Fe(III)-Enhanced Sonochemical Degradation Of Methylene Blue In

Conditions: sonication at 354.5 kHz, 35 W power, solution volume 300 mL, ..... Ince, N. H.; Tezcanli, G.; Belen, R. K.; Apikyan, I. G. Ultrasound as a...
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Environ. Sci. Technol. 2005, 39, 8936-8942

Fe(III)-Enhanced Sonochemical Degradation Of Methylene Blue In Aqueous Solution C L A U D I O M I N E R O , * ,† MIRCO LUCCHIARI,‡ DAVIDE VIONE,† AND VALTER MAURINO† Dipartimento di Chimica Analitica, Universita` di Torino, Via P. Giuria 5, 10125 Torino, Italy, and Consorzio Interuniversitario Nazionale “La Chimica per l’Ambiente” (INCA), Viale della Liberta` 5/12, Mestre (VE), Italy

The sonochemical degradation rate of Methylene Blue (MB) is markedly increased in the presence of Fe(III), a rather inexpensive reagent for the application of sonochemistry to wastewater treatment. The effect of Fe(III) is due to a sonochemically induced Fenton reaction, where both reactants (Fe(II) and H2O2) are sonochemically synthesized. Hydroperoxide/superoxide, generated upon sonochemical processes in aerated solution, is a key species involved in both Fe(III) reduction to Fe(II) and in the production of H2O2. The Fenton reaction between Fe(II) and H2O2 then produces hydroxyl radicals, enhancing the degradation of MB. A further enhancement of the degradation of the substrate in the presence of Fe(III) takes place upon addition of H2O2, which is likely to favor the Fenton process. Interestingly, H2O2 alone, in the absence of Fe(III), has a very limited effect on the sonochemical degradation rate.

Introduction Among the methods aimed at the degradation of pollutants in aqueous solution, much attention has recently been focused on the so-called advanced oxidation processes for water and wastewater decontamination. In these processes various techniques (e.g., photolysis, photocatalysis, Fenton reaction, sonochemistry) are applied to produce reactive species (often, but not always, the hydroxyl radical) with the purpose of inducing the transformation of water-dissolved organic pollutants. A way of generating hydroxyl radicals in aqueous solution is the application of ultrasounds in the frequency range 30-1000 kHz, in which case important sonochemical effects can be observed (1). As a consequence of the sonochemical generation of hydroxyl radicals, degradation of many organic compounds in solutions exposed to ultrasounds has been reported (1-3). Application of ultrasounds to aqueous solutions induces the formation of vapor- and gas-filled microbubbles (4). The phenomenon of acoustic cavitation consists of three stages: bubble nucleation, growth, and implosive collapse. Inside the collapsing microbubbles, temperature and pressure increase up to very high values, inducing the pyrolysis of water vapor and of the volatile organic compounds that can be present in the gas phase (5). Pyrolysis of water vapor yields * Corresponding author phone: +39-011-6707632; fax: +39-0116707615; e-mail: [email protected]. † Universita ` di Torino. ‡ Consorzio Interuniversitario Nazionale “La Chimica per l’Ambiente”. 8936

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hydroxyl radicals and hydrogen atoms. Hydroxyl radicals in particular are very reactive and can transform organic compounds. Reactions can take place in the gas phase, at the gas-liquid interface, and in the solution bulk after transfer of gaseous radicals into the liquid phase (1). Accordingly, the sonochemical degradation of an organic compound can be influenced by gas-phase pyrolysis and oxidation, and by reaction with hydroxyl at the gas-liquid interface and in the aqueous phase. The relative importance of the reported pathways will depend on the compound volatility and reactivity under the different conditions (6). The ultrasound-induced splitting of water molecules will cause reactions 1-10 in the presence of dissolved oxygen (the reported rate constants refer to the solution bulk (6-8)). In reactions 1 and 9, ))) ) ultrasounds. )))

H2O 98 •OH + H• H• + •OH f H2O H• + O2 f HO2• 2 •OH f H2O2

(1)

[k2 ) 2 × 1010 M-1 s-1]

(2)

[k3 ) 1.9 × 1010 M-1 s-1]

(3)

[k4 ) 5.5 × 109 M-1 s-1]

(4)

2 HO2• f H2O2 + O2

[k5a ) 8.3 × 105 M-1 s-1] (5a)

HO2• + •O2- + H+ f H2O2 + O2

[k5b ) 9.7 × 107 M-1 s-1] (5b)

HO2• + •OH f H2O+O2

[k6a ) 7.1 × 109 M-1 s-1] (6a)

O2- + •OH f OH- + O2

[k6b ) 1 × 1010 M-1 s-1] (6b)

•

H2O2 + •OH f H2O + HO2•

[k7 ) 3 × 107 M-1 s-1] (7)

H2O2 + H• f •OH + H2O

[k8 ) 9 × 107 M-1 s-1] (8)

)))

O2 98 2 O

(9)

O + H2O f 2 •OH

(10)

Both reactions 4 and 5 can produce hydrogen peroxide upon water sonication. However, in the presence of even small amounts of hydroxyl scavengers in solution, the relative importance of reaction 4 becomes negligible when compared with reaction 5. In aerated solution, N2 is known to produce nitrite, nitrate, and other species (4). Further processes will therefore take place, involving nitrogen compounds (9-11). Sonochemical degradation has been applied to the abatement of chlorinated pollutants and to the remediation of contaminated soil (12, 13). The main limit to the application of sonochemistry to water and wastewater treatment has been represented so far by the high costs. Interesting alternatives might be the induction of the hydrodynamic cavitation phenomenon (14), and the combination of ultrasounds with other techniques (ozonolysis, direct or sensitized photolysis, catalysis and photocatalysis, etc.) (15, 16). The Fenton reaction (Fe(II) + H2O2) is another way to produce hydroxyl radicals in solution. The main drawback of this technique is represented by the cost of the reactants, in particular Fe(II). For this reason various methods have 10.1021/es050314s CCC: $30.25

 2005 American Chemical Society Published on Web 10/14/2005

been introduced to reduce the rather cheap Fe(III) salts to Fe(II), which resulted in photo-Fenton and electro-Fenton (17) techniques. An approach to achieve better performance in sonochemical degradation processes, exploiting a sonochemically induced Fenton reaction, is the subject of the present paper.

Experimental Section Sonication was carried out with an L3 CommunicationsElac Nautik sonifier equipped with two piezoelectric heads to be used in alternative, and a Dressler Cesar RF power generator (see Supporting Information (SI) for details). The device can produce ultrasounds at 206.0, 354.5, 619.0, and 1063 kHz. The volume of the sonicated solution was 300 mL. The sonochemical degradation of MB was spectrophotometrically monitored on line. The sonication experiments were carried out at pH 2 to keep Fe(III) in its dissolved monomeric forms and thus avoid the formation of Fe(III) polynuclear species and colloids (18, 19). When required, He/O2 mixtures were produced with an Entech model 4560 SL dynamic diluter, connected to pressurized gas containers. A nitrogen atmosphere was obtained by taking N2 directly from the container. Conditioning of the reaction chamber was carried out by bubbling the gas (flow 1 L min-1) into the solution for 20 min before sonication, and blowing above the solution during sonication to prevent air entrance and not to perturb the cavitation phenomena and the spectrophotometric measurements. The analysis of H2O2 (horseradish peroxidase method (20)), nitrate, nitrite, and dissolved oxygen was carried out using standard methods (see SI). Kinetic Calculations. Numerical kinetic calculations were carried out using the Chemical Kinetics Simulator package (IBM, Almaden Research Center, www.almaden.ibm.com/ st/msim/ckspage.html), which runs simulations of complex kinetic systems adopting Monte Carlo techniques. Due to the complexity and multiphase nature of the sonochemical processes, such an approach was not used to simulate the system and obtain numerical values for the rate constants, but as an approximation to compare the relative weight of two or more reactions that occur in the aqueous phase.

Results and Discussion Methylene Blue Sonochemical Degradation. The choice of MB as substrate for sonochemical degradation is motivated by both basic and experimental reasons. MB is a nonvolatile, charged substrate, which is unlikely to be found in the gas phase. MB therefore is unlikely to undergo gas-phase pyrolysis and oxidation, and the sonochemical degradation can be expected to take place mainly in solution. Another reason for the choice is that the rate constant for the reaction between MB and aqueous hydroxyl is high and known from the literature (2.1 × 1010 M-1 s-1; (7)). Furthermore, the use of a colored substrate is required by the on-line spectrophotometric monitoring, to reduce as much as possible the probability of spectral interference by the degradation intermediates. The time evolution of the spectrum of MB alone in solution, upon sonication, and the rationale for the adopted sonication frequency, are reported in the SI, showing that no spectral interference is present for short irradiation times. For the calculation of the initial degradation rates, the reference wavelength for calculating the concentration was the absorption maximum at 660 nm, where there is no interference by the absorption of Fe(III) and H2O2. Effect of Added Fe(III). Figure 1 reports the time evolution of 3.0 × 10-6 M MB at pH 2 by H2SO4, under different conditions (see also Table 1, entries 1, 3, 8, 9, 22, and 24).

FIGURE 1. Time evolution of Methylene Blue (initial concentration 3.0 × 10-6 M) in the presence of (a) 1.0 × 10-3 M H2O2 without sonication; (b) 5.0 × 10-5 M Fe2(SO4)3 without sonication; (c) 5.0 × 10-5 M Fe2(SO4)3 + 1.0 × 10-3 M H2O2 without sonication; (d) sonication without Fe(III) and without H2O2; (e) sonication in the presence of 5.0 × 10-5 M Fe2(SO4)3; (f) sonication in the presence of 5.0 × 10-5 M Fe2(SO4)3 + 1.0 × 10-3 M H2O2. Conditions: sonication at 354.5 kHz, 35 W power, solution volume 300 mL, pH 2, adjusted by addition of H2SO4. The absence of a significant reactivity by H2O2 or Fe(III) alone gives confidence that the degradation of MB requires either sonication, or the interaction of H2O2 and Fe(III) between themselves and/or with ultrasounds. In the presence of Fe(III) + H2O2 without sonication the degradation of MB is accounted for by the Fenton reaction (8, 21). In this case the rate-determining step is the reduction of Fe(III) to Fe(II) by H2O2, HO2•, and •O2- (8, 22). The Fenton reaction between Fe(II) and H2O2 then yields •OH, responsible for MB degradation. The reaction system (reactions 11-15) is reported below (7, 8). For a discussion of the relative role of hydroxyl and Gif-type Fenton chemistry see the SI.

Fe(III) + H2O2 f Fe(II) + HO2• + H+

[k11 e 3 × 10-3 M-1 s-1] (11)

HO2• a H+ + •O2-

[pKa12 ) 4.8; k12 ) 1.58 × 105 s-1; k-12 ) 1 ×

1010 M-1 s-1] (12)

Fe(III) + HO2• f Fe(II) + O2 + H+

[k13 ) 1 × 103 M-1 s-1] (13)

Fe(III) + •O2- f Fe(II) + O2

[k14 ) 1.5 × 108 M-1 s-1] (14)

Fe(II) + H2O2 f Fe(III) + •OH + OH-

[k15 ) 63 M-1 s-1] (15)

At pH 2 reaction 14 strongly prevails over reaction 13. The time evolution of Methylene Blue in the presence of Fe(III) + H2O2 clearly departs from a pseudo-first-order degradation curve. This effect, already observed in the case of phenol under comparable conditions (22), is most likely due to the combination of two phenomena: (i) competition between the substrate and the early degradation intermediates for reaction with hydroxyl, which slows down the rate, and (ii) further reduction of Fe(III) by later intermediates, which accelerates the degradation (23). VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Initial Degradation Rates of Methylene Blue (MB)a no.

conditions

atmosphere

sonicication

rate (mol L-1 s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

MB MB + 1 × 10-5 M Fe(III) MB + 1 × 10-4 M Fe(III) MB + 5 × 10-4 M Fe(III) MB + 1 × 10-3 M Fe(III) MB + 5 × 10-3 M Fe(III) MB + 1 × 10-2 M Fe(III) MB + 1 × 10-3 M H2O2 MB + 1 × 10-4 M Fe(III) MB + HClO4 (pH 2) MB + 1 × 10-4 M Fe(III) + HClO4 (pH 2) MB MB + 1 × 10-5 M Fe(III) MB + 1 × 10-4 M Fe(III) MB + 1 × 10-3 M Fe(III) MB + 8 × 10-5 M Fe(III) + 1 × 10-3 M H2O2 MB + 1 × 10-4 M Fe(III) + 1 × 10-3 M H2O2 MB + 3 × 10-4 M Fe(III) + 1 × 10-3 M H2O2 MB + 8 × 10-4 M Fe(III) + 1 × 10-3 M H2O2 MB MB + 1 × 10-4 M H2O2 MB + 1 × 10-3 M H2O2 MB + 1 × 10-2 M H2O2 MB + 1 × 10-4 M Fe(III) + 1 × 10-3 M H2O2 MB + 1 × 10-4 M Fe(III) + 3 × 10-3 M H2O2 MB + 1 × 10-4 M Fe(III) + 8 × 10-3 M H2O2 MB + 1 × 10-4 M Fe(III) + 1 × 10-2 M H2O2

air air air air air air air air air air air He/O2b He/O2b He/O2b He/O2b air air air air air air air air air air air air

yes yes yes yes yes yes yes no no yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes

(3.48 ( 0.37) × 10-9 (5.71 ( 0.39) × 10-9 (6.78 ( 0.35) × 10-9 (7.31 ( 0.63) × 10-9 (8.91 ( 1.16) × 10-9 (1.14 ( 0.19) × 10-8 (1.48 ( 0.22) × 10-8 undetectable (9.46 ( 0.03) × 10-11 (3.69 ( 0.56) × 10-9 (7.03 ( 1.19) × 10-9 (4.62 ( 0.09) × 10-9 (6.19 ( 0.83) × 10-9 (7.19 ( 1.37) × 10-9 (7.77 ( 1.30) × 10-9 (1.29 ( 0.17) × 10-8 (1.41 ( 0.31) × 10-8 (1.87 ( 0.37) × 10-8 (2.28 ( 0.20) × 10-8 (4.27 ( 0.04) × 10-9 (4.63 ( 0.27) × 10-9 (4.69 ( 0.29) × 10-9 (3.46 ( 0.20) × 10-9 (1.41 ( 0.33) × 10-8 (1.64 ( 0.28) × 10-8 (1.68 ( 0.23) × 10-8 (1.65 ( 0.25) × 10-8

a Initial concentration of Methylene Blue (MB): 3 × 10-6 M. pH 2, adjusted by addition of H SO , unless otherwise reported. Sonication frequency 2 4 (where appropriate) 354.5 kHz, 35 W power. Other conditions are specified in the Table. Initial formation rates calculated as k C0 after fitting of the experimental data with pseudo-first-order equations of the form Ct ) C0 exp(-k×t), where Ct is the concentration of MB at time t, C0 is the initial concentration (3 × 10-6 M), and k is the pseudo-first-order degradation rate constant. Error bounds to the rates ) (σ, related to the goodness of the fit to the experimental data and representing intra-series variance. Entries 1 and 20 report the results of replicate runs, carried out at about two week’s distance (inter-series variance). Variability is around 20% in such a case. b He/O2 atmosphere in 8:2 ratio.

The degradation of MB alone upon sonication is probably due to the formation of hydroxyl radicals. Sonication of an aerated aqueous solution can yield •OH upon reactions 1, 8, 10, and 18. If no initial H2O2 is present in the system, the importance of reaction 8 is minor relative to 1. In aerated solution •NO forms from N2 and O2 at the elevated temperatures of the collapsing microbubbles (9). Upon sonochemical nitrogen fixation, reactions between •NO and HO2• and between H2O2 and HNO2 (9-11) yield peroxynitrous acid, HOONO, and hydroxyl in reaction 18.

HO2• (•O2- + H+) + •NO f HOONO

[k16 ) 6.7 × 109 M-1 s-1] (16)

HNO2 + H2O2 f HOONO

[k17 ) 180 M-1 s-1] (17)

HOONO (>70%) f HNO3 HOONO ( Rate20 for 10-3 M [Fe(III)], and Rate14 ≈ Rate20 for 10-2 M [Fe(III)]. Figure 4 reports the initial degradation rate of MB as a function of the concentration of added Fe(III). The case where no H2O2 has been added to the system will be discussed first (open circles). The degradation rate of MB increases with increasing [Fe(III)] up to at least 0.01 M (see also Table 1, entries 1-7). It is noticeable that the increase is quite steep for low Fe(III) concentration values, and slows down in the range 0.001-0.01 M [Fe(III)]. Fe(III) reduction rate can be expected to increase with increasing [Fe(III)], but at the same VOL. 39, NO. 22, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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time the availability of H2O2 decreases as shown in Figure 2. H2O2 would thus become the limiting Fenton reagent at high [Fe(III)], and the degradation rate would not be enhanced by further increasing [Fe(III)]. Literature data suggest that additional sources of HO2• and Fe(II) might be present in the system. Hydroxyl addition to aromatic substrates (MB and/or its degradation intermediates) is very likely, and often followed by hydrogen abstraction upon reaction with molecular oxygen to give HO2• (31). Moreover, reduction of Fe(III) to Fe(II) might be induced by intermediate species (molecular or radicalic) formed upon MB degradation, in analogy with results reported for the Fenton degradation of Malachite Green and benzene (23, 31). The initial degradation rate of MB as a function of the initial [Fe(III)], without H2O2, under He/O2 atmosphere is reported in Figure 4 (solid semicircles). To avoid the complexity arising from the nitrogen-related species, N2 should be eliminated from the system. Experiments under reduced pressure of pure oxygen are too far from otherwise comparable conditions (4). The replacement of N2 with an inert gas is problematic. Ar and He have the same polytropic ratio, which is higher than that of N2, causing a higher maximum temperature reached upon microbubble collapse. However, N2 and Ar have similar thermal conductivity, while that of He is much higher. This implies that Ar induces the highest sonochemical production of reactive species, and that, conversely, He could behave like N2, but without generation of other interfering species. Actually, the system He/O2 under sonochemical conditions more closely resembles N2/O2 than Ar/O2 (32, 33). It is evident from Figure 4 that no significant difference can be observed between the aerated system and the 8:2 He/O2 atmosphere, under otherwise comparable conditions (see also Table 1, entries 12-15, and compare with 1-7). The presence of a He/O2 atmosphere would completely eliminate the nitrogen-related chemistry, which is a potential source of hydroxyl (see, e.g., reaction 18). The reported data under He/O2 atmosphere therefore suggest that nitrogenrelated species are not important as hydroxyl sources in aerated solution, and do not affect in possible other ways the observed Fe(III) enhancement of the sonochemical degradation of MB. Effect of Added H2O2. The enhancement effect of Fe(III) has been described so far as a sonochemically induced Fenton reaction. In this context one would expect that the addition of H2O2 would further enhance the degradation process. Indeed, Figures 1 and 4 (open squares) indicate that the addition of H2O2 increases the degradation rate of MB in the presence of Fe(III) under sonication. See also Table 1, entries 16-19. To get insight into the role of H2O2 it is however useful to study the effect of its addition in the absence of Fe(III). Practically no effect can be observed for [H2O2] e 1 × 10-3 M, while there is a slight inhibition of degradation for 1 × 10-2 M H2O2 (see Table 1, entries 20-23). Also note that no direct reaction takes place between H2O2 and MB in the absence of sonication (see Figure 1 and Table 1, entry no. 8). H2O2 can be expected to act both as hydroxyl scavenger (reaction 7) and as hydroxyl source (reaction 8) under sonochemical conditions. •OH scavenging by H2O2 would result in slower degradation of the substrate. Considering the literature rate constants for the reactions of hydroxyl with MB and H2O2 (7), under our conditions one would expect H2O2 to become the main hydroxyl scavenger for [H2O2] > 2 × 10-3 M. In the presence of 1.0 × 10-2 M H2O2, the degradation rate of MB would be only one-fifth of that observed in the absence of H2O2. However, the experimental data (Table 1, entries 20-23) indicate that this is clearly not 8940

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the case. Methylene Blue degradation rate decreases only a little when passing from 1.0 × 10-3 M to 1.0 × 10-2 M initial H2O2. It is thus reasonable to hypothesize a nonnegligible role of H2O2 as hydroxyl source, e.g., upon reaction 8 between H2O2 and H•. On the basis of the reported rate constants (7), in the presence of 2.4 × 10-4 M dissolved oxygen and of 1.0 × 10-2 M H2O2, one-fifth of the sonochemically generated H• would react with H2O2 to yield •OH, with the remainder reacting with oxygen in reaction 3. Moreover, H2O2 might generate hydroxyl also by sonochemical splitting (34). As the applied ultrasounds are able to split water molecules, it is unlikely that the less stable compound H2O2 is unreactive under sonochemical conditions. In reaction 21, ))) ) ultrasound: )))

H2O2 98 2 •OH

(21)

Reaction 21 could make H2O2 a competitive hydroxyl source when compared with the sonolysis of the solvent (reaction 1) and of dissolved O2 (reactions 9 and 10). In the hypothesis that H2O2 acts both as a source and as a sink of hydroxyl, and given the known rate constants and the experimental data, it would give an almost negligible contribution to the •OH budget (either production or scavenging) below 1 × 10-3 M initial concentration. Above this concentration the role of H2O2 starts to become significant, with both production and scavenging of hydroxyl being operational. A moderate prevalence of the scavenging effect is observed in the presence of 1.0 × 10-2 M H2O2. By contrast, Figures 1 and 4 indicate that H2O2 substantially increases the degradation rate of MB in the presence of Fe(III), up to 3 × 10-3 M H2O2 (see Table 1, entries 24-27). Considering that H2O2 alone, without Fe(III), has a very limited effect, it is reasonable to attribute the observed enhancement of MB degradation to an enhanced Fenton reaction. The Fenton reaction requires the presence of Fe(II), formed upon Fe(III) reduction, and of H2O2. As H2O2 is now added to the system, its occurrence is no more linked with the sonochemical generation. In the absence of added H2O2 its generation requires the dismutation of HO2•/•O2-, which is also involved in Fe(III) reduction, resulting in a competition that would limit the production of both Fe(II) and H2O2. Indeed, Figure 2 shows that the addition of Fe(III) decreases the availability of H2O2. On the contrary, if H2O2 is added to the system, Fe(III) reduction would not decrease any more its availability, therefore enhancing the Fenton reaction. In the presence of added H2O2, a further process of Fe(III) reduction is reaction 11 between Fe(III) and H2O2. CKS simulations show that reaction 11 gives a minor contribution to Fe(III) reduction, unless [H2O2] is considerably higher than 1.0 × 10-2 M. Moreover, the impact of hydroxyl scavenging by H2O2 is at least partially reduced in the presence of Fe(III). Reaction 7 between H2O2 and hydroxyl consumes •OH but yields HO2•. HO2•/•O2- is then involved in Fe(III) reduction to Fe(II), and Fe(II) yields hydroxyl upon reaction with H2O2. This way the hydroxyl consumed in reaction 7 can be restored in the presence of Fe(III), in particular if [Fe(III)] is sufficiently high to make Fe(III) reduction prevail over HO2•/•O2dismutation. Mechanistic and Applicative Implications. The sonochemical degradation rate of MB is increased in the presence of Fe(III). This effect is due to a Fenton reaction induced by sonochemical processes. The Fenton reaction requires the generation of Fe(II) + H2O2. The main source of H2O2 at pH 2 is the hydroperoxy radical, HO2• (reaction 5a), with a less important contribution by its conjugate base, •O2- (reaction 5b). Important hydroperoxyl sources are the reaction between H• and O2 (reaction 3), and the possible hydrogen abstraction

SCHEME 1. Summary of the Main Pathways Leading to the Sonochemical Generation of Hydroxyl, Hydrogen Peroxide, and Fe(II) in the Presence of Fe(III)a

a The numbers near the arrows refer to the reactions listed in the text. The additional hydroxyl production in the presence of Fe(III) is initiated by the process of oxygen reduction carried out by H•. O2 is activated into the more reactive HO2•/H2O2, with final generation of the oxidizing species •OH through a Fenton process.

that molecular oxygen carries out on organic radicals, such as the hydroxyl-aromatic adducts. The relevant pathways are reported in Scheme 1. The reduction of Fe(III) to Fe(II) can be carried out by •O -, the conjugate base of HO • (reaction 14), by H•, and 2 2 possibly by some transformation intermediates of the organic substrate. The reaction between sonochemically generated Fe(II) and H2O2 then yields •OH, which accelerates MB degradation. The involvement of •O2- in Fe(III) reduction, followed by the reaction between Fe(II) and H2O2, accounts for the reduced sonochemical formation of H2O2 in the presence of Fe(III) (see Figure 2). The substantial effect of added H2O2 in the presence of Fe(III) gives further support to the hypothesis of a Fenton process being operational in the system. It is apparent from Scheme 1 that the additional hydroxyl generation induced by Fe(III) is initiated by the H•-assisted reduction of oxygen The reductive formation of hydroperoxyl from oxygen results in O2 sonochemical activation. The enhancement of the sonochemical degradation by added Fe(III), with and without addition of H2O2, is also interesting from the point of view of the application of sonochemistry to wastewater treatment. Increasing the effectiveness of sonochemical degradation processes by the addition of a quite inexpensive reactant such as Fe(III) would be of help in reducing the cost of sonochemical treatments. Further discussion on this issue is reported in the SI.

Acknowledgments Financial support by Interuniversity Consortium “Chemistry for the Environment” (INCA)-Progetto Sisifo and Universita` di Torino-Ricerca Locale is gratefully acknowledged.

Supporting Information Available Further description of the experimental setup and the experimental conditions, together with the rationale of their choice, and some additional considerations on the reaction pathways. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 16, 2005. Revised manuscript received September 9, 2005. Accepted September 12, 2005. ES050314S