wastewater [6]. The physical and chemical effects of the ultrasound produced cavitation
bubbles in the aqueous solution generates high temperature and pressure within the liquid
during the collapse of the bubbles deals with the destruction of complex compounds [7]. The
effective treatment is caused by the increased rate of generation of ●OH radicals and other
reactive moieties. The degradation kinetics followed by the sonochemical treatment can be of
zero order, first order depend upon the in situ production of ●OH radicals and their degree of
contact with the contaminants present in the wastewater [8]. The limitations of the individual
process can be eliminated by the process integration of ultrasound with the addition of
catalyst. Further the turbulence in the aqueous solution created by the ultrasound helps in
catalyst fragmentation and de-agglomeration thus increasing the activity of the catalyst [9].
Recently the pulsated ultrasound treatment has been studied for improving the efficiency of
the degradation kinetics.
Ultrasound has a uniqueness as the chemical free treatment method utilizing the
generated ●OH radicals i.e., short lived oxidants [10]. Major hinderance is the high
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operational energy being wasted in the continuous mode. The shielding effect of the bubbles
created when these bubble accumulate near the irradiating source [11]. This energy wastage
can be reduced by the several ways like modification of the reactor, integrating the ultrasound
with other techniques and the pulsed mode ultrasound. The pulsed mode ultrasound is usually
operating ultrasonic equipment in cyclic repetition such as ON and OFF mode repeatedly.
This process has been developed to deliver more rapid effects both in time and life cycle of
the generated cavitation bubbles [12]. In recent years the highlights of the ultrasound
performance in degradation of pollutants has been reported as the effective method among
advanced oxidation process.
The technologies employed in degradation studies still produces incomplete
intermediates after the oxidation process [13]. The usage of catalyst helps in transforming
these complex intermediates into harmless compounds [14]. Recently the piezoelectric and
electronic properties of wurtzite ZnO crystals showed the generation of electric dipole where
the wonderful cation-anion displacement occurs promoting the formation of oxidative
radicals very rapidly [15]. When these semiconductor catalysts coupled with the pulsed
ultrasound in the degradation of azo dyes showed a beneficial synergetic effects. This
strategy helped inducing a high efficiency technology to remove the organic pollutant from
wastewater. The aim of this work was to assess the performance of pulsed ultrasound with
ZnO semiconductor catalyst as a part of advanced oxidation process for degradation of azo
dyes, studying the effects of various other parameters like pH, anti-oxidants, dye
concentration and volume on the rate of decolourization. Moreover, the synergetic efficiency
of the combined parameters with pulsed ultrasound process was evaluated.
2. Experimental procedure
2.1. Chemicals
Reactive Red 4 was obtained from Sigma Aldrich (Colour Index No. 18105, EC No. 241-
663-8, molecular weight: 995.21) and was used as supplied. High purity reagents, sulphuric
acid (Merck), sodium hydroxide (Merck), zinc acetate (Merck), ethanol and hydrogen
peroxide solution (30% w/w, Merck) were used for the solution. All the solutions were
prepared using Millipore water. All chemicals and reagents used were of analytical grade and
used without any further purification. Ultrasonic processor (Sonics & Materials, VCX 500)
was used for the experimental study.
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2.2 Synthesis of catalyst
The ZnO particles are prepared in an ultrasonic processor using zinc acetate and sodium
hydroxide as precursors. 0.005 M zinc acetate and 0.02 M sodium hydroxide in the
stoichiometric ratio of 1:2 were dissolved in water [16]. Following the hydrothermal process,
the solution was sonicated for 30 min with an ultrasonic probe. The milky white precipitate
was filtered and washed with ethanol 5 times to remove the impurities. The obtained ZnO
nanoparticle was dried in a vacuum oven at 105°C for 24 hours to remove the moisture.
2.3 Catalyst characterization
The morphology of synthesized catalysts ZnO was investigated using scanning electron
microscopy (SEM) images (JEOL, Hitachi Corporation, Japan). The elemental composition
of the prepared catalyst by the energy-dispersive analysis of X-rays (EDAX) conjugating
with SEM confirms the formation of ZnO. The surface area and pore volume of the
synthesized catalyst were measured using BET (Brunauer–Emmett–Teller) surface area and
particle analyser (ASAP 2020, Micromeritics, USA). The phase purity and composition of
the synthesized catalyst were studied using X-ray diffraction (XRD) (Cu Kα radiation, D8
advance Bruker, Germany). XRD patterns were recorded in the 2θ range between 20° and
80°. By using the Scherrer formula the average crystallite size (D) of the catalyst was
calculated (Eqn. 1).
0.9 λD =
β cos θ, Eqn. 1
where k is the X-ray wavelength and b is the full width at half maximum (FWHM) obtained
at a corresponding 2θ angle. UV–visible diffuse reflectance spectra (DRS) were recorded to
determine the absorption capacity of the catalysts prepared. Specord (Analytikjena) S-600
UV–visible spectrophotometer was used in the wavelength range of 200–800 nm to obtain
the DRS spectra [17].
2.4 Experimental procedure
0.01004 mmol (1000 ppm) concentration of Reactive Red 4 stock solution was prepared with
Millipore water and used for the experimental study. The degradation studies were carried out
using 1000 mL dye solution placed in water cooled jacketed beaker at room temperature 25º
C. Fig 1a. represents the scheme of the experimental setup. The pH of the Reactive Red 4
solution was adjusted either by adding drops of 0.05N sulphuric acid solution or 0.05N
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sodium hydroxide solution. A small dosage in steps of H2O2 concentrations (8.82, 17.64,
26.46, 35.28, 44.1, 52.62, 61.74, 70.56, 79.38, 88.2, 97.02 and 105.84 mmol) was added to
study its effect of anti-oxidants on RR4 dye colour removal. For all the experiments, the
ultrasound was irradiated for an hour and the decolourization of RR4 was monitored using
UV–vis spectrophotometer for every 15 min with intermittent sampling.
2.5 Analytical Procedure
The optical absorption spectra of the Reactive Red 4 dye solutions were determined by
JASCO UV–Visible spectrophotometer equipped with equipped with a photoelectric detector
at the maximum absorption wavelength of 517 nm. The UV–vis spectrum and the chemical
structure of RR4 are shown in Fig. 1b. The aromatic intermediates after degradation studies
were analysed using GC-MS (Perkin Elmer). The GC was equipped with Elite-5 (5%
Diphenyl) Dimethylpolysiloxane series capillary columns of length 30m, inner diameter
0.53mm, film thickness 1.50mm and interfaced directly to the MS. The GC column was
operated at a temperature of 80 oC for 2 minutes then increased to 250
oC at the rate of 15
oC
min-1
. The other experimental conditions were: Electron ionization voltage 70 eV, helium as
carrier gas, injection temperature 280 oC and source temperature 80
oC.
Fig 1a and Fig 1b
3. Results and Discussions
3.1 Effect of pulsed sonication treatment
The experiments were conducted in continuous sonication and pulsed sonication system,
respectively to illustrate the degradation potential of the individual processes. As shown in
Fig.2, the colour hardly decreased when both the processes were applied to 0.01004 mmol
concentration of reactive red 4 solution. It is due to the fact that only a small amount of •OH
radicals were formed in the presence of only sonolysis [18]. It was also confirmed that little
colour removal was achieved when pulsed sonication was applied such that during the ON
and OFF time interval (5 seconds) the dye molecules have been decomposed at the bubble
water surface. Therefore, the decolorization efficiency was also increased from 29% to 36%.
Similarly, the produced •OH radicals were not enough for complete degradation RR4 as mere
ultrasound was inefficient for the complete removal of the recalcitrant pollutant molecules
[19].
Fig 2
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3.2 Effect of pH
The effect of pH on decolourization of RR4 was also performed. Fig. 3 shows that the low
pH favors high decolourization with small differences in the range 2.0-4.0, and lower rate at
alkaline pH [20, 21]. The natural pH of the solution is 6.71, during acidification the dye
would become hydrophobic and easily available for ultrasound treatment. Similar studies
reveal that the degradation kinetics increased for RR4 dye molecules in the acidic medium.
Also at acidic condition the H+ ions will be readily activated, generates more
•OH radicals
during ultrasound treatment. Under alkaline condition there is a gradual decrease in the
degradation rate. This is due to the formation of conjugate base of the dye molecule which
makes the dye molecule more hydrophilic and deactivates the formation of •OH radicals
resulting in lesser degradation. The colour removal rate of the dye is maximum of 42.19%
when the pH is 4 [22]. The colour removal rate was increased little from natural pH to pH 7.0
due to the availability of more •OH radicals than normal solution pH. When compared with
ultrasound (US) treatment alone, the solution pH shows high colour removal rate.
Fig 3
3.3 Effect of H2O2 concentration
The effect of ultrasound with oxidants were evaluated using hydrogen peroxide
addition at different concentration to the RR4 aqueous dye sample at solution pH. In the
presence of ultrasonication the generation of •OH radicals increases according to the
concentration of H2O2 added [23]. Fig. 4 illustrate the decolourization efficiency at different
H2O2 concentration. The increase in •OH radicals generation was because the dispersed H2O2
tended to act as additional nuclei available at the cavitational hot spot for the pyrolysis of
water molecules and formation of •OH radicals and can thus be used to quantify the
effectiveness in generating the desired reactive radicals [24]. Therefore 66% colour removal
due to higher oxidation activities when 8.82 mmol of H2O2 was added. The higher colour
removal of 68% was achieved with the increase of concentration to 17.64 mmol of H2O2 than
with the ultrasound alone. This ensures the formation of the powerful oxidizers like HO2/O2- ,
•OH whose oxidation–reduction potentials are +1.5 and +2.8 eV respectively. Moreover, the
scavenging reaction takes place when the concentration of H2O2 increased further [25] where
the large of number of •OH radicals interact and form H2O and H2O2 where the extinction of
•OH happen resulting in low colour removal rate.
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Fig 4
3.4 Effect of dye concentration
The influence of initial dye RR4 concentration of the sample solution was
investigated with different concentration ranging from 10ppm to 1000ppm on colour removal
rate. As illustrated in Fig. 5, the dye colour removal changed inversely to the initial dye
concentration of RR4 [26]. Higher colour removal rate was obtained for the process study
where the initial dye concentration of 10ppm solution relative lower initial concentration
while the dye concentration was increased 10 times further. For an instance, in 60 min, the
simple pulsed sonication achieved 26% decolorization, while 24% decolorization could be
achieved in the process when the dye solution concentration increased by 10 folds. The
possible explanation might be that the ratio of •OH radicals to dye molecules in the solution
decreased when the concentration of the dye molecules increased 10 folds [27], resulting in
an excess of dye molecules regarding to •OH radicals generated during the sonication
process. As a result, the decolorization rate lowered for the high initial dye concentration than
the lower dye concentration.
Fig 5
3.5 Catalyst Characterization
SEM images were recorded on catalysts synthesized by both continuous, pulsed mode
sonication and shown in Fig.6a and Fig. 6b. We can see the smooth, uniform texture and
agglomerated pseudo spherical structure of ZnO [28, 29]. The agglomeration observed was
due to the van der Waals forces among the gap energy between the particles. The
transformation of surface morphology is from agglomeration to wurtzite relatively due to the
shock wave applied during the sonication [30]. During the pulsed sonication ZnO was
subjected to some changes in its crystallinity [31]. These changes are significant in pulsed
sonication when compared to continuous mode. After the drying in the hot air oven for 24
hours the different structure evolves from both the continuous and pulsed mode of sonication.
TEM and EDAX characterization also confirms the formation of ZnO nanoparticles using
sonication Fig. 6c and Fig. 6d.
Fig 6a, 6b, 6c and 6d
Fig. 7 shows the XRD pattern of the synthesized ZnO catalyst having average diameters. The
structure was subjected to sonication with continuous or pulsed mode yielding agglomerated
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or wurtzite crystal structure.XRD patterns are shown for ZnO samples in Fig. 6. Diffraction
peaks appeared in the pattern are enumerated and listed. The peaks are in accordance with the
standard diffraction pattern of ZnO (JCPDS: 036-1451). To understand the crystallinity, the
peaks at 2θ angles of 31.6, 34.2 and 36.2 were selected. The angular location of these peaks
suggests that the planes of (100), (002) and (101) correspond to the hexagonal crystal lattice
existing along with a slight residual tensile stress [32]. Also the lattice parameters showing
sharp reflections where the presence of Zn(OH)2 has been identified along with the ZnO
amorphous nanoflakes [29, 30]. When the ZnO particles subjected to pulsed sonication the
lattice during the nucleation and growth gains thermal energy for entropic configuration thus
increasing its crystallinity.
Diffuse reflectance spectra (DRS) shows the absorption properties of the catalyst depends
upon the angle of incidence, the refractive index and surface roughness. DRS obtained were
analysed using the (Kubelka–Munk theory) Kubelka–Munk equation at any wavelength Eqn
2
2(1-R)F(R) =
2R Eqn 2
where R represents the absolute reflectance and F(R) as Kubelka–Munk function. Fig. 8
shows the diffused reflectance for ZnO nanopowders [33]. It is clearly seen that the diffused
reflectance increased with increasing wavelengths. The optical transitions in semiconductor
catalyst occurs due to direct and indirect transitions. The optical band gap Eg was calculated
by using the fundamental absorption, such that the electron excitation from the valance band
to the conduction band. The optical band gap was calculated by using the following equation
Eqn 3
n
g
F(R)hυ(αhυ) = = A(hυ-E )
tEqn 3
where A represents an energy-independent constant and Eg as the optical band gap, n -
constant determines the type of optical transitions and for indirect allowed transition, n=2;
and indirect forbidden transition, n=3, for direct allowed transition, n=1/2; for direct
forbidden transition, n=3/2. The optical band gap of the samples was calculated from the
plots of (F(R)hυ/t)2 as a function of photo energy hυ. The optical band gap of ZnO
nanoparticles were found to be 3.19eV further in good agreement with the band gap from
diffused reflectance previously calculated by other researchers equals to 3.22eV [34].
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The variation in the thickness and density of the synthesized catalyst were shown by the BET
adsorption-desorption curve. The obtained curve corresponds to type IV hysteresis loop the
characteristics of mesoporous particles. The surface area of the synthesized ZnO particles
were found to be 3.93 m2/g (Fig. 8), approximately equals to the surface area of ZnO
nanoparticles reported earlier [35]. The ZnO agglomerated assemblies with sufficient surface
area and more mesoporous structures also acts as adsorbents while used for the treatment of
dye pollutants.
Fig 7 and Fig 8
3.6 Effect of catalyst loading
ZnO catalyst upon sonication produce electrons from the conduction band or generate
holes in the valence band (Eqns. 4 and 5) thereby splitting the water molecule as H+ and
●OH
radicals (Eqn. 6). Amount of ZnO concentration plays an essential role in influencing the
generation of ●OH radicals for the degradation processes [36]. In this treatment process,
various concentrations of ZnO were studied to obtain its optimal concentration for the higher
colour removal rate. The results for effect of catalyst concentration on degradation of RR4
are shown in Fig. 9. From the experimental results it reveals that decolorization rate of RR4
increases upto the catalyst concentration 0.3 g/L. Addition of ZnO above 0.35 g/L did not
affect the decolorization kinetics. The decolorization percentage of RR4 started to decrease
slowly when the concentration of ZnO was increased above 0.40 g/L. The reduction in the
colour removal rate was due to the catalytic decomposition effect of ZnO. It is known that the
scavenging of ●OH radical happens when its concentration is increased, they recombine to
form less oxidative OH- ions. The catalytic effect also accordingly decreases as its
concentration was higher. ZnO also undergoes a reaction with hydroxyl ions to form Zn(OH)2
[37] which has lower catalytic activity (lesser redox potential of conduction and valence
band) (Eqn. 7).
US 2+ •-
2ZnO Zn + O→ Eqn 4
US - +
CB VBZnO ZnO(e + h )→ Eqn 5
US2+ + 2+ -
2Zn + H O H +Zn -OH� � ��� � �� Eqn 6
2+ -
2Zn + 2OH Zn(OH)→ Eqn 7
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Fig 9
3.7 Combined effect of pulsed sonolysis, pH, H2O2 and Catalyst
The complete degradation of dyes into CO2 and H2O were studied in this treatment process
and analysed by the COD. The COD values can be related to the concentration of organics
present in the dye. The colour removal from the wastewater cannot be considered as complete
oxidation [38]. Many researchers have reported that there were reaction intermediates present
in the solution after the decolourization studies [39]. In our studies, the intermediates from
RR4 which will form aromatic ring structure intermediates were long lived and more
carcinogenic to the aquatic life. Thus, the optimum operating conditions of pH, initial
concentration of the dye, ZnO catalyst concentration, H2O2 concentration and pulsed
sonication, applied for the degradation of aromatic contents and mineralisation of dye were
analysed. Fig 10 shows that the decolourization percentage of RR4 in the hybrid process
(Combined effect of pulsed sonolysis, pH (4.0), H2O2 (17.64 mmol) and Catalyst (0.35 g/L))
achieved 97% degradation in about 20 minutes of reaction time. Furthermore, it also shows
that 87.5% reduction of COD occurring at a faster reaction rate than colour removal
achieving with in the time interval of 17 minutes. Thus the combined effect prevails to be
more beneficial in COD removal rather than simple decolourization.
Fig 10
3.8 Degradation Pathway of RR4
The formation of intermediates as the chromophores of RR4 evolved as the
degradation of dye proceeds reflect in the UV-vis spectra. The spectral changes are attributed
to the change in the π → π* transition related to the –N=N– group, C–C group, C–N group,
benzene and naphthalene rings of the RR4 molecule [40]. The absorption peak of RR4
decreases rapidly when the degradation reaction proceeds. After the reaction time when there
was no peak in the absorption peak indicating that the azo bonds and the conjugated
molecules of the dye molecule were completed destroyed and converted into simpler
molecules. The intermediates formed during the degradation after the reaction time 60
minutes were analysed by GC/MS. During the degradation process the byproducts like
sulfonic acid, heneicosane, 3-Hexadecene, 1-Octadecene etc were detected. Moreover, there
were several pathways through the degradation process followed [41]. The cleavege of –
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N=N– group, C–C group, C–N group, benzene and naphthalene rings of the RR4 molecule
follow several concurrent pathways by the oxidation of mainly ●OH radicals have been
indicated and the corresponding plausible pathway have been proposed in Fig.11. Eventually
the destruction of RR4 aromatic rings were formed and all the complex molecules were
degraded into smaller molecules.
Fig 11
3.9 Reusability of catalyst for RR4 removal
The reusability and stability of the catalyst material for the degradation reveals the
cost effectiveness. The catalyst effect on degradation of RR4 was carried out with ZnO (0.30
g/L) for 10 ppm concentration at the solution pH 6.71. After the pulsed sonication for the
removal of RR4, the COD was reduced 37%. The catalyst was filtered and treated with 1 M
NaOH and 1 M ethanol then washed repeated 3 times with Millipore water and dried at
120°C successively to regenerate for the next cycle of degradation studies. The degradation
studies were experimented with 4 cycles. Fig. 12 shows the COD values of the repeated
degradation studies using the regenerated catalyst. It revealed that after 3 cycles the catalyst
showed 30% COD reduction as there was a slight decrease in the catalytic activity due to the
lesser number of active sites in the catalyst [42]. The decrease in sites may be due to the
subsequent attack by the dye molecules and reducing the catalytic efficiency. Moreover, the
catalyst stands effective after regeneration and further indicating that the catalyst potential for
wastewater treatment applications.
Fig 12
Conclusion
The ultrasound in pulsed mode alone have negligible effect on the decolourization of RR4.
However, when ultrasound combined with other parameters like pH variation, catalyst
loading, dye concentration and H2O2 addition gave faster decolourization rate. The synergetic
effect of AOP can be explained by the formation of hydroxyl radicals helps in the faster
decolourization kinetics. When the concentration of H2O2 increased there was an increase of
decolourization reaching a maximum (asymptotic value). Moreover, the increase in
antioxidant intuitively describes that the excessive addition can favour the radical scavenging
resulting in decrease in the removal kinetics. Under the hybrid combined experimental
conditions, the dye solution could be completely decolorized within 20 minutes a very short
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reaction time. The rate for colour removal is much faster and similar that of COD removal.
All the aromatic ring, azo bond were cleavaged within 20 min, and ended in simpler
compounds. Finally, the plausible degradation path of RR4 was predicted.
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List of Figures
1a. Schematic representation of experimental setup
1b. Absorbance spectra and structure of RR4 dye
2. Decolourization of RR4 dye solution with the variation mode of ultrasound
3. Decolourization of RR4 dye solution with the variation of initial solution pH
4. Decolourization of RR4 dye solution with the variation of H2O2 Concentration
5. Decolourization of RR4 dye solution with the variation of dye Concentration
6. SEM images of ZnO by A) Continuous mode and B) Pulsed Mode C) EDAX
D)TEM
7. XRD and DRS of ZnO
8. BET Surface area of ZnO
9. Decolourization of RR4 dye solution with the variation of dye Concentration
10. COD and Colour removal by the combined effect
11. Plausible degradation pathway
12. Reusability of the synthesized catalyst
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OH•
radicals
Ultrasound
Probe
)))
Water out
Cooling water in
Fig 1a Model experiment setup
Figure 1b Absorbance Spectra and Structure of RR4
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
200 300 400 500 600 700 800
Absorbance
Wavelength (nm)
y = 0.0138x + 0.0872R² = 0.9921
0
0.5
1
1.5
2
0 20 40 60 80 100
Absorbance
Concentration (ppm)
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Figure 2 Decolourization of RR4 dye solution with the
variation mode of ultrasound [US Irradiation time—1 h, RR4 conc.
10 ppm].
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70
Ct / C0
Time (minutes)
Sonolysis
Pulsed
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Figure 3 Decolourization of RR4 dye solution with the
variation of initial solution pH [US Irradiation time—1 h, RR4 conc.
10 ppm].
0
5
10
15
20
25
30
35
40
45
2 3 4 5 6 7 8 9 10 11
% Decolourization
pH
sonolysis
Pulsed
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Figure 4 Decolourization of RR4 dye solution with the
variation of H2O2 Concentration [US Irradiation time—1 h, RR4 conc.
10 ppm].
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60
Ct / C0
Time (minutes)
8.82 mmol
17.64 mmol
26.46 mmol
35.28 mmol
44.1 mmol
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Figure 5 Decolourization of RR4 dye solution with the
variation of dye Concentration [US Irradiation time—1 h, pH-6.71].
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70
Ct / C0
Time (minutes)
10ppm 50ppm
100ppm 500ppm
1000ppm
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Figure 6 SEM images of ZnO by A) Continuous mode and B) Pulsed Mode C) EDAX
D)TEM
A B
C D
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(202)
(004)
Intensity (a.u.)
(100)
2θ Position
(002)
(101)
(102)
(110)
(103)
(200)
(112)
(201)
Figure 7 XRD and DRS of ZnO
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Figure 8 BET Surface area of ZnO
0
5
10
15
20
25
30
0 0.2 0.4 0.6 0.8 1
Quantity Adsorbed (cm
3/g STP)
Relative Pressure (P/P0)
Desorption
Adsorption
0
0.01
0.02
0.03
0.04
0.05
1 10 100Pore volume (cm
3/g)
Pore width (nm)
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Figure 9 Decolourization of RR4 dye solution with the
variation of dye Concentration [US Irradiation time—1 h, pH-6.71]
0.5
0.6
0.7
0.8
0.9
1
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6
% Decolourization
ZnO Catalyst (g/L)
sonolysis
pulsed
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Figure 10 COD and Colour removal by the combined effect [ Ultrasonic Irradiation –
1h, pH - 4.0, H2O2 - 17.64 mmol and Catalyst - 0.35 g/L]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14 16 18 20 22 24
Ct / C0
Time (minutes)
COD
Colour
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O
NH OH
S
N
-OO
O
S
N
O-
O
O
S
NH
-O
O
O
N N
N NH
Na+ Na+
Na+
Cl
S
O-O
O
Na+
NH2 OH
SO3Na
NH2
SO3Na
NH2
N N
N NH
Cl
SO3Na
SO3Na
ONa
ONa
H2N
NO
SO3Na
O
SO3Na
OH NHCOCH3
H2N
OH NHCOOH
H2N H2N
OH NH2
HO
OH OH
CH3
NH2
COOH
NH2
NH2 OH
OH
OO
HO
HO
OOH OH
OH
O O
HO
H2N
SO3
SO3NH2
N N
N NH2Cl
Figure 11 Plausible degradation pathway
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Figure 12 Reusability of the synthesized catalyst [US Irradiation time—1 h, pH-6.71,
Catalyst - 0.35 g/L]
0
10
20
30
40
1 2 3 4
% COD removal
Cycle
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Graphical abstract
Facile approach for synthesis of stable, efficient and recyclable ZnO through pulsed
sonication and its application for degradation of recalcitrant azo dyes in wastewater
GK Dinesh* and R Saranya**
*Department of Chemical Engineering, Indian Institute of Science Education and Research
Bhopal, Bhopal (M.P.), India
**Polymeric Materials & NanoComposites (PMNC), Department of Physics, Trinity College
Dublin, University of Dublin, Dublin, Ireland.
�
H2O
●OH
H2O
●OH
H2O
●OH
H2O
●OH
H2O
●OH
Cavitation Bubble Interior
Hot Spot Zone
Gas Phase
5000 – 10000 K, 1000 atm
Radical generation
Pyrolysis mechanism
Molecular dissolution
Bubble-liquid Interface:
2000 K, 1 atm
Free radicals formation and
react with parent molecules
Thermal decomposition
�
H2O
●OH
e-
h+H2O
●OH
�
e-
h+
H2O
●OH
�
e-
h+
H2O
●OH
�
e-
h+
H2O
●OH
�
e-
h+
e-
h+
H2O
●OH
�
ZnO
agglomerated
H2O
●OH
On the hybrid process of pulsed sonolysis combined with ZnO catalyst produced more ●OH
radicals for attacking the pollutants thus decolorization of 97% achieved in short time.
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