Rapid Mineralization of Azo-Dye Wastewater by Microwave

Article pubs.acs.org/JPCC

Rapid Mineralization of Azo-Dye Wastewater by Microwave Synergistic Electro-Fenton Oxidation Process Yujing Wang,† Hongying Zhao,†,‡ Junxia Gao,† Guohua Zhao,*,†,‡ Yonggang Zhang,‡ and Yalei Zhang*,‡ †

Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China

‡

ABSTRACT: A method for in situ improvement of the electro-Fenton (EF) oxidation process using microwave radiation was proposed. The microwave enhanced electroFenton (MW-EF) degradation of azo dye wastewater with boron-doped diamond (BDD) anode was carried out in a continuous flow system under atmospheric pressure. The activation effects of microwave were studied through determining the accumulated hydroxyl radicals, electrogenerated H2O2, and the evolution of iron ions and intermediates. The results showed that, under microwave irradiation, both cathode and anode surfaces were activated efficiently. For cathode, the presence of microwave in electro-Fenton oxidation process accelerated the Fe(III)/Fe(II) redox cycles, leading to relatively steady Fe(II) recovery. Moreover, the transduction of electrons was promoted and the electrosynthesis of H2O2 from O2 was greatly accelerated on the cathode. For BDD anode, under microwave, the blocking of electrode surface by intermediates was enormously alleviated and more active sites of producing ·OH were formed on the electrode surface. Consequently, the concentration of ·OH was significantly promoted in MW-EF process, which was 2.3 times of traditional EF method. Further studies indicated that both the formation and degradation rates of intermediates in MW-EF system were obviously increased comparing with EF due to the synergistic effect between microwave irradiation and electro-Fenton. In addition, the removals of TOC and methyl orange concentration, mineralization current efficiency were respectively around 3.1, 1.1, and 3.2 times higher than that without microwave radiation. This work enriched the theory of catalytic oxidation combination technology and developed a new idea for elimination concentrated refractory organic pollutants. The oxidation ability of H2O2 is enhanced by adding Fe2+ ion as catalyst to yield Fe3+ ion and ·OH from Fenton’s reaction (eq 2).10

1. INTRODUCTION Presently, a great development has been achieved in decomposing persistent organic pollutants (POPs) in wastewaters by environmentally-friendly advanced oxidation processes (AOPs), involving various chemical, photocatalytic oxidation, electrocatalytic oxidation, and Fenton oxidation methods based on the in situ generation of hydroxyl radical (·OH).1−5 Among these AOPs, the Fenton oxidation reaction is known as an efficient way to degrade organic pollutants, which can generate hydroxyl radicals during the reaction between Fe2+ and H2O2 in acidic wastewater.6,7 This is a rather easy procedure that can be used to efficiently oxidize a wide variety of organics.8 However, H2O2 is a powerful oxidizer, which is quite corrosive and unstable, inducing inconvenient and hazardous for transport and storage. Meanwhile, the constant supply of Fe2+ is required to maintain the Fenton reaction.9 On the basis of the above deficiency, the electroFenton (EF) technique is developed which can continuously supply H2O2 in an acidic solution via a two-electron reduction of the injected oxygen (eq 1).3,7 O2 + 2H+ + 2e− → H2O2 © 2012 American Chemical Society

Fe2 + + H2O2 → Fe3 + + OH− + ·OH

(2)

In addition, the advantage of electro-Fenton process is the regeneration of Fe2+ by continuous Fe3+ reduction at the cathode from eq 3, thus enhancing the destruction rate of organic pollutants.11 Fe3 + + e− → Fe2 +

(3)

Herein, the surface condition of electrode and the recovery of Fe2+ are the key factors in electro-Fenton reaction. The electrode surface may be passivated during the electro-Fenton process, possibly because the active sites of electrode surface are blocked by initial pollutant and intermediates.12,13 The passivation would decrease the efficiency of H2O2 electrogeneration (eq 1). Moreover, the formation of Fe(III) complexes could inhibit the continuous and effective recovery Received: December 30, 2011 Revised: March 8, 2012 Published: March 8, 2012

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dx.doi.org/10.1021/jp212590f | J. Phys. Chem. C 2012, 116, 7457−7463

The Journal of Physical Chemistry C

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of Fe2+ (eq 3) in electro-Fenton system.9,14 These drawbacks have challenged the scientists to explore novel techniques to prevent the passivation of electrodes and accelerate the regeneration of Fe2+, accordingly improve the oxidation ability of the electro-Fenton process. Microwave is an ultrahigh-frequency electromagnetic wave that promotes several effects on the vibration of chemical bonds, the activation energy and the spatial structure of product.15−17 It has been applied in chemical reactions and in the treatment of environmental pollutants because of its characteristics in the thermal and nonthermal effects. The thermal effect of microwave provides stable homogeneous heat source, and the nonthermal effect of microwave could be enhance the ion mobility and the diffusion of charge carriers to the surface. The applications of microwave combined with other methods such as chemical reaction and electrochemical oxidation have been explored.18−20 Our previous studies also showed that microwave radiation could activate the electrode surface and improve the mass transfer efficiency in electrochemical processes.12,13,21 Therefore, in the hope of enhancing the generation of ·OH and rapid mineralization of organic pollutants, this work was focused on investigating a new method for wastewater remediation which was in situ microwave activated electro-Fenton process. Methyl orange (MO) is a typically biorefractory azoic dye and its derivatives showed much greater toxicity toward humans and animals. It was selected as the target pollutant to study the effect of the enhancement in MO mineralization with microwave activated electro-Fenton method at boron-doped diamond (BDD) electrode. BDD was prepared under microwave condition which confirmed that the BDD electrode would be stable in the presence of microwave radiation.22 Moreover, Brillas et al. has successfully applied BDD electrode in the electro-Fenton process to obtain high electrochemical performance.3,11,23,24 In this work, the degradation of MO in aqueous solution was investigated respectively by microwave radiation (MW), electrochemical oxidation (EC), electro-Fenton (EF), microwave activated electrochemical oxidation (MW-EC), and microwave activated electro-Fenton (MW-EF) methods in a continuous flow system under atmospheric pressure. The main differences of electrochemical oxidation of MO with these five methods were discussed in detail by mineralization current efficiency and kinetic parameter, expecting to explain effects of microwave irradiation. The mechanism of in situ activation of oxidation ability with microwave irradiation was also investigated systematically from the intermediates and the accumulated ·OH in solution.

Figure 1. Schematic diagram of experimental setup. (1) blower pump; (2) thermometer; (3) liquid buffer; (4) liquid flowmeter; (5) DC regulated power supply; (6) cathode: titanium bar; (7) anode: BDD electrode; (8) peristaltic pump; (9) microwave oven; and (10) condenser tube.

order to avoid the high temperature and pressure caused by the microwave radiation, and the buffer pool was connected with condenser tube which was open to the atmosphere, moreover the buffer pool was installed in a constant-temperature bath of 308 ± 5 K, thus the system is carried out in a mild temperature and under atmospheric pressure. The flow rate was 180 mL min−1 and the reactor contained 100 mL of MO solution. 2.3. Oxidation Process of MO. The oxidation of MO process was carried out in electrochemical oxidation equipment. The anode was a 3.0 cm2 BDD thin film deposited on a conductive Si sheet purchased from Switzerland. For simplifying the analysis and gaining insight into the microwave effect on electro-Fenton reaction, we chose Ti as cathode, which shows no microwave absorption, rather than carbonaceous materials with strong microwave adsorption. The titanium foil with the same surface area was used as cathode and the gap between the electrodes was 2 cm. During the electrolysis experiment, the air was bubbled to the solution at 0.02 m3 h−1 through a porous pipe-diffuser and 10 mA cm−2 constant current density (j) was applied and the relevant potential was 3 V. MO solution (100 mL, 300 mg L−1,) was degraded in an aqueous medium containing 0.05 M Na2SO4 as supporting electrolyte and 1 mM Fe2+ as catalyst. In EF and MW-EF process, the pH of the solution kept at 3.0. 2.4. Analytical Procedures. Total organic carbon (TOC) was measured on a Shimadzu TOC-Vcpn analyzer. The determination of H2O2 was performed by using titanium oxysulfate as indicator and UV−vis spectrophotometer (Agilent 8453, Agilent Corporation, U.S.) was used to measure the absorbance of the complex compound solution at the maximum absorption wavelength λ = 409 nm.25 The concentrations of iron were analyzed using the 1,10-phenanthroline method.26 Because of the nonselectivity and high reactivity of ·OH, it is difficult to determine directly its concentrations and the indirect methods must be used. From different indirect detection methods available in the literature,27−29 benzoic

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents. Reagent grade MO (98%) was purchased from Sigma-Aldrich and used without any further purification. All of the other chemicals and organic solvents used were analytical grade. All solutions were prepared with deionized water. 2.2. Experiment Apparatus. A diagram of the experimental setup is shown in Figure 1. A modified domestic microwave oven (frequency 2.45 GHz, output power 127.5 W), which power could be adjusted by a booster, was used to supply microwave energy. Microwave “transparent” Teflon (PTFE) was chosen as the material of the reaction vessel, and a poly column reactor was installed into the microwave oven. The MO solution flowed through the reactor by a peristaltic pump in 7458



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genetic algorithm can be used to simulate the reaction model, and approximately determine the total apparent formation rate constant (kf), and the decay rate constant (kd) by fitting the experimental data.12,34−36

acid was chosen as a probe to target the concentration of ·OH.28 Benzoic acid, known as the radical scavengers, can react with hydroxyl radical in aqueous media with reaction rate constant of 4.2 × 109 M−1s−1.30 This reaction produces phydroxy benzoic acid (p-HBA) as well as o-HBA, m-HBA, and other products. And it was reported that per mole p-HBA was produced quantificationally by 5.87 ± 0.18 mols ·OH.31 It is a well-known semiquantitative method which has been studied and widely used by other groups.32 GC-MS (Agilent 6890GC, Agilent 5973MSD) was used to determine the intermediates qualitatively. The gaseous phase separation of intermediates was performed in the capillary column (Agilent HP-5MS, 30 m × 0.25 mm, 0.25 μm) with injection volume of 1 μL. The temperature of capillary column started from 313.15 to 373.15 K at a rate of 10 K min−1, holds for 2 min, and then increased to 473.15 K at 5 K min−1. The concentrations of MO and its stable degradation products were measured by high-performace liquid chromatogram (HPLC, Agilent HP 1100, U.S.) through comparing the retention time with those of the standard compounds. Samples of 20 μL previously filtered with PTFE fileters of 0.45 μm were injected to the HPLC. Aromatic intermediates were detected and quantified by Agilent Zorbax Eclipse XDB-phenyl column (150 mm ×4.6 mm, 5 μm), and selected UV detector at λ = 465 and 254 nm. Generated small carboxylic acids were detected and quantified by Agilent Zorbax Eclipse XDB-C18 column (150 mm × 4.6 mm, 5 μm) at λ = 210 nm. For these analyses, a mixture of 2/71/27 (v/v) methanol/acetonitrile/water for phenyl column and a 50/50 (v/v) methanol/phosphate buffer (pH = 2.3) for C18 column were employed as mobile phase at a flow rate of 1.0 mL min−1. The mineralization reaction of MO can be expressed as follows:

3. RESULTS AND DISCUSSION 3.1. Enhancement of EF Process by Microwave Action. As is known to all, the amount of ·OH radicals plays an important role for the degradation of contaminations.2,11,21 Figure 2A showed the accumulated ·OH concentration in degradation solution. It depicted that the ·OH radical was very hard to be produced if only with solo microwave at least in our experiment condition. Whereas in the EC, EF, MW-EC, MWEF processes, the concentration of ·OH radical (C·OH) improved with increasing the electrolysis time. The value of C·OH remained the highest in the MW-EF process during the whole reaction. For example, at 180 min, C·OH in the MW-EF process was 70.7 μM, which was 2.7 times and 1.3 times higher than that in the EC (19.3 μM) and EF (30.9 μM) processes, respectively. Obviously, microwave radiation greatly promoted the generation of ·OH radicals and efficiently enhanced the oxidation ability of electro-Fenton system. Furthermore, the increment of ·OH radicals (ΔC·OH) in the EC and EF processes were different between with and without microwave radiation (see inset of Figure 2A). C·OH raise in the EF process was much faster than that in the EC process under microwave radiation. The ΔC·OH between the MW-EF and EF was 1.7 times of that between MW-EC and EC at 180 min, which indicated that microwave irradiation was more conductive to producing ·OH in the EF process comparing to sole EC system. It was attributed to the synergistic effect of microwave radiation, which not just activated BDD electrode surface,12,13,21 more importantly, notably enhanced the activity of Fenton oxidation reaction. The formation of ·OH radicals were therefore in situ increased with the action of microwave radiation, which was further discussed from two aspects as follows. On the one hand, since the conversion of Fe(III) was very crucial to Fenton oxidation reaction,37 the evolution of Fe(III) during the experiment in the EF and MW-EF processes were presented in Figure 2B. For both systems, Fe(III) concentration rapidly increased from zero to steady detected concentrations, which indicated that a rapid oxidation of Fe(II) to Fe(III) with electrogenerated H2O2 via Fenton reaction (eq 2) occurred. Additionally, as shown in Figure 2B, it was found that the accumulated concentration of Fe(III) remained much lower in whole process in the MW-EF than that in the EF, revealing the conversion of accumulated Fe(III) was promoted by microwave. The possible reason was under microwave conditions, temperatures at the electrode surface needed to be consistent with liquid superheating, mass transport could be enhanced and these high temperatureshigh shear force conditions induced by focused microwaves at the electrode surface were beneficially employed to conduct transduction of electrons.38 So in the MW-EF the transduction of electrons was promoted on the cathode by microwave, and then the Fe(III) reduction and the recovery Fe(II) were accelerated. Moreover, as reported in the literatures,3,11 large proportion of Fe(III) complexes would be formed along with the degradation reaction, such as Fe(C2O4)+, Fe(C2O4)2−, Fe(C2O4)33−, and Fe(OH)2+, which might prevent the regeneration of Fe2+ from Fe(III). However, Guinea et al. have already pointed out that the conversion of Fe(III) complexes could be improved under UVA or solar light as energy source.14,37 Thus, it was

2C14 H14N3NaO3S + 58H2O → 28CO2 + 2Na+ + 2SO4 2 − + 3N2 + 144H+ + 142e−

Mineralization current efficiency (MCE) for the treated MO solution is estimated from eq 4:33 MCE =

Δ(TOC)exp Δ(TOC)theor

× 100%

(4)

Where Δ(TOC)exp is the experimental value for TOC removal at time t and Δ(TOC)theor is the theoretically value of TOC removal, calculated by the following: Δ(TOC)theor =

I × t × nc × M × 103 mg·L−1 ne × F × V

where I is the current intensity (A), t is electrolysis time (s), F is Faraday constant (96485 C mol−1), ne is the electron transfer number in the mineralization reaction, nc is the carbon number of the organic compound, M is carbon atomic weight (12 g mol−1), and V is the volume of the sample solution (L). For MO, ne and nc are 142 and 28, respectively. By assuming that the formation and decay of each intermediate was simplified overall sequential reactions, regardless that they might be formed via complicated and multistep processes, an approximate reaction model is postulated. In order to deeply analyze the evolution of intermediates formed during the MO degradation, a numeric 7459



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concentration of 55.8 mg L−1 to 0.2 mg L−1 in the EF process at 20 min, which is 2.9 mg L−1 in the MW-EF process. This result confirms that the Fe(II) regeneration was improved under microwave radiation, which just like UVA/solar light performance. On the other hand, to in-depth understand the effect of microwave in the MW-EF process, the studies on the evolution of H2O2 were also carried out. Figure 2C exhibited the difference of H2O2 concentration (ΔCH2O2) in its corresponding processes with and without microwave. In Figure2C, the ΔCH2O2 between EC and MW-EC was above zero during the whole reaction, revealing that microwave radiation obviously accelerated the H2O2 generation due to the activated electrode. It was known that microwave activation effect at metal electrodes was highly localized due to the working electrode acting as an “antenna”. Under appropriate conditions, microwaves were self-focused at the end of the metal tip into a region within the diffusion layer at the electrode surface/solution interface.39,40 It has reported that microwave pulses induce fast temperature transients within a small zone at the electrode surface but not within the bulk liquid.39 In the high temperature zone at the electrode surface, mass transport was enhanced by (i) the temperature effect on diffusion and (ii) forced convection in the presence of temperature gradients.41−43 The forced convection effect could become particularly strong and then H2O2 formation was strong accelerated in electrochemical process by microwave. But in the EF process, H2O2 was consumed by Fe(II) via Fenton reaction (eq 2). As we can see from Figure 2C the ΔCH2O2 between MW-EF and EF was below zero except that in the first 50 min. This observation can be explained by the improved efficiency of decomposing Fe(III) complexes and enhanced activity of Fenton oxidation reaction by microwave, increasing the consumption of H2O2 (see the comparison in Figure 2C, inset). In other words, microwave could simultaneously favor the formation of H2O2 and the conversion of H2O2 to ·OH in the EF system. Therefore, microwave was more appropriated to combine with the EF technique than the EC for higher oxidation efficiency, which would be further discussed on the subsequent section. 3.2. Acceleration of MO Oxidation by Microwave. The mineralization performance can be illustrated by TOC removal of MO (see Figure 3A). In MW system only with the microwave radiation, less than 1.5% of TOC was removed in 3 h, suggesting that microwave radiation cannot independently enhance the degradation of organics. In the MW-EF process, the TOC removal was up to 36.2% at 30 min, while it took respectively 110, 200, and above 600 min in the MW-EC, EF, and EC process to achieve the same TOC abatement. This indicated that MW-EF method possessed much higher mineralization ability than other processes. Moreover, TOC removal achieved to 95.3% in the MW-EF process after 180 min of electrolysis, while it was hard to reach complete mineralization even in 10 h for other methods. Therefore, microwave radiation effectively enhanced the oxidation ability of EF process. The MW-EF method was supposed to be a superior technology for complete mineralization of MO. In addition, the efficiency of the treatment was evaluated with its mineralization current efficiency (MCE). The evolution of MCE in the EC, EF, MW-EC, and MW-EF processes was exhibited in Figure 3B. As can be seen, the efficiency of EC, EF, MW-EC, and MW-EF processes were 26%, 42%, 58%, and 148% at 30 min, respectively. The average MCE in the MW-EF

Figure 2. Concentration variation with electrolysis time during MO degradation. (MO solutions was in 0.05 M Na2SO4, pH 3.0, 308.15 K and liquid flow is 180 mL min−1. In the EF and MW-EF methods, the starting solution contained 1 mM Fe2+ as catalyst and air flow was 500 mL·min−1, the same as below.) Plots correspond to the following: (A) ·OH [the increment of ·OH is depicted in the inset panel: (a) ·OH concentration difference between MW-EF and EF; (b) ·OH concentration difference between MW-EC and EC]; (B) Fe(III); (C) H2O2 concentration difference: (a) H2O2 concentration difference between ME and E; (b) H2O2 concentration difference between MEF and EF (the corresponding H2O2 concentration evolution in different method is depicted in the inset panel).

reasonable to assume that the microwave radiation, being also one kind of external energy just like UVA/solar light, would accelerate the Fe(III)/Fe(II) redox cycles in EF system. To further confirm this hypothesis, further studies about the evolution of Fe(II) during the EF and MEF processes were carried out. The Fe(II) rapidly decreases from initial 7460



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respectively. Moreover, the data for MO concentration decay were further analyzed by kinetic equations. Good linear plots were obtained when fitted to a pseudofirst-order reaction (see the comparison in Figure3C, inset). The pseudofirst-order rate constant (k) for the MW-EF process were 5.33 × 10−2 min−1 increased by 8.96%, 34.0% and 102%, respectively, compared to those in the MW-EC, EF, and EC process. 3.3. Mechanism and Kinetics of the Intermediates Degradation. The above analysis about the strong enhancement on electro-Fenton oxidation reaction by the microwave indicated that the in situ increased mineralization effect for wastewater was a composite process combining the highly effective activating the surface of electrodes and the accelerating the ferric and ferrous iron cycles. The mechanism of the MWEF process could be concluded in following. As shown in Scheme1, the difference of EF oxidation between with and without microwave irradiation was unambiguously revealed. First, the presence of microwave in electro-Fenton oxidation process promoted the decomposing of Fe(III) complexes and accelerated the Fe (III)/Fe(II) redox cycles, leading to relatively steady Fe(II) recovery. Moreover, the transduction Scheme 1. Possible Mechanism of Microwave Enhanced Electro-Fenton Oxidation Reaction

Figure 3. Time-course evolution of (A) Removal of TOC; (B) Mineralization current efficiency; and (C) MO removal (the corresponding kinetic analysis associated with a pseudo first-order reaction is depicted in the inset panel); (under the same conditions of Figure 2).

was 3.2 times higher than in EF. One of the reasons was the sustained in situ activation of electrode. The other reasons would be the increased efficiency of decomposing Fe(III) complexes and the improved oxidation activity of EF, resulting in more generation of ·OH. All of these factors could lead to higher mineralization current efficiency. Figure 3C displayed the removal of MO during the degradation process. In the MW-EF process, the MO removal reached to 98% at 60 min, which increased by 18.9% compared to that in the EF process. The time required to reach the same value in MW-EC and EC was about 140 and 200 min, 7461



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Figure 4. Evolution of the concentration of the intermediates with electrolysis time during the degradation of MO under the same conditions of Figure 2. Plots correspond to compounds: (A) benzoquinone; (B) hydroquinone; (C) p-nitrophenol (D) 2,5-dinitrophenol; (E) oxalic acid; and (F) maleic acid.

of electrons was promoted and the electrosynthesis of H2O2 from O2 was accelerated on the cathode. Second, under microwave, the blocking of electrode surface by intermediates was alleviated, and the BDD anode surface with more active sites to form ·OH efficiently. Meanwhile, the thermal effect of microwave radiation could increase the temperature of the electrode and the solution, and the diffusion coefficient of MO. Consequently, the amount of ·OH formed from MW-EF reaction was strong increased and the electro-Fenton oxidation was greatly enhanced. The new system possessed strong oxidation ability could mineralize pollutants efficiently. In order to further understand the activation of microwave in electro-Fenton oxidation reaction, a study on the intermediates and their oxidation routes was carried out. The same intermediates were identified for all methods by using GC− MS technique, implying that the oxidation pathway of MO was not changed with microwave irradiation. As shown in Figure 4A−F, the major intermediates were benzoquinone, hydro-

quinone, p-nitrophenol, 2,5-dinitrophenol, oxalic acid, and fumaric acid. But the accumulating concentration of the intermediates detected by HPLC was quite different between with and without microwave irradiation. Compared to other methods, the concentrations of each intermediate in the MWEF process were accumulated more quickly to reach the experimentally measured maximum (Cmax) and then decreased faster too (see Figure 4A−F). In addition, the values of kf and kd, calculated by the numeric genetic algorithm, were also increased faster under microwave radiation. Take hydroquinone as an example, in MW-EF process hydroquinone were quickly accumulated with the Cmax of 7.0 mg L−1 at 30 min and then completely degraded at 120 min. In the EF process, it took 120 min to reach the Cmax of 3.8 mg L−1 and remained 2.3 mg L−1 at 180 min. The kf of hydroquinone was 0.79 × 10−4 min−1 in the EF process while increased to 5.02 × 10−4 min−1 in the MW-EF process. Meanwhile, the kd of hydroquinone also increased to 5.21 × 10−4 min−1 in the MW-EF process from 7462



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0.75 × 10−4 min−1 in the EF process. As known, more of the electrode active sites would be destroyed if the intermediates lasted for longer time in solution. Therefore, higher values of kf and kd in the MW-EF process lead to less fouling of electrode and rapid regeneration of Fe(II). These results corroborated that the intermediates were rapidly oxidized in the MW-EF method, leading to faster destruction of pollutions.

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4. CONCLUSIONS The enhancement of mineralization MO by the EF process with microwave activation in a flow system was studied through accumulated hydroxyl radical, electrogenerated H2O2, the evolution of Fe(III), intermediates, and MO degradation efficiency. The presence of microwave irradiation greatly activated the electrode surface and efficiently accelerated the Fe(III)/Fe(II) redox cycles, leading to relatively steady Fe(II) recovery, which increased the formation of ·OH radicals. The concentration of ·OH was found to be highest in the MW-EF process at any reaction interval, and it was 1.3 times higher in the MW-EF than that in the EF. The major intermediates were benzoquinone, hydroquinone, p-nitrophenol, 2,5-dinitrophenol, fumaric acid, and oxalic acid measured by GC-MS and HPLC, suggesting that the degradation pathway of MO remained the same under microwave irradiation. Nevertheless, in the MW-EF process, the intermediates were generated and decomposed more rapidly, leading to better degradation performance. This work would enrich the theory of advanced oxidation technology and developed a new idea for exploring elimination of highly concentrated refractory organic pollutants.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (86)-21-65988570; fax: (86)-21-65982287; e-mail: g. [email protected] (G.Z.), [email protected] (Y.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported jointly by the National Natural Science Foundation P.R. China (Project No. 21077077), Shanghai Educational Development Foundation (Project No. 2011CG19), and the Program for Young Excellent Talents in Tongji University (Project No. 2010KJ063).

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REFERENCES

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