Degradation Mechanism of Methylene Blue in a Heterogeneous

Dec 14, 2013 - MB, indicating the good reusability and stability of Fc. The reaction rate ... catalyst and the temperature, and the degradation of MB ...
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Degradation Mechanism of Methylene Blue in a Heterogeneous Fenton-like Reaction Catalyzed by Ferrocene Qian Wang, Senlin Tian,* and Ping Ning Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, No.727 South Jingming Road, Chenggong District, Kunming 650500, China ABSTRACT: Ferrocene (Fc) was used to catalyze a heterogeneous Fenton reaction for the degradation of methylene blue (MB). The high catalytic activity and stability of Fc in the Fc/Fenton-like system are confirmed. MB removal percentage could still maintain at 99.50% after Fc was successively used for three cycles and only 1.12% of Fc dissolved with 100% discoloration of MB, indicating the good reusability and stability of Fc. The reaction rate can be accelerated by improving the dosage of the catalyst and the temperature, and the degradation of MB was found to follow the pseudo-first-order kinetic model and the activated reaction energy was calculated to be 82.71 kJ/mol. MB was determined to be degraded through three different pathways simultaneously during the Fc/Fenton-like process, forming Cl−, NO3−, SO42−, and lower molecular weight intermediates: C−N and C−S broke to form the previous rarely reported benzothiazole (determined by liquid chromatography− mass spectrometry), C−N broke to form phenol, and -OH and -HSO3 connected to MB.

1. INTRODUCTION Fenton process is a promising class of advanced oxidation processes (AOPs) used for wastewater treatment.1,2 Traditional Fenton, consisting of H2O2 and ferrous ions, is verified to be rapid and nonselective in contaminants degradation,3,4 for the generation of hydroxyl radicals (•OH).5 •OH is a kind of green oxidant with the oxidation potential of 2.8 eV,6,7 which is much higher than that of 1.78 eV of H2O2.8 The equipment for traditional Fenton is simplistic,9−11 and the optimal condition is verified to be in the range of pH 2−4.12,13 The reactions involved in •OH generation are listed in eqs 1−8:14−16

retard the reaction. Disadvantages, such as the pH window is narrow and the catalysts used in the Fenton process cannot be reused, impeded the wide application of Fenton.17−19 The iron sludge generated after reaction is also a kind of second pollution which may influence water quality.20−22 Since immobilized catalytic systems are usually more chemically robust, more easily separable and recyclable, and so on than their homogeneous counterparts,23 many studies have been made to establish heterogeneous Fenton systems through loading of activates onto various kinds of vectors24−26 to solve the problems mentioned above. But the preparation of catalyst is relatively complex, the catalyst is costly, and energies such as ultrasonic and UV light usually need to be introduced to assist the reaction.27−29 Also, activates loaded on vectors may leach off as reactions proceed, leading to a decrease in catalytic activity. The concentration and pH of the dyeing wastewaters discharged fluctuate in a large range as the seasons change, and they are hard to treat using biological methods.As a kind of cationic dye, methylene blue (MB) can be widely found in dyeing wastewaters. It is of deep color, and comments have been made about its existance in wastewaters discharged from dyeing, feather, textile, and any other industries. MB is of high stability, it is antibiodegradable, and it is hard to degrade using traditional methods. AOPs such as Fenton, electrochemical, and so on are efficient in treating wastewaters containing MB and any other dye molecules, but disadvantages such as the iron sludge generated after the reaction and the introduction of extra energy have limited their application.

Fe 2 + + H 2O2 → •OH + OH− + Fe3 k1 ≈ 70 M−1 s−1

(1)

Fe3 + + H 2O2 → Fe 2 + + H+ + HOO• k 2 ≈ 0.002 − 0.01 M−1 s−1

(2)

Fe3 + + HOO• → Fe 2 + + H+ + O2 k 3 ≈ 1.2 × 10−6 M−1 s−1

(3)

Fe 2 + + •OH → Fe3 + + OH− k4 ≈ 3.2 × 108 M−1 s−1

(4)

•

OH + H 2O2 → H 2O + HOO• k5 ≈ 3.3 × 107 M−1 s−1

Fe 2 + + HOO• → HOO− + Fe3 + •

OH + •OH → H 2O2

(5) (6) (7)

•

OH + organics → products + CO2 + H 2O

Received: Revised: Accepted: Published:

(8)

The generation rate of Fe2+ (0.002−0.01 M−1 s−1) is much slower than that of consumption (70 M−1 s−1), which will © 2013 American Chemical Society

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October 10, 2013 November 27, 2013 December 14, 2013 December 14, 2013 dx.doi.org/10.1021/ie403402q | Ind. Eng. Chem. Res. 2014, 53, 643−649

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C18 column, 2.1 mm × 100 mm, 1.7 μm, and a Waters Quattro Premier XE mass spectrometer equipped with an atmospheric pressure ionization probe. The eluent, consisting of methanol (A) and water with 0.1% formic acid (B), served as the mobile phase in a gradient mode (10% A at 0−5 min, 10%−90% A at 5−15 min, 90% A at 15−20 min) at a flow rate of 0.2 mL/min. The MS detection was performed with electrospray ionization (ESI) in positive mode with 2.8 kV for capillary source voltage, 40.0 V for sampling cone voltage, and 4.0 V for extraction cone voltage, with source temperature of 90 °C and desolvation temperature of 150 °C. The desolvation gas flows at 200 L/h. The mass range of the detector was 40−800 m/z with 1 s/scan.

Ferrocene is a transition metallic compound (Fe(C5H5; Fc)2), which possesses electron donor−acceptor conjugated structure, showing good redox reversible characteristics.30−32 In our previous studies, Fc was verified to be of good catalytic activity and high stability.32 It has been widely used in many kinds of reactions but rarely as a heterogeneous catalyst in the Fenton process before. Considering the characteristics mentioned above, Fc was used in this study to establish a new heterogeneous Fenton to treat MB solution. In this work, the discoloration of MB by a heterogeneous Fenton catalyzed by Fc was investigated. The stability and dissolution of Fc was studied. The role of catalyst dosage and reaction temperature during MB discoloration was discussed as well as the kinetics of the discoloration of MB and the reaction activated energy. Finally, the intermediate products were detected, and the degradation pathway of MB was proposed.

3. RESULTS AND DISCUSSION 3.1. Comparison of Discoloration of MB in Various Degradation Systems. Discolorations of MB in various

2. MATERIALS AND METHODS 2.1. Materials. Ferrocene was purchased from Jingchun Chemical Co. Ltd., Shanghai, and used without further purification. Hydrogen peroxide (30%), alcohol, and NaOH obtained from Sinopharm Co. Ltd., Shanghai, are all of analytical grade. MB bought from Sinopharm Co. Ltd., Shanghai, is BS grade. Sep-pak C18 cartridges packed with C18 silica were bought from Waters, Milford, MA, USA. Acetonitrile, methanol, and dichloromethane bought from Aldrich were all HPLC grade. An imprint for direct identification of the membrane type in syringe filter, 13 mm × 0.45 μm, was bought from Jinteng Experimental Equipment Co. Ltd., in Tianjin, China. 2.2. Experiments for Degradation of MB. The catalytic activity experiments of Fc for degradation of MB in solution were performed at ambient temperature using a 250 mL reactor. The catalyst was dispersed in the reactor containing 100 mL of 10 mg/L MB, and the reactions were initiated with the addition of H2O2 with catalyst dosage ranging from 0.186 to 0.558 g/L and reaction temperature varying from 30 to 60 °C, at pH 4. Solutions in ther reactor were kept stirred throughout the procedure. Samples were withdrawn at predetermined reaction intervals for analysis. 2.3. Analytical Methods. The progress of catalytic degradation of MB was monitored by measuring the absorbance in a UV−visible spectrophotometer (Shimadzu 2450; λ = 662 nm). Inorganic ions in solution were analyzed using an ion chromatograph (IC, Shimadzu with CDD 10 A SP) equipped with IC-SA2 columns (eluents: 0.19078 g of Na2CO3 + 0.14282 g of NaHCO3 in 1 L solution). To identify the intermediates produced during the degradation of MB, after oxidization for 2 and 5 h and filtration, samples were treated using solid-phase extraction method (SPE). Prior to use, the C18 cartridge was rinsed with 10 mL of methanol. The solution was loaded into a Waters Sep-pak C18 cartridge, and the adsorbed intermediates were eluted with CH3CN. The solution was dried at N2 atmosphere and then dissolved in dichloromethane for GC−MS measurement. The intermediates were examined with GC−MS (Agilent 6890) equipped with column DB-5. The column temperature program was as follows: 40 °C (2 min); 40−280 °C (20 °C/min) and 280 °C holding for 15 min; 280−320 °C (10 °C/min). MB degradation products were also identified directly after filtration without any further treatment, using a ultraperformance liquid chromatography/ mass spectrometry (UPLC/MS) system consisting of a Waters Acquity UPLC (PDA detector) equipped with a Waters BEA

Figure 1. Comparison of discolorating efficiencies of 10 mg/L MB under various systems (pH = 4.0, 30 °C, 0.372 g/L Fc, and 23.58 mmol/L H2O2).

Figure 2. Variation of concentration of dissolved Fc in solution during the discoloration of MB.

degradation systems, namely, Fc, H2O2, and Fc + H2O2, were compared to verify the catalytic activity of Fc and the main process in the Fc/Fenton-like system. MB concentrations versus relative reaction time internals were shown in Figure 1. As seen in Figure 1, because of the oxidation effect of H2O2, 7.07%−8.98% of MB can be removed in 5−120 min under the 644

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Figure 3. Influence of catalyst dosage on the discoloration of MB (30 °C, pH = 4, 10 mg/L MB, [H2O2] = 23.58 mmol/L).

Figure 5. Influence of the dosage of H2O2 on the discoloration of MB (30 °C, pH = 4, 10 mg/L MB, 0.372 g/L Fc).

Table 1. Pseudo-First-Order Rate Constants under Different Conditions ka, min−1

condition catalyst dosage (g/L)

reaction temperature (°C)

dosage of H2O2 (mmol/L)

0.186 0.279 0.372 0.558 30 45 60 7.86 15.72 23.58 31.44

26.02 34.03 61.73 83.88 36.74 79.59 725.21 96.94 142.40 145.70 127.77

× × × × × × × × × × ×

R2 −3

10 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

0.976 0.961 0.967 0.933 0.984 0.998 0.910 0.947 0.974 0.994 0.958

Figure 6. Plot of ln(ka) versus 1/T for the estimation of the reaction activated energy.

Figure 4. Influence of reaction temperature on the discoloration of MB (pH = 4, 10 mg/L MB, [H2O2] = 23.58 mmol/L, 0.372 g/L Fc). Figure 7. Inorganic ions detected by ion chromatogram after 50 min reaction.

condition with presence of H2O2. And even in the system of Fc (Fc present only), 5.02% MB can be removed because of the adsorption of Fc particles suspended in solution. Obviously, the removal efficiency provided by only H2O2 is too low to meet the practical needs of treating MB involved wastewater, for the antioxidation character of MB and the low oxidation potential (1.78 eV) of H2O2. Nevertheless, when it comes to the system with both H2O2 and Fc, MB in solution was removed fast in the first 30 min, and nearly 100% of MB was removed in 120 min.

MB degraded by Fc, H2O2, and the heterogeneous Fenton catalyzed by Fc were calculated to be 5.02%, 8.80%, and 86.00%. This result indicated that heterogeneous Fenton catalyzed by Fc played the main role in the discoloration of MB, but not the adsorption of MB onto Fc. Thus, apparently, there are three processes contributing to the removal of MB in 645

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solution,32 including the adsorption of MB on the surface of Fc particles, the homogeneous Fenton process catalyzed by Fc that dissolved in solution, and the heterogeneous Fenton process catalyzed by Fc particles which caused the main degradation of MB, but the heterogeneous Fenton catalyzed by Fc contributed most to the discoloration of MB. 3.2. Stability of Ferrocene in the Heterogeneous Fenton-like Reaction System. To address the stability of catalyst used in tahe Fc/Fenton-like system, experiments were carried out to investigate the Fc concentration in solution versus time under the following conditions: pH = 4, 30 °C, 10 mg/L MB, 0.3 g/L Fc, and [H2O2] = 23.58 mmol/L. The stability of Fc in the heterogeneous Fc/Fenton-like system can be indicated by the results presented in Figure 2. Fc dissolved as the reaction proceeded, and the concentration was tested to be 1.8 × 10−5 mol/L after 2 h, which calculated to be 1.12% of the Fc added at the beginning of the reaction. The Fc particles suspended in solution were filtered out, rinsed with water, dried at room temperature, and further used for the second and third rounds. The results shown in Figure 2 indicate that residual MB

Figure 8. Mass spectrometer for benzothiazole generated in the MB degradation process using the Fc/Fenton-like process.

Figure 9. Proposed degradation pathway of MB by the Fc/Fenton-like process. 646

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and ka are also listed in Table 1. Rate constants ka are 36.74 × 10−3, 79.59 × 10−3, and 725.21 × 10−3 min−1 in correlation with reaction temperatures at 30, 45, and 60 °C. This phenomenon indicated that the reaction of MB degradation in the Fc/Fenton-like process is favored at higher temperature and the reaction is endothermic. 3.3.3. Influence of the Dosage of H 2 O 2 on the Degradation of MB. As the source of •OH, the dosage of H2O2 used may influence the efficiency of the Fc/Fenton-like process obviously. Thus, the effect of the dosage of H2O2 on the discoloration of 10 mg/L MB at pH 4, in the presence of 0.3 g/L Fc with initial MB concentration of 10 mg/L, was investigated, and the result is depicted in Figure 5. Residual MB in solution decreased as the dosage of H2O2 increased from 7.86 to 23.58 mmol/L, but increased as the dosage of H2O2 further increased to 31.44 mmol/L. Reasons maybe that the reaction was favored as more dosage of H2O2 was used; otherwise, too much H2O2 in solution played the role of the precursor of •OH. Experimental plot data in the range of 20− 45 min were fitted to the pseudo-first-order equation, and the coefficients R2 and ka are listed in Table 1. Rate constants ka are 96.94 × 10−3, 142.40 × 10−3, 145.70 × 10−3, and 127.77 × 10−3 min−1 in correlation with 7.86, 15.72, 23.58, and 31.44 mmol/L H2O2. This phenomenon indicated that the optimal dosage of H2O2 is 23.58 mmol/L, and redundant H2O2 in solution will be resistant to the reaction. 3.4. Activated Reaction Energy. Kinetic studies of MB discoloration were studied to quantitatively characterize the apparent discoloration under different conditions. It allows us to know the effects of catalyst concentration, reaction temperature, and H2O2 concentration on the apparent reaction rate and favor the determination of the activated reaction energy (Ea). Provided the known effect of catalyst dosage and temperature on MB discoloration, the reaction activated energy can be calculated from the Arrhennius equation. The ka constant at different temperatures also can be expressed by the Arrhenius equation as

amounts in solution were 0, 0, and 0.5% after 2 h reaction according to the first, second, and third round use of Fc, respectively. It follows that even a small amount of Fc would dissolve in the Fenton-like system during the discoloration of MB; the Fc particles suspended in solution are still of high catalytic activity after been used for three times. Thus, Fc is of high stability in the Fc/Fenton-like system. 3.3. Influence of Operating Parameters on MB Discoloration. 3.3.1. Influence of Catalyst Dosage on the Degradation of MB. As a heterogeneous catalyst in the Fenton process, Fc may have three functions: first, to adsorb directly more or less MB molecules; second, to facilitate the decomposition of hydrogen peroxide and provide hydroxyl radicals for the oxidation of MB; last, because of the dissolved Fc in solution, to make the solution more turbid and also make some contribution to decreasing the transmittance. In order to investigate the influence of the dosage of Fc on the degradation of MB, 10 mg/L MB at pH 4 in the presence of 23.58 mmol/L H2O2 with dosages of Fc varied from 0.186 to 0.558 g/L were tested, and the results are shown in Figure 3. Similar trends were observed with four different dosages of Fc. The transmittance of solution decreased in the first 5 min with 0.186 and 0.279 g/L of Fc, and then MB residual in solution decreased quickly in the range of 5−30 min. Residual MB in solution continuously decreased to 0 with the dosage varied from 0.186 to 0.558 g/L. An increase in catalyst dosage results in a faster removal of MB as expected, due to the fact that more • OH radicals are formed with more catalysts present. To quantitatively characterize the discoloration of MB with various dosages of Fc present, the kinetics of MB discoloration were studied. Reaction plots in the range of 10−30 min were fitted to the pseudo-first-order kinetic equation, and the correlation coefficients R2 and rate constants are listed in Table 1. However, it should be stated that the kinetic study in this work only refers to the apparent reaction kinetics because three or more processes contribute to the overall discoloration of MB. The results indicated that reaction plots fitted the pseudo-first-order kinetics equation well and the reaction rate is influenced obviously by catalyst dosage. The reaction rate ka was calculated to be 26.02 × 10−3, 34.03 × 10−3, 61.73 × 10−3, and 83.88 × 10−3 min−1 in correlation with 0.186, 0.279, 0.372, and 0.558 g/L Fc. The reason is presented as follow: H2O2 decomposition was accelerated as the dose of Fc increased and more hydroxyl radicals generated, leading to the faster discoloration of MB. Thus, the dosage of Fc is an important parameter influencing the discoloration of MB, and the discoloration of MB was favored as the dosage of Fc increased. 3.3.2. Influence of Temperature on the Degradation of MB. Reaction temperature is an important parameter influencing the efficiency of a catalytic oxidation process. The effect of reaction temperature on the discoloration of 10 mg/L MB at pH 4, in the presence of 23.58 mmol/L H2O2 and 0.3 g/ L Fc with initial MB concentration of 10 mg/L was also tested, and the result is depicted in Figure 4. Obviously, the higher reaction temperature would avail the heterogeneous Fenton process catalyzed by Fc, and similar results have been reported before.33,34 As discussed earlier, there are two possible reasons for this phenomenon. One is that more H2O2 decomposes at a faster rate to generate more •OH for the degradation of MB; another is that more energy is provided at higher temperature for the reactant molecules to overcome reaction activated energy. Experimental plot data in the range of 10−20 min were fitted to a pseudo-first-order equation, and the coefficients R2

⎛ −E ⎞ ka = A exp⎜ a ⎟ ⎝ RT ⎠

(9)

The linear form of the equation is

ln(ka) = ln(A) −

Ea RT

(10)

where ka is the rate constant, A is a preexponential factor, Ea is the reaction activation energy (J/mol), R is the gas constant (8.314 J/mol K), and T is the absolute temperature (K). Using the data listed in Table 1, ln(ka) versus 1/T were plotted in Figure 6, and Ea can be obtained from the slope of the linear fitted to the plots. The Ea value obtained in the case of the heterogeneous Fenton process catalyzed by Fc was 82.71 kJ/ mol. 3.5. Degradation Mechanism of MB. In terms of the preceding results, the degradation mechanism of MB was investigated and proposed. The MB solutions were treated by the Fc/Fenton-like system, and the color of the MB solution gradually faded as the reaction proceeded, which indicated that the MB concentration decreased obviously. This phenomenon can be ascribed to the destruction of the whole molecular or the chromophore destruction. In order to clarify the MB degradation mechanism, intermediates and final products 647

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Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, China and the Analysis and Testing Foundation of Kunming University of Science and Technology (Grant 2010150).

generated were detected using IC, GC−MS, and LC−MS technologies and the MB degradation pathway was proposed. Ions generated in the degradation of MB in the Fc/Fentonlike system were analyzed by the analysis of ion chromatogram. Inorganic ions found in water are NO3−, Cl−, and SO42−; Cl− could be detected at the beginning of the reaction, indicating that the S−Cl bond was the first broken in the MB degradation process and the MB molecule oxidized into NO3−, Cl−, and SO42− in the Fc/Fenton-like process. Hydroxyl radical was proved to generate on the surface of the catalyst in our previous study;32 thus, the Fc/Fenton-like process may be described as follows: H2O2 was catalyzed to decompose and hydroxyl radicals were generated on the surface of the catalyst and then transferred to the liquid phase to oxidize MB in solution. Samples were withdrawn at predetermined time intervals and analyzed with GC−MS and LC−MS. Benzothiazole was detected in the reaction, which is rarely mentioned in previous studies. The molecular structure and retention time in LC−MS analysis are listed in Figure 8. Based on the intermediate and final products detected, the degradation mechanism for MB is analyzed and described, as shown in Figure 9. The MB molecule decomposed in three different ways simultaneously. Most Cl− may be ionized during the dissolution of MB and exist in the detached state. N−CH3 with the lowest bond energy of 70.8 kcal/mol35 is first broken and the −CH3 is oxidized to HCHO or HCOOH. C−S and C−N are broken in the following oxidization of the remaining structure; N−CH3 and C−N are broken after the oxidation of Cl−S to SO, and the S−C bond in the remaining structure is broken to form phenol and aniline-2-sulfonic acid; some two -OH and -SO3H connected to C2, C5, C8, and C11 in one MB molecules as the N−CH3 bond is broken, and then -OH is derived. These organic intermediates in solution were further oxidized until finally transformed into CO2, H2O, Cl−, SO42−, and NO3−.

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(1) Ma, Y. S.; Huang, S. T.; Lin, J. G. Degradation of 4-nitrophenol using the Fenton process. Water Sci. Technol. 2000, 42 (3−4), 155. (2) Zhu, N. W.; Gu, L.; Yuan, H. P.; et al. Degradation pathway of the naphthalene azo dye intermediate 1-diazo-2-naphthol-4-sulfonic acid using Fetnon’s reagent. Water Res. 2012, 46, 3859. (3) Santos, M. S. F.; Alves, A.; Madeira, L. M. Paraquat removal from water by oxidation with Fenton’s reagent. Chem. Eng. J. 2011, 175, 275. (4) Ma, Y. S.; Huang, S. T.; Lin, J. G. Degradation of 4-nitrophenol using the Fenton process. Water Sci. Technol. 2000, 42 (3−4), 155. (5) Kang, S. F.; Liao, C. H.; Hung, H. P. Peroxidation treatment of dye manufacturing wastewater in the presence of ultraviolet light and ferrous ions. J. Hazard. Mater. 1999, 65, 317. (6) Stefan, M. I.; Hoy, A. R.; Bolton, J. R. Kinetics and mechanism of the degradation and mineralization of acetone in dilute aqueous solution sensitized by the UV photolysis of hydrogen peroxide. Environ. Sci. Technol. 1996, 30, 2382. (7) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; et al. Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals(HO•/O•−) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17, 513. (8) Adán, C.; Carbajo, J.; Bahamonde, A.; et al. Phenol photodegradation with oxygen peroxide over TiO2 and Fe-doped TiO2. Catal. Today. 2009, 143 (3−4), 247. (9) Walling, C. Fenton’s reagent revised. Acc. Chem. Res. 1975, 8, 125. (10) Zazo, J. A.; Casas, J. A.; Mohedano, A. F. Chemical pathway and kinetics of phenol oxidation by Fenton’s reagent. Environ. Sci. Technol. 2005, 39, 9295. (11) Schrank, S. G.; José, H. J.; Moreira, R.; et al. Applicability of Fenton and H2O2/UV reactions in the treatment of tannery wastewaters. Chemosphere 2005, 60 (5), 644. (12) Utset, B.; Garcia, J.; Casado, J.; et al. Replacement of H2O2 by O2 in Fenton and photo-Fenton ractions. Chemosphere 2000, 41, 1187. (13) Rodrigguez, M. L.; Timokhin, V. I.; Contreras, S.; et al. Rate equation for the degradation of nitrobenzene by ‘Fenton-like’ reagent. Adv. Environ. Res. 2003, 7, 583. (14) Lu, M. C.; Chen, J. N.; Chang, C. P. Oxidation of dichlorvos with hydrogen peroxide using ferrous ions as catalyst. J. Hazard. Mater. 1999, 65, 277. (15) Ma, J. H.; Song, W. J.; Chen, C. C.; et al. Fenton degradation of organic compounds promoted by dyes under visible irradiation. Environ. Sci. Technol. 2005, 39, 5810. (16) Fan, X. Q.; Hao, H. Y.; Shen, X. X.; et al. Removal and degradation pathway study of sulfasalazine with Fenton-like reaction. J. Hazard. Mater. 2011, 190, 493. (17) Duesterberg, C. K.; Mylon, S. E.; Waite, T. D. pH effect on Iron-catalyzed oxidation using Fenton’s reagent. Environ. Sci. Technol. 2008, 42, 8522. (18) Sontakke, S.; Modak, J.; Madras, G. Effect of inorganic ions, H2O2 and pH on the photocatalytic inactivation of Escherichia coli with silver impregnate combustion synthesized TiO2 catalyst. Appl. Catal., B 2011, 106 (3−4), 453. (19) Wang, Q.; Gao, B. Y.; Wang, Y. Effect of pH on Humic acid removal performance in coagulation-ultrafiltration process and the subsequent effects on Chlorine decay. Sep. Purif. Technol. 2011, 80 (3), 549. (20) Hsueh, C. L.; Huang, Y. H.; Wang, C. C. Photoassisted fenton degradation of nonbiodegradable azo-dye(Reactive Black 5) over a novel supported iron oxide catalyst at neutral pH. J. Mol. Catal. A: Chem. 2006, 245 (1−2), 78.

4. CONCLUSION Ferrocene was used in a heterogeneous Fenton process as catalyst in this study. More catalyst dosage and higher temperature favors the degradation of MB, and the degradation of MB with various catalyst dosages at different reaction temperatures in the range of 10−20 min followed the pseudofirst-order kinetic model, and the reaction activated energy calculated from Arrhenius equation was 82.71 kJ/mol. Inorganic ions such as NO3−, Cl−, and SO42− were detected in IC analysis, while intermediates generated in the Fc/Fentonlike system were detected using IC, GC−MS, and LC−MS, and benzothiazole which has rarely been reported was observed in LC−MS analysis with the retention time of 12.58 min. The degradation pathways of MB were proposed, containing three ways that occurred simultaneously.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.:+86 871 5920528. Fax: +86 871 5920528. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (Grants 20607008 and 21077048), the 648

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