Review Ultrasound Catalyzer

Applied Catalysis B: Environmental 29 (2001) 167–176

Review

Ultrasound as a catalyzer of aqueous reaction systems: the state of the art and environmental applications ˙ Apikyan b N.H. Ince a,∗ , G. Tezcanli a , R.K. Belen a , I.G. a

Institute of Environmental Sciences, Boˇgaziçi University, 80815 Bebek-Istanbul, Turkey PISA Textile Dyeing and Finishing Industries, Çobançe¸sme, Güne¸sli-Istanbul, Turkey

b

Received 21 May 2000; received in revised form 26 July 2000; accepted 19 August 2000

Abstract The work presented here is aimed to guide environmental researchers and engineers in ultrasound-based chemical reaction systems, which are emerging as promising alternatives for the removal of refractory organics in treated effluents or natural waters. In accordance, the paper is presented in two parts as: (i) an extensive review of aqueous reaction systems catalyzed by ultrasonic pressure waves, with emphasis on basic theories, physical/chemical principles, state of the art, and figures-of-merits of sonochemistry, and (ii) a summary of lab-scale experimental work using ultrasonic reaction schemes for rendering or catalyzing the destructional removal of organic compounds by oxidation in the bulk liquid upon hydroxylation, and/or by thermal decomposition in the gaseous bubble. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ultrasound; Cavitation; Microbubbles; Sonochemistry; Advanced oxidation process (AOP); Free radicals; Pyrolysis; Thermal decomposition; Bubble-liquid interface

1. Introduction The second half of the last century has witnessed a rapidly deteriorating water environment as the outcome of extravagant use of complex organic compounds, the spent parts of which were discharged into conventionally operated wastewater treatment systems. Research and development in innovative technologies during the last decade have shown that advanced oxidation processes (AOP) are highly promising for the remediation of such contaminated water/effluent systems without generating any sludge or solid material of hazardous character [1–4]. ∗ Corresponding author. Tel.: +90-212-263-1500; fax: +90-212-257-5033. E-mail address: [email protected] (N.H. Ince).

Destruction or mineralization of organic compounds by these processes is based on oxidative degradation by free radical attack, particularly by the hydroxyl radical, which is a far more powerful oxidizing agent than all commonly known oxidants [5]. Free radicals in AOP practices are generated by a variety of methods such as: (i) photochemical irradiation with ultraviolet light (coupled with a powerful oxidizing agent and/or a semiconductor), (ii) Fenton and photo-Fenton catalytic processes, (iii) ␥-radiolysis, (iv) electron beam irradiation techniques and (v) sonolysis [6–13]. Among these, sonolysis is rarely used, despite the very unique and “extreme” conditions generated by ultrasound waves in liquid media, resulting in a remarkably suitable medium for “high energy chemistry”. Under well-established conditions, these “extremes” not only promote the oxida-

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tive destruction of target contaminants via free radical reactions, but also provide an excellent medium for their thermal decomposition in the gas phase. Hence, the production of free radical species by sonolysis extends the goals of AOP beyond aqueous phase oxidative destruction to gaseous decomposition, owing to very special effects generated by the formation and collapse of acoustic cavities in sonicated water. The goal of this paper is to review the principles and state of the art of “ultrasound-based free radical generation”, to induce “sonochemical” reaction conditions in aqueous systems for potential application in the remediation of water environments. (Sonochemistry is defined as chemical reactivity induced and/or catalyzed by intense pressure waves in a liquid medium.) The reason for reviewing so many of basic principles is due to the fact that ultrasonic systems are extremely sensitive and vulnerable to operational parameters, which cannot be controlled without a good knowledge and understanding of physical and chemical phenomena. The work is further aimed to point out some lab-scale applications of ultrasonic systems from the literature, reporting the destructive removal of hazardous substances in water by free radical mediated oxidation processes, and/or by thermal decomposition of the solutes as they diffuse from the bulk liquid to the gaseous bubble interior.

2. Background theories Ultrasound is defined as any sound of a frequency above that to which the human ear has no response (i.e. above 16 kHz). In practice, three ranges of frequencies are reported for three distinct uses of ultrasound [14]: (i) high frequency, or diagnostic ultrasound (2–10 MHz), (ii) low frequency or conventional power ultrasound (20–100 kHz), and medium frequency, or “sonochemical-effects” ultrasound (300–1000 kHz). It is this latter range, where chemical reaction processes are uniquely catalyzed through very “extreme” temperatures and pressures generated by the formation, growth and collapse of cavitation bubbles. When a liquid is exposed to an acoustic field, the pressure waves of the sonic vibrations create a time/frequency dependent acoustic pressure, consisting of alternating compression and rarefaction cycles

[15]. If the applied pressure is equal to the negative pressure developed in the rarefaction cycle of the wave such that the distance between the molecules of the fluid exceeds the critical molecular distance to hold it together, the liquid breaks apart to form cavities made of vapor and gas-filled microbubbles [15,16]. The phenomenon called “acoustic cavitation” consists of at least three distinct and successive stages: nucleation, bubble growth (expansion), and under proper conditions implosive collapse [17]. The first stage is a nucleated process, by which cavitational nuclei are generated from microbubbles trapped in microcrevices of suspended particles within the liquid [17,18]. In the second stage, the bubbles grow and expand in a manner restricted by the intensity of the applied sound wave. With high-intensity ultrasound, a small cavity grows rapidly through inertial effects, whereas at lower intensities the growth occurs through “rectified diffusion”, proceeding in a much slower rate, and lasting many more acoustic cycles before expansion [17]. The third stage of cavitation occurs only if the intensity of the ultrasound wave exceeds that of the “acoustic cavitational threshold” (typically a few watts/cm2 for ordinary liquids exposed to 20 kHz). At this condition, the microbubbles overgrow to the extent where they can no longer efficiently absorb energy from the sound environment to sustain themselves, and implode violently, therefore, in a so called “catastrophic collapse” [15,16,18,19]. During this collapse stage, the temperatures and pressures released are in such extremes that the entrapped gases undergo molecular fragmentation, which is the underlying phenomenon in homogenous sonochemistry [16,20,21]. Furthermore, it has been observed that just before the catastrophic collapse of compressed gas-filled cavities in water, the bubbles produce a flash of light called “sonoluminescence”, as detected by a peak at 310 nm and a broad continuum throughout the visible [21,22]. The spectrum of sonoluminescent water was associated with the formation of high-energy species (e.g. excited hydroxyl radicals) from molecular fragmentation of compressed gases, rather than with black body radiation [19,21,23]. Hence, like photochemistry, sonochemistry involves the introduction of very large amounts of energy in a short period of time, but the type of molecular excitation is thermal, unlike the electronic excitation felt

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by molecules in photochemical processes [17]. It is further claimed that sonochemistry lies in between “high-energy” and “molecular” physics, requiring, therefore, the use of microscopic description of matter [18]. At present, no consensus exists for the physical explanation of the collapse phase, except that “extreme and non-equilibrium” conditions exist during the implosion [16,24]. The most highly favored explanation is that given by the “hot spot theory”, which suggests that the collapse is so rapid that the compression of the gas and vapor inside the bubble is adiabatic [25,26]. Consequently, the temperatures and pressures within a collapsing microbubble can reach values as high as 4200–5000 K and 200–500 atm, respectively, just before fragmentation [16,17]. The localized “hot spot” generated by the rapid collapse of acoustic cavities is very short lived (<10 ␮s), implying the existence of extremely high heating and cooling rates in the vicinities of 1010 K/s [14,17,21]. Hence, sonochemistry generated by short-lived cavitational heating is found similar to shock-tube chemistry or multi-photon infrared laser photolysis, but it is unique in that acoustically induced cavitational heating occurs only at condensed phases of the fluid [24,27]. Less popular than the “hot spot theory” are the “convergent shock-wave” and the “electrical perturbation” models, which are specifically preferred for explaining the luminescence of a single bubble [28,29]. The emission of light from a single bubble during collapse was also modeled by Hickling [30] and Le Point, Le Point-Mullie [24], who compared the phenomenon with “water freezing” and “microplasma formation” from tiny electrified jets projected inside the cavity, respectively. Finally, in the “new electrical model” proposed by Marquilis [31], the distortion and splitting of a cavity into small entities were compared to the negative picture of the liquid jet distortion in aeresol sprays.

3. Physicochemical aspects of sonochemistry A great majority of sonochemical systems with potential industrial applications are heterogeneous, where enhancement of chemical reactivity is associated with the physical effects of ultrasound such as heat and mass transfer, surface activation, and phase

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mixing [17,18,32]. Sonocatalysis of liquid–liquid heterogeneous reactions is based on the mixing effect of acoustic streaming, which promotes the emulsification of non-miscible liquids by enhancing reaction rates upon increased interfaces [18]. When the system is made of a solid–liquid biphasic medium, catalysis is a consequence of the disruption of the solid by the jetting phenomenon associated with the collapse of cavitation bubbles [18]. It is important to note that many of such effects are observed when the heterogeneous medium is irradiated with low frequency, or power ultrasound at the 20–100 kHz range [18]. Homogenous sonochemistry, however, is induced directly by the outcome of “extreme” conditions in collapsing microbubbles [18]. Such extremes are reported to produce very unique catalytic effects, arising from inherent advantages of the system such as: (i) the ability to generate high-energy species and (ii) the mimicry of autoclave reaction conditions (i.e. high temperatures and pressures) on a microscopic scale [17]. These effects start in the cavities, which are made of microbubbles filled with vapor of the liquid medium and/or dissolved volatile solutes and gases diffused into them [14]. During the collapse of these cavities in pure aqueous systems, gaseous water molecules entrapped in expanded microbubbles are fragmented as in pyrolysis to generate highly reactive radical species, such as hydroxyl radicals [33]. The formation of these radicals in sonicated water was demonstrated in various laboratories, using combined spin trapping and EPR techniques, the Weissler reaction, fluorescence measurements from 2-hydroxy-terephthalate produced by hydroxylation of terephthalate ion, DMPO trapping, and sonoluminescence measurements based on the oxidative degradation of luminol to aminophtalate by sonolytically produced hydroxyl radicals [34–40]. In non-aqueous organic solvents or aqueous media containing volatile organic gases and solutes, cavitational collapse not only results in the fragmentation of water molecules to hydroxyl and hydrogen radicals, but also in the formation of organic radicals, as confirmed by experimental studies with ESR spectroscopy [41]. The hydroxyl radicals generated by water sonolysis may either react in the gas phase or recombine at the cooler gas–liquid interface and/or in the solution bulk to produce hydrogen peroxide and water as

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Fig. 1. Possible sites of chemical reactions in homogenous reaction media.

shown in [33,42]: H2 O → • OH + • H

(1)

• OH

(2)

+ • H → H2 O

2• OH → H2 O

(3)

2• OH → H2 O2

(4)

2• H → H2

(5)

If the solution is saturated with oxygen, peroxyl and more hydroxyl radicals are formed in the gas phase (upon the decomposition of molecular oxygen), and the recombination of the former at the cooler sites (interface or the solution bulk) produces additional hydrogen peroxide, as shown in [43,44]: O2 + • H → • O2 H

(6)

O2 → O + O

(7)

O + H2 O → • OH + • OH

(8)

•O H 2

(9)

+ • O2 H → H2 O2 + O2

Experience in homogenous sonochemistry has shown that there are three potential sites for chemical reactions in ultrasonically irradiated liquids, as illustrated in Fig. 1: (i) the cavitation bubble itself, (ii) the interfacial sheath between the gaseous bubble and the surrounding liquid, and (iii) the solution bulk [45]. In water or effluent treatment practices, organic pollutants may be destroyed either at the first two sites upon combined effects of pyrolytic decomposition and hydroxylation, or in the solution bulk via oxidative degradation by hydroxyl radicals and hydrogen peroxide. The extent of oxidation in the latter site is limited by the quantity of uncombined hydroxyl radicals

available in solution, which in turn is a matter of the life time and collapse duration of the bubbles, as well as the geometry of the reactor. The more important cavity effects are reported to occur when the frequency of the wave is equal to the resonating frequency of the bubble [46]. The resonance radius of a bubble excited by low frequency waves is reported to be ∼170 ␮m (at 20 kHz), and the cavities entrapping such bubbles are said to be “stable” or long lived, with average life times of ∼10 ␮s [15,44,47]. In this kind of cavitation, the collapse stage is delayed till after the elapse of a number of compression and rarefaction cycles, during which sufficient volumes of volatile solutes and solvent vapors in the liquid may flow into the gas phase [15]. The delayed growth and long collapse duration of gas-filled bubbles allow radical scavenging and recombination reactions at the interfacial sheath (as shown by Eqs. (2)–(5) and 8), thus inhibiting the mass transfer of hydroxyl and other reactive species into the solution [48]. Hence, low frequency ultrasound is expected to induce destructive effects only for hydrophobic solutes, which easily diffuse into the cavity bubbles to undergo pyrolytic destruction inside the collapsing bubble, or hydroxylation and thermal decomposition at its interfacial sheath, where pressure gradients and temperatures are still high enough to induce thermal effects. On the contrary, the resonance radii of bubbles excited by medium frequency (300–1000 kHz) ultrasound waves are extremely small (4.6 ␮m at 500 kHz), giving rise to very short-lived (0.4 ␮s on the average) and mainly void or vapor-filled “transient” cavitations [15,27]. The pressures and temperatures developed in such cavities are much higher than found in “stable” cavities, and larger energies are released into the sur-

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rounding liquid during their more rapid and violent collapse [46,49,50]. Furthermore, such cavitations are so short lived and the collapse is so rapid that the time for appreciable degrees of radical scavenging reactions in the hot bubble or at its interface is insufficient. As a consequence, medium frequency ultrasound waves are highly effective for catalyzing advanced oxidation processes, which are aimed to destroy non-volatile organic solutes in solution. The destructive effect of ultrasound in this condition is due to the high probability of hydroxyl radical transfer into the surrounding liquid during the collapse stage of acoustic cavitations [24,35,48]. It is obvious, then that the selection of the right frequency range (the region of conventional power ultrasound, or that of “sonochemical effects” ultrasound) is of utmost significance for achieving appreciable degrees of decontamination. The choice is based primarily on the physicochemical properties of the contaminating species, such as vapor pressure (or Henry’s constant), solubility and octanol–water partition coefficient [50,51]. Hydrophobic chemicals with high vapor pressures have a strong tendency to diffuse into the gaseous bubble interior, and the most effective site for their destruction, therefore, is the bubble-liquid interface and/or the bubble itself [52–54]. Hence, aqueous solutions contaminated with volatile pollutants should preferably be exposed to power ultrasound (whereby long-lived “stable” cavities are generated) for thermal and oxidation effects in the gas phase and the gas–liquid interface [45,53–55]. In contrast, hydrophilic compounds with low vapor pressures and low concentrations tend to remain in the bulk liquid during irradiation, due to the repulsive forces exerted to-and-from the slightly hydrophobic bubble surfaces. The major reaction site for these chemicals, therefore, is the liquid medium, where they may be effectively destroyed by oxidative degradation, provided that sufficient quantities of hydroxyl radicals are ejected into the solution during cavity collapse. As stated previously, maximum radical transfer into the bulk medium occurs when the collapse is “transient”, or when sonication is carried out via medium frequency ultrasound waves. Moreover, at this frequency and at high concentrations of such solutes, an additional destructive pathway via thermal decomposition was observed, as demonstrated by the formation of pyrolysis products along with hydroxy-

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lated intermediates during sonolysis at 300–500 kHz [56,57]. The extent of destruction by pyrolytic fragmentation of non-volatile contaminants is directly related to their concentration and hydrophobicity, which dictates their ability to migrate towards the bubble and/or to accumulate at the bubble–liquid interface [33]. Consequently, the probable site for thermal decomposition of non-volatile solutes is the interfacial bubble sheath, at which solutes may accumulate via adsorptive processes during the formation and growth of acoustic cavities. At appreciable concentrations, the adsorptive tendency of non-volatile solutes on non-polar surfaces of cavity bubbles was verified by the exhibition of saturation type kinetics, typical of Langmuirian behavior, which is commonly proposed for describing photocatalytic process kinetics [33].

4. Figures-of-merits for optimizing sonochemical reaction systems The main concern of scientists and engineers working with ultrasonic systems is to accomplish maximum reaction yields and/or maximum pollutant destruction at optimal conditions. Research and development in sonochemical systems exposed the significance of two basic strategies for maximizing reaction efficiencies: (i) optimization of power and reactor configuration and/or (ii) enhancement of cavitation [32,51]. The first strategy requires a mechanistic approach with features like: (i) selection of the transducer (piezoelectric or magnetic material that converts electrical impulses to mechanical vibrations) and generator (probe types for low-frequency, and plate-types for high frequency effects), (b) configuration and dimensioning of the reaction cell, and (iii) optimization of the power efficiency (i.e. the effective power density delivered to the reaction medium). These aspects have been thoroughly described and reviewed by others in the last decade [14,32,51,58,59]. The second strategy, i.e. the enhancement of cavitation to maximize chemical reactivity involves the addition of different gases and solids into the system to test and compare their effectiveness in increasing reaction yields and/or reaction rates. Aqueous media free of impurities are well known with their tremendously high cavitation threshold, so that any kind of interference to create “liquid defects” in the structure is reported to favor cavitation events

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[32]. The easiest way of creating impurities in sonicated water is by saturating the solution with a soluble gas, which speeds up the initialization of cavity formation via provision of excess nuclei [32], while enhancing the collapse conditions by increasing the temperature within cavity bubbles [35,40]. However, since the first effect of cavitation is degassing, the solution will rapidly be free of dissolved gases if gas entrainment is ceased during sonication. It is common practice, therefore, to bubble the liquid incessantly with a gas throughout the sonication period to maintain a constant gas flow into the bubbles so as to sustain the “extreme” conditions of collapse. The selection of this gas is also of significance, because the final temperature of a collapsing bubble is closely related (by a power function) to a gas parameter, called the “polytropic γ ratio”, i.e. the ratio of specific heats (Cp /Cv ) of the ambient gases entrapped in the bubble [26,40]. The nature of the saturating gas is further important, due to the inverse relationship between thermal conductivity of a gas and the temperature build-up in a cavity bubble [35,51]. Sonoluminescence studies with rare gases have shown that as the thermal conductivity decreases in the order: Xe < Kr < Ar < Ne < He, the amount of heat loss to the surrounding liquid (due to heat conduction in the gas) also decreases, the collapse approaching perfect adiabatic conditions [40,51]. Hence, sonolytic radical yields increase with increasing effective temperature of collapsing bubbles, i.e. with increasing γ ratios, but decrease with increasing thermal conductivities of ambient

gases [51]. Furthermore, rare gases are more effective than diatomic gases (and air), owing to higher γ ratios obtained with monatomic gases in water [20,40,51]. However, despite the equal γ ratios of Argon and Helium in water, much higher yields of pyrolysis products were detected with the former, as attributed to its 10-fold lower thermal conductivity [47]. The addition of solid catalysts, such as glass beads, ceramic disks, SiO2 , Al2 O3 and talc into the reaction medium is another common method for enhancing cavitation effects. The presence of such material is reported to be especially useful for micronization of species (in ultrasonic cell disruption), and for the abrasion, activation and alteration of the chemical properties of catalyst surfaces during ultrasonic irradiation of liquid media [32].

5. Applications in environmental remediation The preliminary steps before application are the selection of material, equipment and sonicator, followed by the design and configuration of the reactor. For best results, it is very important that these features are resolved by collaborative work between the researcher and manufacturer through exchange of ideas, definition of goals, and discussion of expected drawbacks and outputs. Hence, experimental schemes are usually goal-specific, but may be broadly generalized in terms of the operational frequency, which determines the type of the sonicating device suitable with the prop-

Fig. 2. A typical batch reactior using a horn type sonicator to emit low-frequency ultrasound waves into the reaction medium.

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Fig. 3. A typical reaction scheme using a plate type transducer to emit high-frequency ultrasound waves into solution from the bottom of a batch reactor.

erties of reaction medium and target contaminants. In accordance, typical reactor schemes used in lab-scale applications with low and medium frequency irradiation to render ultrasonic destruction of volatile and non-volatile substrates are suggested in Figs. 2 and 3, respectively. Generation of sonochemical reaction conditions in environmental remediation processes provides pollutant destruction either directly via activating thermal decomposition reactions, or indirectly by the production and/or enhancement of hydroxyl radical yields in advanced oxidation processes. Many of such studies are focused on parametric and kinetic analyses of contaminant degradations, and comparison of reaction efficiencies with those conducted in the absence of ultrasound. The input concentrations are generally low (10−9 to 10−3 moles/l), as typical of refractory organics found in natural waters and conventionally treated effluents. Most commonly tested chemicals are those that contain substituted aromatic groups, like phenol, nitrophenol, chlorophenol, azo-benzene and toluene. In addition, reports of some specific studies with chlorinated solvents, herbicides, substituted ethers, natural organic matter, surfactants, reactive textile dyestuff, and chlorofluorocarbons are available in the literature. A brief summary of some of the reported work is given below. Irradiation of dilute solutions of phenol with low and high frequency ultrasound showed that degrada-

tion was much faster with high frequency ultrasound (487 kHz), the pathway being advanced oxidation, as verified by the formation of typical oxidation intermediates (hydroquinone, cathecol and benzoquinone) and the lack of pyrolysis products [35,60]. In another study by Drijvers et al. [54], the decomposition of phenol and trichloroethylene (TCE) were investigated under the combined effect of sonolysis at 520 kHz and chemical oxidation with hydrogen peroxide, using solid catalysts such as Al2 O3 , ZnO, Ni2 O3 and CuO. The authors reported that while TCE was not at all effected by the addition of H2 O2 and solid catalysts, the degradation of phenol was largely enhanced by the presence of H2 O2 /CuO. They attributed this different behavior to the difference in the hydrophobicity of the two compounds. A study involving the sonochemical decomposition of p-nitrophenol (p-NP) in the presence and absence of strong scavengers of hydroxyl radicals such as humic acid reported that the rate of degradation was not significantly affected by the concentration of hydroxyl scavengers below a threshold value [61]. Hence, it was suggested that pyrolysis is the main reaction channel for the decay of this compound during sonication. The result was consistent with previous work showing that p-NP decays via first-order reaction kinetics near the hot interface of cavity bubbles due to its thermal instability at temperatures over 160◦ C [53]. The authors of the referred study have underlain the pH sen-

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sitivity of the decomposition reactions by addressing the decrease in the rate of degradation of p-NP upon increased concentrations of phosphate and bicarbonate ions [53]. They claimed that at pH values higher than pKa of p-NP, the molecule becomes negatively charged and is repulsed by bubble surfaces, thus being displaced away from the interfacial region where the main decomposition reactions are expected to occur. Serpone et al. [33] studied the kinetics of 2-, 3-, and 4-chlorophenol decomposition in air equilibrated media by low frequency ultrasound irradiation. They reported that reaction products and kinetics were parallel to those observed in heterocatalytic oxidation of these compounds with semiconductor particles. Sonochemical treatment of wastewaters contaminated with benzene and toluene in a “parallel plate near field acoustic processor” was shown by Thoma et al. [62] to be highly effective for the destruction of both compounds within reasonable energy requirements. The authors further found that the degradation of parent molecules followed first-order reaction kinetics, and the rate constants in each case were inversely proportional to the initial concentration of the compound. Petrier et al. [35] studied the degradation of pentachlorophenate in argon, air and oxygen saturated aqueous solutions under 20 and 530 kHz ultrasonic irradiation. They reported that the higher frequency was considerably more effective, and the degradation was faster when the solution was bubbled with argon than when bubbled with O2 or air. Kang and Hoffmann [63] studied the kinetics and mechanism of sonolytic destruction of methyl tert-butyl ether (MTBE) under the effect of ozone gas and ultrasonic irradiation at 205 kHz. They reported that ozone accelerates the first-order degradation rate of MTBE by sonolysis at decreasing initial concentrations of the compound. They also observed that the presence of carbonate and bicarbonate ions in solution as potential competitive reagents for hydroxyl radicals did not lower the rate of MTBE degradation, concluding that the degradation occurred at the interface of the cavitation bubble, not in the liquid bulk. David et al. [64] reported that the systematic herbicides chlorpropham and 3-chloroaniline are destroyed more effectively at 482 kHz than at 20 kHz. Chemical analyses of effluent samples showed that while the mechanism of destruction for chlorpropham was py-

rolytic decomposition inside the gaseous bubble, 3chloroaniline was destroyed mainly by radical mechanisms and oxidative degradation in the solution bulk. The degradation of fulvic acid (as a representative of natural organic matter) under the combined effect of low frequency ultrasonic irradiation and ozonolysis was studied by monitoring the time rate of change in total organic carbon content of the solution during contact [65]. The combination was reported to provide a significant advantage as total mineralization of organic carbon, which could be achieved neither by ozone nor ultrasound alone. It was concluded that the ultrasound/ozone process extends the application of sonochemical techniques to the catalysis of advanced oxidation processes for the removal of refractory organic electrolytes in natural water. Alegria et al. [52] studied the mechanism and site of degradation of non-volatile surfactants by ultrasonic irradiation at 50 kHz, using argon as the saturating gas. They reported that surfactants orient themselves radially in the interfacial region with their polar head groups pointing to the bulk solution. This was verified by the detection of pyrolysis products and the observation of a strong inhibition of hydrogen peroxide production with increasing surfactant concentration, as opposed to the sonolysis of solutions containing non-volatile non-surfactants. Vinodgobal and Kamat [13] reported the results of three hydroxyl radical mediated oxidation reactions (photocatalysis, ␥-radiolysis and sonolysis) for the degradation of reactive dye, Acid Orange-7 under the effect of saturation with O2 . They noted the similarity of reaction pathways in all three processes, as made evident from the single identifiable intermediate produced in all experiments, and concluded that textile azo dyes are effectively destroyed by advanced oxidation, or any hydroxyl radical mediated reaction pathways. In another study, the authors investigated the ultrasonic mineralization of a textile dye, Remazol Black-B at 640 kHz under a stream of O2 gas [57]. They reported that dye destruction starts in the solution bulk with the rupture of the azo-bond by • OH attack, and complete decolorization is accompanied by total mineralization, provided that sufficient contact is allowed. The authors concluded that ultrasonic remediation should be considered as a solution alternative to the reuse and/or recycle of textile dyeing mill effluents as process water.

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Joseph et al. [66] investigated the effect of Fenton’s Reaction on the degradation of azo-benzene and some related azo dyes at 500 kHz. As in the previous work of Vinodgobal and Kamat [57], these authors also reported that the first step in the reaction scheme was the cleavage of the azo-double bond upon hydroxyl radical attack. Fenton’s Reaction at optimal Fe(II) concentrations was found to induce a threefold increase in the reaction rates. Finally, they reported that saturating the solution with Ar instead of O2 accelerated the reactions by 10%. Cheung and Kurup [67] studied the sonochemical destruction of two chlorofluoro-carbons, CFC11 and CFC113 in dilute aqueous solutions under the effect of power ultrasound at 20 kHz. They observed that the reactions were governed by first-order kinetics, and the observed rate constants for both compounds were found to decline slightly with an increase in solution temperature from 5 to 10◦ C. The result being in agreement with temperature effects observed with other chlorinated hydrocarbons [68] was attributed to the decrease in the cavitation intensity with an increase in solvent vapor pressure. The authors further reported that the rapid rate of degradation was not affected by the very slight degree of parent compound volatilization (5%) during the experiments. Gonze et al. [69] investigated the effect of ultrasonic irradiation (500 kHz) as a pre-treatment operation to reduce the toxicity, or to improve the biodegradability in untreated effluents prior to secondary treatment. Experiments were carried out with synthetic effluents contaminated with sodium pentachlorophenate, and acute toxicity tests were conducted using Vibrio fischeri and Daphnia magna as test organisms. The authors reported that ultrasound is a highly effective pre-treatment tool by virtue of its potential to reduce the toxicity of untreated wastewaters, while rapidly improving their biodegradability. Finally, there is supportive evidence that power ultrasound provides ultimate bacterial destruction in infected waters, and the rate of the process increases with solids addition into the reaction medium [70].

6. Conclusions Catalysis of chemical reactivity by ultrasonic pressure waves is an emerging technology for water and

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effluent remediation, owing to the advantages of “high energy” chemistry induced by “very extreme” conditions achieved during the rapid collapse of cavitation bubbles. The ability of sound waves to catalyze the decomposition of refractory organic compounds in water must be considered a major advantage over currently practiced advanced treatment technologies, where intensive chemical and energy inputs are required for acceptable degrees of destruction. Furthermore, sonolysis adds a unique dimension to AO systems, with the ability of ultrasound waves to be transmitted perfectly through opaque systems, unlike that of ultraviolet light. If environmental applications of ultrasonic techniques emerge as post-treatment schemes for destructive removal of refractory compounds in effluent streams, they will mostly require the use of medium-frequency ultrasound, since such chemicals are usually macromolecules with complex molecular structures and hydrophilic properties. It is fortunate that reactor systems designed for medium frequency irradiation are relatively easier to maintain than those operated with power ultrasound, due to the drawbacks associated with the latter as noise and cavitational erosion. Such problems, however, may be overcome by sound-proof material and the proper selection, configuration and maintenance of the equipment.

Acknowledgements The work presented here is part of a State funded project DPT98K120900. The authors thank the Prime Ministry of Turkey for the funding. References [1] L.C. Bauman, M.K. Stenstrom, Water Res. 24 (8) (1990) 949. [2] K. Kusakabe, S. Aso, T. Wada, J. Hayashi, S. Moroka, K. Isomuta, Water Res. 25 (10) (1991) 1199. [3] N.H. Ince, M.I. Stefan, J.R. Bolton, J. Adv. Oxid. Technol. 2 (3) (1997) 442. [4] N.H. Ince, Water Environ. Res. 70 (6) (1998) 1161. [5] O. Legrini, E. Oliveros, A.M. Braun, Chem. Rev. (1993) 671. [6] N. Serpone, E. Pelizetti (Eds.), Photocatalysis, Fundementals and Applications, Wiley, New York, 1989. [7] C. Morrison, J. Bandara, J. Kiwi, J. Adv. Oxid. Technol. 1 (2) (1996) 160. [8] M.R. Hoffman, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69.

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