Effect of Bulk Temperature and Frequency on the Sonolytic

Mar 28, 2011 - In this case, the degradation of 1,4-dioxane was least effective at both frequencies, clearly indicating that the increase in the solut...
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Effect of Bulk Temperature and Frequency on the Sonolytic Degradation of 1,4-Dioxane with Fe0 Hyun-Seok Son,# Sung-Keun Kim,† Jong-Kwon Im,† Jeehyeong Khim,‡ and Kyung-Duk Zoh*,† #

Department of Applied Chemistry, Konkuk University, Chungju 380-702, Korea Department of Environmental Health, School of Public Health, Seoul National University, Seoul, 151-742, Korea ‡ School of Civil, Environmental and Architectural Engineering, Korea University, Seoul, 136-701, Korea †

ABSTRACT: The sonolysis of 1,4-dioxane in the absence and the presence of zero-valent iron (Fe0) was investigated by altering bulk temperature and ultrasonic frequency conditions. In the absence of Fe0, the sonolytic degradation efficiency of 1,4-dioxane was higher at 170 kHz as compared to that at 35 kHz. As bulk temperature increased, sononlytic degradation efficiency of 1,4-dioxane decreased at both frequencies; however, the rate decrease by the increase in temperature was smaller at 170 kHz than at 35 kHz. Adding methanol as a radical scavenger remarkably decreased the sonolytic degradation rate at 170 kHz within 60 min in all bulk temperatures. In contrast, the decrease in the degradation rate by adding methanol at 35 kHz was observed after 60 min only at 10 °C, but not at 20 and 30 °C. These results indicate that both bulk temperature and sonication frequency should be considered to optimize the sonolytic degradation. The degradation efficiency was significantly enhanced in the presence of Fe0 especially at 170 kHz, indicating that Fe0 can facilitate hydroxyl radical production by Fenton-type reaction mechanism during sonication.

’ INTRODUCTION 1,4-Dioxane (C4H8O2), an emerging water contaminant, is currently used as a solvent in manufacturing processes and is found in fumigants, automotive coolant, cosmetics, and personal care products such as deodorants, shampoos, toothpastes, and mouthwashes.1 The U.S. EPA and the International Agency for Research on Cancer (IARC) have classified 1,4-dioxane as a probable human carcinogen.2 Physical methods such as adsorption and air-stripping for removing 1,4-dioxane from water are ineffective because of its higher water solubility (4.31  105 mg L1).3,4 1,4-Dioxane is also resistant to microbial degradation due to its resistant heterocyclic structure.5 Sonolytic irradiation method recently has received increasing attention for the destruction of organic pollutants in waters and wastewaters.6,7 Sonolytic degradation in aqueous phase involves several reaction pathways and zones such as pyrolysis inside the bubble and/or at the bubbleliquid interface, and hydroxyl radical-mediated reactions at the bubbleliquid interface and/ or in the liquid bulk.8 The formation, growth, and collapse of cavitation bubbles drive sonochemical reactions in aqueous solutions. The collapse can occur inside a cavitation bubble and at the bubblewater interface.9,10 The temperature inside the bubble and at the bubblewater interface can reach 5200 and 1900 K, respectively,11 and H2O molecules under these extreme conditions are thermally disintegrated to hydrogen and hydroxyl radicals (H 3 and OH 3 ).12 The cavitation in the sonication process can be altered by the bulk temperature and ultrasound frequency.13 In particular, the increase of the bulk temperature in the solution can lower the threshold of bubble production, increase vapor pressure, and decrease surface tension, which can weaken bubble intensity.14 Jiang et al.15 recently reported that, while the degradation efficiency of 4-chlorophenol decreased with increased bulk r 2011 American Chemical Society

temperature at lower frequency (20 kHz), it increased at higher frequency condition (500 kHz) until 40 °C. This result indicates that the effect of the bulk temperature is dependent on the inherent nature of the bubble such as whether it is transient or stable, formed from sonication.8 Other recent studies have shown that the addition of oxidants such as Fe2þ and Fe0 can increase OH radical generation during sonolytic degradation, thereby increasing the degradation rate of 1,4-dioxane.16,17 In this study, the effects of the bulk temperature and sonication frequency on the sonolytic degradation of 1,4-dioxane were examined to try and understand the sonolytic reaction mechanism. The possible mechanism for generating OH radical in the sonolysis of 1,4-dioxane in the presence of Fe0 was also investigated.

’ EXPERIMENTAL SECTION Materials. A 1,4-dioxane solution was prepared by diluting a 1000 mg L1 stock solution, which was prepared by dissolving reagent-grade 1,4-dioxane (ACS grade; Aldrich, Milwaukee, WI) in Nanopure deionization water (R = 18 MΩ cm1; Barnstead Co., Ltd., Dubuque, IA). Iron powder (325 mesh; Alfa Aesar, Ward Hill, MA), methanol (HPLC grade; J.T. Baker, Sugar Land, TX), and methylene chloride (HPLC grade; J.T. Baker) were used as received. Experimental Setup. We used a bath-type sonicator (Flexonic-1000; Mireasonication, Seoul, Korea) consisting of a reactor made of Pyrex, a generator, a transducer, and a temperature controller. Figure 1 shows the scheme of the sonication Received: September 3, 2010 Accepted: March 2, 2011 Revised: January 13, 2011 Published: March 28, 2011 5394

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Figure 1. Scheme of sonication reactor: (a) Oscillator (35/72/100/172 kHz), (b) sonicator (bath type), (c) reactor, and (d) chiller.

system with temperature control system. Sonolysis of 1,4-dioxane at fixed bulk temperatures (10, 20, and 30 °C) was performed with a chiller for controlling the solution temperature at frequencies of 170 and 35 kHz. The power indicating the amplitude of sonic wave can affect the cavitation in the sonolysis reaction. In the study, ultrasonic power, which is defined as the electrical energy that is being delivered to the acoustic transducer, was set to 15.4 ( 1.2 W at both 170 and 35 kHz to remove the effect induced by the power difference.18 The experiment without temperature control was also performed without a chiller. The reactor (1 L) was closed by tightening the lid of the reactor using a clamp during the reaction. Blank experiments were performed with 1,4-dioxane only in the solution without stirring, sonication, and adding Fe0. The maximum loss of 1,4-dioxane in the blank condition was only 0.67%, indicating that there is no volatilization loss. During sonolysis reaction in the presence of Fe0, the continuous stirring by agitator (70 ( 5 rpm) equipped in the reactor was conducted to guarantee the homogeneous suspension and dispersion iron in the solution at both frequencies. Because of the moderate stirring, the difference in the degradation efficiency of 1,4dioxane with or without stirring in sonication-only reaction was minimal (data now shown), indicating that the stirring during sonolysis in the presence of Fe0 did not disturb the cavitation in the sonolysis. Argon (Ar, 99.999%) was added to the solution to minimize the effect of oxygen included in air. The solution was injected with argon gas continuously during reaction. To qualitatively estimate the production of OH radical during sonication, 49 mM of methanol (MeOH), a radical scavenger, was added to the solution at the start of the sonolytic reaction.19 The initial pH of the 1,4-dioxane solution was around 5.6 in the absence and presence of Fe0, respectively. The pH was not controlled in any of the experiments. All experiments in the study were performed in triplicate. Analysis. Before analysis, an aqueous sample of 1,4-dioxane was extracted by liquidliquid extraction using a methylene chloride solvent (sample: solvent = 20:1, v/v), and the mixture was centrifuged at 3000 rpm for 2 h. The extract was then analyzed using a gas chromatograph (HP 6890; Hewlett-Packard, Palo Alto, CA) equipped with a HP-5MS 5% phenyl methyl siloxane capillary column (30 m  0.25 mm  0.25 μm) and a mass selective detector (HP 5973) under the following conditions: split ratio, 10:1; carrier gas (He, 99.999%), 1.2 mL min1; electron ionization, 71 eV; and mass scan range, 35250 m z1. The temperature was held at 60 °C for 5 min and then increased to 250 °C at a rate of 10 °C min1, followed by a 3 min hold at 250 °C for the oven and 150 °C for the inlet. The injection

Figure 2. Effect of the bulk temperature on the sonolytic degradation of 1,4-dioxane (1,4-D) at (a) 170 kHz and (b) 35 kHz.

volume into GC was 10 μL, the calibration range was 0.11 μg L1, and the limit of detection (LOD) and limit of quantification (LOQ) were 0.0298 and 0.0903 μg L1, respectively. The solution pH was measured using a pH analyzer (model 52A, Orion, Reno, NV). To estimate the degree of Fe0 oxidation, the concentration of ferrous ion (Fe2þ) in aqueous solution was determined with a UVvis spectrophotometer (BioMate 3; Thermo Spectronic, Rochester, NY) using a phenanthroline method.19 To measure the concentration of Fe2þ, 2 mL of HCl was mixed with the sample, followed by the addition of 10 mL of ammonium acetate buffer and 4 mL of 1,10-phenanthroline monohydrate (5 mM), for a final volume of 100 mL; after mixing, the absorbance of the sample was analyzed at 510 nm.

’ RESULTS AND DISCUSSION Effect of the Bulk Temperature on the Sonication-Only Reaction. Figure 2 shows the effect of the bulk temperature on

the degradation of 1,4-dioxane by sonolysis under two different 5395

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Table 1. Change of Pseudo-First-Order Rate Constants (kobs) According to the Reaction Time in the Sonolysis of 1,4-Dioxane in the Absence or Presence of Methanol rate constant (kobs  103 min) 10 °C frequency 170 kHz

35 kHz

a

time

20 °C

30 °C

no MeOH

with MeOH

no MeOH

with MeOH

no MeOH

with MeOH

overall time

2.58

2.35

2.49

2.08

1.49

1.16

initial timeb

2.6

2.13

1.94

1.7

1.26

1.32

extend timec overall timea

2.99 0.91

2.12 0.59

2.92 0.49

1.69 0.49

1.69 0.30

0.61 0.30

initial timeb

0.98

0.98

0.34

0.34

0.24

0.24

extend timec

0.82

0.38

0.61

0.61

0.32

0.31

a

Overall time: 0180 min. b Initial time: 060 min. c Extend time: 60180 min.

sonication frequencies in the presence of temperature control. As the bulk temperature of the aqueous solution increased, the degradation efficiency of 1,4-dioxane decreased at both frequencies. Figure 2 also shows that, regardless of the bulk temperature, the degradation efficiency of 1,4-dioxane was higher at 170 kHz than at 35 kHz. The cavitation procedure includes the production of cavity through the repetition of extension and compression of ultrasonic wave, and the collapse of grown cavity.15 At lower frequencies, the cycle of its extension and compression is slower, resulting in the lower intensity and larger size of cavity than at higher frequencies. Also, higher reaction temperature can increase the frequency of the collision among cavities. Therefore, the production degree of OH radical generated by the sonolysis of H2O through the cavitaiton procedure can be reduced at lower frequency and higher temperature. Figure 2 also shows the degradation of 1,4-dioxane by sonolysis at 170 and 35 kHz in the condition without solution temperature control (no temperature control). In the absence of temperature control, the solution temperature increased from 24 °C at the start of the reaction to 69 and 56 °C after 60 min of the reaction at 170 and 35 kHz, respectively. In this case, the degradation of 1,4-dioxane was least effective at both frequencies, clearly indicating that the increase in the solution temperature decreased the degradation of 1,4-dioxane. The sonolytic degradation of 1,4-dioxane was fitted to the pseudo-first-order rate model based on the data in Figure 2 (Table 1). Interestingly, while the difference in the pseudo-firstorder rate (kobs) of 1,4-dioxane at 170 kHz between 10 °C (2.58  103 min1) and 20 °C (2.49  103 min1) was insignificant, the degradation rate at 35 kHz was enhanced significantly from 0.49  103 to 0.91  103 min1 with the decrease in temperature from 20 to 10 °C, respectively (Table 1). At low frequency (35 kHz), due to the large number of cavitation bubbles formed, it is expected that an increase in temperature will lead to an increase in the possibility of coalescence among the bubbles, resulting in some of the bubbles losing their activity.15 Therefore, the rate of 1,4-dioxane degradation decreases by the increase of the temperature. However, the rate decrease of 1,4-dioxane degradation was lower at 170 than at 35 kHz. This can be explained as the enhancement in the generation rate of OH and HO2 radicals at higher frequency (170 kHz). The cavitation bubbles formed by sonolysis have a more gaseous (stable) nature at higher frequency.15,20,21 The increase of aqueous temperature certainly

increases the number of cavitation bubbles on sonolysis and thus the rate of production of radicals (OH and HO2 radicals), although it results in a lowering of the cavitation threshold.15 This result implicates that the effect of increasing bulk temperature is dependent on the sonication frequency. On the basis of the Arrhenius equation (k = AeEa/RTmax), Suslick et al.22 derived a simple expression (eq 1) for the dependency of sonochemical degradation rates (k) on the bulk temperature of sonicated media T0: ln k ¼ ln A 

Ea Pν RT0 Pa ðγ  1Þ

ð1Þ

where γ is the ratio of specific heats, Pa is the acoustic pressure at the beginning of collapse, and Pν is the vapor pressure of solvent at T0. As per eq 1, lower degradation efficiencies were observed at elevated temperatures, which result in the increase of Pν. Sonolysis of 1,4-Dioxane with Fe0. Figure 3a shows that the sonolytic efficiency at 170 kHz in the presence of Fe0 increased with an increase of bulk temperature from 10 to 20 °C, and then decreased at 30 °C. However, in case of 35 kHz, as compared to Figures 3b and 2b, the sonolytic efficiency increased slightly (5%) in all bulk temperatures with the addition of Fe0. It was recently reported that many pollutants such as chlorinated compounds, nitrate, phosphate, and chloride were reduced by the oxidation of Fe0 by electrostatic interaction and adsorption.23 The reductive degradation of chlorinated compound and nitrobenzene by the oxidation of Fe0 was also reported.24,25 However, even though the oxidation reaction of Fe0 to Fe2þ is thermodynamically spontaneous, the higher activation energy for the oxidation of Fe0 to Fe2þ makes the reaction rate very slow.26 Therefore, the removal of pollutants in Fe0-only reaction should be accomplished in the condition with providing external energy such as sonication energy. In the sonolysis with Fe0, the following synergic effects are expected: (1) improvement of heterogenic characteristics of bubble, (2) enhancement of OH radical generation by Fentontype reaction with H2O2 produced by OH radical in the sonolysis, and (3) increase in the active sites for the cavitation of Fe0 surface and the increase in the movement of 1,4-dioxane into the active sites. The pitting and cracking of the active site on Fe0 surface is caused by ultrasound. The cavities on the Fe0 formed by ultrasound will enhance the stability of the bubble and generate the turbulence on imploding the bubble, resulting in the increase in the abatement rate of target compound.27,28 Therefore, the 5396

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Figure 4. The production of Fe2þ with and without ultrasonication (US) (sonication condition: 170 kHz, [Fe0]initial = 1 g L1, bulk temperature = 20 °C).

Many researchers also agree that H2O2 is generated by the recombination of OH radicals in the interface of bubble and bulk solution during sonolysis as shown in eq 3.27,28 Therefore, Fenton-like reaction can be induced by the reaction between Fe2þ and H2O2 in the combined process of sonication with Fe0 (eq 4).

Figure 3. Degradation efficiency of 1,4-dioxane (1,4-D) in the sonolytic reaction with Fe0 at (a) 170 kHz and (b) 35 kHz ([Fe0] = 0.5 g L1).

synergy effect in the combined reaction of sonication with Fe0 can be observed by the increase of the production of OH radical, and the mass transfer onto the active site of Fe0, which promote both the chemical and the heterogeneous reactions between solid phase (Fe0) and 1,4-dioxane. Fenton-type Reaction Induced in the Combined Reaction of Sonication and Fe0. Furthermore, Fe0 can be oxidized by the radicals such as OH radical produced by the sonolysis of H2O (eq 2). Fe0 þ 2OH 3 f Fe2þ þ 2OH

ð2Þ

Figure 4 shows the result of production of Fe2þ from Fe0 in the absence or presence of sonication. This result shows that the oxidation of Fe0 to Fe2þ was significantly enhanced in the presence of sonication. The oxidation of Fe0 to Fe2þ by ultrasound is very limited because the activation energy for the oxidation of Fe0 into Fe2þ is quite high (129.7 kJ mol1).29 Therefore, as shown in Figure 4, the rapid increase of the oxidation of Fe0 is mostly achieved by OH radical (eq 2).

OH 3 þ OH 3 f H2 O2

ð3Þ

Fe2þ þ H2 O2 f Fe3þ þ OH þ OH 3

ð4Þ

Figure 5 shows the effect of Fe0 concentration on the sonolytic degradation rate of 1,4-dioxane at 170 kHz. The degradation rate (kobs) of 1,4-dioxane increased proportionally with the increase in the concentration of Fe0, indicating that the degradation of 1,4-dioxane is closely related to the oxidation of Fe0 during sonication. 1,4-Dioxane is known to be primarily transformed at the bubblewater interface because the materials inside the bubbles are dissolved gases, water molecules, and volatile compounds.17,29 The increase of the degradation rate in the presence of Fe0 indicates that the degradation of 1,4-dioxane occurs at the bubblesolution interface where OH radical is released from collapsing bubble and generated by Fenton-type reaction. Furthermore, the heterogeneous materials such as Fe0 in the sonolysis can act as a site for the cavitation as well as induce the asymmetrical collapse of the bubbles.27 In fact, Sunartio et al.31 reported that the asymmetrical collapse of the bubbles in the sonolysis is likely to be a dominant pathway to create the cavitation. If the surface area of Fe0 increases by the ultrasonic cleaning effect,27,28 the active sites for cavitation will increase by pitting and cracking of the metal surface on collapsing bubble. This phenomenon can facilitate the transportation of 1,4-dioxane between bubble and bulk solution, thus resulting in the increase of 1,4-dioxane degradation. Effect of Adding Methanol on the Sonolytic Degradation of 1,4-Dioxane. Table 1 shows the effect of adding methanol on the sonolysis of 1,4-dioxane at different frequencies. At 170 kHz, 5397

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Figure 5. Effect of Fe0 concentration on the reaction rate in the combined reaction of sonication with Fe0 at bulk temperature of 10, 20, and 30 °C (reaction condition: 170 kHz).

the degradation rate decreased in the presence of methanol in all bulk temperatures. At 35 kHz, the inhibition effect of methanol was only observed at 10 °C only, but not at 20 and 30 °C (Table 1), indicating that the production of OH radical at lower frequency (35 kHz) is not effective at higher bulk temperature. The bubbles formed in sonication at 35 kHz are unstable (transient) as compared to those formed at 170 kHz. The temperature increase in bulk solution increases the possibility of the coalescence between the bubbles, resulting in the increase of the resonance size of bubble. Additionally, the collision between bubbles will increase by the increase of bulk temperature.15 As a consequence, transient bubbles (35 kHz) were more easily collapsed than stable bubbles (170 kHz). Riesz et al.12 reported that radicals such as OH radicals can be produced inside and at the bubblewater interface of stable cavitation bubbles. The generation of radicals in the sonication may be closely related to the characteristics of cavitation bubbles determined by the ultrasonic frequency. Because of the transient characteristics of bubbles at 35 kHz, the possibility for the recombination between OH and H radical increases in the longer expansion period of bubble at lower frequency (35 kHz).9 This implicates that the transportation to the interface of bubble solution of OH radical will become difficult at lower ultrasonic frequency(35 kHz). Therefore, the oxidation degree of 1,4dioxane by OH radical in the interface can be enhanced under conditions of higher frequencies. On the basis of the result shown in Table 1, it can be concluded that 1,4-dioxane was mainly decomposed by OH radical, which agreed with the result of our previous study.17 Previous studies reported that the addition of alcohol in sonication with low frequency can prevent the coalescence of bubbles by adsorption into the surface of bubble, resulting in an increase in the number of active bubbles.30,32,33 However, this enhancement by methanol addition is difficult to achieve due to low concentration of methanol (49 mM) used in this study. In this concentration, methanol cannot have the significant coalescence effect on bubbles due to the hydrophilic property into water. In addition, the adsorption of methanol into the surface of bubble can inhibit

Figure 6. The changes in the pseudo-first-order rate constants (kobs) in the sonolytic degradation of 1,4-dioxane in the presence of Fe0 with and without methanol at (a) 170 kHz and (b) 35 kHz.

the oxidation of 1,4-dioxane by OH radical released from bubbles due to the scavenging effect by methanol, which has high secondorder rate constant with OH radical (9.7  108 s1 M1).34 Therefore, on the basis of the result shown in Table 1 indicating that 1,4-dioxane is mainly decomposed by OH radical, the decrease of 1,4-dioxane degradation by the addition of methanol may occurs by the scavenging effect of OH radical by the volatile compound. Interestingly, the difference in the degradation rate in the presence or absence of methanol increased after 60 min (post time) of reaction time (Table 1). In sonication with methanol, while the decrease in the rate constant at 35 kHz was only observed after 60 min at 10 °C, at 170 kHz it was the highest at 20 °C. It means that the temperature effect for the production of OH radical during sonication is dependent on the sonic frequency. Figure 6 along with Table 1 show the effect of methanol on the sonolytic degradation rate of 1,4-dioxane in the combined reaction of sonication with Fe0. At 170 kHz, the highest degradation rate of 1,4-dioxane was observed at 10 °C in the 5398

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Table 2. Possible Reaction Mechanism in the Sonication Reaction in the Presence of Fe0 within bubble

interface between bubble and bulk solution

(1) H2O(l) f H2O(g)

(10) Fe0 f Fe2þ •

(2) O2 f O þ O

(8) 2OH f H2O2

(3) H2O f OH• þ H•

(9)a 2HO2• f H2O2 þ O2

a

surface of Fe0

(4) OH• þ H• f H2O

V

(5)a O2 þ H• f HO2•

(11) Fe2þ þ H2O2 f OH• þ OH þ Fe3þ

(6)a OH• þ O f HO2• (7) H• þ H2O f H2 þ OH• a

HO2 radical induced by O2 (reactions 2, 5, 6, and 9) was not expected in this study due to the experiment condition with argon purging.

OH radicals can be generated by the decomposition of water vapor molecules in the bubbles from sonication reaction (reaction 3). However, HO2 radical induced by O2 (reactions 2, 5, 6, and 9) cannot be produced in this study due to argon purging of the solution. H2O2 only can be generated by only the recombination of OH radicals by the decomposition of water vapor molecules in the bubbles (reaction 8). Therefore, Fentonlike reaction may occur in sonication in the presence of Fe0 by reaction 11 in Table 2. Figure 7 shows that the pH of the solution decreased in sonication-only, but slightly increased in Fe0-only reaction. As is also shown in Figure 7, the pH in the combined reaction of sonication with Fe0 clearly increased from 5.6 to 6.5, indicating the production of hydroxide ion (OH) in the combined reaction as shown in eq 4. This result can justify that the enhancement of 1,4-dioxane degradation in the combined reaction is mainly achieved due to the production of OH radical as in Fenton reaction.

Figure 7. The change of pH during sonication-only, Fe0-only, and the combined reaction (sonication condition: 170 kHz, [Fe0]initial = 1 g L1, bulk temperature = 20 °C).

sonication-only reaction (Table 1), but at 20 °C in the combined reaction of sonication with Fe0. As shown in Figure 6, the OH radical scavenging effect by methanol was more significant in the combined reaction than in the sonication-only reaction, indicating that OH radical is more effectively produced in the combined reaction. Because 1,4-dioxane (bp 101 °C) is the semivolatile compound, the compound will be effectively decomposed within a bubble at higher temperature. The temperature effect is available in the stable bubble condition, which is induced at higher frequency.9,15,23 This result implicates that the bubble is more stable (gaseous) in the combined reaction of sonication with Fe0 than in the sonication-only reaction. As was also shown in Table 1, OH radical in the sonication at 35 kHz was not effectively produced except at 10 °C. However, the slight decrease in the degradation rate by methanol addition at every temperature was observed in the combined reaction at 35 kHz, as shown in Figure 6b. This result indicates that the generation of OH radical in the combined reaction is enhanced as compared to the sonication-only reaction due to the increase of bubble activity and/or the supplementary production of radicals caused by Fenton-like reaction.17 Possible Sonication Mechanism in the Presence of Fe0. Table 2 shows the possible reactions in the sonication based on the results of this and other studies.15,23,31,35 As shown in Table 2,

’ CONCLUSIONS This study examined the effect of bulk temperature and sonication frequency on the sonication in the absence and presence of Fe0 for the degradation of 1,4-dioxane. The increase in bulk temperature decreased the sonolytic rate of 1,4-dioxane at both 170 kHz (higher frequency) and 35 kHz (lower frequency); however, the difference was less prominent at 170 kHz as compared to at 35 kHz. The production of OH radical increased more at 170 kHz than at 35 kHz. The addition of Fe0 in the sonication enhances the degradation efficiency of 1,4-dioxane by facilitating the release of OH radical produced by bubble collapsing as well as increasing the generation of H2O2, inducing Fenton-like reaction. On the basis of the results, it was found that the combined reaction of sonication with Fe0 will be more suitable technology to abate the recalcitrant pollutants as compared to the sonication-only process. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ82-2-880-2737. Fax: þ82-2-762-2888. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was partly supported by the Ministry of Environment of the Republic of Korea (Project No. 071-071-116) and the Korea Science and Engineering Foundation (Project No. R01-2007-000-20886-0). 5399

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