Enhanced decolorization of methyl orange using zero-valent copper nanoparticles under assistance of hydrodynamic cavitation

Article abstract of DOI:10.1016/j.ultsonch.2014.05.025

The rate of reduction reactions of zero-valent metal nanoparticles is restricted by their agglomeration. Hydrodynamic cavitation was used to overcome the disadvantage in this study. Experiments for decolorization of methyl orange azo dye by zero-valent copper nanoparticles were carried out in aqueous solution with and without hydrodynamic cavitation. The results showed that hydrodynamic cavitation greatly accelerated the decolorization rate of methyl orange. The size of nanoparticles was decreased after hydrodynamic cavitation treatment. The effects of important operating parameters such as discharge pressure, initial solution pH, and copper nanoparticle concentration on the degradation rates were studied. It was observed that there was an optimum discharge pressure to get best decolorization performance. Lower solution pH were favorable for the decolorization. The pseudo-first-order kinetic constant for the degradation of methyl orange increased linearly with the copper dose. UV-vis spectroscopic and Fourier transform infrared (FT-IR) analyses confirmed that many degradation intermediates were formed. The results indicated hydroxyl radicals played a key role in the decolorization process. Therefore, the enhancement of decolorization by hydrodynamic cavitation could due to the deagglomeration of nanoparticles as well as the oxidation by the in situ generated hydroxyl radicals. These findings greatly increase the potential of the Cu⁰/hydrodynamic cavitation technique for use in the field of treatment of wastewater containing hazardous materials.

Full text of DOI:10.1016/j.ultsonch.2014.05.025

Accepted Manuscript
Enhanced decolorization of methyl orange using zero-valent copper nanoparti-
cles under assistance of hydrodynamic cavitation
Pan Li, Yuan Song, Shuai Wang, Zheng Tao, Shuili Yu, Yanan Liu
PII:
S1350-4177(14)00184-9
DOI:
http://dx.doi.org/10.1016/j.ultsonch.2014.05.025
ULTSON 2621
Reference:
Ultrasonics Sonochemistry
To appear in:
Received Date:
Revised Date:
Accepted Date:
17 February 2014
27 May 2014
27 May 2014
Please cite this article as: P. Li, Y. Song, S. Wang, Z. Tao, S. Yu, Y. Liu, Enhanced decolorization of methyl orange
using zero-valent copper nanoparticles under assistance of hydrodynamic cavitation, Ultrasonics Sonochemistry
(2014), doi: http://dx.doi.org/10.1016/j.ultsonch.2014.05.025
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Enhanced decolorization of methyl orange using zero-valent copper
nanoparticles under assistance of hydrodynamic cavitation
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Pan Li 1, Yuan Song2, Shuai Wang2, Zheng Tao3, Shuili Yu1,*, Yanan Liu3,*
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1School of Environmental Science and Engineering, State Key Laboratory of Control
and Resource Reuse, the Collaborative Innovation Center of Advanced Technology
and Equipment for Water Pollution Control and the Collaborative Innovation Center
for Regional Environmental Quality, Tongji University, 1239 Siping Road, Shanghai,
P.R. China
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2School of Environmental Science and Engineering, Tongji University, 1239 Siping
Road, Shanghai, P.R. China
3School of Environmental Science and Engineering, Dong Hua University, 2999
Renmin Rd. North, Songjiang District, Shanghai, 201620 China
*Authors to whom correspondence should be addressed
Shui Li, email: ysl@tongji.edu.cn; phone: +86-21-65982708
Yanan Liu, email: liuyanan@dhu.edu.cn; phone: +86-21-67792538
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ABSTRACT
The rate of reduction reactions of zero-valent metal nanoparticles is restricted by
their agglomeration. Hydrodynamic cavitation was used to overcome the disadvantage
in this study. Experiments for decolorization of methyl orange azo dye by
zero-valent copper nanoparticles were carried out in aqueous solution with and
without hydrodynamic cavitation. The results showed that hydrodynamic cavitation
greatly accelerated the decolorization rate of methyl orange. The size of nanoparticles
was decreased after hydrodynamic cavitation treatment. The effects of important
operating parameters such as discharge pressure, initial solution pH, and copper
nanoparticle concentration on the degradation rates were studied. It was observed that
there was an optimum discharge pressure to get best decolorization performance.
Lower solution pH were favorable for the decolorization. The pseudo-first-order
kinetic constant for the degradation of methyl orange increased linearly with the
copper dose. UV-vis spectroscopic and Fourier transform infrared (FT-IR) analyses
confirmed that many degradation intermediates were formed. The results indicated
hydroxyl radicals played a key role in the decolorization process. Therefore, the
enhancement of decolorization by hydrodynamic cavitation could due to the
deagglomeration of nanoparticles as well as the oxidation by the in situ generated
hydroxyl radicals. These findings greatly increase the potential of the
Cu0/hydrodynamic cavitation technique for use in the field of treatment of wastewater
containing hazardous materials.
Keywords: Zero-valent copper, nanoparticle, hydrodynamic cavitation, hydroxyl
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radicals, agglomeration, azo dye
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1. Introduction
Over 100,000 commercially available dyes are extensively used in different
industries, and more than 0.7 million tons of synthetic dyes are produced annually
worldwide [1, 2]. During the dyeing process, approximately 15% of the dye is lost
and is released through industrial effluents [2, 3]. Azo dyes which are characterized
by the presence of nitrogen double bonds (─N═N─) represent the largest class of
synthetic dyes used in the textile processing, paper making, printing and cosmetic
industries [3, 4]. The release of azo dyes into the environment through colored dye
effluents creates serious environmental pollution problems due to their intense color,
toxicity, carcinogenicity and mutagenicity [5]. Hence, the treatment of azo dye
wastewaters is becoming a matter of great concern. Azo dye wastewaters with high
concentration are difficult to treat using conventional activated sludge treatment
methods because of their synthetic origins and complex aromatic molecular structures
[6]. Treatment though physical separation processes such as adsorption, coagulation,
membrane filtration, etc., is basically contaminants transfer from wastewater to
another waste. Advanced oxidation processes (AOPs), such as ozonation, Fenton’s
oxidation, photocatalytic oxidation, sonolysis appear to be more appropriate for
treating wastewaters that containing azo dyes [7-10]. This is because hydroxyl
radicals generated in AOPs are very strong and non-selective oxidants with the ability
to rapidly decompose many dissolved compounds in water matrix. But at the present
level of their development, the AOPs have not been widely employed in large-scale
industries because of their high cost and complex system.
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In recent years, metal-containing micro/nano particles, especially zero-valent iron
(ZVI), have drawn considerable attention for the remediation of contaminated land,
surface and groundwater. Zero-valent metals are efficient reducing agents and they
have been proven effective in reducing azo dyes [11, 12]. The mechanism is well
known as the reductive cleavage of the azo group as shown in Scheme 1 [11, 12].
Zero-valent metals, such as iron and copper are good electron donors. The azo dye
molecules accept electrons from the metals and transform into transitional products
when combining with H+.
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Scheme 1
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The degradation reaction of azo dyes using zero-valent iron is found to occur on the
surface of ion metal. The mass transfer of dye chemicals to the iron surface is the
controlling step of the heterogeneous reaction [12, 13]. Therefore, the degradation rate
can be increased by decreasing particle size to nano-scale. However, commercial
grade zero-valent metal nanoparticles are always agglomerates rather than single
primary nanoparticles, which results in loss of the reactivity of surface site[14]. Many
attempts were taken to improve the efficiency of zero-valent metal reducing system.
Acoustic cavitation appears to deagglomerate particles and improve mass transfer
through the collapse of cavities or microbubbles [15-17].
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Hydrodynamic cavitation is known as a kind of cavitation which usually induced
by pressure differentials in a flowing liquid by introducing constrictions in the flow
[18-20]. Cavities can be generated due to the large pressure decrease. These bubble
cavities subsequently collapse when the pressure suddenly recovers. As cavities
collapse, a given bubble can reach temperatures as high as 5000K and pressures as
high as 100 MPa (so-called “hot spots”). The drastic environmental change triggers
high-speed micro-jets and free radical generation due to decomposing water vapor
and noncondensable gases inside the bubble [21-23]. As compared to acoustic
cavitation, hydrodynamic cavitation is more energy efficient and more suitable to
large scale application [18, 22, 24]. Recently, water jet cavitation is reported to be an
efficient method to modify the surface properties at the nano levels[25]. The use of
hydrodynamic cavitation in combination with Fenton process results in a good
synergistic effect in degradation of refractory contaminants [26-29]. We previously
reported that free-radicals could be generated by the collapse of microbubbles [30,
31].
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In the current study, the combination of zero-valent copper nanoparticles with
hydrodynamic cavitation for the decolorization of azo dye was investigated. Copper is
known to be a mild hydrogenation catalyst and it is also widely used as a catalyst in
other AOPs, such as wet-air oxidation. Accordingly it was selected in the study. The
decolorization of methyl orange, used here as a model azo dye, was investigated
under various conditions. The purpose of this study is to determine possible
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enhancement of decolorization of azo dye by the combined process, with the aim of
advancing the technical application of the zero-valent metal reduction technique.
2. Material and methods
2.1 Reagents
Cu0 was obtained from Shanghai Chao Wei Nano Technology Co., Ltd, China.
Morphological and elemental analyses of the Cu0 nanoparticles were performed using
a
scanning electron microscope (SEM) (S-360, Cambridge, UK) with
energy-dispersive X-ray (EDX) at 20 kV. Particle size was analyzed with Malvern
Mastersizer 3000 particle analyzer (Malvern Instruments Ltd., United Kingdom). It
was found that the Cu0 nanoparticles were agglomerated without pretreatment and
they had an average diameter of approximately 18 µm (Fig. 1). The SEM-EDX
spectrum showed that copper was the major species, accounting for 97.7% of the
mass of the sample (Fig. 2).
Methyl orange, methanol, hydrochloric acid and sodium hydroxide were
purchased from Sinopharm Chemical Teagent Co., Ltd., China. All chemicals used
were analytical grade or higher.
2.2 Hydrodynamic cavitation reactor
Figure 3 shows a schematic diagram of the hydrodynamic cavitation reactor used
in the current study. The system consists of a water tank with a 10 L capacity, a
centrifugal pump (0.37 kW, 20NPD04S, Shanghai Nikuni Pumps co., Ltd., China) and
an adjustable orifice valve acting as a cavitation device. The sectional area of flow at
the valve is variable. The intake side of the pump was connected to the bottom of the
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water tank while the discharge side of the pump was connected to the bottom of the
water tank, forming a closed loop circuit. Pressure gauges were provided at
appropriate places to measure the pressures. The temperature was maintained constant
by a heat-exchanger coil inside the water tank.
2.3 Decolorization experiments
For the decolorization experiment, 5 L of aqueous solution of 10 mg L-1 methyl
orange was prepared in the water tank. Hydrodynamic cavitation began by switching
on the pump and controlling the valves. After switching on the pump, water could be
sucked in by the pump and then discharged through the orifice valve at a pressure of
0.2-0.5 MPa. The solution was circulated at a flow rate of 5.8-14 L min-1. The
decolorization experiments were initiated after the addition of the desired amount of
Cu0 nanoparticles to the water tank. The dye solution was periodically sampled by
pipettes (5 mL), and the total sampling volume did not exceed 5% of the total solution
volume. Membrane filtration (pore size of 0.22 μm) and solution pH adjustment to
pH=6.5 were conducted immediately after sampling to terminate the reaction. The
liquid temperature in the tank was maintained at 293±1 K using a chiller. The initial
pH of the solution was adjusted using diluted sodium hydroxide solution (5.0 mol L-1
NaOH) or diluted hydrochloric acid solution (5.0 mol L-1 HCl) and was measured by
a pH meter (SevenEasy S20, Mettler-Toledo International Inc., China). The dosage of
Cu0 nanoparticles was varied between 20 mg L-1 and 200 mg L-1.
2.4 Analysis
The concentration of methyl orange in the treated dye solution was determined
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based on the constructed calibration curve at the absorption wavelength of 460 nm
and expressed as the molar ratio to the initial concentration. The UV-vis spectrum
during the dye degradation was measured at 200-800 nm using a UV-vis spectrometer
(UV765, Shanghai Precision & Scientific Instrument Co., LTD, China). The
intermediates formed during the degradation of methyl orange were analyzed using
FT-IR (Nicolet 5700, Thermo Electron Scientific Instruments Co., Ltd., UK).
2.5 Kinetic analysis
Colored dye degradation by zero valent metals is generally assumed to be a
first-order reaction. Therefore, the reaction kinetics of methyl orange degradation was
modeled using a pseudo-first-order rate equation, Eq. (1):
ꢈ
C = ꢀꢁ × ꢂꢃꢄ
(1)
ꢅꢆꢇ
where kobs denotes the observed first-order reaction rate constant, t the reaction time,
C0 the initial concentration of methyl orange, and C the concentration of methyl
orange at time t. When the reaction occurs in the water/bubble/Cu0 interphase, the
phase transfer is limited, which makes it difficult to reach 100% degradation
efficiency. Thus, the residual dye concentration is modified as Eq. (2):
ꢈ
C − Cꢉꢊꢋ = (Cꢁ − Cꢉꢊꢋ) × α × eꢃꢄ
(2)
ꢅꢆꢇ
where Cmin is the ultimate residual methyl orange concentration after treatment and
αꢌis the variation coefficient for the ideal first-order kinetics (1.0 denotes ideal
first-order kinetics; a larger deviation from 1.0 indicates less fit with the first-order
kinetics). Cmin and kobs can be obtained by non-linear regression.
3. Results and discussion
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3.1 Decolorization of methyl orange with and without hydrodynamic cavitation in the
presence of Cu0 nanoparticles
Methyl orange was treated by hydrodynamic cavitation alone, Cu0 nanoparticle
alone and Cu0 nanoparticle reduction coupled with hydrodynamic cavitation. The
degradation efficiency was illustrated in Fig. 4. The dosage of Cu0 nanoparticle was
40 mg L-1, the initial pH of solution was 3.0, and the pump discharge pressure was 0.4
MPa. Only 6.8% of methyl orange removal was obtained using hydrodynamic
cavitation alone. It has been known that hydrodynamic cavitation can produce ·OH
radicals due to the creation of hotspots (local areas of high temperature and pressure)
[31]. But the quantities of ·OH generated by hydrodynamic cavitation only were too
low to be of practical use in decolorization.
Cu0 nanoparticle alone test was conducted under mechanical stirring. Cu0
nanoparticles were added to 1 L of 10 mg L-1 methyl orange solution at varying
mixing rates (100, 500 and 1000 rpm) in a 2-L beaker without hydrodynamic
cavitation exposure. Only 20% of the methyl orange was decolorized during the
20-min test period (Fig.4), and no apparent difference in degradation rate was
observed at the varied mixing rates. It suggested that only mechanical stirring hardly
deagglomerated metal nanoparticles.
Figure 4 shows that the degradation efficiency was increased sharply to 83%
when introducing hydrodynamic cavitation to Cu0 nanoparticle treatment (Cu0/HC).
This means that the degradation of the dye by Cu0 was elevated by approximately
3.15 times. showing the synergistic effect compared to the individual Cu0 reduction
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and hydrodynamic cavitation. The synergistic coefficient ( R ) was defined as
ꢄ
ꢏ
ꢅꢆꢇ,ꢍꢎ
ꢏ
R = ꢄ
(3)
ꢅꢆꢇ,ꢍꢎ /ꢐꢍ
ꢏ
ꢏ
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where ꢑꢒꢓꢔ,ꢕꢖ and ꢑꢒꢓꢔ,ꢕꢖ /ꢗꢕ are the pseudo-first-order reaction constants for the
degradation reaction with Cu0 and Cu0/HC, respectively. The synergistic coefficient
was 4.25, which indicates good synergistic effect between hydrodynamic cavitation
and Cu0 nanoparticle.
We also determined whether methyl orange could be removed through physical
adsorption by Cu0 nanoparticles. The dye solution (1 L) was filtered using a 0.22-μm
membrane after the decolorization treatment by the Cu0/HC process. Acidification
with 1 mol L-1 HCl was performed to dissolve any particles left on the membrane. The
UV-vis spectrum of the acidic solution was measured at 200-650 nm. Neither UV nor
visible absorption was observed for the acidic solution, which suggested that methyl
orange was not removed by Cu0 nanoparticle adsorption.
The SEM picture of Cu0 samples after hydrodynamic cavitation was shown in
Fig 1. The size of Cu0 nanoparticles was apparently reduced after hydrodynamic
cavitation after 20-min of continuous treatment. We also measured zeta potential of
Cu0 samples, which is an important factor in understanding dispersion stability of
nanoparticles in water. The zeta potential of Cu0 samples decreased from -13.0 mV to
-17.6 mV after cavitation treatment. Consequently, it is suggested that hydrodynamic
cavitation could effectively promote the dispersion of Cu0 nanoparticles in water.
3.2 Effects of solution pH on the decolorization
To identify the optimum operational conditions, we performed experiments in
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which the pH of the dye solution was varied from alkaline to strongly acidic. The
changes in the concentration of methyl orange with time are illustrated in Fig. 5. The
Cu0 nanoparticle dosage was 40 mg L−1 and the pump discharge pressure was 0.4
MPa. No perceptible changes in methyl orange concentration occurred when the pH
was not adjusted (pH= 6) or when NaOH solution was added (pH= 10 and 12).
However, obvious decolorization was observed when HCl was added to the solution,
and the methyl orange was degraded at a greater rate when the pH was decreased
from 4.0 to 3.0, which suggested that the decolorization process favors acidic
conditions and the lower solution pH contributes to a higher degradation rate. This
supports the fact that H+ ions in the solution are directly involved in the reduction
reaction [12, 32]. When azo dye molecules collision with zero valent copper at lower
pH, Cu0 accepts electrons oxidizing to Cu2+. At the same time, azo molecules combine
with H+ and turn into unstable transitional compounds (Scheme 1). Consequently, the
change of pH strongly influences the decolorization of the azo dye.
3.3 Effects of pump discharge pressure on the decolorization
The pump discharge pressure was changed from 0.2 MPa to 0.5 MPa and the
effect on decoloriaztion are depicted in Fig. 6. The dosage of Cu0 nanoparticle and the
initial pH of solution was kept constant at 40 mg L-1 and 3.0, respectively. It was
found that the extent of decolorization increased with an increase in the pressure from
0.2 MPa to 0.4 MPa and then decreased. The maximum reaction rate constant was
observed at 0.4 MPa discharge pressure.
Bubble dynamics studies have indicated that cavitational intensity generated at
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the collapse of the bubble increases with higher discharge pressure of the system. The
condition of cavitation can be evaluated by a dimensionless number, namely,
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cavitation number which is calculated as follows:
ꢀꢘ = ꢙꢃꢙ
(4)
ꢚ
ꢛ
ꢜ
ꢜ
ꢝꢘ
ꢏ
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where P is the recovered downstream pressure (N/m2), Pv is the vapor pressure of the
liquid at the operating temperature (N/m2), v0 is the velocity of the liquid at the orifice
valve (m/s), and ρ (g/m3)is the density of the liquid. The cavitation number decreased
from 1.3 to 0.55 when the discharge pressure increased from 0.2 MPa to 0.5MPa in
the present system. The decrease in the cavitation number results in the increase of the
number of cavities formed. But excess cavities at high pressures are liable to coalesce
resulting in large bubbles which could decrease cavitational intensity [19]. Similar
results have been reported in optimizing the operation parameters for hydrodynamic
cavitation reactors [19, 33]. Mishra and Gogate [26] investigated the degradation of
Rhodamine B using orifice and venturi cavitation reactors and observed the reaction
rate reached maximum at the inlet pressure of 4.8 atm. Gogate and Pandit [34] have
modified the mathematical modeling of hydrodynamic cavitation and demonstrated
that there is at an optimum pressure at which the cavitation intensity could reach
maximum.
3.4 Effects of catalyst dosage on the decolorization
We next investigated the effects of Cu0 nanoparticle dosage on methyl orange
decolorization by the Cu0/HC process under acidic pH conditions (pH= 3). The pump
discharge pressure was kept at 0.4 MPa. The color removal efficiency significantly
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increased with the Cu0 nanoparticle dosage in the methyl orange solution (Fig. 7). The
Cu0 doses were 10, 40, 60, 140 and 200 mg L-1, which corresponded to rate constants
(kobs) of 0.0906, 0.344, 0.859, 1.49 and 1.08 min-1, respectively. A regression analysis
indicated that the observed first-order rate constant and the Cu0 nanoparticle dose had
a nicely linear relationship (R2= 0.95) (Fig. 7). As the degradation of methyl orange
by Cu0/HC occurs on the surface, it is anticipated that the heterogeneous reaction rate
would increase with the increase of copper surface active sites. Accordingly, the
observed first-order rate constant linearly increases with the increase of copper dosage.
Nevertheless, they did not show a linear relationship when the Cu0 dosage was
excessive (over 140 mg L-1). It is supposed that the active surface area of unit mass
copper decreased due to the aggregation or overlapping of copper particles[35].
3.5 Discussion on the reaction pathways
3.5.1 UV-vis spectra change during treatment
The UV-vis spectra analysis was conducted to investigate the dye degradation
and evolution of intermediates during the Cu0/HC treatment. Figure 8 illustrates the
UV-vis spectrum changes of the dye solution as a function of reaction time. As can be
observed from these spectra, before the degradation, the absorption spectrum of
methyl orange in water was characterized by one main band in the visible region, with
a maximum absorption at 465 nm, and by another band in the ultraviolet region
located at 273 nm. The peak at 273 nm was associated with “benzene-like” structures
in the molecule [36], and that at 465 nm originated from an extended chromophore
comprising both aromatic rings connected through the azo bond (─N=N─). During
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the first 10 min of Cu0/HC treatment, the absorption of the visible band at 465 nm
decreased rapidly, while the absorption at 273 nm increased and reached a peak at 10
min. During the next 20 min, the absorption at 273 nm decreased gradually. The decay
of the absorbance at 273 nm was considered to be evidence of aromatic fragment
degradation in the dye molecule and its intermediates [36].
3.5.2 FT-IR spectra changes after treatment
The methyl orange degradation and formation of intermediates was also
evidenced by changes in the FT-IR spectra. Figure 9 illustrates the typical FT-IR
spectra in the region of 500–4000 cm-1 for the dye at the initial conditions and after 1
h of treatment (pH: 3.0, pump pressure: 0.4 MPa, dosage: 40 mg L-1). The
characteristic peaks of the azo bond (−N=N−, at 1440 and 1519 cm-1) and the benzene
ring (at 1400-1600 cm-1) of methyl orange disappeared, and the antisymmetric and
symmetric stretching vibrations of –CH3 (three peaks at 2800-3000 cm-1) clearly
disappeared. The characteristic peaks at 1197 and 1038 cm-1, attributed to the
−SO
asymmetric stretching vibration of the
3Na group [37], also clearly decreased.
The results indicated that the azo bond and the benzene ring were broken and that
desulfuration and demethylation occurred with the decolorization of methyl orange.
The breakage of chromophore band in methyl orange could form anilines and
benzenesulfonic acids.
Meanwhile, several new peaks at 1417, 1585 and 1720
cm-1 were generated after 1 h of Cu0/HC treatment, which were attributed to the
antisymmetric and symmetric stretching vibrations of the C−ꢞO groups in the salts of
carboxylic acids. Further oxidation of the aromatic intermediates could produce
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carboxylic acids, such as acetic acid and formic acid.
3.5.3 Effects of •OH scavenging on decolorization
The degradation experiment was performed in the presence of methyl alcohol,
a type of •OH scavenger, to investigate the role of •OH. Methyl alcohol is capable of
deactivating •OH and its derivatives. Methanol reacts with •OH and, to a lesser extent,
with hydrogen radicals (•H) with the second-order rate constants 9.7×108 mol−1 s−1
and 2.6×106 mol−1 s−1, respectively [38]. Methanol was added to the methyl orange
solution during the decolorization treatment by the Cu0/HC process. The
concentration of methanol was varied in the range of 0.1 to 1.0 mL L-1, while the Cu0
nanoparticle dose and the initial solution pH were maintained at 40 mg L-1 and 3,
respectively. It was found that the decolorization rate decreased significantly with the
addition of methanol (Fig. 10). When methanol concentration was 1 mL L-1, the
degradation efficiency only achieved 18% during a 20-min treatment period. It is
notable that the degradation efficiency after adding enough •OH scavenger decreased
to a level as low as that obtained during only Cu0 treatment. These results indicated
that ∙OH was mainly responsible for the dye degradation process by Cu0/HC.
The degradation reaction of azo dyes using ZVI is well known to be a
hydrogenation reaction occurring on the surface of metal iron. Therefore, the
─N
─
chromophore ( =N ) in the methyl orange molecule may convert to anilines when
they are absorbed on copper surface (Eq. (5)). Meanwhile, hydroxyl radicals are
generated by the extremely high temperature caused by the adiabatic compression of
cavities in association with hydrodynamic cavitation. Copper may enhance the
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generation of free radicals (Eq. (6)). The ∙OH could attack the chromophore (─N=N─)
in the methyl orange molecule through the addition of electrons and transfer reactions
that yielded hydroxylated products (Eq. (7)).
ꢏꢌꢌ
ꢌꢌꢌꢢꢣ
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Rꢟ − N = N − Rꢠ +Hꢡ ꢤꢥꢥꢦ Rꢟ − NHꢠ + Rꢠ − NHꢠ
(5)
(6)
(7)
ꢏ
HꢠO ꢤꢢꢣꢥꢥ,ꢥꢧꢥꢨꢥꢩꢥꢪꢥꢫꢥꢩꢥꢨꢥꢋꢥꢬꢥꢉꢥꢊꢥꢭꢥꢌꢭꢥꢬꢥꢮꢥꢊꢈꢥꢬꢥꢈꢥꢊꢫꢦꢋ ∙ OH
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Dye + ∙ OH ꢯ dye − OH
4. Conclusions
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The degradation of methyl orange by copper nanoparticles was investigated in
the presence and absence of hydrodynamic cavitation. It was shown that the
decolorization efficiency of the process was markedly enhanced by the exposure of
hydrodynamic cavitation. We investigated that hydrodynamic cavitation could
effectively promote the dispersion of nano Cu0. The decolorization process was
greatly affected by the solution pH, the discharge pressure and the copper dose. The
decolorization rate increased when the pH of the dye solution was decreased from 4 to
3. The pseudo-first-order kinetic constant for the degradation of methyl orange
increased linearly with the copper dose. UV-vis spectroscopic and Fourier transform
infrared (FT-IR) analyses confirmed that many degradation intermediates were
formed. It is testified that hydroxyl radicals played an important role in the
decolorization process by Cu0/HC. Our results provide valuable information for the
treatment of azo dye wastewaters.
Acknowledgments
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This work was supported by the National Natural Science Foundation of China
(51208366 ), the Foundation of the Science and Technology Commission of Shanghai
Municipality, China (12ZR1451000) and the Foundation of the State Key Laboratory
of Pollution Control and Resource Reuse, China (PCRRY12001).
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21

(a)
(b)
Figure 1. Size change and SEM images of Cu0 nanoparticles (a) before
and (b) after hydrodynamic cavitation treatment

Figure 2. EDX spectrum of Cu0 nanoparticles

Cooling pipe
Tank
Orifice valve
Cooling circulator
Valve
Pressure gauge
Pressure gauge
Figure 3. Schematic of the hydrodynamic cavitation system

1.0
0.8
0.6
0.4
0.2
0.0
Only HC
Nanocopper under mechenical stirring
Nanocopper with HC
0
5
10
Time [min]
15
20
25
Figure 4. Decolorization of methyl orange by nano Cu0 with and without hydrodynamic cavitation

1.0
0.8
0.6
0.4
0.2
0.0
pH=3.0
pH=3.5
pH=4.0
pH=6.0
pH=10.0
pH=12.0
regression lines
0
5
10
15
20
25
Time [min]
Figure 5. Effect of pH on the decolorization of methyl orange by Cu0/HC

0.4
0.3
0.2
0.1
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0.1
0.2
0.3
0.4
0.5
Pressure [MPa]
P=0.2 MPa Cv=1.3
P=0.3 MPa Cv=0.76
P=0.4 MPa Cv=0.64
P=0.5 MPa Cv=0.55
Regression lines
0
5
10
Time [min]
15
20
25
Figure 6. Effect of pump discharge pressure on the decolorization of methyl orange by Cu0/HC

2.0
1.5
1.0
0.5
0.0
-0.5
1.2
0.8
0.4
0.0
0
40
80
120
Dosage [mg/L]
10 mg/L
20 mg/L
30 mg/L
40 mg/L
60 mg/L
100 mg/L
140 mg/L
0
5
10
15
20
Time [min]
Figure 7. Effect of Cu0 nanoparticle dosage on the decolorization of methyl orange by Cu0/HC

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0 min
5 min
10 min
60 min
0 min
10 min
60 min
5 min
200
300
400
Wave length [nm]
500
600
Figure 8. UV-visible absorption spectral changes of methyl orange during Cu0/HC treatment