Ferrocene-Catalyzed Heterogeneous Fenton-like Degradation of

Article pubs.acs.org/IECR

Ferrocene-Catalyzed Heterogeneous Fenton-like Degradation of Methylene Blue: Influence of Initial Solution pH Qian Wang, Senlin Tian,* and Ping Ning Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming, 650500, China ABSTRACT: Ferrocene (Fc) was introduced to establish a heterogeneous Fenton-like system for the treatment of Methylene Blue (MB). Discoloration of MB was used to monitor the contributions of adsorption, homogeneous, and heterogeneous Fenton in the heterogeneous Fenton-like process. The dissolution and electrochemical character of Fc were investigated, as well as the effect of initial solution pH on the apparent removal of MB and processes that occurred in the system. pH 3 was verified to be the optimal condition for Fc and showed good redox reversibility. 100% discoloration of MB and 63.5% removal of COD were reached at pH 3, with only 0.2% Fc dissolved. Fc was found to play roles of catalyst as well as adsorbent in the Fc/H2O2 system. Three processes including adsorption, Fenton catalyzed by Fc dissolved in solution (homogeneous Fenton), and Fc particles suspended in solution (heterogeneous Fenton) worked simultaneously. The efficiencies of the homogeneous Fenton and heterogeneous Fenton were up to the adsorption to some extent. The efficiency of homogeneous Fenton catalyzed by Fc dissolved in solution was about 76.66 times that of heterogeneous Fenton catalyzed by Fc particles at pH 5, but the bulk of MB was degraded in the heterogeneous Fenton catalyzed by Fc particles. quality.15 Some of these heterogeneous Fenton catalysts cannot work efficiently, for the low fruit loaded on the vectors.16,17 Herein, extra energies, such as electricity,18 ultraviolet,19−21 ultrasonic,22,23 etc., need to be introduced to assist in the Fenton process. As a kind of organic transition metallic compound, ferrocene (Fe(C5H5)2) is highly stable and nontoxic, and it has been widely used in many fields in recent years due to the rich chemistry of the iron(II) center.24 The electron donor− acceptor conjugated structure25−27 in the Fc molecule made it of good redox reversible characteristics and high catalytic capacity. Several investigations have been made on wastewater treatment through heterogeneous Fenton system by immobilizing Fc on silica,28 SBA-15,29 SWCNT,30 MCM-41,31 and any other vectors, but Fc has rarely been used in the Fenton reaction as a catalyst directly. In this study, Fc was used as a catalyst to establish a new Fc/Fenton system, and was used in the treatment of MB. H2O2 was used in the Fc/Fenton system as in traditional Fenton and any other heterogeneous Fenton systems. For the good stability and catalysis activity, Fc can be cycled, which makes the cost of Fc/Fenton not much higher than that of the Fenton and Fenton-like systems commonly used. In our previous studies,32,33 Fc was verified to be of good catalytic activity and high stability. H2O2 can be catalyzed to transform into ·OH for the oxidation of MB in solution. The degradation pathway was proposed, and some fresh intermediates appeared during the degradation of MB, which have rarely been detected. It is important to make a thorough inquire of the processes that exist in the heterogeneous system, the mechanisms, and the optimal conditions. The aim of this work

1. INTRODUCTION Water pollution, especially dyeing wastewater, has drawn considerable attention for several decades, for the deep color, toxicity, and high COD content.1 Dyeing wastewater is hard to treat using biochemical methods, for the variation of concentration and pH in a wide range. Thus, chemical oxidation technologies have been considered a promising method for two decades due to their ability to destroy toxic and biologically refractory organic contaminants in aqueous solutions.2 Advanced oxidation processes (AOPs) are technologies, which involve generating highly reactive oxygen species, primarily in the form of the highly reactive and nonselective hydroxyl radical (·OH).3,4 The oxidation potential of hydroxyl radicals has been estimated as +2.8 and +2.0 eV at pH 0 and pH 14, respectively.5,6 It is of the highest oxidation potential only with the exception of the fluorine atom.7 Most organic pollutants could be oxidized into low molecule acids,8−11 even CO2 and H2O directly by OH. Fenton has drawn considerable interesting since being discovered. Traditional Fenton system comprised of hydrogen peroxide (H2O2) and ferrous iron (Fe2+) is a kind of efficient AOP commonly in use today.12,13 No special equipment is needed, and iron salts and hydrogen peroxide used in Fenton are considered economical and environmental friendly. However, drawbacks of traditional Fenton need to be taken into account, such as the need for a large amount of Fe salts and H2O2, and the narrow pH window for application. Also, catalysts used in the Fenton process were hard to separate; thus, sludge containing metals generated at the end of the reaction were considered as a kind of second pollution, which indicated a waste of chemical reagents, on the other side.14 The sludge is usually treated by precipitation and redissolution. To eliminate such problems from homogeneous Fenton processes, some efforts have been made to develop heterogeneous Fenton catalysts. The leaching of the active metals cannot be completely avoided and may partly influence the water © 2014 American Chemical Society

Received: Revised: Accepted: Published: 6334

January 9, 2014 March 23, 2014 March 28, 2014 March 28, 2014 dx.doi.org/10.1021/ie500115j | Ind. Eng. Chem. Res. 2014, 53, 6334−6340

Industrial & Engineering Chemistry Research

Article

oxalated in solution reacted with H2O2 in solution to generate pertitantic acid. The dosage of pertitantic acid in solution (determined at 400 nm) was the certain dosage of H2O2. Hydroxyl radicals in solution were detected by a photometric technique.32,36 Hydroxyl radicals generated in solution can only survival a very short time, and the lifetime is less than 10−4 s. It is hard to be detected directly. The production of ·OH from H2O2/Fe2+ process was assayed by the oxidation of 1,10phenanthroline-Fe2+ to 1,10-phenanthroline-Fe3+ by Jin34. The decrease of 1,10-phenanthroline-Fe2+ (ΔC) was the certain dosage of ·OH generated. The accuracy of the determination of OH radical in the system is 10−7 g/L. Cyclic voltammograms and currents under various initial solution pH values were detected using the device shown in Figure 1. The electrochemical experiments were performed in a

was to investigate the influence of initial solution pH on the efficiency of the Fc/H2O2 system, and the processes progressed simultaneously in the multiphase system. The effect of initial solution pH values on the reversible redox property of Fc was also detected using an electrochemical workstation. Also, the effect of initial solution pH on the decomposition of H2O2, generation of hydroxyl radicals, and dissolution of Fc was probed in detail. The influence of initial solution pH on the removal of MB in various processes and the dissolution of Fc were also studied to understand the correlations between the processes in the system, as well as the influence of pH on each process to propose a proper mechanism and screen the optimum technological conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. Hydrogen peroxide, Methylene Blue, ethanol, Al2O3, Li2SO4, and Fc were purchased from Jingchun Ltd. Co., Shanghai, China, and used directly without further purification. Other reagents were all obtained from Sinopharm Ltd. Co., Shanghai, China. All chemicals used in this study were of analytical grade. All of the solutions used were prepared daily, with deionized water made from the Millipore system with a resistivity of 18.25 MΩ cm−1. N2 was applied by Messer Co. Ltd., Kunming, Yunnan, China. 2.2. Procedures. Both the adsorption and the catalytic oxidation experiments were performed in 500 mL glass vessels. A water cycling system was used to keep the temperature in the range of 30 ± 1 °C. The initial solution pH values were adjusted with 0.1 mol/L H2SO4 and 0.1 mol/L NaOH. The adsorption of MB was investigated by putting the solutions in contact with a given amount of Fc particles until equilibrium; during adsorption experiments, slurry composed of dye solution and catalyst suspension was stirred magnetically and placed in the dark for the adsorptive removal of MB. Samples were picked up and detected after filtration. A given amount of H2O2 was added into filtrations that were obtained after adsorption for 2 h, to initiate the homogeneous Fenton catalyzed by dissolved Fc in solution. Samples were withdrawn at predetermined time intervals, and detected immediately. Experiments were designed to look into the apparent removal of MB using the Fc/H2O2 system. The initial solution pH values were adjusted before the addition of Fc. All solutions were thoroughly stirred to make them well-distributed before the addition of the reagents. Fenton reactions were started with the addition of H2O2. Samples were withdrawn at selected intervals, and the pH was increased to 12 to terminate the reaction, with NaOH (2 mol/L). Finally, the reaction solution was filtered to separate the catalyst particles out for analysis. Experiments were carried out at least in duplicate, and all results were expressed as a mean value. The error between replicate runs was maintained to within 3%. 2.3. Analysis. The concentrations of Methylene Blue and dissolved Fc in solution were detected by a UV−visible spectrophotometer (Shimadzu, 2450) at 66034 and 442 nm,32 respectively, corresponding to the maximum absorbance of MB and Fc. All samples were filtered through a 0.45 μm filter film, and the pH was adjusted to above 10 to quench the reaction before analysis. COD was analyzed by using a Jiangfen HH-6 COD analyzer, Jiangsu, China. H2 O2 in solution was determined using the potassium (IV) oxalate method.35 A predetermined dosage of potassium(IV) oxalated was added into the solution to make it 8.0 mmol/L, and potassium(IV)

Figure 1. Electrochemical cell.

super constant temperature trough. N2 was purged to expel dissolved oxygen in solution, and the whole process was kept at N2 atmosphere. Platinum electrodes were used as working electrodes and auxiliary electrodes, while glassy carbon electrode was used as reference electrode. 0.2 mol/L Li2SO4 was chosen as the supporting electrolyte.

3. RESULTS AND DISCUSSION 3.1. Influence of Initial Solution pH on the Discoloration and Mineralization of MB. Figure 2(1) shows the apparent removal of MB in the Fc/Fenton-like system with initial solution pH varied from 2 to 9. It can be seen that pH made a decisive influence on the removal of Methylene Blue in the Fc/H2O2 system. Residual MB in solution approached a plateau as the initial solution pH increased from 2 to 3. Nearly 100% MB removal was achieved at pH 3, and the removal efficiencies of MB at pH 4 and 5 were both determined to be 99.82%. The removal efficiency of MB decreased to 41.75% at pH 6, and varied in the range of 40.48−41.75% with the initial solution pH further increasing from 6 to 9. The result was similar to that of the traditional Fenton process. Thus, it can be concluded that the Fc/H2O2 system progressed well under acidic conditions, and pH 3 was optimal. Experiments with pH varied from 2 to 5 were carried out to investigate the influence of initial solution pH on the kinetics of the apparent removal of MB, and the results are shown in Figure 2(2). The rate of the removal of MB varied as the initial solution pH changed, and the fastest rate was obtained at pH 3, and slowed as the initial solution pH departed from 3. Chemical oxygen demand (COD) is occupied as an indicator to measure the dosage of pollutants in water in this study. A 6335

dx.doi.org/10.1021/ie500115j | Ind. Eng. Chem. Res. 2014, 53, 6334−6340


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L MB, under conditions of 0.372 g/L Fc, [H2O2]/[MB] = 3.17, 30 °C. As can be seen from Figure 3, the influence of initial solution pH on COD removal is not so remarkable, and the most COD removal was obtained at pH 3, which agreed well with that of traditional Fenton. The removal of COD was much faster at the beginning of the reaction, and then slowed at 20 min. After 120 min, residual COD in solution was determined to be 38.03%, 36.24%, 36.90%, and 38.71% at pH 2, 3, 4, and 5, respectively, indicating that some organic intermediates were generated, and the intermediates and degradation pathway have been proposed in our previous study.33 MB degraded in three pathways simultaneously, and more than 20 intermediates were generated. The fresh benzothiazole was detected as involved in the degradation of MB. The discoloration rate of MB was much slower at pH 2, for Fc+ was stable under acidic conditions and the recycling of Fc/ Fc+ was then greatly inhibited (eqs 1 and 2),10 leading to a reduction of the catalytic capacity.15 Conversely, OH− in solution could also impede the reaction, for the deduction of the recycling redox capacity of Fc as depicted in Figure 6. In addition, the oxidation potential of ·OH was much smaller at pH 5 than under strong acidic condition.35,37 Fc+ + H 2O → Fc + ·OH + H+

(1)

2Fc+ + H 2O2 → 2Fc + O2 + 2H+

(2)

higher COD value means more serious pollution, for 100% discoloration may not mean the whole removal of organics. Dyes may degrade into intermediates with low molecules, which might be more toxic and refractory than the initial pollutant itself,33 instead of H2O and CO2 directly. Figure 3 displays the effect of initial pH on the COD removal of 10 mg/

3.2. Contributions of Various Processes in the Fc/H2O2 System. Theoretically, there are three processes contributing to the removal of MB in the Fc/H2O2 system, including adsorption, heterogeneous Fenton catalyzed by Fc particles, and homogeneous Fenton catalyzed by dissolved Fc in solution. To qualitatively evaluate the contributions of the three processes to the discoloration of MB, adsorption of MB onto the surface of Fc, catalytic oxidation, and the heterogeneous and homogeneous Fenton processes were all taken into account. In Fc/H2O2 system, the treatment of MB was quite effective even under the neutral condition (as shown in Figure 2). The operation process could be eased and the cost could be lowered; thus, experiments were designed to be operated at pH 5 in the following study. As shown in Figure 4 and Table 1, in the adsorption followed by homogeneous Fenton process (A+C), the removal efficiency

Figure 3. Effect of initial solution pH on the mineralization of MB in the presence of 0.372 g/L Fc and [H2O2]/[MB] = 3.17, 30 °C.

Figure 4. Investigation on the contributions of adsorption, homogeneous Fenton, and heterogeneous Fenton in the Fc/H2O2 system to MB removal.

Figure 2. Effect of initial solution pH on the discoloration of 10 mg/L MB in the presence of 0.372 g/L Fc and [H2O2]/[MB] = 3.17: (1) residual MB in solution after 2 h reaction with initial solution pH 2−9; and (2) variation of the concentration of MB.

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respectively. This is due to the fact that MB mainly exists as MB+ in solution, while Fc ↔ Fc+ occurs on the surface of Fc particles, and the formed Fc+ leached into solution. H+, as well as Fc+, in solution may resist the approaching of MB+ to Fc particles and OH− decreased the surface charge of Fc particles; thus, the adsorption of MB+ was also inhabited, and the optimal condition turns out to be pH 3. In the following 2 h, MB was oxidized to decompose by the dissolved Fc involving homogeneous Fenton. The dosage of catalyst as well as the catalytic activity influenced the degradation efficiency. Dissolution of Fc in solution led to a higher substrate-to-catalyst ratio, and the Fenton reaction was proved to have occurred at the surface of Fc.38 The total active surface area of the Fc molecules was much larger than that of the Fc particles suspended in solution in a given amount. Thus, higher efficiency could be achieved as Fc dissolved in solution. As depicted in Figure 7, the dissolution of Fc was influenced by initial solution pH obviously, and most favored at pH 3, which is in the same trend as the adsorptive removal of MB. This phenomenon is due to the following reasons: the concentration of MB around the Fc particles was higher than that of the solution, the difference between the adsorption capacities at equilibrium and at any time, the catalytic process occurred on the surface of the catalyst,29 and Fc+ generated leached into the solution much more easily. As shown in Figure 6, Fc exhibited

Table 1. Removal Efficiency of MB Due to Different Processes under Various Initial Solution pH Values initial solution pH

removal efficiency due to adsorption (%)

removal efficiency due to homogeneous Fenton (%)

removal efficiency due to heterogeneous Fenton (%)

apparent removal efficiency (%)

2 3 4 5

10.87 18.31 15.83 12.05

10.90 20.99 18.86 11.20

72.84 60.71 64.74 72.90

93.60 100 99.43 96.15

of MB was determined to be 23.2% at 4 h, in which 12.0% was reduced during the adsorption held in the dark for 2 h. The apparent 96.1% removal of MB was obtained in 2 h in the Fc/ H2O2 system. Three processes including the adsorption of MB onto Fc, the homogeneous Fenton catalyzed by dissolved Fc, and the heterogeneous Fenton catalyzed by Fc particles occur simultaneously (A/C). 72.9% MB in solution was calculated to be degraded in the heterogeneous Fenton catalyzed by Fc particles. The results indicated that adsorption, homogeneous, and heterogeneous Fenton processes all contributed to the removal of MB, and the heterogeneous Fenton process played the main role. The efficiencies of the homogeneous and heterogeneous Fenton versus the dosages of catalysts (dissolved Fc and solid Fc particles) were compared. As shown in Figure 7, the concentration of dissolved Fc in solution was detected to be approximately 1 × 10−5 mol/L after 2 h, accounting for 0.2% of the total Fc added in the Fc/H2O2 system. As 11.2% MB was degraded in the homogeneous Fenton and 72.9% in the heterogeneous Fenton, the catalytic potential of the dissolved Fc was calculated to be 76.66 times the Fc particles suspended in solution. The degradation of MB was determined by both the dosage and the catalytic activity of catalysts in the two Fenton systems; thus, the Fc dissolved in solution made a limited contribution in accelerating the degradation of MB. 3.3. The Influence of Initial Solution pH on Various Processes. The effects of pH on the adsorption and the homogeneous Fenton were investigated, and the results are shown in Figure 5 and Table 1. 18.31% adsorptive removal of MB was obtained at pH 3 after 2 h, and the adsorptive removal of MB decreased as the initial solution pH departed from 3. The adsorptive removals of MB were determined to be 10.87%, 15.83%, and 12.05% under the conditions of pH 2, 4, and 5,

Figure 6. Effect of pH on the ratio of oxidation peak current on reduction.

the best redox reversible potential at pH 3, for the ratio of Ipc/ Ipa was most approaching 1 at pH 3, and the ratio decreased as the initial solution pH departed from 3. The reduction potentials of Fc were determined to be 0.342, 0.359, 0.331, and 0.293 at pH 2, 3, 4, and 5, respectively. It means that Fc dissolved in solution is of good redox reversibility at pH 3.16 On the basis of the characters of Fc mentioned above, pH 3 was verified to be the optimal condition for homogeneous Fenton catalyzed by dissolved Fc. The removal efficiencies of MB in homogeneous Fenton were determined to be 10.90%, 20.99%, 18.86%, and 11.20% as the initial solution pH varied from 2 to 5. Heterogeneous catalytic oxidation contributed most to the degradation of MB, and the efficiencies were calculated to be 71.84%, 60.71%, 65.34%, and 72.90% at pH 2, 3, 4, and 5, respectively. 100% removal of MB can be reached only at pH 3 in Fc/H2O2 system. Because of the adsorption of MB on the surface of Fc, which decreased the substrate-to-Fc particle ratio,

Figure 5. Effect of pH values on the removal of MB in the adsorption followed by homogeneous catalytic oxidation process. 6337



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thus, quantities of active sites for the heterogeneous Fenton are not available. MB was oxidized to decompose more efficiently in the homogeneous Fenton process catalyzed by dissolved Fc. A conclusion was induced that adsorption, homogeneous, and heterogeneous Fenton processes contributed simultaneously to the removal of MB, and the distributions of the three processes were up to the adsorption to some extent. 3.4. Influence of Initial Solution pH on the Dissolution of Fc. To qualitatively evaluate the influence of initial solution pH on the dissolution of Fc, concentrations of Fc were measured at selected intervals as pH varied from 2 to 5, and the results are shown in Figure 7. The initial solution pH made a

Figure 8. pH effect on the decomposition of H2O2 in the homogeneous catalytic process.

Figure 7. Effect of initial solution pH on the solubility of Fc.

remarkable influence on the dissolution of Fc. The concentrations of Fc dissolved in solution at 120 min were detected to be 1.65 × 10−5, 2.15 × 10−5, 1.8 × 10−5, and 1.0 × 10−5 mol/L according to pH 2, 3, 4, and 5, respectively, indicating that the dissolution of Fc was accelerated at pH 3 and concentrations of dissolved Fc decreased as the initial solution pH values departed from 3. Reasons for this phenomenon have already been explained in section 3.3. 3.5. Influences of Initial Solution pH on the Decomposition of H2O2 and Generation of Hydroxyl Radicals in the Homogeneous Catalytic Process. Hydroxyl radicals generated from the decomposition of H2O2 play the role of oxidant in the Fc/Fenton system; the decomposition of H2O2 and the generation of ·OH were both influenced by initial solution pH as shown in Figures 8 and 9. H2O2 decomposed since being added, residual H2O2 in solution was detected to be 64.1%, 44.9%, 48.7%, and 51.3%, and the concentrations of ·OH were detected to be 2.58 × 10−6, 6.72 × 10−6, 5.82 × 10−6, and 3.02 × 10−6 g/L at pH 2, 3, 4, and 5, respectively, at 120 min, which is consistent with the apparent removal of MB and the efficiencies of the homogeneous Fenton. The oxidation potential of ·OH was reported as 2.8 V at pH 0, and decreased to 2.0 as the pH increased to 14.5,6 This phenomenon can be addressed in two perspectives: First, homogeneous Fenton was most available at pH 3, due to the good redox reversibility and the 0.2% dissolution of Fc. The efficiency of homogeneous Fenton catalyzed by dissolved Fc is calculated to be 76.66 times the heterogeneous Fenton catalyzed by Fc particles suspended in solution; the decomposition of H2O2 and the generation of ·OH were

Figure 9. Effect of pH values on the generation of hydroxyl radicals in the homogeneous catalytic process.

favored under that condition. Second, excess H+ or OH− in solution would retard the generation of ·OH, as shown in eqs 1−3:11 Fc + H 2O2 → Fc+ + ·OH + OH−

(3)

4. CONCLUSIONS The multiphase system comprised of Fc and H2O2 was established to deal with MB in solution. The effect of initial solution pH on the mechanism of the removal of MB was investigated in detail. The three processes including adsorption, heterogeneous Fenton catalyzed by Fc particles, and homogeneous Fenton catalyzed by Fc dissolved in solution occurred simultaneously in the Fc/H2O2 system. Heterogeneous Fenton played the main role in the removal of MB, but the efficiency of homogeneous Fenton was calculated to be about 76.66 times the heterogeneous Fenton. The contributions of the homogeneous and heterogeneous Fenton were up to the adsorption process to some extent. pH 3 was the optimal condition for the adsorptive removal of MB, the dissolution of Fc was accelerated under this condition, and Fc is of good redox reversibility at pH 3. Thus, the generation of ·OH from the decomposition of H2O2 was favored, leading to 100% discoloration and 63.75% COD removal at pH 3. 6338



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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

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REFERENCES

This work was financially supported by the National Science Foundation of China (20607008; 21077048; 21277046), the Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, China, Key Project of Chinese Ministry of Education (210202), and the Analysis and Testing Foundation of Kunming University of Science and Technology (2010150).

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