Environmental Technology
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Enhanced degradation of metronidazole by
heterogeneous sono-Fenton reaction coupled
ultrasound using Fe3O4 magnetic nanoparticles
Yang Hu, Guan Wang , Mingzhi Huang, Kairong Lin, Yuqiang Yi, Zhanqiang
Fang, Pengjun Li & Kangming Wang
To cite this article: Yang Hu, Guan Wang , Mingzhi Huang, Kairong Lin, Yuqiang Yi, Zhanqiang
Fang, Pengjun Li & Kangming Wang (2017): Enhanced degradation of metronidazole by
heterogeneous sono-Fenton reaction coupled ultrasound using Fe3O4 magnetic nanoparticles,
Environmental Technology, DOI: 10.1080/09593330.2017.1374470
To link to this article: http://dx.doi.org/10.1080/09593330.2017.1374470
Accepted author version posted online: 31
Aug 2017.
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Publisher: Taylor & Francis & Informa UK Limited, trading as Taylor & Francis Group
Journal: Environmental Technology
DOI: 10.1080/09593330.2017.1374470
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Enhanced degradation of metronidazole by heterogeneous sono-Fenton reaction
coupled ultrasound using Fe3O4 magnetic nanoparticles
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Yang Hua, Guan Wangc,d, Mingzhi Huanga, Kairong Lina,b*, Yuqiang Yi c,d, Zhanqiang Fangc,d**,
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Pengjun Lia, Kangming Wanga
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a
Department of Water Resource and Environment, Sun Yat-Sen University, Guang Zhou 510275, China
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Key Laboratory of Water Cycle and Water Security in Southern China of Guangdong High Education, Guangzhou
510275, China
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School of Chemistry and Environment, South China Normal University, Guangzhou 51006,Guangdong, China
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Guangdong Technology Research Centre for Ecological Management and Remediation of Water System,
Guangzhou 51006, China
Metronidazole (MNZ), one of the most commonly used nitroimidazole antibiotics in the world, poses a serious
threat to human life and health. In this study, an enhanced sono-Fenton process for the degradation of MNZ is
presented. The catalytic capacity of nano-Fe3O4 in systems comprising ultrasound + Fe3O4 + H2O2, and the
influential parameters such as H2O2, nano-Fe3O4 doses and pH for the Sono-Fenton process, was investigated. The
results showed that the nano-Fe3O4 particles appeared to be roughly spherical in shape, with an average size of
10–20 nm. It was found that •OH radicals were rapidly generated due to the catalytic activity of the nano-Fe3O4.
MNZ could be degraded within a wide pH range, from 3 to 9, and the degradation efficiencies were considerably
enhanced by ultrasound. When the MNZ concentration was fixed at 20 mg/L, the nano-Fe3O4 dosage at 500 mg/L,
the pH at 3 and the reaction temperature at 30°C, the removal efficiencies of MNZ were above 98% after 5 h. It is
indicated that Fe3O4 magnetic nanoparticles were synthesized as heterogeneous catalysts to effectively degrade
MNZ, and the observed stability and recyclability demonstrated that nano-Fe3O4 was promising for the treatment
of wastewater contaminated with antibiotics.
Keywords : Metronidazole; nano-Fe3O4; Heterogeneous sono-Fenton; degradation
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Introduction
Antibiotics are a class of drugs used to prevent and treat bacterial infections, with strong inhibitory
and lethal effects on bacteria, moulds, mycoplasma and many other pathogenic microorganisms
[1,2]. Owing to the misuse of antibiotics, they have been detected in surface water and
groundwater, especially in rural drinking water [3,4].This situation might lead to an increased
selection pressure for the development and spread of antibiotic-resistant bacteria and resistance
genes, which is regarded as one of the three greatest threats to human health by the World Health
Organization [5-7]. The use of antibiotics is a detrimental factor for the survival and reproduction
of bacteria. Bacteria need to adapt to the environment and produce resistance, at this point, we
refer to the use (or choice) of antibiotics as antibiotic selection pressure [8,9]. Antibiotic-resistant
bacteria and resistance genes are difficult to treat, leading to increased morbidity, mortality and
cost of treatment [10]. They represent a serious threat to human life and health in rural areas.
Therefore, the removal of antibiotics from water is essential. Many methods, including ozonation,
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the Fenton and photo-Fenton processes, UV irradiation, nanofiltration and absorption processes,
have been used to remove antibiotics from wastewater [11].
Metronidazole (MNZ), which belongs to the class of pharmaceutical and personal care
products (PPCPs), is a nitroimidazole derivative. It is used to treat infections caused by anaerobic
bacteria and protozoa, such as Trichomonas and Giardia lamblia; however, MNZ has been
suspected of biological carcinogenicity and mutagenicity [12]. Ré conducted a comet assay to
evaluate the genotoxicity of MNZ. The results showed that MNZ could destroy the DNA of
human lymphocytes. Lanzky reported the acute toxicity of MNZ to freshwater and marine
organisms [13].MNZ is highly soluble in water, and resistant to biodegradation. Traditional
sewage treatment methods do not effectively remove it, resulting in its accumulation in the
ecological environment. The high concentration of MNZ in the effluent of sewage treatment plants
has led to adverse effects on the human and ecological environment.
The Fenton process is commonly used worldwide to eliminate antibiotic contaminants.
However, the method suffers several drawbacks, such as the need for large quantities of Fe2+,
which can result in large-scale sedimentation [14]. Moreover, the Fenton reaction requires acidic
conditions, with the pH value generally adjusted to 3, which can increase the cost of the process
[15-19]. With the development of Fenton-like technology, the main drawbacks of the traditional
Fenton process have been overcome to some extent. In recent years, nanoscale Fe3O4 magnetic
particle technology has received growing attention for its high efficiency in eliminating a variety
of pollutants, mainly organic matter [20-23]. However, the aggregation of nano-Fe3O4 particles
during the reaction reduces their surfactivity and dispersibility in aqueous solution, thus reducing
their catalytic activity [21] [23] .It is well known that the application of ultrasound (US) to
aqueous solutions can alleviate mass-transfer limitations and provide an additional cavitation
effect, which is beneficial for degradation in heterogeneous catalytic systems [24] .Therefore, a
system comprising US + Fe3O4 + H2O2 may overcome the defects of the traditional Fenton
reaction. This paper reports the synthesis of nano-Fe3O4 particles, an investigation of an
ultrasound-assisted Fe3O4 + H2O2 system for the degradation of MNZ. To evaluate the degradation
efficiency of MNZ, the effects of different parameters, including the nano-Fe3O4 dosage, initial
MNZ concentration and pH, on the degradation were measured.
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Chemicals and apparatus
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Chemicals: Metronidazole (purity of > 98%) from Aladdin Industrial; 10 mol/L H2O2; 30%
NH3·H2O; 0.01 mol/L HCl; CH3CN (chromatographically pure); 75% H2SO4; FeCl3·6H2O and
FeSO4·7H2O (both the latter as analytical reagents).
Apparatus: high-performance liquid chromatography (HPLC), HP1100, Shimadzu, Japan;
total organic carbon (TOC) analyser, TOC 5000, Shimadzu, Japan; constant-temperature
ultrasonic cleaner; UV-Vis spectrophotometer, Japan; vacuum-drying oven; horizontal
constant-temperature oscillator; electronic analytical balance; digital pH meter, PHS-3B, INESA
Scientific Instrument Co., Ltd; constant-flow pump, HL001, Shanghai Precision instrument
factory.
Materials and methods
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Methods
(1) Preparation of nano-Fe3O4: FeCl3·6H2O (8.20 mmol) and FeSO4·7H2O (8.20 mmol) were
dissolved in 30 mL 0.01 mol/L HCl aqueous solution and heated to 80°C; then, 40 mL of 3.0
mol/L ammonia solution was added dropwise into the heated Fe2+/Fe3+ solution at a flow rate of
5.23 mL/min at 80°C under magnetic stirring. After 3 h reaction, the generated black nano-Fe3O4
particles were collected by magnetic separation, washed five times with deionized water at neutral
pH, and then dried and ground into powder. The proposed reaction for the formation of
nano-Fe3O4 is shown in the following equation:
Fe2+ + 2Fe3+ + 8NH3·H2O = Fe3O4↓ + 8NH4+ + 4H2O
(1)
(2) Characterisation of nano-Fe3O4: The surface, morphology and size of the nano-Fe3O4
particles were investigated by scanning electron microscopy (SEM; JSM-6330F, JEOL, Japan) and
transmission electron microscopy (TEM; JEM-2100HR, JEOL, Japan). Brunnaer–Emmett–Teller
(BET) surface area analysis of nano-Fe3O4 was performed using an ASAP2020M surface analyser
(Micromeritics Instrument Corp., USA) by the nitrogen adsorption–desorption method. The
chemical elements of the nanoparticles were determined using X-ray photoelectron spectroscopy
(XPS, ESCALAB 250 Thermo-VG Scientific, USA).
(3) Experimental procedures: Seven 250 mL conical flasks were taken as the reaction vessels,
and 200 mL MNZ solution (20 mg/L) was added into each conical flask. The pH values of the
reaction systems were regulated by hydrochloric acid and sodium hydroxide solution. Then,
specific amounts of nano-Fe3O4 and H2O2 at different ratios were added to the reaction solutions
separately. After that, the reaction vessels were placed into a constant-temperature ultrasonic
cleaner at 30°C, and the reactions commenced. Samples were withdrawn at specific times, and the
MNZ concentrations in the samples were determined by HPLC equipped with a UV detector at
318 nm. A Diamonsil (R) C18 column (5 μm, 250 mm length × 4.6 mm ID) was used, and the
mobile phase was composed of a mixture of acetonitrile and ultrapure water (20/80, v/v). The flow
speed was set at 1.0 mL/min, and 20 μL injections were used.
Results and discussion
Characterisation of nanoscale catalyst
The TEM and SEM images (Fig. 1) showed that the nano-Fe3O4 particles prepared by the above
method had a favourable size distribution, with an average size of 10–20 nm. The rough surface
structures, with large numbers of depressions, indicated an abundance of active sites on the
surfaces of the nano-Fe3O4 particles. Because of the magnetic and intermolecular forces, the
nano-Fe3O4 particles tended to adopt the form of chain structures. The BET surface area of the
nano-Fe3O4 was 44.6 m2/g. Fig. 2 shows that the pore size distribution of the prepared nano-Fe3O4
was narrow. It can be concluded that the nanoparticles were homogeneous in form. The XPS
patterns of the as-prepared Fe3O4 magnetic nanoparticles are shown in Fig. 3. According to the
results of the XPS analysis, Fe, O, Cl and C were present on the surface of nano-Fe3O4. Moreover,
in the spectrum of Fe 2p there are peaks at 710.67 and 724.42 eV, corresponding to Fe 2p3/2 and
2p1/2, respectively, indicating the existence of Fe3O4 [25-27]; in addition, the peak at 94 eV was
also attributed to the existence of Fe3O4 [28]. It was clearly evident that the iron oxide obtained in
the preparation mainly existed in the form of Fe3O4. The existence of the elements carbon and
chlorine may have resulted from the residual ethanol used during the pretreatment.
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Synergistic effect between the sonochemical and catalytic degradation of MNZ
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The degradation of MNZ was conducted in systems with the following compositions: US + Fe3O4
+ H2O2; US + Fe3O4; US + H2O2; US; Fe3O4 + H2O2; Fe3O4; and H2O2, with initial pH values of
5.79 (Fig. 4). By analysing the final removal rate of the reaction and the reaction rate constant
obtained from pseudo-first-order kinetics, the optimal process was identified, and this process was
shown to involve a synergistic effect [29].[29] The decomposition curves of MNZ under the
different processes are shown in Fig. 4.
The initial pH of the MNZ solutions was 5.79. Comparing the decomposition curves of the
different processes, it is clear that Fe3O4, H2O2 or US, when used separately, had no significant
effects on MNZ decomposition. However, when adding US to assist the chemical decomposition
processes, the final removal rates were greatly improved. After 5 hours in the US + Fe 3O4 + H2O2
system, the removal rate of MNZ was close to 98%, which is much higher than the removal rates
achieved by single-factor US, Fe3O4 and H2O2. This was followed by the US + H2O2, US + Fe3O4
and Fe3O4 + H2O2 systems, which with 46%, 38% and 36%, respectively, also achieved higher
rates than any of the single factors. The calculated rate constants in descending order were
kobsUS+Fe3O4+H2O2 > kobsUS+H2O2 > kobsUS+Fe3O4 > kobsFe3O4+H2O2. Therefore, among
the variety of processes studied, the US + Fe3O4 + H2O2 system exerted the strongest effect on the
removal of MNZ.
In summary, the use of US greatly improved the removal rates because of a synergistic effect
between US and the various chemical processes. We propose two possible mechanisms: (1) the
mechanical effect of US may promote the dispersion of nano-Fe3O4 particles, which prevents their
agglomeration and improves and maintains the activity of Fe3O4 in the reaction process; (2) US
may promote the decomposition of H2O2 into living radicals, which actively degrade MNZ
[30,31] .
While Chakma S.[32-34] studied the physical mechanism of the hybrid sono-Fenton process
with identification of links between individual mechanism of the sonolysis and Fenton process.
The synergy between sonolysis and Fenton process is revealed to be negative. The dissolved
oxygen in the medium is found to play an important role in decolorization through conservation of
oxidizing radicals. They also found that shock waves generated by cavitation bubbles cause
desorption of dye molecules from catalyst surface and reduce the probability of dye–radical
interaction, thus reducing the net utility of photochemically generated •OH radicals towards dye
decolorization.).
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Effect of H2O2 concentration on degradation of MNZ
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H2O2 is a source of the hydroxyl free radical (•OH); therefore, the dosage of H2O2 has a significant
effect on Fenton-like systems. The solutions used in the following experiment contained 20 mg/L
MNZ. To study the effect of the concentration of H2O2 on the degradation of MNZ, the pH values
were fixed at 7, the nano-Fe3O4 dosage at 500 mg/L and the reaction temperature at 30°C. The 5-h
degradation of MNZ in US + Fe3O4 + H2O2 systems was conducted with varying concentrations of
H2O2. The results of first-order kinetic fitting are shown in Fig. 5.
Fig. 5 shows that the degradation rate of MNZ increased from 56.7% to 95% as the
concentration of H2O2 in the reaction system was increased from 39.8 to 234.3 mmol/L. From the
curves of the first-order kinetic fitting, it can be seen that the reaction rate constant (kobs) was only
Effects of important parameters on degradation of MNZ
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1.86 × 10−3 in the absence of H2O2. Under this condition, MNZ was still largely resistant to
degradation after 5 h. This demonstrated that there was insufficient •OH in the US + Fe 3O4 system
for MNZ degradation to occur. With the increase of the H2O2 concentration, the degradation rate
grew. With the fixed dosage of nano-Fe3O4, as the dosage of H2O2 was increased from 0 to 234.3
mmol/L, kobs increased steadily from 1.86 × 10−3 to 1.01 × 10−2, namely a 5.58-fold increase.
From these results, we conclude that in this dosage range, the amount of H2O2 adsorbed on the
surface of the Fe3O4 catalyst increased as a function of the H2O2 concentration, thus providing a
higher reactant concentration for the generation of •OH.
However, while the degradation rate of MNZ increased with the increase of the concentration
of H2O2, it can be predicted that any excess radicals would react with each other, rather than
taking part in the oxidation of MNZ. The self-reaction of •OH would produce a certain amount of
HO2•, but the oxidative capacity of that species is weaker than •OH [35]. Furthermore, the excess
H2O2 would compete with MNZ for adsorption on the nano-Fe3O4 surfaces [31,36], and this
competitive adsorption might reduce the concentration of MNZ around the catalyst surface, with a
negative effect on the rate of degradation. This effect requires further study in systems with much
higher dosages of H2O2.
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Effect of nano-Fe3O4 dosage on degradation of MNZ
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As a peroxidase-like catalyst, the nano-Fe3O4 accelerated the decomposition of H2O2 into •OH
radicals; thus, the amount of nano-Fe3O4 was an important factor in the reaction system. The
solutions used in the following experiment contained 20 mg/L MNZ, while the concentration of
H2O2 was 157.4 mmol/L, the pH was 5.79 and the reaction temperature was 30°C. The 5-h
degradation of MNZ in US + Fe3O4 + H2O2 systems was conducted with varying concentrations of
Fe3O4. The results of first-order kinetic fitting are shown in Fig. 6.
The degradation rate of MNZ increased as the concentration of nano-Fe3O4 in the reaction
systems was increased from 100 to 500 mg/L, while the kobs value increased from 5.2 × 10−3 to 6.7
× 10−3. The degradation rate was highest (86.5%) when the catalyst concentration was 500 mg/L.
When the dosage was further increased to 1000mg/L, the removal rate sharply decreased to 72.9%,
and the kobs to 4.5×10−3. From the four groups of curves of pseudo-first-order kinetic data (Fig. 6),
it can be seen that the degradation of MNZ in the US + Fe3O4 + H2O2 system was consistent with
pseudo-first order-kinetics. Compared with the removal rate of the US + H2O2 process, it can be
seen that the reaction rate constant increased significantly after the addition of nano-Fe3O4. This
demonstrated that within a certain range, the addition of catalyst improved the contact probability
between Fe2+ and H2O2. The catalytic activity thus increased, and a large amount of •OH was
created to promote the degradation of MNZ [37] .At dosages above 500 mg/L, a quenching
reaction occurred between •OH and •OH[38-40]. The concentration of •OH in the solution was
therefore reduced, so that the degradation rate fell. In this experiment, the optimum dosage of
Fe3O4 to maximise the degradation rate of MNZ was therefore 500 mg/L [41] .
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Effect of the pH
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Previous studies have found that the pH can affect the surface-charge properties, adsorption
behaviour and electron-transfer ability of the catalyst, which all affect the catalytic degradation.
Thus, it is necessary to study the extent and the kinetics of the degradation reaction in a wide
range of pH conditions, to explore the influence of the pH on the degradation. In the following
experiments, the MNZ concentration was fixed at 20 mg/L, the H2O2 dosage at 157.4 mmol/L, the
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nano-Fe3O4 dosage at 500 mg/L and the reaction temperature at 30°C. The pH values were
adjusted to 3, 5, 5.79, 7 and 9 using sodium hydroxide and hydrochloric acid. The results of
first-order kinetic fitting are shown in Fig. 7.
Fig. 7 shows that the pH significantly affected the degradation rate of MNZ. When the pH
was 3, the degradation rate was highest, reaching 98% after 5 h, and kobs was 1.4 × 10−2; when
the pH was 5, the degradation rate was still very high, which was 1.25 × 10−2; when the pH was
5.79 and 7, kobs decreased to approximately 7 × 10−3 and 6 × 10−3, respectively; and when the pH
was 9, the degradation rate decreased rapidly, with a kobs of only 3.1 × 10−3. This dependence on
pH is similar to that of the traditional Fenton reaction. We propose that, as the reaction system
changed from acidic to neutral and then alkaline pH, there was a sharp decrease in the
concentration of Fe in the oxidation state Fe2+, thus hindering the activity of the catalyst [42].[42]
At the same time, H2O2 was more easily decomposed into H2O and O2, which reduced the
generation of •OH [43,44]. In addition, because the isoelectric point of nano-Fe3O4 is 7, it is easily
protonated under acidic conditions, allowing the catalyst and the MNZ to combine into a complex
compound that assists catalysis. For these reasons, the degradation rate of MNZ was higher under
more acidic conditions.
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Effect of radical scavengers on MNZ removal
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To confirm the existence of hydroxyl free radicals, tertiary butanol (t-BuOH), a strong radical
scavenger, was added into the solutions at various doses immediately before the reaction [45]. The
results are shown in Fig. 8. In the reaction system not containing hydroxyl radical scavengers, the
removal rate was 98.2%, and was close to completion after 8 h. The presence of t-BuOH markedly
decreased the removal rate from 98.2% (without t-BuOH) to 42.2% (0.1 M t-BuOH) and 36.4% (1
M t-BuOH). These results implied that one of the main factors of the degradation of MNZ was the
ability of nano-Fe3O4 to catalyse the production of •OH radicals from H2O2.
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Mineralisation of MNZ under optimised conditions
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The US + Fe3O4 + H2O2 system with a 500 mg/L dosage of nano-Fe3O4 and a 234.3 mmol/L
dosage of H2O2 was used as a standard system to investigate the mineralisation of MNZ. Ruixiong
Huang et al studied heterogeneous sono-Fenton catalytic degradation of bisphenol A by Fe3O4
magnetic nanoparticles under neutral condition [31]. The optimum concentrations of Fe3O4 MNPs
and H2O2 are 585 mg/L and 160 mmol/L , respectively. Augustine Chioma Affam et al studied
optimization of Fenton treatment of amoxicillin and cloxacillin antibiotic aqueous solution[46].
The optimum operating conditions for Fenton treatment of AMX and CLX antibiotic aqueous
solution at pH 3 were H2O2 /COD molar ratio 2, H2O2 /Fe2+ molar ratio 76, and reaction time
90min for 78.98, 72.96, and 81.18% removal of COD, TOC, and NH3–N, respectively. The results
displayed in Fig. 9 show that the removal of MNZ was very efficient under these conditions. As
the reaction proceeded, the removal of MNZ increased markedly. However, the decomposition of
MNZ was not accompanied by any substantial removal of the total organic carbon (TOC removal),
which remained very low at approximately 8%. This showed that the MNZ was converted into
other organic compounds with the same carbon count as MNZ. The reaction products were not
adsorbed onto the nano-Fe3O4, and the TOC of the solution before and after the reaction remained
essentially unchanged.
The links between the ultrasound and Fenton
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In order to further evaluate the synergistic effects among various factors in US+Fe3O4+H2O2
system. The synergistic index f (synergistic, index) was used to evaluate the synergistic effects of
the technological conditions [29]. For the decomposition reaction, the synergistic index is
calculated by the decomposition rate constant under the action of each single factor (or simple
system), and the larger the f value, the greater the synergy between the individual factors. The
synergistic index was calculated as follows:
f=
kobsA B
kobsA  kobsB
（2）
Among them, f was the synergistic index of kobsA and kobsB；kobsA and kobsB were the rate
constants of MNZ decomposition reaction in A and B systems, respectively. The results were
shown in Tab.1.
As you can see from the table, f1 obtained a maximum value of 6.85. It was shown that US,
Fe3O4 and H2O2, the three factors, acted together to form US+Fe3O4+H2O2, the system produced a
strong synergy. Meanwhile, f2 obtained the maximum value, which was close to f1. It showed that
the combination of US and Fe3O4+H2O2 produced a strong synergistic effect, and ultrasonic
treatment can greatly improve the decomposition effect of MNZ on Fe3O4+H2O2 process.
Besides，f3 and f4 also obtained high value. It indicated that H2O2 and US+Fe3O4, Fe3O4 and
US+H2O2 also show strong synergistic effects, the combination of any factor could greatly
increase the decomposition rate constant of MNZ in a simple system composed of two other
factors. To sum up, US+Fe3O4+H2O2 system had a strong synergistic effect in the decomposition
of MNZ, and had a great advantage in the decomposition of MNZ.
Transformation of MNZ by US + Fe3O4 + H2O2
A reaction solution was analysed from initiation to termination by scanning UV-Vis spectroscopy,
and the results are shown in Fig. 10. As can be seen from the figure, the characteristic UV
absorption peak of MNZ at 318 nm gradually decreased in intensity and eventually disappeared as
the reaction proceeded. Thus, as the reaction time of H2O2 and MNZ increased, the residual
concentration of MNZ gradually decreased. At the same time, another peak was observed at 240
nm. This was most likely the characteristic absorption peak of H2O2, decreasing in intensity as the
reaction time proceeded. These results showed that H2O2 reacted chemically with MNZ.
Fig. 11 shows the variation of the HPLC chromatograms of MNZ with reaction time. As
shown in Fig. 11, the retention time (Rt) of MNZ was 5.3 min, and the peak height decreased as
the reaction proceeded, indicating that the residual concentration of MNZ decreased with
increasing reaction time. Meanwhile, several chromatographic peaks appeared at Rt = 1.7, 2.4 and
5.4 min. These chromatographic peaks were most likely the characteristic peaks of the
intermediates or final products of MNZ. The time dependence of the peaks at Rt = 1.7 and 5.4 min
was not straightforwardly linear: their intensity increased during the first 2 h of reaction time, then
decreased after 2 h, then showed another increase, and finally another decrease. The behaviour of
these peaks indicated that they probably represented intermediates. The peak at Rt = 2.4 min
increased in intensity with increasing reaction time. This peak evidently represented the final
products.
The UV-Vis and HPLC spectra of the reaction between H2O2 and MNZ indicated the
formation of new products during the process of removal, which confirmed the involvement of
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H2O2 in the removal of MNZ.
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Conclusions
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A series of comparative experiments was conducted, which demonstrated that a synergistic effect
occurred in a heterogeneous Fenton-like system with the assistance of ultrasound (US) at a range
of initial pH values. It was also shown that in the presence of US, nano-Fe3O4 exhibited superior
catalytic activity, and the degradation rate of MNZ was greatly enhanced. The factors influencing
the removal efficiency of MNZ also included the dosage of nano-Fe3O4, the dosage of H2O2 and
the initial pH of the solution. Considering the chemical properties of MNZ, and the experimental
results obtained when adding a hydroxyl radical scavenger to the US + H2O2 + Fe3O4 system, the
mechanism of the decomposition of MNZ in this system was concluded to be oxidation by •OH
radicals.
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Acknowledgements
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317
The authors acknowledge financial support by the water conservancy science and technology innovation project of
Guangdong Province (Grant No. : 2015-19). The authors declare that they have no conflict of interest.
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Table and Figure captions
Table1 The synergistic index of US+Fe3O4+H2O2 system
Figure 1 (a) SEM images of nano-Fe3O4; (b) TEM images of nano-Fe3O4
Figure 2 BET analysis results for nano-Fe3O4
Figure 3 XPS patterns of nano-Fe3O4
Figure 4 Degradation of MNZ in different systems
Figure 5 Effect of different H2O2 concentrations on degradation of MNZ in US + Fe3O4 + H2O2
system. The concentration of nano-Fe3O4 is 500 mg/L, pH = 5.79.
Figure 6 Effect of nano-Fe3O4 on degradation of MNZ in US + Fe3O4 + H2O2 system. The
concentration H2O2 is 157.4 mmol/L, pH = 5.79.
Figure 7 Effect of pH on the degradation of MNZ in US + Fe3O4 + H2O2 system. The dosage of
nano-Fe3O4 is 500 mg/L and the concentration of H2O2 is 157.4 mmol/L.
Figure 8 Effect of addition of t-BuOH on the degradation of MNZ. The dosage of nano-Fe3O4 is
500 mg/L and the concentration of H2O2 is 157.4 mmol/L.
Figure 9 TOCremoval in the degradation of MNZ under optimised conditions.
Figure 10 Variation of UV-Vis spectra of MNZ with reaction time.
Figure 11 Variation of HPLC chromatogram of MNZ with reaction time.
Table1. The synergistic index of US+Fe3O4+H2O2 system
Synergistic index
Formula
f1
f=
kobsUS  Fe3O4  H 2O2
Synergistic index value
6.85
kobsUS  kobsFe3O4  kobsH 2O2
f2
f=
f3
f=
f4
f=
kobsUS  Fe3O4  H 2O2
7.64
kobsUS  kobsFe3O4  H 2O2
kobsUS  Fe3O4  H 2O2
5.20
kobsH 2O2  kobsFe3O4 US
kobsUS  Fe3O4  H 2O2
kobsFe3O4  kobsUS  H 2O2
440
11
5.00
Figure 1.
(a) SEM images of nano-Fe3O4; (b) TEM images of nano-Fe3O4
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Figure 2.
BET analysis results for nano-Fe3O4
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Figure 3.
XPS patterns of nano-Fe3O4
14
100
Fe3O4
H2O2
Fe3O4+H2O2
80
US
US+Fe3O4
Removal Rate (%)
US+H2O2
US+Fe3O4+H2O2
60
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0
0
464
465
466
467
468
50
100
150
200
250
300
T(min)
Figure 4.
Degradation of MNZ in different systems .The concentration of H2O2 is 157.4 mmol/L, the
concentration of nano-Fe3O4 is 500 mg/L, pH = 5.79.
15
3.0
0.009
kobs(min-1)
2.8
2.6
0.006
2.4
0.003
2.2
50
2.0
1.8
ln(CO/C)
B
100
150
200
H202 dose(mmol/L)
39.8mmol/L
79.3mmol/L
157.4mmol/L
234.3mmol/L
1.6
1.4
1.2
1.0
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0.6
0.4
0.2
0.0
0
469
470
471
472
473
50
100
150
200
250
300
T(min)
Figure 5. Effect of different H2O2 concentrations on degradation of MNZ in US + Fe3O4 + H2O2 system. The
concentration of nano-Fe3O4 is 500 mg/L, pH = 5.79.
16
2.2
0.007
-1
kobs(min )
2.0
1.8
1.6
ln(CO/C)
1.4
0.006
0.005
0
200 400 600 800 1000
Fe3o4 dose(mg/L)
1.2
100mg/L
250mg/L
500mg/L
1000mg/L
1.0
0.8
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0.4
0.2
0.0
0
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491
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493
50
100
150
200
250
300
T(min)
Figure 6. Effect of nano-Fe3O4 on degradation of MNZ in US + Fe3O4 + H2O2 system. The concentration of
H2O2 is 157.4 mmol/L, pH = 5.79.
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507
Figure 7. Effect of pH on the degradation of MNZ in US + Fe3O4 + H2O2 system. The dosage of nano-Fe3O4 is
500 mg/L and the concentration of H2O2 is 157.4 mmol/L.
18
Omol/L t-BuOH
0.1mol/L t-BuOH
1mol/L t-BuOH
100
Removal Rate (%)
80
60
40
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0
0
508
509
510
511
512
513
514
515
50
100
150
T(min)
200
250
300
Figure 8. Effect of addition of t-BuOH on the degradation of MNZ. The dosage of nano-Fe3O4 is 500 mg/L and
the concentration of H2O2 is 157.4 mmol/L.
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MNZ
TOC
100
Removal Rate (%)
80
60
40
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0
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519
50
100
150
200
250
T(min)
Figure 9.
TOCremoval in the degradation of MNZ under optimised conditions.
20
300
30min
60min
120min
180min
240min
300min
3.0
2.5
A
2.0
1.5
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1.0
0.5
0.0
200
520
521
522
300
400
500
600
wavelength(nm)
Figure 10.
Variation of UV-Vis spectra of MNZ with reaction time.
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800
30min
60min
120min
180min
240min
300min
30
Peak Area
25
20
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5
0
523
524
1
Figure 11.
2
3
4
T(min)
5
6
Variation of HPLC chromatogram of MNZ with reaction time.
525
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