Ultrasonics Sonochemistry 17 (2010) 78–83

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Ultrasonics Sonochemistry

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Sono-enhanced degradation of dye pollutants with the use of H2O2 activatedby Fe3O4 magnetic nanoparticles as peroxidase mimetic

Nan Wang, Lihua Zhu *, Mingqiong Wang, Dali Wang, Heqing Tang *

College of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 March 2009Received in revised form 17 June 2009Accepted 25 June 2009Available online 28 June 2009

Keywords:Synergistic effectUltrasonicCatalysisRhodamine BHydrogen peroxide

1350-4177/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.ultsonch.2009.06.014

* Corresponding authors. Tel.: +81 27 87543432; faE-mail addresses: lhzhu63@yahoo.com.cn (L. Zh

(H. Tang).

Sono-enhanced degradation of a dye pollutant Rhodamine B (RhB) was investigated by using H2O2 as agreen oxidant and Fe3O4 magnetic nanoparticles (MNPs) as a peroxidase mimetic. It was found thatFe3O4 MNPs could catalyze the break of H2O2 to remove RhB in a wide pH range from 3.0 to 9.0 andits peroxidase-like activity was significantly enhanced by the ultrasound irradiation. At pH 5.0 and tem-perature 55 �C, the ultrasound-assisted H2O2–Fe3O4 catalysis removed about 95% of RhB (0.02 mmol L�1)in 15 min with a apparent rate constant of 0.15 min�1 for the degradation of RhB, being 6.5 and 37.6 foldsof that in the simple catalytic H2O2–Fe3O4 system, and the simple ultrasonic US-H2O2 systems, respec-tively. The beneficial synergistic behavior between Fe3O4 catalysis and ultrasonic was demonstrated tobe dependent on Fe3O4 dosage, H2O2 concentration, pH value and temperature. As a tentative explana-tion, the observed significant synergistic effects was attributed to the positive interaction between cav-itation effect accelerating the catalytic breakdown of H2O2 over Fe3O4 nanoparticles, and the function ofFe3O4 MNPs providing more nucleation sites for the cavitation inception.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Advanced oxidation processes (AOPs) have attracted increasingattention in the treatment of hazardous organic pollutants, be-cause these processes can generate strong oxidizing oxygen spe-cies such as �OH radicals. As a green oxidant, H2O2, acting as aresource of reactive oxygen species, is very important in the oxida-tive degradation of organic pollutants. However, because H2O2

decomposes fairly slowly at room temperature, it is necessary toenhance the generation of reactive oxygen species for effectivelyremoving organic pollutants in the H2O2 systems.

As one of AOPs, sonochemical technology is widely used for thedestruction of non-biodegradable pollutants, in which organic pol-lutants are degraded by sonochemically generated hydroxyl radi-cals and/or pyrolysis near cavitation bubbles [1]. A lot of organicpollutants have been tried to be degraded with a sonochemicalprocess, but ultrasonic (US) processes are limited by a low degra-dation rate in most cases, especially for the removal of non-volatiledyes, due to these refractory hydrophilic molecules can hardly ap-proach the gas–liquid interface [2]. It is estimated that the degra-dation rate of organic pollutants in a conventional sonochemicalprocess should be promoted by 10–100 times for its applicationin the practical treatment of industrial wastewaters [3]. Some

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approaches have been explored for this purpose. For example, anaddition of a small amount of carbon tetrachloride is found to in-crease the ultrasonic decolorization of methyl orange by about100 times [4]. The presence of H2O2 is observed to result in a highersonolysis removal of phenolics by a factor of 20–30% [5]. However,this increase is very slight, and the simple US-H2O2 process cannotbe used in the practical treatment of refractory organic pollutants,due to its limited degradation rate.

The introduction of catalysts is another way to drive the decom-position of H2O2 for the oxidation of organic contaminants. In thewell-known Fenton process, H2O2 is activated by Fe(II)/Fe(III) toproduce strong oxidizing �OH radicals. When the Fenton processis used to treat organic pollutants, it may have the drawbacks ofa narrow pH range for working, and Fe sludge disposal and/orregeneration [6]. Being independent of the presence of organic sub-strates, moreover, Fe(II) ions can catalyze the decomposition ofH2O2 even in the dark, resulting in low efficiency for the utilizationof H2O2 [7]. Thus, various Fenton-like processes are developed byimmobilizing Fe(II)/Fe(III) on resin [7], coordinating Fe(II)/Fe(III)with organic ligands [8], and developing iron (hydro)oxides [9].Another group of catalysts for the activation of H2O2 is peroxidaseenzymes. Several commercial peroxidases such as horseradish per-oxidase (HRP) are used to remove certain phenolic compounds anddyes from aqueous solutions [10,11]. Bhunia reported that HRPshowed the substrate specificity toward different dyes and all thetested dyes had very slow degradation rates compared to phenols[11]. This, along with the instability and high cost, limits the poten-

N. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 78–83 79

tial application of peroxidase enzymes in the treatment of waste-waters, especially the dye effluents [12]. Recently, Gao et al. re-ported that Fe3O4 magnetic nanoparticles (MNPs) exhibit anenzyme mimetic activity and developed an immunosorbent assayfor the detection of mouse IgG and carcinoembryonic antigen byusing Fe3O4 MNPs as a replacement of HRP [13,14]. The peroxi-dase-like nature of Fe3O4 MNPs allowed the development of spec-trophotometric methods for the H2O2 detection by using Fe3O4

MNPs as catalyst [15,16]. Zhang et al. used H2O2–Fe3O4 MNPs todegrade phenol in a tube with 125 lL phenol solution, and foundthat the removal of phenol achieved about 85% within 3 h [17].In our laboratory, it was confirmed that some organic pollutantscould be partially degraded in the H2O2–Fe3O4 MNPs system, butthe degradation rate was very slow. This hints that the H2O2-acti-vation ability of Fe3O4 MNPs is not so strong for the application inthe treatment of hazardous organic pollutants, and it is necessaryto increase the H2O2-activation ability of Fe3O4 MNPs.

As mentioned above, the degradation of organic pollutants inboth the US-H2O2 and H2O2–Fe3O4 MNPs systems is required tobe promoted for a practical treatment of organic pollutants. Whenthe two systems are combined into an integrated one, it is ex-pected that a synergistic effect between the sonochemical and cat-alytic reactions may markedly accelerate the degradation oforganic pollutants, because ultrasonic irradiation generates bene-fits in heterogeneous catalytic systems by decreasing mass transferlimitations and providing the additional cavitation effect [18]. Thisidea may be supported by the enhanced degradation of dyes in US-Fenton [19] and US-Fenton-like [20] processes, and in an ultra-sound-irradiated TiO2 photocatalytic system [21]. Therefore, themain objective of the present work is focused on the integrationof sonolysis and catalysis by using Fe3O4 MNPs as a heterogeneouscatalyst, where Rhodamine B (RhB) as a typical dye pollutant.

2. Experiments

2.1. Materials

RhB, FeCl3�6H2O, FeSO4�7H2O, NH3�H2O and H2O2 (30%, w/w)were purchased from Sinopharm Chemical Reagent Co. Ltd. (Chi-na). Horseradish peroxidase (HRP, specific activity of 300 unitsmg�1, RZ P 3) was obtained from Tianyuan Biologic EngineeringCorp. (China) and N,N-diethyl-p-phenylenediamine sulfate (DPD)was from Aldrich. All the used chemical reagents were of analyticalgrade. Double distilled water was used throughout experiments.The solution pH was adjusted using diluted H2SO4 and NaOHsolutions.

2.2. Preparation of Fe3O4 nanoparticles

Fe3O4 nanoparticles were synthesized by a modified co-precip-itation method [16]. FeCl3�6H2O (2.22 g) and FeSO4�7H2O (2.28 g)were dissolved in 30 mL 0.01 mol L�1 HCl aqueous solution andheated to 80 �C. The warmed Fe(II)/Fe(III) solution was added drop-wise into 40 mL of 3.0 mol L�1 ammonia solution at 80 �C undermagnetic stirring. After 3 h reaction, the generated black Fe3O4

nanoparticles were collected by magnetic separation, washed withwater to neutral pH, and then re-dispersed to 100 mL of water andstored for use (referred to as the Fe3O4 MNPs stock solution with aconcentration of 12.5 g L�1).

2.3. Degradation procedure

An ultrasonic system with a 6-mm-diameter titanium probeoperating at 20 kHz was used for ultrasonic irradiation experi-ments (Ningbo Scientz Biotechnology Co. Ltd., Zhejiang, China).

The tip of the titanium probe was placed about 2.0 cm below thesurface of the solution during sonication, and the power beingtransferred to the solution was measured as 6.0 W by means ofthe calorimetric method [4]. Sonication was applied in pulsedmode (2s on/1s off) helping to minimize bulk temperature in-crease, and the sonication time was expressed as the sum ofpulse-on times. During the degradation, the reactor was immersedinto a water bath at 25 ± 1 �C.

For typical experimental runs, Fe3O4 MNPs was dispersed into50 mL of RhB aqueous solution, followed by shaking for 15 min.After the adsorption–desorption equilibrium was attained, thedegradation of RhB was initiated by rapid adding H2O2 to the reac-tor and immediately turning on the ultrasonic. At given time inter-vals, 2 mL aliquots of the reaction solution were sampled, andimmediately centrifuged at 14,000 rpm for 15 min with an EBA-21 centrifugal (Hettich, Germany) to remove Fe3O4 MNPs. Theresultant solution was used for the analysis of related chemicalspecies in it.

2.4. Analysis

RhB concentration was analyzed by measuring the absorbanceof the solution at 554 nm on a Cary 50 UV–Vis spectrophotometer(Varian, USA). The formation of �OH radicals was monitored byusing terephthalic acid (TA) as a fluorescence probe. This fluores-cence method is a very specific detector with high sensitivity forthe quantitative measurement of �OH radicals, in which TA easilyreacts with �OH to form highly fluorescent 2-hydroxy terephthalicacid (TAOH) [22]. The fluorescence spectra were recorded on a JAS-CO FP6200 spectrofluorometer.

To evaluate the amount of H2O2 adsorbed over Fe3O4 nanopar-ticles, Fe3O4 MNPs was dispersed into 50 mL of H2O2-containingRhB-free solution and then mechanically stirred in the dark for1 h. After being centrifuged and filtered, the H2O2 concentrationin the solution was determined with a spectrophotometric method[23], where DPD was oxidized by H2O2 in the presence of HRP toform a colored product DPD�+, which had an absorption maximumat 551 nm. It is worthy noting that when Fe3O4 MNPs were addedinto a solution of H2O2, the H2O2 concentration was slightly de-creased in the first several minutes and then kept almost constant,but the H2O2 concentration was decreased to a much greater ex-tent as the immersion time was increased in the simultaneouspresences of both Fe3O4 MNPs and RhB. This indicates that the ini-tial decreasing of H2O2 concentration in the RhB-free Fe3O4–H2O2

system is attributed to its adsorption but not its decompositionover Fe3O4 MNPs, due to the lack of RhB.

3. Results and discussion

3.1. Synergistic effect between the sonochemical and catalyticdegradation of RhB

In control experiments, we confirmed that almost no degrada-tion of RhB was observed when only H2O2 was added into the solu-tion or when the RhB solution was irradiated only with ultrasoundbut without the addition of H2O2. Then, the sonolysis of RhB wasinvestigated in the absence and presence of Fe3O4 MNPs and/orH2O2. As shown in Fig. 1, RhB is degraded slowly in the presenceof H2O2 under ultrasonic irradiation (curve 1), indicating that thethermal decomposition of H2O2 in the cavitation bubble increasesslightly the formation of reactive radicals. Because the low volatilityand high solubility of H2O2 in water limits its concentration in thecavitation bubbles, resulting in a very limited ultrasonic degrada-tion rate of RhB [19]. In the simple catalytic system of H2O2–Fe3O4

MNPs without ultrasonic irradiation (curve 3), the degradation rate

0 20 40 600.0

0.2

0.4

0.6

0.8

1.0

c / c

0

Time /min

123

4

Fig. 1. Degradation of RhB in systems of (1) US-H2O2, (2) US-Fe3O4, (3) H2O2–Fe3O4,and (4) US-H2O2–Fe3O4. Other conditions: initial concentration (c0) of RhB,0.02 mmol L�1; c0 of H2O2, 40 mmol L�1; Fe3O4 MNPs load, 0.5 g L�1; pH, 5.0;temperature, 25 �C.

0.0 0.2 0.4 0.6 0.8 1.00.00

0.01

0.02

0.03

0.04

k /m

in-1

Fe3O

4 loading /g L

-1

Fig. 2. Effect of Fe3O4 MNPs loading on the rate constant k of RhB degradation inUS-H2O2–Fe3O4 MNPs system.

80 N. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 78–83

of RhB is increased to an extent, relative to that in the system ofH2O2-US. This is attributed to the intrinsic peroxidase mimeticactivity of Fe3O4 MNPs. It is reported that due to this intrinsic perox-idase mimetic activity, Fe3O4 MNPs can activate H2O2 and are usedfor the H2O2 detection and phenol decomposition [13–17]. How-ever, the degradation rate of RhB in this simple catalytic system isnot so high for the practical treatment of RhB as a typical pollutant.It is exciting that the degradation of RhB is significantly promotedwhen this simple catalytic system is under ultrasonic irradiation(curve 4). Within 60 min for the ultrasonic irradiation, the removalof RhB achieves 90% in the US-H2O2–Fe3O4 MNPs system, beingmuch greater than that (less than 20%) in the US-Fe3O4 MNPs sys-tem (curve 2). The addition of Fe3O4 MNPs enhances the sonochem-ical degradation of RhB in US-H2O2 system, possibly because thepresence of particles provide more nucleation sites favoring the for-mation of cavities and consequently increase the ultrasonic effi-ciency [24]. Nevertheless, it has been found that neither the addedTiO2 (Degussa P25) nor SiO2 nanoparticles have observable influ-ences on the RhB degradation in US-H2O2 system. This implies thatonly Fe3O4 MNPs exhibit a significant enhancing effect on the sono-catalytic degradation of RhB, being ascribed to its intrinsic proper-ties (more details will be discussed in Sections 3.2.4 and 3.3).Since the RhB degradation is quite slow in both systems of H2O2–Fe3O4 MNPs (simple catalytic system, curve 3) and US-H2O2 (simpleultrasonic system, curve 1), the very fast degradation of RhB in theUS-H2O2–Fe3O4 MNPs system demonstrates that there is a strongsynergistic effect between the sonochemical and catalytic degrada-tion of RhB.

Under all the tested conditions, the RhB degradation was ob-served to approximately follow a pseudo-first-order reaction inkinetics, which can be expressed as ln (ct/c0) = kt + y, where y is aconstant, t is reaction time (min), k is the apparent rate constant(min�1), and c0 and ct are the RhB concentrations (mmol L�1) attime of t = 0 and t = t, respectively. The fitting of the data in Fig. 1gives that the rate constant k in different systems is increased inthe order of US-H2O2 (0.0034 min�1) < US-Fe3O4 (0.0046 min-1) < H2O2–Fe3O4 (0.0055 min�1) < US-H2O2–Fe3O4 (0.035 min�1).By comparing with the k values in the simple catalytic H2O2–Fe3O4 MNPs system and the simple ultrasonic US-H2O2 system,the value of k in US-H2O2-Fe3O4 MNPs system is increased by 6.4and 10.3 times, respectively. Such a great increasing indicates thatthe combination of Fe3O4 MNPs with ultrasonic irradiation exhibitsa strongly synergistic effect on the RhB degradation.

3.2. Effects of important parameters on degradation of RhB

3.2.1. Effect of the loading of Fe3O4 MNPsFig. 2 illustrates the RhB degradation in the US-H2O2–Fe3O4

MNPs system with various loads of Fe3O4 MNPs. Compared withthe US-H2O2 system (i.e., the zero load of Fe3O4 MNPs), the addi-tion of Fe3O4 MNPs significantly enhanced the degradation ofRhB, because Fe3O4 MNPs act as peroxidase-like catalyst to acceler-ate the decomposition of H2O2, resulting in the easier generation ofstrong oxidizing radical species. When the Fe3O4 MNPs concentra-tion is increased from 0 to 0.5 g L�1, the rate constant k of RhB deg-radation is greatly increased from 0.0034 to 0.035 min�1. Beyondthe Fe3O4 loading of 0.5 g L�1, k approaches an almost constantvalue.

3.2.2. Effect of H2O2 concentrationFig. 3 gives the degradation of RhB in the US-H2O2–Fe3O4 MNPs

system with different initial concentrations of H2O2. When no ini-tial H2O2 is present, RhB is hardly degraded. This indicates that thepresence of Fe3O4 MNPs alone cannot produce enough amounts ofreactive radical species under the ultrasonic irradiation. The addi-tion of H2O2 enhances the RhB degradation. As the H2O2 concentra-tion is increased from 6.0 to 40.0 mmol L�1, the rate constant k ofRhB degradation is greatly increased up to 0.010 and 0.035 min�1

in the US-H2O2–Fe3O4 MNPs system (curve 1), and at the sametime the adsorption of H2O2 on the surface of Fe3O4 MNPs is in-creased from 0.40 to 3.22 mmol g�1 (curve 3). Similar dependenceof the degradation rate on the H2O2 concentration is observable inthe H2O2–Fe3O4 MNPs system in the absence of ultrasonic irradia-tion (curve 2). The similarity between the H2O2 concentrationdependences of the degradation rate and the H2O2 adsorption inthe system indicates that the adsorption of H2O2 on Fe3O4 MNPstakes a primarily important role in the degradation process ofRhB in the US-H2O2–Fe3O4 MNP system. Meanwhile, the similaritybetween the H2O2 concentration dependences of the degradationrates in the H2O2–Fe3O4 MNPs and US-H2O2–Fe3O4 MNPs systemsshows that the ultrasonic irradiation will not markedly change themechanism of the RhB degradation in the H2O2–Fe3O4 MNPsystem.

By comparing curves 1 and 2 in Fig. 3, it is found that at eachconcentration of H2O2, the synergistic effect between the sono-chemical and catalytic degradation of RhB are significant becausethe k values in the US-H2O2–Fe3O4 system are much greater thanthat in the H2O2–Fe3O4 system. It is noted that the k values tend

0

5

10

0 20 40 600.00

0.02

0.04

Ads

orbe

d H

2O2 /m

mol

g-1

3

2

k /m

in-1

c0 of H

2O

2 /mmol L-1

1

Fig. 3. Effect of H2O2 concentration on the rate constant k of RhB degradation in thesystems of (1) US-H2O2–Fe3O4 and (2) H2O2–Fe3O4. (3) Adsorbed amount of H2O2 onFe3O4 MNPs (0.5 g L�1).

20 40 600.00

0.05

0.10

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0.20

o

k /m

in-1

1

2

3

(a)

N. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 78–83 81

to be decreased slightly in both the US-H2O2–Fe3O4 and H2O2–Fe3O4 systems, when the H2O2 concentration is more than40 mmol L�1, suggesting the excessive adsorption of H2O2 on thesurface of Fe3O4 MNPs is unfavorable to the RhB degradation. Thisis ascribed to two reasons. One is that the excessive adsorption ofH2O2 on the surface of Fe3O4 MNPs is unfavorable to the competi-tive adsorption of the dye pollutant molecules on the Fe3O4 MNPs,limiting the degradation of RhB in the heterogeneous catalytic sys-tem. The other is that at high concentrations the excessive H2O2

may act as the radicals scavenger due to the combination of thereactive radicals generated at higher concentrations of H2O2, less-ening the reactive radicals available for substrate oxidation [3].

3.2.3. Effect of pHThe Fe3O4-catalyzed RhB degradation was investigated in the

absence and presence of ultrasound irradiation at different initialsolution pH values. It is easily seen from Fig. 4 that the simple cat-alytic system of H2O2–Fe3O4 without ultrasound irradiation worksat pH values of a wide range from 2 to 9, with a favorable rangefrom pH 3 to pH 9 (curve 2), because the catalyst becomes lesschemically stable at pH values lower than about pH 2. Althoughthe simple catalytic system of H2O2–Fe3O4 can degrade RhB at

4 6 8 100.00

0.01

0.02

0.03

0.04

k /m

in-1

pH

1

2

Fig. 4. Effect of pH on the rate constant k of RhB degradation in (1) US-H2O2–Fe3O4

and (2) H2O2–Fe3O4 systems.

pH values of a wide range, the RhB degradation rate is indeed veryslow. When the ultrasonic irradiation is applied onto the H2O2–Fe3O4 system, the resultant US-H2O2–Fe3O4 system yields muchenhanced degradation of RhB at each of the tested pH values (curve1). The much enhanced degradation of RhB at pH values rangingfrom pH 3 to pH 9 is very favorable to the treatment of practicalwastewaters, because it is unnecessary to pre-adjust the solutionpH. It is noted that relative to degradation at pH values from 3 to9, the RhB degradation becomes slower at pH 10. This is partlydue to the decreasing of the adsorption of H2O2 on the catalyst cov-ered with FeðOHÞ3�6 [17] and partly due to the fast decompositionof H2O2 into H2O and O2 at strong basic conditions. Both of theabove effects are detrimental to the catalytic crack of H2O2 byFe3O4 MNPs to produce reactive radical species.

3.2.4. Effect of temperatureThe influence of temperature on the catalytic activity of Fe3O4

MNPs in the absence and presence of ultrasound irradiation wasshown in Fig. 5a. Like previous reports [15,16], the catalytic degra-dation of RhB in the H2O2–Fe3O4 system without ultrasonic irradi-ation is promoted as temperature is increased (curve 2 in Fig. 5a).This also means that Fe3O4 MNPs possess the advantage of muchmore robust to high temperatures than the nature enzyme HRP,which showed a decreasing activity toward the removal of phenolas the increasing temperature [25]. The values of k in the H2O2–Fe3O4 system are rapidly increased from 0.0018 min�1 at 15 �C to

T / C

3.0 3.2 3.4 3.6

-6

-4

-2

0

2

T -1 /10-3K-1

ln k

1

(b)

Fig. 5. (a) Effect of temperature on the degradation rate constant k of RhB in(1) US-H2O2–Fe3O4, (2) H2O2–Fe3O4 and (3) H2O2-US systems. (b) The plots of ln kagainst T�1 in the systems of (1) US-H2O2–Fe3O4 and (2) H2O2–Fe3O4.

0

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10

15

00

100

200

300

IV

I

II

I F /a

.u.

Time /min

III

k/m

in-1

I II III IV

15105

Fig. 6. Rate constants for the generation of �OH radicals in the systems of (I) US-Fe3O4, (II) H2O2–Fe3O4; (III) US-H2O2; and (IV) US-H2O2–Fe3O4. The inset gives thekinetics of �OH generation in the four systems.

82 N. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 78–83

0.023 min�1 at 55 �C, with a factor of 12.7 times. When the ultra-sonic irradiation is exerted on the H2O2–Fe3O4 system, the temper-ature dependence of the degradation of RhB is almost the same asthat without the ultrasonic irradiation. The values of k in the US-H2O2–Fe3O4 system are increased from 0.012 min�1 at 15 �C to0.15 min�1 at 55 �C, with a factor of 12.5 times (curve 1 inFig. 5a). As the control, the RhB degradation in the US-H2O2 systemwithout the catalyst is very slow at all the tested temperatures,with k values of only 0.0034 ± 0.0003 min�1 at the temperaturerange from 15 �C to 55 �C (curve 3 in Fig. 5a). At any a tested tem-perature, the k value is increased significantly in the order of US-H2O2 < H2O2–Fe3O4 < US-H2O2–Fe3O4, implying that the RhB deg-radation in the presence of H2O2 is more strongly promoted byusing Fe3O4 MNPs as peroxidase mimetic than that by using ultra-sonic as assisted energy, and the simultaneous introduction ofFe3O4 MNPs and ultrasonic irradiation exhibits a significant syner-gistic effect.

Fig. 5b gives the plot of ln k against 1/T. The curve for the US-H2O2–Fe3O4 (ln k = 15.2–5.60 � 103/T) system run well parallelwith that for the H2O2–Fe3O4 (ln k = 13.0–5.48 � 103/T) system,indicating that the degradation of RhB in both systems has similarapparent Arrhenius activation energy (Ea). The data fitting givesthe values of Ea as 45.7 kJ mol�1 for the H2O2–Fe3O4 system and46.6 kJ mol�1 for the US-H2O2–Fe3O4 system, respectively, beingslightly greater than that (32.0 kJ mol�1) for the reported US-Fen-ton system (10 mmol L�1 FeSO4 + 6 mmol L�1 H2O2) [26], becausethe bound Fe ion on Fe3O4 MNPs limits its effective attack toH2O2 relative to the free ions in homogeneous Fenton system. Be-cause the RhB degradation is very slow in the US-H2O2 system andthe errors are very significant in the measurements of the verysmall k values in this system at lower temperatures, it is difficultto obtain the Ea value for the RhB degradation in the US-H2O2 sys-tem in our laboratory. However, the decomposition of H2O2 is theprimarily important step in the RhB degradation in the US-H2O2

system. It is known that the bond breakage of HO–OH and H–OOHinto �OH and HO2

� radicals requires very high energies of 213.8 and374 kJ mol�1, respectively [27]. Thus, we may roughly take213.8 kJ mol�1 as the apparent Arrhenius activation energy forthe RhB degradation in the US-H2O2 system. By comparing withsuch a great Ea value, we can find that the presence of Fe3O4 MNPsgreatly decreases the apparent Arrhenius activation energy, there-by enhances the RhB disappearance. Furthermore, the almost sameEa values in both the H2O2–Fe3O4 and US-H2O2–Fe3O4 systemsindicate that the ultrasonic irradiation does not change the intrin-sic reaction mechanism of RhB degradation in the H2O2–Fe3O4 sys-tem. In other words, the ultrasonic irradiation does not change therate determining step in the RhB degradation by H2O2 catalyzedwith Fe3O4 MNPs. This may account for the similar trends observedfor the influences of pH value and H2O2 concentration on the RhBdegradation in the H2O2–Fe3O4 system in the absence and presenceof ultrasonic irradiation (Figs. 3 and 4). However, the sonochemicaleffect enhances the mass transfer on the surface of Fe3O4 nanopar-ticles, leading to much increased collision proportionality ofthe reactants. Accordingly, the value of apparent pre-exponen-tial factor (i.e., 4.0 � 106) in the Arrhenius equation for theUS-H2O2–Fe3O4 system is about 10 times that (4.4 � 105) for theH2O2–Fe3O4 MNPs system, which can account for that the k valueof RhB degradation is much greater in the US-H2O2–Fe3O4 than inH2O2–Fe3O4 under similar experimental conditions.

3.3. Tentative explanation to synergistic effect of catalysis andsonochemistry

To clarify the synergistic effect of the Fe3O4 catalysis and sono-chemistry, a fluorescence technique is used to detect of the pro-duced �OH radicals, in which the added terephthalic acid will

react with �OH to yield a strongly fluorescent product 2-hydroxyterephthalic acid (TAOH). The fluorescence intensity of generatedTAOH is proportional to the amount of formed �OH radicals [22].From the inset of Fig. 6, it is found that there are well linearcorrelations between the fluorescence intensity (i.e., the amountof generated �OH radicals) and reaction time in all the tested sys-tems. Thus, the apparent rate constant (k�OH) of generated �OHcould be calculated from the slope. Fig. 6 gives the values of k�OH

as 0.18, 1.4, 5.1 and 14.9 min�1 for in the systems of US-Fe3O4,H2O2–Fe3O4, US-H2O2 and US-H2O2–Fe3O4, respectively. It iscertainly observed that much more �OH radicals are formed inthe systems containing H2O2 than that without H2O2, implyingthat the generation of �OH radicals is mainly ascribed to theH2O2-involving reactions. Among the tested systems, theUS-H2O2–Fe3O4 system has the largest k�OH value, suggesting thatthe ultrasonic irradiation can further promote the �OH generationfrom the decomposition of H2O2 catalyzed by Fe3O4 MNPs, andconsequently accelerating the RhB degradation.

According to the above discussions, a synergistic mechanism forthe ultrasound-assisted RhB degradation in H2O2–Fe3O4 MNPs sys-tems is proposed. H2O2 molecules are adsorbed on the surface ofFe3O4 MNPs, and the surface-adsorbed H2O2 molecules are acti-vated by Fe(II)/Fe(III) to generate reactive oxygen species such as�OH radicals, which further destruct the adsorbed RhB and/or dif-fuse into the solution to attack RhB molecules near the Fe3O4/solu-tion interface. When the Fe3O4 dispersion containing H2O2 and RhBis exposed to ultrasound, the cavitation effect produces high tem-peratures, diminishes the agglomeration of Fe3O4 MNPs, and in-creases the rate of mass transfer [28], resulting in acceleratingthe effective breakdown of H2O2 at the Fe3O4 surface to producemore reactive radicals, and thereby greatly increasing the RhB deg-radation. Furthermore, Fe3O4 MNPs provide more nucleation sitesfavoring the formation of cavities and increase the overall cavita-tion intensity [24]. Thus, a markedly synergistic effect of Fe3O4

MNPs and ultrasound is observed for the RhB degradation in thepresence of H2O2, due to the positive interaction between sono-chemical effects and Fe3O4 MNPs.

4. Conclusion

As a green oxidant, H2O2 may have applications in treatments ofdye pollutants. Because the breakage of H2O2 into radicals requiresan energy as high as 213.8 kJ mol�1, the dye pollutant RhB is hardly

N. Wang et al. / Ultrasonics Sonochemistry 17 (2010) 78–83 83

degraded by using H2O2 alone. To enhance the RhB degradationwith H2O2 as oxidant, the combination of Fe3O4 MNPs as peroxi-dase mimetic and ultrasound irradiation was investigated. It wasobserved that there is a marked synergistic effect between thesonochemical and catalytic degradation of RhB. In comparison withthe degradation of RhB in the presence of only H2O2, the introduc-tion of either Fe3O4 MNPs or ultrasound irradiation promoted theRhB degradation. This is attributed to the peroxidase-like catalyticactivity of Fe3O4 MNPs as peroxidase mimetic and the sonochem-icall effect of the ultrasonic irradiation. When the ultrasonic irradi-ation and Fe3O4 MNPs catalyst were simultaneously used at pH 5.0and temperature 55 �C, the RhB degradation was much moregreatly accelerated in the system of US-H2O2–Fe3O4 MNPs, withthe k value of 0.15 min�1 being about 37.6 and 6.5 times that inthe systems of US-H2O2 and H2O2–Fe3O4 MNPs, respectively. Sucha strong synergistic effect is ascribed to that the ultrasonic irradi-ation generates benefits in Fe3O4 heterogeneous catalysis byincreasing mass transfer and providing cavitation effect, andFe3O4 nanoparticles favor the cavitation inception by offeringnucleation sites. Because of its wide working pH range (pH 3–9)and high degradation efficiency, the ultrasound-assisted Fe3O4

MNPs catalysis with H2O2 as a green oxidant may become a prom-ising approach for the removal of organic pollutants.

Acknowledgements

Financial supports from the National Science Foundation of Chi-na (Grants Nos. 20877031 and 20677019) are gratefullyacknowledged.

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