Enhanced Degradation of Chlorobenzene in Aqueous Solution Using Microwave-Induced Zero-Valent Iron and Copper Particles 1
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C h i e n - L i L e e , C h i h - J u G . J o u * , H. Paul W a n g
ABSTRACT: Microwaves were applied to reduce the activation energy of chlorobenzene in aqueous solution and enhance its removal using nanoscale zero-valent iron (Fe ) or zero-valent copper (Cu ) particles as dielectric media. When Fe and Cu particles absorb microwave energy, the electrical potential difference causes the metal electrons to rotate faster, thus producing more heat. The microwave-irradiated metal particles reduced the chlorobenzene activation energy by 6.1 kJ/mol (13.3 kJ/mol versus 19.4 kJ/mol) for Fe and 5.4 kJ/mol (15.8 kJ/mol versus 21.4 kj/ mol) for Cu and enhanced the chlorobenzene removal 4.1 times (82.8% versus 20.4%) for Fe and 3.7 times (72.1% versus 19.5%) for C u . The Fe" has a higher standard reduction potential than C u : it is capable of removing more chlorobenzene than Cu (82.8% versus 72.1%). Using the microwave-induced nano-scale iron or copper particle is effective in treating toxic organic substances, as demonstrated in this study. Water Environ. Res., 82, 642 (2010). 0
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KEYWORDS: microwave, zero-valent iron, zero-valent copper, chlo robenzene, activation energy (Ea). doi: 10.2175/106143009X12529484816033
Introduction Recently, chemical reduction of hazardous compounds using zero-valent metals has been studied intensively (Choe et al., 2001). For example, zero-valent iron (ZVI) has been used successfully to treat various groundwater and soil contaminants, including a large variety of organic (e.g., halogenated organic solvents, azoaromatics, and nitroaromatics) and inorganic (e.g., arsenic and chromium) compounds (Lien and Zhang, 2007; Scherer et al., 1997; X u et al., 2006; X u , Zhou, He, and Hao, 2005; X u , Zhou, He, and Wang, .2005). The dechlorination of chlorinated organic solvents using Z V I generally is recognized as a surface-mediated reaction (Kim and Carraway, 2000; Schäfer et al., 2003; X u et al., 2005). The reaction involves the transfer of two electrons directly on the Z V I particle surface, as follows (Dombek et al., 2001):
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Department of Safety, Health and Environmental Engineering, National Kaohsiung First University of Science and Technology. Taiwan. 2
Department of Environmental Engineering, National Cheng Kung University, Taiwan. * Department of Safety. Health and Environmental Engineering, National Kaohsiung First University of Science and Technology, Taiwan; e-mail:
[email protected]. 642
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Therefore, in addition to the reactivity of individual chlorinated hydrocarbons, the source, quality, purity, and freshness of nanoscale Z V I (Fe ) also influence the rate of the chlorobenzene reduction reaction (Cheng and Wu, 2000; Lin and Lo, 2005). There are several disadvantages associated with the use of Z V I particles, as follows: 0
(1) Iron oxides are formed easily on the iron surface to reduce the reactivity, (2) High iron surface activity is difficult to maintain, and (3) Irons of different sources may have various activities (Lin et al., 2004; Zhang et al., 1998). Nano-scale copper particles have a longer shelf life and are more stable in aqueous solution than Z V I . thus avoiding some unwanted chemical reactions (Lin and Lo, 2005). Microwaves (MWs) are a form of electromagnetic radiation, with frequencies ranging between 300 M H z and 300 G H z (Appleton et al., 2005; Jones et al., 2002; Tai and Jou, 1999; Yuan et al., 2006). The principles underlying the characteristic heating by microwave radiation are dipole orientation (dipole rotation) and ionic conduction (Hidaka et al., 2007). When irradiated by microwaves, the microwaves absorbed by a solution will cause the polar molecules to rotate rapidly in the solution, to bring about a thermal effect, which will reduce the activation energy of the solution system and weaken the various chemical bonds (Zhang et al., 2007). Recently, microwave heating technology has attracted the interest of many researchers in various technological and scientific fields, because microwave radiation is known to enhance various reaction kinetics, as a result of its rapid and homogeneous heating (Menéndez et al., 2002; Park et al., 2000; Takashima et al., 2008). It has been used for disinfecting soil (Mavrogianopoulos et al., 2000); immobilizing heavy metals in contaminated soil, including chromium (Tai and Jou, 1999) and lead (Jou, 2006); and remediating polychlorinated biphenyl-contaminated sites (Goi et al., 2006). Microwaves also are combined with granular activated carbon or Z V I for treating pentachlorophenol (Jou, 2008; Jou and Wu, 2008) and improving the efficiency of the titanium dioxide (TiO2) photocatalyst (Jou et al., 2008), to achieve savings on energy consumption, while improving treatment efficiencies (Liu et al., 2004).
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The objectives of this study were to 0
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(1) Compare the reactivity of Fe and zero-valent copper (Cu ) toward a chlorobenzene aqueous solution, (2) Investigate the effect of temperature on the chlorobenzene reduction rate and further evaluate the activation energy. (3) Investigate the influence of microwave radiation on the chlorobenzene reduction rate and chlorobenzene activation energy, and (4) Examine the distribution of major reaction byproducts.
Materials and Methods Materials. A 100-mg/L chlorobenzene solution was prepared by dissolving 905 µL of 99.9% pure chlorobenzene (GR Reagent, O.P.S Co., Ltd.) in 99.9% methanol (GR Reagent, O.P.S Co., Ltd.) to form a 2000-mg/L stock solution. Approximately 200 µL of the stock solution then was diluted with deionized water (18.2 M Q , Millipore Co., Billerica, Massachusetts) to form the 100-mg/L chlorobenzene working solution. The F e °and Cu nano-particles were from the Conyuan Biochemical Technology Ltd. Company (Taiwan). The specific surface areas of the Fe and Cu nanoparticles were 55.8 and 44.8 m /g, respectively, as measured using a Brunauer-Emmett-Teller isotherm surface analyzer. 0
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Figure 1—A plot of ln(C/C ) versus time for the dechlorination of chlorobenzene by Fe (line with circles) and Cu (lines with diamonds) particles. O
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Thermo Fisher Scientific, Waltham, Massachusetts) using a potassium bromide (KBr) window was used to scan the sample 8 times between 400 and 4000 c m ; the resolution was 0.5. - 1
Batch Experiments. The study was initiated by placing 40 mL of 100-mg/L chlorobenzene solution in 40-mL boron-silica serum vials with Teflon-coated screwed caps, to eliminate headspace. After the addition of 1 g of Fe or C u , the sample vials were placed in a constant-temperature water bath that was mounted on a 50-rpm reciprocal shaker. The sample could be controlled constant at 25, 40, 50, and 60 C for carrying out the chlorobenzene decomposition tests at different reaction periods (i.e., 30, 60, 90, 120, 150, 180, and 240 minutes). 0
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In the chlorobenzene degradation study using microwave radiation, a modified household microwave oven (Sampo Co., Taiwan) operated at 2.45 G H z with a maximum power of 650 W was used for generating the microwave energy. A n 80-mL boronsilica glass column reactor (Seton Finite Co., Taiwan) (lowenergy-loss and heat-resistant [up to 700 C]) was used to hold the sample: it was installed in the microwave oven for carrying out the experiment. The top of the reactor end was connected to a vacuum gas-collecting bag (Ren-Dah Enterprise Co., Taiwan) for captur ing all organic substances that may escape with the tail gas during the reaction period. The microwave energy was set at 250 W to irradiate the prepared chlorobenzene sample according to the following operating conditions: microwave irradiation time = 20 seconds, microwave interruption irradiation time = 120 sec onds, total irradiation time = 300 seconds, and number of cycles = 15. A l l experiments were repeated at least 3 times; the average was used to calculate the chlorobenzene removal efficiency. Analysis Methods. A n HP 6890 gas chromatographer (GC) equipped with a capillary column (HP-5MS) and coupled with an HP 5973 mass selective detector (MSD) (Hewlett-Packard Asia Pacific Led., Singapore) was used for identifying and quantifying intermediates and final products. The carrying gas (helium) flowrate was maintained constant at 1.5 mL/min. The oven temperature was programmed to vary from 70 to 260 C at a ramp rate of 30 C/min: it was then held at 260 C for 5 minutes. Images produced by a scanning electron microscope (SEM) (S-2500, Hitachi, Japan) were used to observed the changes of surface structure, and Fourier transformation infrared (FTIR, Nicolet iS10, July 2010
Results and Discussion Chlorobenzene Degradation by Nano-Scale Zero-Valent Iron and Copper Particles. The degradation rate of chlorinated solvents in contact with highly pure Fe or Cu in a closed, wellmixed, and anaerobic batch system follows a pseudo-first-order rate model, with respect to the aqueous phase concentration, as described in eq 2 (Cheng and Wu, 2000; Su and Puis, 1999). Generally, the rate of degradation is proportional to the exposed iron or copper surface area. Therefore, the degradation rate, with respect to the quantity of Fe or Cu surface area, is described by eq 3, as follows (Liou et al., 2005): 0
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Where C = CO = k = k =
chlorobenzene concentration (mg/L), initial chlorobenzene concentration (mg/L), observed rate constant (h-1), surface area normalized reaction rate constant (L/h-m ), p = surface area concentration of Fe or Cu (m /L of solution), and t = time (hours). sa
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The linear relationship between the negative logarithmic value of C /C, or In (C /C), versus reaction time (t) is shown in Figure 1. The pseudo-first-order rate constants (k) are 1.0 X 10 ( m i n ) for Fe particles and 9.0 X 10 ( m i n ) for C u . The rate of degradation is proportional to the exposed iron or copper surface area. Therefore, the reaction rate constant, k, needs to be normalized, with respect to metal activity per unit surface area, based on the surface area and the mass concentration of iron and copper particles. For specific surface areas of 55.8 m /g (Fe ) and 44.8 m /g (Cu ), eq 2 is used to calculate values of the surface O
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area normalized reaction rate constants (k ). They are 7.2 X 10 (L/h·nT ) for Fe and 8.1 X 10 ( L / h · m ) for Cu", revealing that Fe has a slightly faster chlorobenzene degradation rate than Cu [1.0 X 10 versus 9.0 X 10 ( m i n ) ] . The results of using the statistical least squares to analyze the correlation between Fe and Cu degradation curves show that, for Fe , the R and standard deviation values are 0.98 and 5.5 X 1 0 , respectively, whereas the R and standard deviation values are 0.99 and 2.5 X 1 0 , respectively, for C u . sa
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In F e - H O systems, the reductive transformation by Fe is a surface-mediated process, which requires close contact between the substance and iron surface for electron transfer to take place (Noubactep, 2007). When the oxidation reaction to dechlorinate organic substances is proceeding in the aqueous solution Z V I provides electrons (Clark and Rao, 2003; L i n and Lo, 2005). Theoretically, a metal that has a stronger reduction potential is more favorable to the dechlorination reaction (Cheng and Wu, 2000). The Fe has a higher standard oxidation potential than Cu (Fe Fe E = +0.447 V versus C u C u E = -0.34 V ) ; the former releases electrons more easily than C u , to reduce the chlorinated organic compounds. Additionally, when the Fe serves as a donor of electrons; it oxidizes itself at room temperature (25°C) to form F e to provide 2 electrons. Afterwards, the oxidation reaction from F e to F e will only provide 1 electron. When Cu is oxidized to form C u , only 1 electron is produced. When absorbing the microwave energy, the extra electron produced by Fe will speed up its oxidation rate, and the electron movement is made faster to produce more friction heat. 2
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Figure 2 — A plot of In(ksa) versus 1/Tfor the estimation of activation energy: dechlorination of chlorobenzene by Fe and Cu particles (temperatures = 25, 40, 50, and 60°C). 0
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activation energy (kJ/mol), molar gas constant [8.314 JV(mol-K)], absolute temperature (K), and pre-exponential factor [ L / ( h m ) ] . .
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In addition, Fe reacts with water to produce hydrogen and hydroxide ions. Two hydrogen ions will combine to form hydrogen bubbles ( H + H H ) ; thus, they will not be involved effectively in the reduction reaction. Further, the hydrogen bubbles will accumulate on the metal surface to interfere with the reduction reaction (Cheng and Wu, 2000). Additionally, hydroxide ions raise the solution pH to enhance the formation of hydroxide on the iron surface, which inhibits the dechlorination reaction of the Z V I (Lavine et al., 2001). The Temperature Effect. The reaction rates obtained for various reaction temperatures are useful to examine the reaction mechanism. For heterogeneous reactions, the effect of temperature on reaction rates can be used to distinguish whether the ratelimiting step involves chemical reactions at the surface or the diffusion of a reactant (Lien and Zhang, 2007), because the slowest reaction step requires the greatest activation energy (Ea). Low Ea values typically indicate that the rate-limiting step is reactant diffusion or the diffusion-controlled processes, whereas higher Ea values show chemical-reaction-controlled processes (Su and Puis, 1999). For example, the activation energies for diffusion-controlled reactions in solution are between 10 and 20 kj/mol (Liou et al., 2005), and the chemical-reaction-controlled reactions have larger activation energies (>29 kJ/mol) (Lien and Zhang, 2007). The rates of chlorobenzene degradation with Fe particles at various temperatures can be fitted with the Arrhenius equation, as follows (Su and Puls, 1999): +
+
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Therefore, a plot of In k versus 1/T would result in a linear relationship with the slope equal to —Ea/R and the intercept equal to In A. sa
Figure 2 gives the Arrhenius plots of In k versus 1/7 for the chlorobenzene dechlorination, with both Fe" and C u " particles. The Arrhenius behavior is followed in the temperature range from 25 to 60°C, and the slope of the plots was assigned as a ratio of the activation energy to the ideal gas constant. Consequently, the activation energies of dechlorination depredated by Fe°and Cu° particles are approximately 19.4 and 21.4 kJ/mol, respectively. Additionally, the statistical least-squares method was used to sa
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Figure 3—Chlorobenzene removal efficiency for the Fe (solid line with circles), Cu (solid line with diamonds), and fluidized (dashed line with triangles) particles (irradiated with 250 W of microwave energy for 300 seconds). 0
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(a) Virgin Fe (Mag = 100 K X )
(b) Fe subject to 250 W microwave application for 300 sec (Mag = 100 KX)
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(c) Virgin Cu (Mag = 100 KX)
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(d) Cu subject to 250 W microwave application for 300 sec (Mag = 100 KX)
Figure 4—SEM images showing changes of surface structure for Fe and Cu before and after being used as the medium in the microwave treatment for chlorobenzene: (a) virgin Fe (magnification = 100 X 10 magnification), (b) Fe subject to 250 W of microwave energy for 300 seconds (magnification = 100 X 10 magnification), (c) virgin Cu (magnification = 100 x 10 magnification), and (d) Cu subject to 250 W of microwave application for 300 seconds (magnification = 100 X 10 magnification). 0
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evaluate the plot of In k versus 1/T curves for Fe and Cu particles. The results show that, for Fe , R = 0.98 and, for C u , R = 0.99. Effect of Microwave Radiation on Chlorobenzene Degradation. Microwave irradiation will induce ion migration and dipole rotation, causing the dielectric molecules in the solution to vibrate, thus raising the dielectric medium temperature (Appleton et al., 2005; K u et al., 2001; Venkatesh and Raghavan, 2004). The heat can change the thermodynamics characteristics further, reduce the activation energy of the reaction system, and weaken the chemical bond intensities of various molecules. Moreover, the existence of microwave-absorbent media can augment these effects (Zhang et al., 2007). sa
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On the other hand, in F e - H O systems, Fe" reacts with water to produce hydroxide ions and visible hydrogen gas. When the microwave radiation penetrates the solution to reach the surface of ZVI particles, it is absorbed by the latter. Hydrogen bubbles begin to appear on the iron particle surface, which resuspends settled iron particles in the solution, leading to the fluidization of iron particles. The fluidized iron particles effectively will provide more contact with the contaminant to enhance the energy absorption, reaction rate, and removal efficiency. When the sample was shaken well to fluidize all F e ° particles in the solution and then placed in the microwave oven for chlorobenzene degradation at 250 W of microwave irradiation for 300 seconds, Figure 3 shows 92.8% chlorobenzene removal efficiency and 11.4 kJ/mol 2
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Figure 6—Schematic showing the reactions at the surface of Fe or Cu particles. 0
Figure 5—The FTIR C O absorption of C O produced by F e and C u with peaks between 2290 and 2390 c m . 2
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activation energy for the fluidized particles, with 10.0% higher chlorobenzene removal and 6.1 kJ/mol lower activation energy. For Cu particles, the enhanced hydrogen bubble generation is not obvious, indicating that the microwave irradiation did not refluidize the deposited Cu particles. In addition, the activated energy was 13.3 kJ/mol for Fe and 15.8 kJ/mol for C u . The chlorobenzene removal efficiencies were 82.8% for Fe and 72.1 % for C u . Laboratory data also demonstrated that the microwave energy reduced the activation energy of the reaction system (the activation energy was decreased by 6.1 and 5.4 kJ/ mol) and enhanced the chlorobenzene removal 4.1 times (82.8% versus 20.4%) for Fe and 3.7 times (72.1% versus 19.5%) for C u . Additionally, the value of Ea for Fe and Cu was low enough for the oxidation of chlorobenzene to be considered as a typical mass transport-limiting reaction. The results of the statistical least-squares analyses show R and standard deviation values of 0.99 and 8.8 X 10 respectively, for suspended Fe , and 0.98 and 1.4 X 1 0 . respectively, for C u . Additionally, the Fe particles were fluidized in the solution and, on F e or Cu , compounds were formed on the particle surface. Figures 4a and 4b reveal that the appearance of Fe particles did not show much change; the microwave-irradiated Fe particles appear loose. The Cu particles show similar changes, as shown in Figures 4c and 4d. 0
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from the absorbed microwave inside the copper particle results in a rapid temperature rise, and interior heat is transmitted easily to the copper particle surface. Hence, local hotspots are more likely to occur on the copper particle surface, which favors chloroben zene mineralization to produce CO . On the other hand, the chloride concentration in the original solution was 2.9 mg/L (2.9 ppm); it increased to 7.7 mg/L (7.7 ppm) for Fe and 6.3 mg/L (6.3 ppm) for Cu after the microwave treatment. Hence, a suggested pathway for chlorobenzene degra dation in the presence of microwaves by Fe or Cu is shown in Figure 6. The proposed mechanism pathway is as follows: 2
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When a microwave penetrates the solution to reach the surface of Fe or Cu particles, it increases the metal oxidation by creating more active sites on the surface to react with chlorobenzene. However, the non-uniform absorption of microwave energy by Fe or Cu particles may produce local hotspots to convert chlorobenzene into CO . 0
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Reaction Mechanisms. In microwave treatment, the tail gas was collected and then analyzed qualitatively using G C / M S D . The results also confirm that benzene was the major end product. Additional analyses using the FTIR show that strong absorption peaks appeared between 2320 and 2380 c m , and other weak absorption peaks appeared at 665 to 670, 3598 to 3630, and 3703 to 3730 cm . These results strongly suggest that the species identified is carbon dioxide (CO ). The non-uniform absorption of microwave energy by Fe and Cu particles may produce localized hotspots, with a higher temperature than at other locations. Hence, chemical reactions occur more easily, and the high-temperature oxidation causes mineralization of benzene to form CO . Figure 5 reveals that copper has stronger absorption peaks than iron between 2320 and 2380 c m , meaning that more CO is produced with copper than iron, because copper has a lower heat capacity [0.473 versus 0.385 J/(g·°C)] and higher heat conduc tivity than iron [0.4 versus 0.072 W/(mm-C)]. The heat generated - 1
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Conclusions In this study, laboratory data demonstrate that microwave irradiation integrated to Fe or Cu microwave absorbents can enhance the removal of chlorobenzene and reduce the activation energy of the reaction system. In the presence of a 250-W microwave for 300 seconds, the results show that better chlorobenzene removal can be obtained with microwave irradi ation. The microwave-induced Fe and Cu particles results show that the activation energy was decreased by 6.1 kJ/mol (13.3 kJ/ mol versus 19.4 kJ/mol for Fe ) and 5.4 kJ/mol (15.8 kJ/mol versus 21.4 kJ/mol for Cu ), and the chlorobenzene removal was enhanced 4.1 times (82.8% versus 20.4%) for Fe and 3.7 times (72.1% versus 19.5%) for Cu . Additionally, the activation energy levels for Fe and Cu are low enough for the reaction to be a typical mass-transport-limiting reaction. Because the degradation of chlorinated organic solvents by Z V I is a surface-mediated reaction, when microwave radiation penetrates the chlorobenzene solution to reach the Fe or Cu particles, the surface activity and area increase and iron oxidizes, thus reducing the activation energy of the reaction system and enhancing the chlorobenzene removal. 0
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Lee el al. Submitted for publication June 15, 2009; revised manuscript submitted December 28, 2009; accepted for publication January 4, 2010.
Liou, Y . H . ; L o . S. L . : L i n , C. J.; Kuan, W . H . ; Weng, S. C . (2005) Chemical Reduction of an Unbuffered Nitrate Solution Using Catalyzed and Uncatalyzed Nanoscale Iron Particles. J. Hazard. Mater., B127, 102-110.
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TITLE: Enhanced Degradation of Chlorobenzene in Aqueous Solution Using Microwave-Induced Zero-Valent Iron and Copper Particles SOURCE: Water Environ Res 82 no7 Jl 2010 The magazine publisher is the copyright holder of this article and it is reproduced with permission. Further reproduction of this article in violation of the copyright is prohibited.
