Ultrasonics Sonochemistry 21 (2014) 991–996
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MnO2/CeO2 for catalytic ultrasonic degradation of methyl orange He Zhao a, Guangming Zhang a,⇑, Quanling Zhang a,b a b
School of Environment & Natural Resource, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China Master Erasmus Mundus Mamaself Universit Rennes, 1 Campus de Beaulieu, 35042 Ernnes Cedex, France
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 3 December 2013 Accepted 6 December 2013 Available online 13 December 2013 Keywords: MnO2/CeO2 Methyl orange Catalytic ultrasonic degradation Cavitation effect Pre-adsorption
a b s t r a c t Catalytic ultrasonic degradation of aqueous methyl orange was studied in this paper. Heterogeneous catalyst MnO2/CeO2 was prepared by impregnation of manganese oxide on cerium oxide. Morphology and specific surface area of MnO2/CeO2 catalyst were characterized and its composition was determined. Results showed big differences between fresh and used catalyst. The removal efficiency of methyl orange by MnO2/CeO2 catalytic ultrasonic process was investigated. Results showed that ultrasonic process could remove 3.5% of methyl orange while catalytic ultrasonic process could remove 85% of methyl orange in 10 min. The effects of free radical scavengers were studied to determine the role of hydroxyl free radical in catalytic ultrasonic process. Results showed that methyl orange degradation efficiency declined after adding free radical scavengers, illustrating that hydroxyl free radical played an important role in degrading methyl orange. Theoretic analysis showed that the resonance size of cavitation bubbles was comparable with the size of catalyst particles. Thus, catalyst particles might act as cavitation nucleus and enhance ultrasonic cavitation effects. Measurement of H2O2 concentration in catalytic ultrasonic process confirmed this hypothesis. Effects of pre-adsorption on catalytic ultrasonic process were examined. Preadsorption significantly improved methyl orange removal. The potential explanation was that methyl orange molecules adsorbed on catalysts could enter cavitation bubbles and undergo stronger cavitation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction It is estimated that about 15% of dyes produced in the world are released into environment during their synthesis and processing [1]. The release of dyes causes serious environmental problems. Azo dyes are the most frequently used among all dyes. But azo dyes and their intermediate products, such as aromatic amines, are toxic, carcinogenic and mutagenic to aquatic life and humans [2,3]. Thus degradation of these dyes is necessary before they were discharged into the environment. Due to limited effects of traditional methods on degradation of azo dyes, many researchers have studied advanced oxidation processes (AOPs) to treat dye wastewater in recent years. These techniques can degrade harmful organic compounds by highly reactive free radical oxidation and other mechanisms. Among all AOPs, ultrasonic process is very effective and has received increasing attentions. This technique showed good degradation efficiency on colored effluents [4,5]. The main mechanism of ultrasonic process is acoustic cavitations. The acoustic cavitation is consisted of formation (nucleation), rapid growth (expansion) and violent collapse (implosion) of cavitation bubbles in a liquid [6]. Cavitation bubbles are generated during rarefaction cycle ⇑ Corresponding author. Tel.: +86 13520956445. E-mail address: [email protected] (G. Zhang). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.002
where negative acoustic pressure is sufficient to pull water molecules separate from each other to create tiny microbubbles [7]. The collapse of these cavitation bubbles can generate high temperatures (up to 5000 K) and high pressures (above 1000 atm). Under such extreme conditions, the local generated ‘‘hot spots’’ can convert water molecules to highly reactive species, including hydroxyl, hydrogen, hydroperoxide free radicals, and mineralize organic compounds [8,9]. These free radicals are also capable of degrading organic pollutants. However, ultrasound alone has limited efficiency on degrading azo dyes [10]. A potential solution is catalytic ultrasonic process. Catalytic ultrasonic process combines ultrasonic treatment with catalysts such as TiO2, CuO/ZnO and are effective in degradation of Rhodamine B and phenolic compounds [10,11]. Manganese dioxide is a typical catalyst used in oxidation of organic pollutants in wastewater. It is also a good adsorbent due to its high surface areas and surface charges [12,13]. Because of its strong oxidation and adsorption ability, manganese dioxide has excellent removal efficiency of organic matters. Manganese dioxide-ultrasonic process reached 80% degradation efficiency of phenol in 120 min [14] and excellent decolorization and TOC removal on azo dye acid red B [12]. Cerium oxides are another group of effective catalysts in oxidation of organic matters, such as phenol, cyclohexane and cyclohexanone [15–17]. They are especially effective for dye removal since
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they have large pore size that can easily accommodate dye molecules [18]. Furthermore, cerium oxides can improve the efficiency of catalysts after being composited with other metals. Studies showed that cerium oxides promoted oxygen storage and mobility, formed surface and bulk vacancies, thus improved the redox properties of catalyst [19–21]. Because manganese dioxide has good degradation efficiency of dyes and cerium oxides have excellent composite effects, we propose a new catalyst, MnO2 supported by CeO2, for azo dye degradation in catalytic ultrasonic process. In this paper, a new catalyst, MnO2/CeO2, was prepared and characterized. Its efficiency of azo dyes degradation in catalytic ultrasonic process was tested. Mechanisms of MnO2/CeO2 catalytic ultrasonic degradation process were studied. Methyl orange, a typical azo dye, was chosen as the target pollutant in this paper.
t efficiencyð%Þ ¼ C 0CC , where ‘C0’ is the initial concentration of 0 methyl orange and ‘Ct’ means the final concentration of methyl orange. In the pre-adsorption catalytic ultrasonic process, methyl orange solution with catalyst was stirred at 300 rpm for different time. Then the mixture was ultrasonic irradiated for different time. After reaction, the mixture was centrifuged at 3000 rpm for 5 min, and supernatant was used for methyl orange detection. Methyl orange was detected using a Pu Xi TU-1900 UV–vis spectrophotometer. The detection wavelength was at 456 nm.
3. Results and discussion 3.1. Characterization of MnO2/CeO2 catalyst
2. Experimental 2.1. Materials All chemicals used in this work were analytical grade reagents. These chemicals were used without further purification. Cerous nitrate, manganous nitrate and methyl orange were purchased from Tianjin Guangfu Fine Chemical Research Institute, Tianjin Guangfu Technology Development Co. LTD. and Xilong Chemical Industry Co. LTD. respectively. Solutions were prepared with water purified by a Millipore Milli Q UV Plus system. 2.2. Catalyst preparation and characterization A precipitation method was applied to obtain CeO2 material using NaOH and Ce(NO3)2 as starting reagents. Mixture of 150 ml of 0.2 mol/L NaOH and 150 ml of 0.065 mol/L Ce(NO3)2 was stirred at 300 rpm. The reaction temperature was 35 °C. After 4 h reaction, the solution was centrifuged at 12,000 rpm for 10 min to separate the products. The settled products were washed thrice using distilled water to remove byproducts, then were dried at 100 °C in an air atmosphere. After 3 h light yellow CeO2 was obtained [4,5]. The cerium oxide was dipped in manganese nitrate solution for 2 h, then the mixture was filtered. The solid was calcined for 2 h in 400 °C. Finally, MnO2/CeO2 catalyst was obtained. Specific surface area and porosity of catalyst were measured by nitrogen adsorption/desorption isotherms at 77 K. The model of the instrument was 3H-2000PS2, BeiShiDe Instrument-S&T. The specific surface area was obtained by Brunauer–Emmet–Teller (BET) method. And the total pore volume and average pore size of samples were calculated by Barrett–Joyner–Halenda (BJH) method. Surface morphology of samples was investigated using a Hitachi S 4700 scanning electron microscope (SEM) analyzer at different scales and magnifications. Secondary electron detector was used in SEM. Composition of catalyst was determined by energy dispersive spectrometer (EDS). 2.3. Experimental runs Ultrasonic irradiation was performed with a JY92-II ultrasonic generator (Ningbo Xinzhi Technology Co., China) with an operating power of 65 W and frequency of 24 kHz. All experiments were conducted in 150 ml glass beakers. Each time, 100 ml of 20 mg/L methyl orange solution was tested and the solution pH was adjusted to 2.3. In catalytic ultrasonic experiments and pre-adsorption experiments, the additive amount of MnO2/CeO2 was 0.1 g. In catalytic ultrasonic process, the efficiencies of different ultrasonic irradiation time were tested. Degradation efficiency of methyl orange dye was defined as follows: Degradation
The specific surface area, total pore volume and average pore size of catalysts before and after use were measured using BET. The fresh catalyst had a specific surface area of 71.1002 m2/g, a total pore volume of 0.1321 ml/g and an average pore size of 7.431 nm. The used catalyst had a specific surface area of 77.2496 m2/g, a total pore volume of 0.1284 ml/g and an average pore size of 6.652 nm. After reaction, catalyst had small changes. The specific surface area increased by 8.6% and total pore volume and average pore size decreased. In general, the adsorption ability was improved after ultrasonic irradiation. The morphology of fresh and used MnO2/CeO2 was observed through SEM images. It was found that the prepared catalyst had a diverse shape and the size was in the range of 20–130 lm. Fig. 1(a) is the SEM image of fresh MnO2/CeO2 catalyst. Fig. 1(b) shows the SEM image of used MnO2/CeO2 catalyst which had obvious difference with fresh catalyst. For fresh catalyst, lots of catalyst particles with flat surface were observed. After using, the catalyst surface became rough and the structure became dispersed and catalyst particle with flat surface could hardly be found. Changes between fresh and used catalysts were because of mechanical effects of ultrasound. Ultrasound generated high speed liquid microjets and shockwaves towards the solid surface [22–24]. Those microjets reached hundreds of meters per second, and shockwaves yielded high pressure. The surface and structure of catalyst had big changes after used due to impacts of microjets and shock waves. Chemical composition of catalyst was determined by EDS. Results of fresh catalyst showed that mass percent of Mn and Ce were 27.1% and 58.8% respectively, and the molar ratio between Mn and Ce was about 1.2:1. Results of used catalyst showed that mass percent of Mn and Ce were 21.5% and 61.2%, and the molar ratio between Mn and Ce was about 1:1. Clearly, content of Mn in catalyst decreased after ultrasonic irradiation. That was because most Mn was on the surface of catalyst, and might fall from catalyst due to microjets and shock waves in ultrasonic process. So the amount of Mn in catalyst decreased after ultrasonic irradiation.
3.2. Methyl orange removal efficiency of MnO2/CeO2 catalytic ultrasonic process The catalytic activity of MnO2/CeO2 catalyst was evaluated in terms of removal efficiency of methyl orange. The results are shown in Fig. 2. Clearly, the degradation of ultrasonic irradiation was very poor, and the highest removal efficiency was only 10.2% after 30 min. The removal efficiency of MnO2/CeO2 reached 70% after 30 min due to adsorption. After ultrasonic irradiation combined with MnO2/CeO2 catalyst, the removal efficiency of methyl orange was improved significantly. 99% removal efficiency could be obtained after 60 min. MnO2/CeO2 catalytic ultrasonic process exhibited excellent efficiency in degradation of methyl orange.
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The removal efficiency was much higher than those of ultrasonic irradiation alone and catalyst alone. 3.3. Potential mechanisms of MnO2/CeO2 catalytic ultrasonic process Researches showed that there were two major mechanisms in ultrasonic degradation of pollutants, namely oxidation by oxidative free radicals and pyrolysis in ‘‘hot spots’’ during cavitation. Among free radicals, hydroxyl free radical is the most important one. The effectiveness of pyrolysis depends on the generation and collapse of cavitation bubbles, which is significantly impacted by catalyst. So it was necessary to determine the role of hydroxyl free radical and to analysis relationship between catalyst and cavitation bubbles. 3.3.1. Effects of hydroxyl free radical Hydroxyl free radical oxidation is a key mechanism in all AOPs. Previous researches proved that ultrasound could generate hydroxyl free radical, which might oxidize methyl orange due to its high redox potential. To quantify the effects of hydroxyl free radical on methyl orange degradation, hydroxyl free radical scavenger, tertbutanol (TBA), was added to the solution. TBA has a fast reaction rate with hydroxyl free radical (6 108 M1s1) [25] and was commonly used as indicator of hydroxyl free radical reactions [26,27]. Fig. 3 shows effects of TBA addition on the removal efficiency of methyl orange. The addition of TBA had obvious inhabitation of removal efficiency. When ultrasonic irradiation time was 10 min, the degradation efficiency decreased by 25.2%. When ultrasonic radiation time was 60 min, the degradation efficiency decreased by 20.2%. With more ultrasonic irradiation time, inhabitation by TBA decreased due to hydroxyl free radical generated with more reaction time. When the concentration of TBA was higher than 20 mmol/L, degradation efficiency was not further inhibited. The big decline of degradation efficiency illustrated that hydroxyl free radical mechanism acted an important role in methyl orange degradation in catalytic ultrasonic process.
Fig. 1. SEM images of fresh and used MnO2/CeO2 catalyst. (a) Fresh catalyst; (b) used catalyst. The magnification is 2500 times. Surface of fresh catalyst is flat. After used, the surface turns rough and structure becomes dispersed. Huge changes occur between fresh and used catalysts.
3.3.2. The impacts of MnO2/CeO2 catalyst on cavitation Cavitation effect determines the efficiency of ultrasonic degradation process. The strength of cavitation effect depends on the quantity of cavitation bubbles. More cavitation bubbles cause stronger cavitation effects, leading to higher degradation efficiency. To generate more cavitation bubbles, there should be more particles acting as nucleus of cavitation bubbles. It was reported that heterogeneous catalyst contained in solution could be potential nucleus of cavitation bubbles [28]. Once the catalyst particles sizes were on the same order of magnitude with the resonance size of cavitation bubbles, catalyst particles could act as nucleus of cavitation bubbles [29]. Therefore, in MnO2/CeO2 catalytic ultrasonic process, the catalyst size was an important parameter that affected cavitation effects. Suitable size enabled catalyst particles to act as cavitation nucleus, promote formation and collapse of cavitation bubbles, and enhance cavitation effects. The resonance size of cavitation bubbles at given ultrasound frequency can be estimated by [28]:
f ¼
Fig. 2. Methyl orange removal efficiency of MnO2/CeO2 catalytic ultrasonic process. Ultrasonic irradiation alone and catalyst alone have limited removal efficiency of methyl orange. After MnO2/CeO2 combined with ultrasonic irradiation, methyl orange removal becomes faster and more effectively.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2r 3c 2r q P 0 þ a aq 2pa
:
Where ‘f’ means ultrasound frequency, c means the ratio of heat capacities of saturating gas at constant pressure and volume, a is the radius of bubbles, P0 is the ambient pressure, r is the surface tension and q is the density of surrounding medium. In this experiment, f was 24 kHz, c was 1.39 for air, surface tension (r) was negligible, and water density was 1.0 g/cm3. The resonance size of cavitation bubbles in water was calculated to be 0.136 mm. The
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Table 1 Researches on degradation MO. Catalyst
Type
Effects
Reaction time
Rutile and anatase TiO2
Sono-catalytic
150 min
Ag/TiO2 MWCNT/TiO2 Mesoporous titania nanoparticles Au/TiO2 ZnO Fe2O3–CeO2–TiO2/c-Al2O3 Pt–Bi/C
Sonophoto-catalytic Photo-catalytic Photo-catalytic
100% and 41.36% Degradation of MO in presence of retile and anatase TiO2, respectively. 100% Degradation of MO in presence of 60 mg/L Ag/TiO2 93% Degradation of MO in presence of MWCNT/TiO2 98% Degradation of MO in presence of 1 g/L catalyst (pH 2.0)
Photo-catalytic Photo-catalytic Catalytic wet air oxidation (CWAO) Electrochemical degradation process
100% Degradation of MO 100% Degradation of MO in presence of 1 g/L ZnO 98.09% Of color and 96.08% of total organic carbon(TOC) can be removed 95.6% Degradation of MO
120 min 80 min 2.5 h 180 min
average size of catalyst particles was about 0.096 mm calculated from SEM images, which was close to the resonance size of cavitation bubbles. So MnO2/CeO2 catalyst could act as the nucleus of cavitation bubbles. With nucleus available, more cavitation bubbles were generated in the catalytic ultrasonic process. Collapse of cavitation bubbles led to hydrolyzation of water molecule and generation of various free radicals including hydroxyl free radical [30,31]. Typical reactions are listed in Eqs. (1-6), in which ‘‘)))’’ denotes ultrasonic irradiation:
H2 OþÞÞÞ ! OH þ H
ð1Þ
O2 þÞÞÞ ! 2 O
ð2Þ
O þ H2 O ! 2 OH
ð3Þ
OH þ H ! H2 O
ð4Þ
2 OH ! H2 O þ O
ð5Þ
2 OH ! H2 O2 :
ð6Þ
Water molecule and dissolved oxygen molecule are converted into free radicals such as hydroxyl free radical, hydrogen free radical and oxygen free radical in the cavitation bubbles (reactions 1–3). These free radicals can recombine to form H2O, O, H2O2, which are released into the bulk solution (reactions 4–6). Among these products, H2O2 is the most one and its amount is closely
Fig. 3. Effects of the t-butanol (TBA) addition on removing methyl orange. With 10 mmol/L of TBA, degradation efficiencies decrease 25.2% and 20.2% after 10 and 60 min ultrasonic irradiation. Degradation efficiency of methyl orange keeps stable with more than 20 mmol/L of TBA.
120 min 100 min 45 min
correlated with the amount of hydroxyl free radical. Therefore, hydroxyl free radical concentration can be reflected with hydrogen peroxide concentration. Fig. 4 shows the formation of hydrogen peroxide during ultrasonic process and catalytic ultrasonic process. Clearly, MnO2/ CeO2 catalyst generated no hydrogen peroxide. Ultrasonic irradiation generated few hydrogen peroxide. MnO2/CeO2 catalytic ultrasonic process, on the other hand, generated lots of hydrogen peroxide and the concentration reached 6.48 mg/L after 30 min. High concentration of hydrogen peroxide indicated high concentration of hydroxyl free radical in the solution. Such a significant increase of hydrogen peroxide formation showed clearly the synergistic effect of catalytic ultrasonic process. Furthermore, the composition of MnO2/CeO2 catalyst also contributed to methyl orange degradation Combination of MnO2 and CeO2 could form a redox couple since Mn and Ce have various oxidation states [32], Electron transfer in the redox couple increased the surface electric charge, then improved the catalytic activity [33]. The catalyst surface could react with oxygen molecule to generate oxygen free radical [34], which then accelerates the formation of hydroxyl free radical (reaction 3). In summary, both theoretic comparison of resonance size of cavitation bubbles and catalyst sizes and experimental detection of hydrogen peroxide proved that MnO2/CeO2 catalyst might act as the nucleus for cavitation bubbles, enhance the cavitation effects, and promote hydroxyl free radical formation. Furthermore, catalyst composition might also enhance surface electron transfer and promote hydroxyl free radical generation.
Fig. 4. H2O2 generation in catalytic ultrasonic process. Ultrasonic irradiation alone and MnO2/CeO2 alone generate little H2O2. MnO2/CeO2 combined with ultrasonic irradiation significantly increases generation of H2O2.
H. Zhao et al. / Ultrasonics Sonochemistry 21 (2014) 991–996
Fig. 5. Langmuir adsorption isotherm of MnO2/CeO2.in adsorption of methyl orange. The correlation is above 0.99 indicating MnO2/CeO2 adsorption of methyl orange in good accordance with Langmuir adsorption isotherm.
3.4. Impact of pre-adsorption on catalytic ultrasonic process MnO2/CeO2 catalyst had good adsorption ability for methyl orange. It could adsorb 68.1% of total methyl orange in 20 min (Fig. 1). The adsorption isotherm was shown in Fig. 5. The adsorption process of methyl orange by MnO2/CeO2 was in accordance with Langmuir adsorption isotherm. The adsorption of methyl orange was monolayer adsorption. And methyl orange was adsorbed uniformly on the surface of catalyst. Through adsorption, the amount of methyl orange on the catalyst increased a lot, which might influence the catalytic ultrasonic degradation of methyl orange. In order to find out the effects of adsorption on catalytic ultrasonic process, pre-adsorption was done. Results are shown in Fig. 6. The degradation efficiency had big differences with different pre-adsorption time followed by ultrasonic irradiation. With 1 min’ pre-adsorption, the removal efficiency could reach 42% after 1 min ultrasonic irradiation, 15.5% higher than the control. With 10 min’ and 20 min’ pre-adsorption, the removal efficiency could reach 77.5% and 81.5% after 1 min ultrasonic irradiation. The effect of pre-adsorption on MnO2/CeO2 catalytic ultrasonic process might be explained as follows. Methyl orange molecules were adsorbed on the catalyst surfaces, entered cavitation bubbles as catalyst acted as cavitation nucleus, and underwent cavitation
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effects. The ‘‘hot spot’’ effect was the strongest on cavitation nucleus, very high pressure and temperature could directly mineralize methyl orange molecules in the cavitation bubbles. More methyl orange molecules on the cavitation nucleus meant better mineralization. Similarly, the hydroxyl free radicals concentration was also the highest in the cavitation bubbles, more methyl orange molecules on the cavitation nucleus led to better oxidation by hydroxyl free radical. Due to MnO2/CeO2 catalyst acting as nucleus of cavitation bubbles, the concentration of hydroxyl free radical was even higher around the catalyst than in the bulk. Therefore, catalytic ultrasonic degradation of methyl orange became faster with pre-adsorption. With pre-adsorption, the methyl orange degradation mainly occurred on the catalyst surface rather than in the bulk solution [34]. The change of TOC was also investigated. TOC decreased along with the reaction time but the degradation efficiency of TOC was less than that of methyl orange. The results complied with previous findings [35,36]. Some methyl orange could be degraded into carbon dioxide while some were degraded into intermediate products such as aliphatic acids and aldehydes. These intermediate products proceeded at a much slower reaction rate [37,38]. So the reduction of TOC resulted from degradation of methyl orange but was less effective. Methyl orange degradation using catalytic ultrasonic process was studied by few researchers (Table 1). Hydroxyl free radical oxidation of methyl orange was proposed in catalytic ultrasonic process by Wang [39]. In this paper, the hydroxyl free radical oxidation of methyl orange was proven and quantitative analyzed. Moreover, enhancement by catalyst on cavitation effects was studied and increase of degradation efficiency with pre-adsorption was found and explained. 4. Conclusions In this work, MnO2/CeO2 catalyst was successfully prepared using an impregnation method. The catalyst had a specific surface area of 71.1002 m2/g, a total pore volume of 0.1321 ml/g, an average pore size of 7.431 nm, a molar ratio between Mn and Ce of about 1.2:1. After catalytic ultrasonic process, the morphology, size, and composition of catalyst changed significantly. 99.1% of methyl orange could be removed in catalytic ultrasonic process in 60 min, much higher than the effect of ultrasonic degradation alone. Hydroxyl free radical played an important role in oxidation of methyl orange. MnO2/CeO2 catalyst significantly enhanced the cavitation effects due to the closeness of cavitation bubbles’ resonance size and catalyst’s size. High concentration of hydrogen peroxide in catalytic ultrasonic process also proved the enhancement of cavitation effects. The increase of degradation efficiency in catalytic ultrasonic process by pre-adsorption was observed and explained. With pre-adsorption, more methyl orange molecules entered cavitation bubbles and underwent stronger cavitation effects. In summary, MnO2/CeO2 catalytic ultrasonic process has shown great potential in treating dye wastewater. Acknowledgment Authors are grateful for financial support from Basic Research funds in Renmin University of China from the Center Government (12XNLI01). References
Fig. 6. Degradation efficiency of methyl orange in catalytic ultrasonic process with different pre-adsorption time. The degradation efficiency of methyl orange is significantly enhanced by pre-adsorption. With pre-adsorption, catalytic ultrasonic process degrades methyl orange faster and more effectively.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
MnO2/CeO2 for catalytic ultrasonic degradation of methyl orange He Zhao a, Guangming Zhang a,⇑, Quanling Zhang a,b a b
School of Environment & Natural Resource, Renmin University of China, 59 Zhongguancun Street, Beijing 100872, China Master Erasmus Mundus Mamaself Universit Rennes, 1 Campus de Beaulieu, 35042 Ernnes Cedex, France
a r t i c l e
i n f o
Article history: Received 27 July 2013 Received in revised form 3 December 2013 Accepted 6 December 2013 Available online 13 December 2013 Keywords: MnO2/CeO2 Methyl orange Catalytic ultrasonic degradation Cavitation effect Pre-adsorption
a b s t r a c t Catalytic ultrasonic degradation of aqueous methyl orange was studied in this paper. Heterogeneous catalyst MnO2/CeO2 was prepared by impregnation of manganese oxide on cerium oxide. Morphology and specific surface area of MnO2/CeO2 catalyst were characterized and its composition was determined. Results showed big differences between fresh and used catalyst. The removal efficiency of methyl orange by MnO2/CeO2 catalytic ultrasonic process was investigated. Results showed that ultrasonic process could remove 3.5% of methyl orange while catalytic ultrasonic process could remove 85% of methyl orange in 10 min. The effects of free radical scavengers were studied to determine the role of hydroxyl free radical in catalytic ultrasonic process. Results showed that methyl orange degradation efficiency declined after adding free radical scavengers, illustrating that hydroxyl free radical played an important role in degrading methyl orange. Theoretic analysis showed that the resonance size of cavitation bubbles was comparable with the size of catalyst particles. Thus, catalyst particles might act as cavitation nucleus and enhance ultrasonic cavitation effects. Measurement of H2O2 concentration in catalytic ultrasonic process confirmed this hypothesis. Effects of pre-adsorption on catalytic ultrasonic process were examined. Preadsorption significantly improved methyl orange removal. The potential explanation was that methyl orange molecules adsorbed on catalysts could enter cavitation bubbles and undergo stronger cavitation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction It is estimated that about 15% of dyes produced in the world are released into environment during their synthesis and processing [1]. The release of dyes causes serious environmental problems. Azo dyes are the most frequently used among all dyes. But azo dyes and their intermediate products, such as aromatic amines, are toxic, carcinogenic and mutagenic to aquatic life and humans [2,3]. Thus degradation of these dyes is necessary before they were discharged into the environment. Due to limited effects of traditional methods on degradation of azo dyes, many researchers have studied advanced oxidation processes (AOPs) to treat dye wastewater in recent years. These techniques can degrade harmful organic compounds by highly reactive free radical oxidation and other mechanisms. Among all AOPs, ultrasonic process is very effective and has received increasing attentions. This technique showed good degradation efficiency on colored effluents [4,5]. The main mechanism of ultrasonic process is acoustic cavitations. The acoustic cavitation is consisted of formation (nucleation), rapid growth (expansion) and violent collapse (implosion) of cavitation bubbles in a liquid [6]. Cavitation bubbles are generated during rarefaction cycle ⇑ Corresponding author. Tel.: +86 13520956445. E-mail address: [email protected] (G. Zhang). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.12.002
where negative acoustic pressure is sufficient to pull water molecules separate from each other to create tiny microbubbles [7]. The collapse of these cavitation bubbles can generate high temperatures (up to 5000 K) and high pressures (above 1000 atm). Under such extreme conditions, the local generated ‘‘hot spots’’ can convert water molecules to highly reactive species, including hydroxyl, hydrogen, hydroperoxide free radicals, and mineralize organic compounds [8,9]. These free radicals are also capable of degrading organic pollutants. However, ultrasound alone has limited efficiency on degrading azo dyes [10]. A potential solution is catalytic ultrasonic process. Catalytic ultrasonic process combines ultrasonic treatment with catalysts such as TiO2, CuO/ZnO and are effective in degradation of Rhodamine B and phenolic compounds [10,11]. Manganese dioxide is a typical catalyst used in oxidation of organic pollutants in wastewater. It is also a good adsorbent due to its high surface areas and surface charges [12,13]. Because of its strong oxidation and adsorption ability, manganese dioxide has excellent removal efficiency of organic matters. Manganese dioxide-ultrasonic process reached 80% degradation efficiency of phenol in 120 min [14] and excellent decolorization and TOC removal on azo dye acid red B [12]. Cerium oxides are another group of effective catalysts in oxidation of organic matters, such as phenol, cyclohexane and cyclohexanone [15–17]. They are especially effective for dye removal since
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they have large pore size that can easily accommodate dye molecules [18]. Furthermore, cerium oxides can improve the efficiency of catalysts after being composited with other metals. Studies showed that cerium oxides promoted oxygen storage and mobility, formed surface and bulk vacancies, thus improved the redox properties of catalyst [19–21]. Because manganese dioxide has good degradation efficiency of dyes and cerium oxides have excellent composite effects, we propose a new catalyst, MnO2 supported by CeO2, for azo dye degradation in catalytic ultrasonic process. In this paper, a new catalyst, MnO2/CeO2, was prepared and characterized. Its efficiency of azo dyes degradation in catalytic ultrasonic process was tested. Mechanisms of MnO2/CeO2 catalytic ultrasonic degradation process were studied. Methyl orange, a typical azo dye, was chosen as the target pollutant in this paper.
t efficiencyð%Þ ¼ C 0CC , where ‘C0’ is the initial concentration of 0 methyl orange and ‘Ct’ means the final concentration of methyl orange. In the pre-adsorption catalytic ultrasonic process, methyl orange solution with catalyst was stirred at 300 rpm for different time. Then the mixture was ultrasonic irradiated for different time. After reaction, the mixture was centrifuged at 3000 rpm for 5 min, and supernatant was used for methyl orange detection. Methyl orange was detected using a Pu Xi TU-1900 UV–vis spectrophotometer. The detection wavelength was at 456 nm.
3. Results and discussion 3.1. Characterization of MnO2/CeO2 catalyst
2. Experimental 2.1. Materials All chemicals used in this work were analytical grade reagents. These chemicals were used without further purification. Cerous nitrate, manganous nitrate and methyl orange were purchased from Tianjin Guangfu Fine Chemical Research Institute, Tianjin Guangfu Technology Development Co. LTD. and Xilong Chemical Industry Co. LTD. respectively. Solutions were prepared with water purified by a Millipore Milli Q UV Plus system. 2.2. Catalyst preparation and characterization A precipitation method was applied to obtain CeO2 material using NaOH and Ce(NO3)2 as starting reagents. Mixture of 150 ml of 0.2 mol/L NaOH and 150 ml of 0.065 mol/L Ce(NO3)2 was stirred at 300 rpm. The reaction temperature was 35 °C. After 4 h reaction, the solution was centrifuged at 12,000 rpm for 10 min to separate the products. The settled products were washed thrice using distilled water to remove byproducts, then were dried at 100 °C in an air atmosphere. After 3 h light yellow CeO2 was obtained [4,5]. The cerium oxide was dipped in manganese nitrate solution for 2 h, then the mixture was filtered. The solid was calcined for 2 h in 400 °C. Finally, MnO2/CeO2 catalyst was obtained. Specific surface area and porosity of catalyst were measured by nitrogen adsorption/desorption isotherms at 77 K. The model of the instrument was 3H-2000PS2, BeiShiDe Instrument-S&T. The specific surface area was obtained by Brunauer–Emmet–Teller (BET) method. And the total pore volume and average pore size of samples were calculated by Barrett–Joyner–Halenda (BJH) method. Surface morphology of samples was investigated using a Hitachi S 4700 scanning electron microscope (SEM) analyzer at different scales and magnifications. Secondary electron detector was used in SEM. Composition of catalyst was determined by energy dispersive spectrometer (EDS). 2.3. Experimental runs Ultrasonic irradiation was performed with a JY92-II ultrasonic generator (Ningbo Xinzhi Technology Co., China) with an operating power of 65 W and frequency of 24 kHz. All experiments were conducted in 150 ml glass beakers. Each time, 100 ml of 20 mg/L methyl orange solution was tested and the solution pH was adjusted to 2.3. In catalytic ultrasonic experiments and pre-adsorption experiments, the additive amount of MnO2/CeO2 was 0.1 g. In catalytic ultrasonic process, the efficiencies of different ultrasonic irradiation time were tested. Degradation efficiency of methyl orange dye was defined as follows: Degradation
The specific surface area, total pore volume and average pore size of catalysts before and after use were measured using BET. The fresh catalyst had a specific surface area of 71.1002 m2/g, a total pore volume of 0.1321 ml/g and an average pore size of 7.431 nm. The used catalyst had a specific surface area of 77.2496 m2/g, a total pore volume of 0.1284 ml/g and an average pore size of 6.652 nm. After reaction, catalyst had small changes. The specific surface area increased by 8.6% and total pore volume and average pore size decreased. In general, the adsorption ability was improved after ultrasonic irradiation. The morphology of fresh and used MnO2/CeO2 was observed through SEM images. It was found that the prepared catalyst had a diverse shape and the size was in the range of 20–130 lm. Fig. 1(a) is the SEM image of fresh MnO2/CeO2 catalyst. Fig. 1(b) shows the SEM image of used MnO2/CeO2 catalyst which had obvious difference with fresh catalyst. For fresh catalyst, lots of catalyst particles with flat surface were observed. After using, the catalyst surface became rough and the structure became dispersed and catalyst particle with flat surface could hardly be found. Changes between fresh and used catalysts were because of mechanical effects of ultrasound. Ultrasound generated high speed liquid microjets and shockwaves towards the solid surface [22–24]. Those microjets reached hundreds of meters per second, and shockwaves yielded high pressure. The surface and structure of catalyst had big changes after used due to impacts of microjets and shock waves. Chemical composition of catalyst was determined by EDS. Results of fresh catalyst showed that mass percent of Mn and Ce were 27.1% and 58.8% respectively, and the molar ratio between Mn and Ce was about 1.2:1. Results of used catalyst showed that mass percent of Mn and Ce were 21.5% and 61.2%, and the molar ratio between Mn and Ce was about 1:1. Clearly, content of Mn in catalyst decreased after ultrasonic irradiation. That was because most Mn was on the surface of catalyst, and might fall from catalyst due to microjets and shock waves in ultrasonic process. So the amount of Mn in catalyst decreased after ultrasonic irradiation.
3.2. Methyl orange removal efficiency of MnO2/CeO2 catalytic ultrasonic process The catalytic activity of MnO2/CeO2 catalyst was evaluated in terms of removal efficiency of methyl orange. The results are shown in Fig. 2. Clearly, the degradation of ultrasonic irradiation was very poor, and the highest removal efficiency was only 10.2% after 30 min. The removal efficiency of MnO2/CeO2 reached 70% after 30 min due to adsorption. After ultrasonic irradiation combined with MnO2/CeO2 catalyst, the removal efficiency of methyl orange was improved significantly. 99% removal efficiency could be obtained after 60 min. MnO2/CeO2 catalytic ultrasonic process exhibited excellent efficiency in degradation of methyl orange.
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The removal efficiency was much higher than those of ultrasonic irradiation alone and catalyst alone. 3.3. Potential mechanisms of MnO2/CeO2 catalytic ultrasonic process Researches showed that there were two major mechanisms in ultrasonic degradation of pollutants, namely oxidation by oxidative free radicals and pyrolysis in ‘‘hot spots’’ during cavitation. Among free radicals, hydroxyl free radical is the most important one. The effectiveness of pyrolysis depends on the generation and collapse of cavitation bubbles, which is significantly impacted by catalyst. So it was necessary to determine the role of hydroxyl free radical and to analysis relationship between catalyst and cavitation bubbles. 3.3.1. Effects of hydroxyl free radical Hydroxyl free radical oxidation is a key mechanism in all AOPs. Previous researches proved that ultrasound could generate hydroxyl free radical, which might oxidize methyl orange due to its high redox potential. To quantify the effects of hydroxyl free radical on methyl orange degradation, hydroxyl free radical scavenger, tertbutanol (TBA), was added to the solution. TBA has a fast reaction rate with hydroxyl free radical (6 108 M1s1) [25] and was commonly used as indicator of hydroxyl free radical reactions [26,27]. Fig. 3 shows effects of TBA addition on the removal efficiency of methyl orange. The addition of TBA had obvious inhabitation of removal efficiency. When ultrasonic irradiation time was 10 min, the degradation efficiency decreased by 25.2%. When ultrasonic radiation time was 60 min, the degradation efficiency decreased by 20.2%. With more ultrasonic irradiation time, inhabitation by TBA decreased due to hydroxyl free radical generated with more reaction time. When the concentration of TBA was higher than 20 mmol/L, degradation efficiency was not further inhibited. The big decline of degradation efficiency illustrated that hydroxyl free radical mechanism acted an important role in methyl orange degradation in catalytic ultrasonic process.
Fig. 1. SEM images of fresh and used MnO2/CeO2 catalyst. (a) Fresh catalyst; (b) used catalyst. The magnification is 2500 times. Surface of fresh catalyst is flat. After used, the surface turns rough and structure becomes dispersed. Huge changes occur between fresh and used catalysts.
3.3.2. The impacts of MnO2/CeO2 catalyst on cavitation Cavitation effect determines the efficiency of ultrasonic degradation process. The strength of cavitation effect depends on the quantity of cavitation bubbles. More cavitation bubbles cause stronger cavitation effects, leading to higher degradation efficiency. To generate more cavitation bubbles, there should be more particles acting as nucleus of cavitation bubbles. It was reported that heterogeneous catalyst contained in solution could be potential nucleus of cavitation bubbles [28]. Once the catalyst particles sizes were on the same order of magnitude with the resonance size of cavitation bubbles, catalyst particles could act as nucleus of cavitation bubbles [29]. Therefore, in MnO2/CeO2 catalytic ultrasonic process, the catalyst size was an important parameter that affected cavitation effects. Suitable size enabled catalyst particles to act as cavitation nucleus, promote formation and collapse of cavitation bubbles, and enhance cavitation effects. The resonance size of cavitation bubbles at given ultrasound frequency can be estimated by [28]:
f ¼
Fig. 2. Methyl orange removal efficiency of MnO2/CeO2 catalytic ultrasonic process. Ultrasonic irradiation alone and catalyst alone have limited removal efficiency of methyl orange. After MnO2/CeO2 combined with ultrasonic irradiation, methyl orange removal becomes faster and more effectively.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2r 3c 2r q P 0 þ a aq 2pa
:
Where ‘f’ means ultrasound frequency, c means the ratio of heat capacities of saturating gas at constant pressure and volume, a is the radius of bubbles, P0 is the ambient pressure, r is the surface tension and q is the density of surrounding medium. In this experiment, f was 24 kHz, c was 1.39 for air, surface tension (r) was negligible, and water density was 1.0 g/cm3. The resonance size of cavitation bubbles in water was calculated to be 0.136 mm. The
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Table 1 Researches on degradation MO. Catalyst
Type
Effects
Reaction time
Rutile and anatase TiO2
Sono-catalytic
150 min
Ag/TiO2 MWCNT/TiO2 Mesoporous titania nanoparticles Au/TiO2 ZnO Fe2O3–CeO2–TiO2/c-Al2O3 Pt–Bi/C
Sonophoto-catalytic Photo-catalytic Photo-catalytic
100% and 41.36% Degradation of MO in presence of retile and anatase TiO2, respectively. 100% Degradation of MO in presence of 60 mg/L Ag/TiO2 93% Degradation of MO in presence of MWCNT/TiO2 98% Degradation of MO in presence of 1 g/L catalyst (pH 2.0)
Photo-catalytic Photo-catalytic Catalytic wet air oxidation (CWAO) Electrochemical degradation process
100% Degradation of MO 100% Degradation of MO in presence of 1 g/L ZnO 98.09% Of color and 96.08% of total organic carbon(TOC) can be removed 95.6% Degradation of MO
120 min 80 min 2.5 h 180 min
average size of catalyst particles was about 0.096 mm calculated from SEM images, which was close to the resonance size of cavitation bubbles. So MnO2/CeO2 catalyst could act as the nucleus of cavitation bubbles. With nucleus available, more cavitation bubbles were generated in the catalytic ultrasonic process. Collapse of cavitation bubbles led to hydrolyzation of water molecule and generation of various free radicals including hydroxyl free radical [30,31]. Typical reactions are listed in Eqs. (1-6), in which ‘‘)))’’ denotes ultrasonic irradiation:
H2 OþÞÞÞ ! OH þ H
ð1Þ
O2 þÞÞÞ ! 2 O
ð2Þ
O þ H2 O ! 2 OH
ð3Þ
OH þ H ! H2 O
ð4Þ
2 OH ! H2 O þ O
ð5Þ
2 OH ! H2 O2 :
ð6Þ
Water molecule and dissolved oxygen molecule are converted into free radicals such as hydroxyl free radical, hydrogen free radical and oxygen free radical in the cavitation bubbles (reactions 1–3). These free radicals can recombine to form H2O, O, H2O2, which are released into the bulk solution (reactions 4–6). Among these products, H2O2 is the most one and its amount is closely
Fig. 3. Effects of the t-butanol (TBA) addition on removing methyl orange. With 10 mmol/L of TBA, degradation efficiencies decrease 25.2% and 20.2% after 10 and 60 min ultrasonic irradiation. Degradation efficiency of methyl orange keeps stable with more than 20 mmol/L of TBA.
120 min 100 min 45 min
correlated with the amount of hydroxyl free radical. Therefore, hydroxyl free radical concentration can be reflected with hydrogen peroxide concentration. Fig. 4 shows the formation of hydrogen peroxide during ultrasonic process and catalytic ultrasonic process. Clearly, MnO2/ CeO2 catalyst generated no hydrogen peroxide. Ultrasonic irradiation generated few hydrogen peroxide. MnO2/CeO2 catalytic ultrasonic process, on the other hand, generated lots of hydrogen peroxide and the concentration reached 6.48 mg/L after 30 min. High concentration of hydrogen peroxide indicated high concentration of hydroxyl free radical in the solution. Such a significant increase of hydrogen peroxide formation showed clearly the synergistic effect of catalytic ultrasonic process. Furthermore, the composition of MnO2/CeO2 catalyst also contributed to methyl orange degradation Combination of MnO2 and CeO2 could form a redox couple since Mn and Ce have various oxidation states [32], Electron transfer in the redox couple increased the surface electric charge, then improved the catalytic activity [33]. The catalyst surface could react with oxygen molecule to generate oxygen free radical [34], which then accelerates the formation of hydroxyl free radical (reaction 3). In summary, both theoretic comparison of resonance size of cavitation bubbles and catalyst sizes and experimental detection of hydrogen peroxide proved that MnO2/CeO2 catalyst might act as the nucleus for cavitation bubbles, enhance the cavitation effects, and promote hydroxyl free radical formation. Furthermore, catalyst composition might also enhance surface electron transfer and promote hydroxyl free radical generation.
Fig. 4. H2O2 generation in catalytic ultrasonic process. Ultrasonic irradiation alone and MnO2/CeO2 alone generate little H2O2. MnO2/CeO2 combined with ultrasonic irradiation significantly increases generation of H2O2.
H. Zhao et al. / Ultrasonics Sonochemistry 21 (2014) 991–996
Fig. 5. Langmuir adsorption isotherm of MnO2/CeO2.in adsorption of methyl orange. The correlation is above 0.99 indicating MnO2/CeO2 adsorption of methyl orange in good accordance with Langmuir adsorption isotherm.
3.4. Impact of pre-adsorption on catalytic ultrasonic process MnO2/CeO2 catalyst had good adsorption ability for methyl orange. It could adsorb 68.1% of total methyl orange in 20 min (Fig. 1). The adsorption isotherm was shown in Fig. 5. The adsorption process of methyl orange by MnO2/CeO2 was in accordance with Langmuir adsorption isotherm. The adsorption of methyl orange was monolayer adsorption. And methyl orange was adsorbed uniformly on the surface of catalyst. Through adsorption, the amount of methyl orange on the catalyst increased a lot, which might influence the catalytic ultrasonic degradation of methyl orange. In order to find out the effects of adsorption on catalytic ultrasonic process, pre-adsorption was done. Results are shown in Fig. 6. The degradation efficiency had big differences with different pre-adsorption time followed by ultrasonic irradiation. With 1 min’ pre-adsorption, the removal efficiency could reach 42% after 1 min ultrasonic irradiation, 15.5% higher than the control. With 10 min’ and 20 min’ pre-adsorption, the removal efficiency could reach 77.5% and 81.5% after 1 min ultrasonic irradiation. The effect of pre-adsorption on MnO2/CeO2 catalytic ultrasonic process might be explained as follows. Methyl orange molecules were adsorbed on the catalyst surfaces, entered cavitation bubbles as catalyst acted as cavitation nucleus, and underwent cavitation
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effects. The ‘‘hot spot’’ effect was the strongest on cavitation nucleus, very high pressure and temperature could directly mineralize methyl orange molecules in the cavitation bubbles. More methyl orange molecules on the cavitation nucleus meant better mineralization. Similarly, the hydroxyl free radicals concentration was also the highest in the cavitation bubbles, more methyl orange molecules on the cavitation nucleus led to better oxidation by hydroxyl free radical. Due to MnO2/CeO2 catalyst acting as nucleus of cavitation bubbles, the concentration of hydroxyl free radical was even higher around the catalyst than in the bulk. Therefore, catalytic ultrasonic degradation of methyl orange became faster with pre-adsorption. With pre-adsorption, the methyl orange degradation mainly occurred on the catalyst surface rather than in the bulk solution [34]. The change of TOC was also investigated. TOC decreased along with the reaction time but the degradation efficiency of TOC was less than that of methyl orange. The results complied with previous findings [35,36]. Some methyl orange could be degraded into carbon dioxide while some were degraded into intermediate products such as aliphatic acids and aldehydes. These intermediate products proceeded at a much slower reaction rate [37,38]. So the reduction of TOC resulted from degradation of methyl orange but was less effective. Methyl orange degradation using catalytic ultrasonic process was studied by few researchers (Table 1). Hydroxyl free radical oxidation of methyl orange was proposed in catalytic ultrasonic process by Wang [39]. In this paper, the hydroxyl free radical oxidation of methyl orange was proven and quantitative analyzed. Moreover, enhancement by catalyst on cavitation effects was studied and increase of degradation efficiency with pre-adsorption was found and explained. 4. Conclusions In this work, MnO2/CeO2 catalyst was successfully prepared using an impregnation method. The catalyst had a specific surface area of 71.1002 m2/g, a total pore volume of 0.1321 ml/g, an average pore size of 7.431 nm, a molar ratio between Mn and Ce of about 1.2:1. After catalytic ultrasonic process, the morphology, size, and composition of catalyst changed significantly. 99.1% of methyl orange could be removed in catalytic ultrasonic process in 60 min, much higher than the effect of ultrasonic degradation alone. Hydroxyl free radical played an important role in oxidation of methyl orange. MnO2/CeO2 catalyst significantly enhanced the cavitation effects due to the closeness of cavitation bubbles’ resonance size and catalyst’s size. High concentration of hydrogen peroxide in catalytic ultrasonic process also proved the enhancement of cavitation effects. The increase of degradation efficiency in catalytic ultrasonic process by pre-adsorption was observed and explained. With pre-adsorption, more methyl orange molecules entered cavitation bubbles and underwent stronger cavitation effects. In summary, MnO2/CeO2 catalytic ultrasonic process has shown great potential in treating dye wastewater. Acknowledgment Authors are grateful for financial support from Basic Research funds in Renmin University of China from the Center Government (12XNLI01). References
Fig. 6. Degradation efficiency of methyl orange in catalytic ultrasonic process with different pre-adsorption time. The degradation efficiency of methyl orange is significantly enhanced by pre-adsorption. With pre-adsorption, catalytic ultrasonic process degrades methyl orange faster and more effectively.
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