Environ Sci Pollut Res (2015) 22:11170–11174 DOI 10.1007/s11356-015-4774-2
SHORT RESEARCH AND DISCUSSION ARTICLE
Real-time detection of hydrogen peroxide using microelectrodes in an ultrasonic enhanced heterogeneous Fenton process catalyzed by ferrocene Jun Lin 1 & Qing Xin 1 & Xiumin Gao 1
Received: 15 January 2015 / Accepted: 25 May 2015 / Published online: 2 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Microelectrodes were used for real-time detection of hydrogen peroxide (H2O2) in a heterogeneous sono-Fenton system with ferrocene as the catalyst. The working mechanism of reactive blue 13 decolorization in a heterogeneous sono-Fenton system was investigated. Ultrasonic irradiation showed no effect on decolorization when used alone and did not enhance decolorization in the H2O2 system (43.0 % for H2O2 vs. 48.1 % for US+H2O2). However, a system with the presence of Fenton-like reagents achieved complete decolorization. Decolorization was greatly accelerated by the addition of ultrasonic irradiation. Thorough decolorization was achieved in 20 min in the heterogeneous sono-Fenton system, which was 30 min faster than in the heterogeneous Fenton system. Based on the data collected by microelectrodes, accelerated decomposition of H2O2 was also observed. Ultrasonic irradiation aided the ferrocene catalyst in liberating more •OH from Fenton reactions, leading to the faster decolorization. Keywords Microelectrode . Ferrocene . Hydrogen peroxide . Heterogeneous Fenton . Ultrasonic treatment
Introduction Ultrasonic-assisted Fenton treatment has been widely studied in recent years (Cailean et al. 2014; Chakma and Moholkar Responsible editor: Angeles Blanco * Xiumin Gao [email protected] 1
Electronics and Information College, Hangzhou Dianzi University, Xiasha Campus, Hangzhou 310018, People’s Republic of China
2013; Dukkanci et al. 2014). The synergistic mechanism was concluded to improve the generation and utilization of radicals (Basturk and Karatas 2014). Compared to the traditional sonoFenton process, ultrasonic irradiation was found to cause greater enhancement effects in heterogeneous Fenton-like systems by not only improving mass transfer and mixing level in the solid-liquid phase but also by increasing the active catalyst surface area by reducing the size of catalyst particles and continuously removing by-products on the catalyst surface (Elshafei et al. 2014; Zhong et al. 2011; Zhou et al. 2009a). Due to its ability of catalyst recycling (Huang et al. 2012), the heterogeneous sono-Fenton process is considered an outstanding technology in wastewater treatment and has received significant attention (Ning et al. 2014). Many iron-containing heterogeneous catalysts including zero-valent iron (Zhou et al. 2009b), ferric ions supported by activated carbon fibers (Yuyuan et al. 2013), and nano-sized iron oxides (Elshafei et al. 2014) have been tested as substitutes of ferrous reagents in heterogeneous Fenton-like systems. Although rapid degradation of target pollutants was achieved, most of the catalysts were found to leach Fe2+ or Fe3+ in acidic surroundings (Wang et al. 2013). Iron leaching can cause not only a loss of catalyst but also potential environmental issues. As an organic transition-metallic compound, ferrocene (Fc) is highly stable and nontoxic (Nie et al. 2008). It possesses electron donor-acceptor conjugated structure, which shows good redox reversible characteristics and high catalytic capacity (Li et al. 2009). Fc has been used as an excellent homogeneous catalyst for hydroxylation of aromatic compounds such as benzene (Li et al. 2004). However, few studies have treated it as a heterogeneous catalyst, especially in Fenton-like processes. Affected by both ultrasonic irradiation and Fenton-like reactions, hydrogen peroxide can be broken down and generate radicals with high oxidation capacity. The variation of
Environ Sci Pollut Res (2015) 22:11170–11174
hydrogen peroxide content in the system affects the interaction of ultrasonic and Fenton-like reactions. Thus, the concentration of hydrogen peroxide is one of the key factors to the synergism of the heterogeneous sono-Fenton process. There are several ways to determine the concentration of hydrogen peroxide including spectrophotometry (Papoutsakis et al. 2015), iodometric titration (Segura et al. 2012), and using glucose oxidase (Maezono et al. 2011). However, all these methods involve sampling and chemical addition, potentially adding error to the results. Also, the sampling data may not fully represent the dynamic interaction process. Microelectrodes, commonly used in applied microbiology (Paredes et al. 2014) and biomedical engineering (Kongsuphol et al. 2014), are a perfect solution for this problem. They can be put directly into the system and provide realtime detection of hydrogen peroxide content. With a tip size smaller than 10 μm, microelectrodes consume very little target reagent and minimize disturbance caused by the measurements. In the work presented herein, the heterogeneous sonoFenton system with Fc as the catalyst was fully investigated. In order to evaluate the working mechanism, the decolorization of reactive dye was tested with both the Fenton system and sono-Fenton system for comparison. With the help of microelectrodes, real-time detection of hydrogen peroxide was maintained during the experiments, and the working mechanism of the heterogeneous sono-Fenton process was studied.
Materials and methods Materials
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incorporated to polarize the electrodes and measure current. The electrode reactions are listed below: H2 O2 → O2 ðgÞ þ 2 Hþ þ 2 e− Hg2 Cl2 ðsÞ þ 2e‐ → 2HgðlÞ þ 2Cl‐ To facilitate the electrode reactions, the platinum working electrode was polarized at +0.8 Vagainst the SCE. The current is proportional to the H2O2 concentration in the solution. The concentration of hydrogen peroxide was measured using the calibration curve (Fig. 1).
Experiment procedure The batch experiments were carried out to evaluate the capacity of dye decolorization for five systems: an ultrasonic only system (US system), a H2O2 only system (H2O2 system, 6 mM H2O2), a US+H2O2 system, a heterogeneous Fenton system (FC+H2O2 system, 6 mM H2O2 and 200 mg/L Fc), and a heterogeneous sono-Fenton system (US+ FC+H2O2 system). The experiment setup is shown in Fig. 2. A 200-mL flask was used as the reactor and immersed in an ultrasonic bath that contained 1.5 L water (KQ3200E, Kunshan Ultrasonic Instruments Co. Ltd., Jiangsu, China). The ultrasonic irradiation was operated at a fixed frequency of 40 kHz and power of 150 W. A magnetic stirrer with a setting of 300 r/min was employed during the experiments when ultrasonic irradiation was not used. For in situ measurements, the HPM was held by a micromanipulator and put into the liquid phase directly. One hundred milliliters of a 50-mg/L RB13 solution was prepared, and its pH was adjusted to 3.5. The HPM was polarized and the signal was recorded immediately after Fenton-like reagents were added to the solution. Samples were taken for further testing.
C. I. reactive blue 13 (RB13, CAS 12236-84-9) was selected as the reactive dye and obtained from Alfa Aesar. The pH of the solution was adjusted using diluted sodium hydroxide (NaOH) and/or hydrochloric acid (HCl). Hydrogen peroxide (30 %, w/w) and ferrocene was purchased from SigmaAldrich Company.
Self-made hydrogen peroxide microelectrodes (HPMs) were used to measure the concentration of H2O2. The HPMs are amperometric-type sensors. They are made of a platinum wire that is inserted into a glass capillary and covered with a cellulose acetate membrane (Lewandowski and Beyenal 2007). The tip of the microelectrode is less than 10 μm. A standard calomel electrode (SCE) was used as a counter electrode. A picoammeter (Keithley Instrument Inc., Model 6487) was
y=0.1604x-0.0117 2 R =0.9986
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Real-time detection of hydrogen peroxide with microelectrodes
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11172 Fig. 2 Experiment setup: a experiments without ultrasonic; b experiments with ultrasonic
Environ Sci Pollut Res (2015) 22:11170–11174 Micromanipula tor
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The concentration of RB13 was measured using a UV/Vis spectrophotometer (GBC Cintra 303). The maximum absorbance of RB13 (580 nm wavelength) was used for all measurements. The pH of the solution was determined by a digital pH meter (Mettler Toledo™ FE20 FiveEasy™ Benchtop pH Meter).
Results and discussion Ultrasonic irradiation has the ability to degrade many organic pollutants. Its effect results from acoustic cavitation which leads to production of radicals, pyrolytic decomposition, and supercritical water oxidation (Srivastava et al. 2014). Its capability, however, greatly depends on its frequency. Researchers have found that medium frequency ultrasound (300–600 kHz) is more favorable for sonochemical effects compared to low frequencies (20–100 kHz) (Tezcanli-Guyer et al. 2003). Therefore, medium frequencies are usually more effective in degrading hydrophilic and non-volatile organics such as dyes (Eren 2012). Our experimental results were consistent with these findings. Low frequency ultrasound (40 kHz) used in US system did not show any decolorization of RB13 (Fig. 3), and no accumulation of H2O2 was observed (Fig. 4). The H2O2 system showed a higher decolorization rate. H2O2 alone has the ability to decolorize RB13, as it is one of the most powerful oxidizers in acidic solutions. Yet, the oxidation process progressed slowly as only 43.0 % decolorization was achieved in 50 min. Slow H2O2 consumption was also detected using microelectrodes, which confirmed the RB13 decolorization results. The US+H2O2 system was also tested as one of the control experiments. The US+H2O2 system had a slightly higher decolorization rate than the H2O2 system (48.1 %). Due to the effect of acoustic cavitation, ultrasonic irradiation could enhance the H2O2 decomposition into reactive radicals (Huang et al. 2012). Thus, the concentration of H2O2 decreased faster
in the US+H2O2 system during the first 10 min of the experiment than in the H2O2 system (Fig. 4). However, the confinement of radicals (mainly •OH) to the close vicinity of the bubbles can result in the recombination of •OH to produce H2O2 (Sivasankar and Moholkar 2009). This decreases the rate of H2O2 consumption in the US+H2O2 system, maintaining a similar H2O2 consumption rate as the H2O2 system. Since the confined •OH radicals have very low probability of interaction with RB13, enhanced decolorization by ultrasonic irradiation is negligible. Thus, the consumption of H2O2 in both systems remained similar at the end of the experiments. The effects of the Fc+H2O2 system and the US+Fc+H2O2 system on RB13 decolorization were compared (Fig. 5). Realtime detection of H2O2 was maintained using microelectrodes (Fig. 6). In both systems, Fc showed good catalytic activity, which largely promoted the oxidation capacity and led to thorough decolorization. As a heterogeneous catalyst, Fc has an inherent drawback: The catalytic activity is hindered by uneven particle distribution and mass transfer of reagents from liquid phase to the surface of solid particles (Ince et al. 2001).
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Fig. 3 Decolorization of RB13 in US, H2O2, and US+H2O2 system
Environ Sci Pollut Res (2015) 22:11170–11174
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However, its solid form makes catalyst recycling much easier. By simply filtering and drying, Wang et al. used one batch of Fc to remove methylene blue for 3 cycles. No deactivation was observed and the recovery rate reached 98.7 % (Wang et al. 2013). More importantly, the lost Fc was just dissolved in the liquid phase, retaining its original, highly stable structure (Wang et al. 2014). This conclusion was confirmed in our experiments, as no iron ions were identified in the liquid phase after experimentation was completed. No iron leaching distinguishes Fc from other heterogeneous catalysts, making it an environmental friendly alternative. In contrast to previous experiments, ultrasonic irradiation showed significant enhancement effects on RB13 decolorization in the presence of Fenton-like reagents. The rate of decolorization in the US+Fc+H2O2 system was much higher than the sum of the rates of the US system and the Fc+H2O2 system, respectively. It was evident that
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Fig. 4 Real-time detection of H2O2 using microelectrodes in US, H2O2, and US+H2O2 system
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Fig. 5 Decolorization of RB13 in Fc+H2O2 and US+Fc+H2O2 system
Fig. 6 Real-time detection of H2O2 using microelectrodes in Fc+H2O2 and US+Fc+H2O2 system
there was a synergistic effect in the combined sono-Fenton process. Complete decolorization took 50 min in the heterogeneous Fenton system but was shortened by 30 min with the addition of ultrasound. The time history of decolorization was fitted to different kinetic models. Based on regression coefficients (R2), the decolorization in both systems was found to follow pseudo-first-order kinetics (R2 > 0.96). The reaction rate constant for the US+Fc+H2O2 system (k′=0.2173) was much larger than for the Fc+ H2O2 system (k′=0.0977). The concentration of H 2 O 2 also decreased much faster when ultrasonic irradiation was applied. This demonstrates that H2O2 decomposition, the first step of Fenton reactions, is accelerated by ultrasound. Given its low efficiency of breaking down H2O2 molecules and generating reactive radicals, the role of ultrasound in the US+Fc+H2O2 system was mainly physical. Similar to traditional mechanical agitation, ultrasonic irradiation made the solid particle distribution more even. Moreover, the ultrasonic propagation and cavitation bubbles could produce micro-streaming and micro-turbulence, resulting in intense micro-mixing. It enhanced the mass transfer rate of the heterogeneous Fenton system, which was beneficial for •OH generation and reaction with RB13. With a similar dosage of Fenton reagents, the ultrasonic bath in the US+Fc+H2O2 system (150 W×20 min) used more electrical energy than the magnetic stirrer in the Fc+H2O2 system (10 W×50 min) during experimentation. Energy was wasted in the ultrasonic bath, as only a small portion was actually used for decolorization. Future work is necessary to improve the energy utilization efficiency of the ultrasonic system. However, a heterogeneous sono-Fenton system with ferrocene as the catalyst may lead to exciting possibilities for future process design in wastewater treatment.
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Conclusion Microelectrodes were used for real-time detection of H2O2 in a heterogeneous sono-Fenton system with ferrocene as the catalyst. The working mechanism of RB13 decolorization in the heterogeneous sono-Fenton system was fully investigated by comparing five systems (US system, H 2 O 2 system, USH2O2 system, Fc+H2O2 system and US+Fc+H2O2 system). Poor decolorization was found in the US system, H2O2 system, and US+H2O2 system. The addition of Fc into the system largely promoted the oxidation capacity and led to thorough RB13 decolorization. Not only does Fc display excellent reversible redox properties for catalyzing heterogeneous Fenton reactions, but its highly stable and nontoxic structure makes Fc environmental friendly as well. With the help of ultrasonic irradiation, RB13 was completely decolorized in 20 min in the US+Fc+H2O2 system, which was 30 min faster than in the Fc+H2O2 system. Based on the microelectrode data, accelerated decomposition of H2O2 was also observed in US+Fc+H2O2 system. Ultrasonic irradiation aided the Fc catalyst in liberating more •OH radicals from Fenton reactions by improving the mass transfer of the heterogeneous Fenton system, which lead to faster RB13 decolorization. Acknowledgments Financial support for this work was provided by the Zhejiang Province Welfare Technology Applied Research Project (No. 2014C31137) and 151 Talent Project of Zhejiang Province (12-2008).
References Basturk E, Karatas M (2014) Advanced oxidation of Reactive Blue 181 solution: a comparison between Fenton and Sono-Fenton Process. Ultrason Sonochem 21:1881–1885 Cailean D, Teodosiu C, Friedl A (2014) Integrated sono-Fenton ultrafiltration process for 4-chlorophenol removal from aqueous effluents: assessment of operational parameters (part 1). Clean Techn Environ Policy 16:1145–1160 Chakma S, Moholkar VS (2013) Physical mechanism of sono-Fenton process. AICHE J 59:4303–4313 Dukkanci M, Vinatoru M, Mason TJ (2014) The sonochemical decolourisation of textile azo dye Orange II: effects of Fenton type reagents and UV light. Ultrason Sonochem 21:846–853 Elshafei GMS, Yehia FZ, Dimitry OIH, Badawi AM, Eshaq G (2014) Ultrasonic assisted-Fenton-like degradation of nitrobenzene at neutral pH using nanosized oxides of Fe and Cu. Ultrason Sonochem 21:1358–1365 Eren Z (2012) Ultrasound as a basic and auxiliary process for dye remediation: a review. J Environ Manag 104:127–141 Huang RX, Fang ZQ, Yan XM, Cheng W (2012) Heterogeneous sonoFenton catalytic degradation of bisphenol A by Fe3O4 magnetic nanoparticles under neutral condition. Chem Eng J 197:242–249 Ince NH, Tezcanli G, Belen RK, Apikyan IG (2001) Ultrasound as a catalyzer of aqueous reaction systems: the state of the art and environmental applications. Appl Catal B Environ 29:167–176
Environ Sci Pollut Res (2015) 22:11170–11174 Kongsuphol P, Ng HH, Pursey JP, Arya SK, Wong CC, Stulz E, Park MK (2014) EIS-based biosensor for ultra-sensitive detection of TNFalpha from non-diluted human serum. Biosens Bioelectron 61: 274–279 Lewandowski Z, Beyenal H (2007) Fundamentals of biofilm research. CRC Press, Boca Raton, 452 p., 2 p. of plates pp Li L, Shi JL, Yan JN, Zhao XG, Chen HG (2004) Mesoporous SBA-15 material functionalized with ferrocene group and its use as heterogeneous catalyst for benzene hydroxylation. Appl Catal A Gen 263: 213–217 Li YW, Xue LL, Li H, Li ZF, Xu B, Wen SP, Tian WJ (2009) Energy level and molecular structure engineering of conjugated donor-acceptor copolymers for photovoltaic applications. Macromolecules 42: 4491–4499 Maezono T, Tokumura M, Sekine M, Kawase Y (2011) Hydroxyl radical concentration profile in photo-Fenton oxidation process: generation and consumption of hydroxyl radicals during the discoloration of azo-dye Orange II. Chemosphere 82:1422–1430 Nie YL, Hu C, Qu JH, Hu XX (2008) Efficient photodegradation of Acid Red B by immobilized ferrocene in the presence of UVA and H(2)O(2). J Hazard Mater 154:146–152 Ning XA, Chen H, Wu JR, Wang YJ, Liu JY, Lin MQ (2014) Effects of ultrasound assisted Fenton treatment on textile dyeing sludge structure and dewaterability. Chem Eng J 242:102–108 Papoutsakis S, Miralles-Cuevas S, Gondrexon N, Baup S, Malato S, Pulgarin C (2015) Coupling between high-frequency ultrasound and solar photo-Fenton at pilot scale for the treatment of organic contaminants: an initial approach. Ultrason Sonochem 22:527–534 Paredes J, Becerro S, Arana S (2014) Label-free interdigitated microelectrode based biosensors for bacterial biofilm growth monitoring using Petri dishes. J Microbiol Methods 100:77–83 Segura Y, Martinez F, Melero JA, Molina R, Chand R, Bremner DH (2012) Enhancement of the advanced Fenton process (Fe-0/H2O2) by ultrasound for the mineralization of phenol. Appl Catal B Environ 113:100–106 Sivasankar T, Moholkar VS (2009) Physical insights into the sonochemical degradation of recalcitrant organic pollutants with cavitation bubble dynamics. Ultrason Sonochem 16:769–781 Srivastava P, Goyal S, Patnala PK (2014) Degradation of reactive, acid and basic textile dyes in the presence of ultrasound and rare earths Lanthanum and Praseodymium. Ultrason Sonochem 21:1994–2009 Tezcanli-Guyer G, Alaton IA, Ince NH (2003) Sonochemical destruction of textile dyestuff in wasted dyebaths. Color Technol 119:292–296 Wang Q, Tian SL, Cun J, Ning P (2013) Degradation of methylene blue using a heterogeneous Fenton process catalyzed by ferrocene. Desalin Water Treat 51:5821–5830 Wang Q, Tian SL, Ning P (2014) Degradation mechanism of methylene blue in a heterogeneous Fenton-like reaction catalyzed by ferrocene. Ind Eng Chem Res 53:643–649 Yuyuan Y, Lie W, Lijie S, Shun Z, Zhenfu H, Yajun M, Wangyang L, Wenxing C (2013) Efficient removal of dyes using heterogeneous Fenton catalysts based on activated carbon fibers with enhanced activity. Chem Eng Sci 101:424–431 Zhong X, Xiang LJ, Royer S, Valange S, Barrault J, Zhang H (2011) Degradation of CI Acid Orange 7 by heterogeneous Fenton oxidation in combination with ultrasonic irradiation. J Chem Technol Biotechnol 86:970–977 Zhou T, Lim TT, Lu XH, Li YZ, Wong FS (2009a) Simultaneous degradation of 4CP and EDTA in a heterogeneous ultrasound/Fenton like system at ambient circumstance. Sep Purif Technol 68:367–374 Zhou T, Lu XH, Wang J, Wong FS, Li YZ (2009b) Rapid decolorization and mineralization of simulated textile wastewater in a heterogeneous Fenton like system with/without external energy. J Hazard Mater 165:193–199
SHORT RESEARCH AND DISCUSSION ARTICLE
Real-time detection of hydrogen peroxide using microelectrodes in an ultrasonic enhanced heterogeneous Fenton process catalyzed by ferrocene Jun Lin 1 & Qing Xin 1 & Xiumin Gao 1
Received: 15 January 2015 / Accepted: 25 May 2015 / Published online: 2 June 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Microelectrodes were used for real-time detection of hydrogen peroxide (H2O2) in a heterogeneous sono-Fenton system with ferrocene as the catalyst. The working mechanism of reactive blue 13 decolorization in a heterogeneous sono-Fenton system was investigated. Ultrasonic irradiation showed no effect on decolorization when used alone and did not enhance decolorization in the H2O2 system (43.0 % for H2O2 vs. 48.1 % for US+H2O2). However, a system with the presence of Fenton-like reagents achieved complete decolorization. Decolorization was greatly accelerated by the addition of ultrasonic irradiation. Thorough decolorization was achieved in 20 min in the heterogeneous sono-Fenton system, which was 30 min faster than in the heterogeneous Fenton system. Based on the data collected by microelectrodes, accelerated decomposition of H2O2 was also observed. Ultrasonic irradiation aided the ferrocene catalyst in liberating more •OH from Fenton reactions, leading to the faster decolorization. Keywords Microelectrode . Ferrocene . Hydrogen peroxide . Heterogeneous Fenton . Ultrasonic treatment
Introduction Ultrasonic-assisted Fenton treatment has been widely studied in recent years (Cailean et al. 2014; Chakma and Moholkar Responsible editor: Angeles Blanco * Xiumin Gao [email protected] 1
Electronics and Information College, Hangzhou Dianzi University, Xiasha Campus, Hangzhou 310018, People’s Republic of China
2013; Dukkanci et al. 2014). The synergistic mechanism was concluded to improve the generation and utilization of radicals (Basturk and Karatas 2014). Compared to the traditional sonoFenton process, ultrasonic irradiation was found to cause greater enhancement effects in heterogeneous Fenton-like systems by not only improving mass transfer and mixing level in the solid-liquid phase but also by increasing the active catalyst surface area by reducing the size of catalyst particles and continuously removing by-products on the catalyst surface (Elshafei et al. 2014; Zhong et al. 2011; Zhou et al. 2009a). Due to its ability of catalyst recycling (Huang et al. 2012), the heterogeneous sono-Fenton process is considered an outstanding technology in wastewater treatment and has received significant attention (Ning et al. 2014). Many iron-containing heterogeneous catalysts including zero-valent iron (Zhou et al. 2009b), ferric ions supported by activated carbon fibers (Yuyuan et al. 2013), and nano-sized iron oxides (Elshafei et al. 2014) have been tested as substitutes of ferrous reagents in heterogeneous Fenton-like systems. Although rapid degradation of target pollutants was achieved, most of the catalysts were found to leach Fe2+ or Fe3+ in acidic surroundings (Wang et al. 2013). Iron leaching can cause not only a loss of catalyst but also potential environmental issues. As an organic transition-metallic compound, ferrocene (Fc) is highly stable and nontoxic (Nie et al. 2008). It possesses electron donor-acceptor conjugated structure, which shows good redox reversible characteristics and high catalytic capacity (Li et al. 2009). Fc has been used as an excellent homogeneous catalyst for hydroxylation of aromatic compounds such as benzene (Li et al. 2004). However, few studies have treated it as a heterogeneous catalyst, especially in Fenton-like processes. Affected by both ultrasonic irradiation and Fenton-like reactions, hydrogen peroxide can be broken down and generate radicals with high oxidation capacity. The variation of
Environ Sci Pollut Res (2015) 22:11170–11174
hydrogen peroxide content in the system affects the interaction of ultrasonic and Fenton-like reactions. Thus, the concentration of hydrogen peroxide is one of the key factors to the synergism of the heterogeneous sono-Fenton process. There are several ways to determine the concentration of hydrogen peroxide including spectrophotometry (Papoutsakis et al. 2015), iodometric titration (Segura et al. 2012), and using glucose oxidase (Maezono et al. 2011). However, all these methods involve sampling and chemical addition, potentially adding error to the results. Also, the sampling data may not fully represent the dynamic interaction process. Microelectrodes, commonly used in applied microbiology (Paredes et al. 2014) and biomedical engineering (Kongsuphol et al. 2014), are a perfect solution for this problem. They can be put directly into the system and provide realtime detection of hydrogen peroxide content. With a tip size smaller than 10 μm, microelectrodes consume very little target reagent and minimize disturbance caused by the measurements. In the work presented herein, the heterogeneous sonoFenton system with Fc as the catalyst was fully investigated. In order to evaluate the working mechanism, the decolorization of reactive dye was tested with both the Fenton system and sono-Fenton system for comparison. With the help of microelectrodes, real-time detection of hydrogen peroxide was maintained during the experiments, and the working mechanism of the heterogeneous sono-Fenton process was studied.
Materials and methods Materials
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incorporated to polarize the electrodes and measure current. The electrode reactions are listed below: H2 O2 → O2 ðgÞ þ 2 Hþ þ 2 e− Hg2 Cl2 ðsÞ þ 2e‐ → 2HgðlÞ þ 2Cl‐ To facilitate the electrode reactions, the platinum working electrode was polarized at +0.8 Vagainst the SCE. The current is proportional to the H2O2 concentration in the solution. The concentration of hydrogen peroxide was measured using the calibration curve (Fig. 1).
Experiment procedure The batch experiments were carried out to evaluate the capacity of dye decolorization for five systems: an ultrasonic only system (US system), a H2O2 only system (H2O2 system, 6 mM H2O2), a US+H2O2 system, a heterogeneous Fenton system (FC+H2O2 system, 6 mM H2O2 and 200 mg/L Fc), and a heterogeneous sono-Fenton system (US+ FC+H2O2 system). The experiment setup is shown in Fig. 2. A 200-mL flask was used as the reactor and immersed in an ultrasonic bath that contained 1.5 L water (KQ3200E, Kunshan Ultrasonic Instruments Co. Ltd., Jiangsu, China). The ultrasonic irradiation was operated at a fixed frequency of 40 kHz and power of 150 W. A magnetic stirrer with a setting of 300 r/min was employed during the experiments when ultrasonic irradiation was not used. For in situ measurements, the HPM was held by a micromanipulator and put into the liquid phase directly. One hundred milliliters of a 50-mg/L RB13 solution was prepared, and its pH was adjusted to 3.5. The HPM was polarized and the signal was recorded immediately after Fenton-like reagents were added to the solution. Samples were taken for further testing.
C. I. reactive blue 13 (RB13, CAS 12236-84-9) was selected as the reactive dye and obtained from Alfa Aesar. The pH of the solution was adjusted using diluted sodium hydroxide (NaOH) and/or hydrochloric acid (HCl). Hydrogen peroxide (30 %, w/w) and ferrocene was purchased from SigmaAldrich Company.
Self-made hydrogen peroxide microelectrodes (HPMs) were used to measure the concentration of H2O2. The HPMs are amperometric-type sensors. They are made of a platinum wire that is inserted into a glass capillary and covered with a cellulose acetate membrane (Lewandowski and Beyenal 2007). The tip of the microelectrode is less than 10 μm. A standard calomel electrode (SCE) was used as a counter electrode. A picoammeter (Keithley Instrument Inc., Model 6487) was
y=0.1604x-0.0117 2 R =0.9986
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Real-time detection of hydrogen peroxide with microelectrodes
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11172 Fig. 2 Experiment setup: a experiments without ultrasonic; b experiments with ultrasonic
Environ Sci Pollut Res (2015) 22:11170–11174 Micromanipula tor
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The concentration of RB13 was measured using a UV/Vis spectrophotometer (GBC Cintra 303). The maximum absorbance of RB13 (580 nm wavelength) was used for all measurements. The pH of the solution was determined by a digital pH meter (Mettler Toledo™ FE20 FiveEasy™ Benchtop pH Meter).
Results and discussion Ultrasonic irradiation has the ability to degrade many organic pollutants. Its effect results from acoustic cavitation which leads to production of radicals, pyrolytic decomposition, and supercritical water oxidation (Srivastava et al. 2014). Its capability, however, greatly depends on its frequency. Researchers have found that medium frequency ultrasound (300–600 kHz) is more favorable for sonochemical effects compared to low frequencies (20–100 kHz) (Tezcanli-Guyer et al. 2003). Therefore, medium frequencies are usually more effective in degrading hydrophilic and non-volatile organics such as dyes (Eren 2012). Our experimental results were consistent with these findings. Low frequency ultrasound (40 kHz) used in US system did not show any decolorization of RB13 (Fig. 3), and no accumulation of H2O2 was observed (Fig. 4). The H2O2 system showed a higher decolorization rate. H2O2 alone has the ability to decolorize RB13, as it is one of the most powerful oxidizers in acidic solutions. Yet, the oxidation process progressed slowly as only 43.0 % decolorization was achieved in 50 min. Slow H2O2 consumption was also detected using microelectrodes, which confirmed the RB13 decolorization results. The US+H2O2 system was also tested as one of the control experiments. The US+H2O2 system had a slightly higher decolorization rate than the H2O2 system (48.1 %). Due to the effect of acoustic cavitation, ultrasonic irradiation could enhance the H2O2 decomposition into reactive radicals (Huang et al. 2012). Thus, the concentration of H2O2 decreased faster
in the US+H2O2 system during the first 10 min of the experiment than in the H2O2 system (Fig. 4). However, the confinement of radicals (mainly •OH) to the close vicinity of the bubbles can result in the recombination of •OH to produce H2O2 (Sivasankar and Moholkar 2009). This decreases the rate of H2O2 consumption in the US+H2O2 system, maintaining a similar H2O2 consumption rate as the H2O2 system. Since the confined •OH radicals have very low probability of interaction with RB13, enhanced decolorization by ultrasonic irradiation is negligible. Thus, the consumption of H2O2 in both systems remained similar at the end of the experiments. The effects of the Fc+H2O2 system and the US+Fc+H2O2 system on RB13 decolorization were compared (Fig. 5). Realtime detection of H2O2 was maintained using microelectrodes (Fig. 6). In both systems, Fc showed good catalytic activity, which largely promoted the oxidation capacity and led to thorough decolorization. As a heterogeneous catalyst, Fc has an inherent drawback: The catalytic activity is hindered by uneven particle distribution and mass transfer of reagents from liquid phase to the surface of solid particles (Ince et al. 2001).
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Fig. 3 Decolorization of RB13 in US, H2O2, and US+H2O2 system
Environ Sci Pollut Res (2015) 22:11170–11174
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However, its solid form makes catalyst recycling much easier. By simply filtering and drying, Wang et al. used one batch of Fc to remove methylene blue for 3 cycles. No deactivation was observed and the recovery rate reached 98.7 % (Wang et al. 2013). More importantly, the lost Fc was just dissolved in the liquid phase, retaining its original, highly stable structure (Wang et al. 2014). This conclusion was confirmed in our experiments, as no iron ions were identified in the liquid phase after experimentation was completed. No iron leaching distinguishes Fc from other heterogeneous catalysts, making it an environmental friendly alternative. In contrast to previous experiments, ultrasonic irradiation showed significant enhancement effects on RB13 decolorization in the presence of Fenton-like reagents. The rate of decolorization in the US+Fc+H2O2 system was much higher than the sum of the rates of the US system and the Fc+H2O2 system, respectively. It was evident that
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Fig. 4 Real-time detection of H2O2 using microelectrodes in US, H2O2, and US+H2O2 system
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Fig. 5 Decolorization of RB13 in Fc+H2O2 and US+Fc+H2O2 system
Fig. 6 Real-time detection of H2O2 using microelectrodes in Fc+H2O2 and US+Fc+H2O2 system
there was a synergistic effect in the combined sono-Fenton process. Complete decolorization took 50 min in the heterogeneous Fenton system but was shortened by 30 min with the addition of ultrasound. The time history of decolorization was fitted to different kinetic models. Based on regression coefficients (R2), the decolorization in both systems was found to follow pseudo-first-order kinetics (R2 > 0.96). The reaction rate constant for the US+Fc+H2O2 system (k′=0.2173) was much larger than for the Fc+ H2O2 system (k′=0.0977). The concentration of H 2 O 2 also decreased much faster when ultrasonic irradiation was applied. This demonstrates that H2O2 decomposition, the first step of Fenton reactions, is accelerated by ultrasound. Given its low efficiency of breaking down H2O2 molecules and generating reactive radicals, the role of ultrasound in the US+Fc+H2O2 system was mainly physical. Similar to traditional mechanical agitation, ultrasonic irradiation made the solid particle distribution more even. Moreover, the ultrasonic propagation and cavitation bubbles could produce micro-streaming and micro-turbulence, resulting in intense micro-mixing. It enhanced the mass transfer rate of the heterogeneous Fenton system, which was beneficial for •OH generation and reaction with RB13. With a similar dosage of Fenton reagents, the ultrasonic bath in the US+Fc+H2O2 system (150 W×20 min) used more electrical energy than the magnetic stirrer in the Fc+H2O2 system (10 W×50 min) during experimentation. Energy was wasted in the ultrasonic bath, as only a small portion was actually used for decolorization. Future work is necessary to improve the energy utilization efficiency of the ultrasonic system. However, a heterogeneous sono-Fenton system with ferrocene as the catalyst may lead to exciting possibilities for future process design in wastewater treatment.
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Conclusion Microelectrodes were used for real-time detection of H2O2 in a heterogeneous sono-Fenton system with ferrocene as the catalyst. The working mechanism of RB13 decolorization in the heterogeneous sono-Fenton system was fully investigated by comparing five systems (US system, H 2 O 2 system, USH2O2 system, Fc+H2O2 system and US+Fc+H2O2 system). Poor decolorization was found in the US system, H2O2 system, and US+H2O2 system. The addition of Fc into the system largely promoted the oxidation capacity and led to thorough RB13 decolorization. Not only does Fc display excellent reversible redox properties for catalyzing heterogeneous Fenton reactions, but its highly stable and nontoxic structure makes Fc environmental friendly as well. With the help of ultrasonic irradiation, RB13 was completely decolorized in 20 min in the US+Fc+H2O2 system, which was 30 min faster than in the Fc+H2O2 system. Based on the microelectrode data, accelerated decomposition of H2O2 was also observed in US+Fc+H2O2 system. Ultrasonic irradiation aided the Fc catalyst in liberating more •OH radicals from Fenton reactions by improving the mass transfer of the heterogeneous Fenton system, which lead to faster RB13 decolorization. Acknowledgments Financial support for this work was provided by the Zhejiang Province Welfare Technology Applied Research Project (No. 2014C31137) and 151 Talent Project of Zhejiang Province (12-2008).
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