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REVIEWS Sonochemistry: Environmental Science and Engineering Applications Yusuf G. Adewuyi* North Carolina A and T State University, Department of Chemical Engineering, Greensboro, North Carolina 27411

Sonochemical engineering is a field involving the application of sonic and ultrasonic waves to chemical processing. Sonochemistry enhances or promotes chemical reactions and mass transfer. It offers the potential for shorter reaction cycles, cheaper reagents, and less extreme physical conditions, leading to less expensive and perhaps smaller plants. The amount of things that can be accomplished with sonochemistry is, at this stage, only limited by the minds of those working in this exciting field. Existing literature on sonochemical reacting systems is chemistryintensive, and applications of this novel means of reaction in environmental remediation and pollution prevention seem almost unlimited. For example, environmental sonochemistry is a rapidly growing area that deals with the destruction of organics in aqueous solutions. However, some theoretical and engineering aspects are not fully understood. This paper reviews the field comprehensively by combining the existing knowledge from chemistry with insights into the pathways and kinetic analysis of environmental sonochemical reacting systems and with challenges for large-scale applications. The review is intended to advance our understanding and outline directions for future research. Contents 1. Introduction 2. Theory 2.1 Fundamentals of Ultrasound 2.2 Factors Affecting Aqueous Sonochemical Processes 2.3 Fundamentals of Sonochemical Reactions 3. Types of Pollutants 4. Prior Literature 4.1 Aromatic Compounds 4.2 Chlorinated Aliphatic Hydrocarbons 4.3 Explosives 4.4 Herbicides and Pesticides 4.5 Organic Dyes 4.6 Organic and Inorganic Gaseous Pollutants 4.7 Organic Sulfur Compounds 4.8 Oxygenates and Alcohols 4.9 Other Organic Compounds 4.10 Other Environmental Applications 5. Discussion 5.1 Reaction Pathways and Kinetics 5.2 Effects of Water Quality 5.3 Sonication Byproducts and Toxicity Effects 5.4 Efficiency and Scale-up Issues

6. Concluding Remarks 7. Literature Cited 4681 4682 4682 4682 4683 4686 4686 4686 4700 4702 4703 4704 4704 4706 4706 4706 4707 4707 4708 4708 4709 4709

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1. Introduction When ultrasonic (or sonic) energy at high powers more than 1/3 W/cm2 for water at room temperaturesis applied to a liquid, a “cold boiling” termed cavitation takes place. Simply put, cavitation is the formation, growth, and sudden collapse of bubbles in liquids.1,2 Ultrasonic vibration reduces the thickness of liquid films, enhances gas transfer, and reduces bubble coalescence, which increases the interfacial area for gas transfer.3-6 For example, the diffusion of liquids through porous media is enhanced by ultrasound. Ultrasound can be used to separate gases because lighter molecules in an ultrasonic field will travel further than heavier ones. Ultrasonic energy is also used to remove contaminants from air and to break down toxic compounds in water and soil.6 Nearly half of the 189 hazardous air pollutants (“air toxics”) regulated by the Clean Air Act Amendment (CAAA) of 1990 are volatile organic compounds (VOCs). This diverse list includes common solvents or halogenated aliphatic compounds, such as methylene chloride, chloroform, and trichloroethylene, all of which are mineralized by ultrasonic irradiation. 7-14 The term mineralization implies the final products of degradation reactions, which are carbon dioxide, short-chain organic acids, and/or inorganic ions. Benzene, well-known for its resistance to the action of strong * Phone: 336-334-7564.Fax: 336-334-7904.E-mail: adewuyi@ ncat.edu.

10.1021/ie010096l CCC: $20.00 © 2001 American Chemical Society Published on Web 10/04/2001

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Table 1. Some Advanced Oxidation Technologies (i) Fenton-Type Reactions Fe2+ + H2O2 f •OH + Fe3+ + OH(ii) Ozone-Peroxide-UV Systems O3 + -OH f O2- f •OH 3O3 + UV (<400 nm) f 2 •OH H2O2 + UV (<400 nm) f 2 •OH H2O2 + O3 f 2•OH H2O2 + O3 + UV f •OH (iii) Semiconductor Oxides-UV Systems TiO2 + hv f TiO2 (h+ + e-) H+ + OH- f •OH (iv) Radiolysis (High-Energy Beams): H2O f e-aq + H• + •OH + (H2, H2O2, H3O+) (v) Wet Oxidation (WO) Systems: RH + O2 f R• + HO2• RH + HO2• f R• + H2O2 H2O2 + M f 2 OH• RH + OH• f R• + H2O R• + O2 f ROO• ROO• + RH f ROOH + R• (vi) Sonolysis (Ultrasound) H2O f H• + •OH

oxidants, succumbs under ultrasonication in aqueous medium.15 Sonochemical oxidation techniques involve the use of sonic or ultrasonic waves to produce an oxidative environment via cavitation that yields localized microbubbles and supercritical regions in the aqueous phase.16 The collapse of these bubbles leads to surprisingly high local temperatures and pressures. Locally, the high temperature and pressure may reach up to and above 5000 K and 1000 atm, respectively.17,18 These rather extreme conditions are very short-lived but have shown to result in the generation of highly reactive species including hydroxyl (OH•), hydrogen (H•) and hydroperoxyl (HO2•) radicals, and hydrogen peroxide.19-23 These radicals are capable of initiating or promoting many fast reduction-oxidation (REDOX) reactions. These reactions with inorganic and organic substrates are fast and often near the diffusion-controlled rate.24-25 Sonochemistry is an example of advanced oxidation processes (or AOPs).26,27 As shown in Table 1, AOPs owe their enhanced reactivity, as least in part to the generation of reactive free radicals, the most important of which is the excited hydroxyl radical (•OH). A number of studies have documented the role of sonochemistry in homogeneous and heterogeneous chemistry.28-30 Although the phenomena of sonochemistry has been recognized for many years and despite its recent advances, the mechanisms of homogeneous and heterogeneous sonochemistry are not fully understood. It is only recently that applications in synthesis and pollution control have prompted interest in industrial scale operation.31,32 Thompson and Doraiswamy33 and Moholkar et al.34 recently reviewed the fundamentals and science and engineering aspects of ultrasound and its applications to organic synthesis. Luche28 discusses the use of sonochemistry in organometallic synthesis, biphasic systems, catalytic reactions, and organic electrochemistry and the practical considerations for process optimization. However, the applications of this novel means of reaction in environmental remediation and pollution prevention seem almost unlimited. Sonication improves mass transfer and chemical reaction and is expected to reduce or eliminate chemical usage, resulting in minimal sludge and disposal problems. This paper reviews the field of environmental sonochemistry comprehensively by combin-

ing the existing knowledge from chemistry with insights into the pathways and kinetic analysis of environmental sonochemical reacting systems and with challenges for long-term reliability and economical scaleup. 2. Theory 2.1. Fundamentals of Ultrasound. Ultrasound are waves at frequencies above those within the hearing range of the average person, i.e., at frequencies above 16 kHz (16 000 cycles per second).1,2 Ultrasonic energy (high frequency sound waves) produces an alternating adiabatic compression and rarefaction of the liquid media being irradiated. In the rarefaction part of the ultrasonic wave (when the liquid is unduly stretched or “torn apart”), microbubbles form because of reduced pressure (i.e., sufficiently large negative pressures). These microbubbles contain vaporized liquid or gas that was previously dissolved in the liquid. The microbubbles can be either stable about their average size for many cycles or transient when they grow to certain size and violently collapse or implode during the compression part of the wave. The critical size depends on the liquid and the frequency of sound; at 20 kHz, for example, it is roughly 100-170 µm. The implosions are the spectacular part of sonochemistry. The energy put into the liquid to create the microvoids is released in this part of the wave, creating high local pressures up to 1000 atm and high transitory temperatures up to 5000 K.17-21 This energy-releasing phenomena of the bubble formation and collapse is simply called cavitation or (“cold boiling”), or for the case described above, acoustic cavitation.1,35-37 Cavitation can also be achieved by throttling a valve downstream from a pump. When pressure at an orifice or any other mechanical constriction falls below the vapor pressure of liquid, cavitations are generated which then collapse downstream with a recovery of pressure, giving rise to high temperature and pressure pulses. Cavitation achieved from this mechanism is termed hydrodynamic cavitation.38 2.2. Factors Affecting Aqueous Sonochemical Processes. Sonochemistry is complicated by the fact that the nature or the physicochemical properties of the solvent, solute, or gas in the bubble can have dramatic effect on the cavitational collapse.37 Cavities are more readily formed when using solvents with high vapor pressure (VP), low viscosity (µ), and low surface tension (σ); however, the intensity of cavitation is benefited by using solvents of opposite characteristics (i.e., low VP; high µ, σ, and density, F). The intermolecular forces in the liquid must be overcome in order to form the bubbles. Thus, solvents with high densities, surface tensions, and viscosities generally have higher threshold for cavitation but more harsh conditions once cavitation begins.35 There are several properties of gases that can affect sonochemical activities.5 The heat capacity ratio (Cp/Cv) or polytropic ratio (γ) of the gas in the bubble affects the amount of heat released and, hence the final temperature produced in an adiabatic compression and the cause of reaction. Assuming adiabatic bubble collapse, the maximum temperatures and pressures within the collapsed cavitation bubbles (eqs 1 and 2) are predicted by Noltingk and Nepprias from approximate solutions of Rayleigh-Plesset equations.39-40

Tmax ) To

[

]

Pa(γ - 1) Pv

(1)

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{

Pmax ) Pv

}

Pa(γ - 1) Pv

[γ/γ-1]

(2)

where To ) ambient (experimental) temperature or temperature of bulk solution, Pv ) pressure in the bubble at its maximum size or the vapor pressure of the solution, Pa ) pressure in the bubble at the moment of transient collapse (i.e., acoustic pressure), and γ ) polytropic index of the cavity medium.5 As seen from these equations, higher temperatures and pressures are generated with monatomic gases with higher γ than those with polyatomic gases with lower γ. Another parameter that affects cavitational collapse is the thermal conductivity of the gas. Although compression is adiabatic in the sonochemical process, small amounts of heat are transferred to the bulk liquid. A gas with low thermal conductivity reduces heat dissipation from cavitation site following adiabatic collapse and should favor higher collapse temperature compared with high thermal conductivity gas. In addition, the gas with the higher thermal conductivity reduces the temperature achieved in a collapse. The solubility of the gas in the liquid used is also an important aspect; the more soluble the gas, the more likely it is to diffuse into the cavitation bubble. Dissolved gases form the nuclei for cavitation. Soluble gases should result in the formation of larger number of cavitation nuclei and extensive bubble collapse since these gases are readily forced back to the aqueous phase. As expected from sonochemical reactions, lowering temperature increases the rate of reaction unlike most chemical reacting systems. This is attributed to the lowering of the solvent vapor pressure which increases the intensity of cavitation. At low vapor pressure, less vapor has an opportunity to diffuse into the bubble and thus cushion the cavitational collapse, therefore making the implosion more violent. Also, as liquid temperature decreases, the amount of gas dissolved increases and the vapor pressure of the liquid decreases. Very volatile solvents lead to relatively high pressures in the bubble and also “cushion” the collapse. In some cases, the increase of temperature may favor the reaction kinetics to a point and further increase in reaction temperature leads to a decrease in the reaction rate.41 In the case of a progrssive planar or spherical wave, the acoustic (or sound) intensity (in W m-2) is directly related to acoustic pressure by eq 3: 2

Pa I) 2Fc

(3)

where F is the density of the fluid (e.g., water) and c is the speed of sound in the fluid (1500m/s in water). The acoustic power (W) represents the intensity emitted by a given surface. The term Fc represents the acoustic impedance (Z) of the medium. Values of Z for air, water, benzene, and ethanol are 400, 1.5 × 106, 1.1 × 106, and 0.95 × 106, kg m-2 s-1 respectively.28 The literature points to the conclusion that some increase in the pressure of the system should increase the reaction rate due to the magnified effect of cavitation implosions. However, as pressure increases, the intensity must be increased to obtain cavitation in the first place. Too much pressure reduces the rate of reaction by decreasing the frequency or efficiency of bubble formations.37 An increase in ultrasound intensity means an increase in the acoustic amplitude (i.e., Pa). The collapse time,

the temperature, and the pressure on collapse are all dependent on acoustic amplitude; the cavitation bubble collapse will be more violent at higher acoustic amplitudes. An increase in intensity will thus result in greater sonochemical effects in the collapsing bubble.5,28 The power delivered to a system depends to some extent on the frequency level. In most cases, as the power is increased, the reaction rate also increases. At a critical power level, increasing the power will decrease the rate of reaction.42 Power input to the system is dependent on the amplitude. By increasing the amplitude, the power is also increased.42,43 Sonochemical activity rises with increasing intensity to an optimum above which efficiency falls. According to Raleigh, the main condition of effective action for ultrasonic cavitation is that the time of cavity collapse should be smaller than half the ultrasonics period (τ < T/2), as shown in eq 4.44,45 For a bubble under constant external pressure (hydrostatic) from an initial or maximum radius, Rmax, to some final radius, the relation is given by

τ ≈ 0.915Rmax

( ) F Ph

1/2

<

T 2

(4)

where τ ) time of cavitation bubble collapse, Rmax ) the maximum radius of cavitation bubble, T ) ultrasonic period, Ph ) hydrostatic pressure, and F ) density of liquid.28,44-45 When the acoustic power increases and simultaneously increases amplitude of vibration, the maximum radius of the cavity bubble also increases, as well as its time of collapse, τ, and this bubble is not able to collapse within time equal half of the period. That is, before the sound field reverses itself, and the rarefaction phase begins acting on the collapsing bubble. Frequency has significant effect on the cavitation process because it alters the critical size of the cavitational bubble.46-51 At very high frequencies, the cavitational effect is reduced because either (i) the rarefaction cycle of the sound wave produces a negative pressure which is insufficient in its duration and/or intensity to initiate cavitation or (ii) the compression cycle occurs faster than the time for the microbubble to collapse. Lower frequency ultrasound produces more violent cavitation, leading to higher localized temperatures and pressures. However, current research indicates that in reactions such as oxidations, higher frequencies may lead to higher reaction rates. This is due to the fact that higher frequency may actually increase the number of free radicals in the system because although cavitation is less violent, there are more cavitation events and thus more opportunities for the free radicals to be produced. Francony and Petrier observed the ultrasonic degradation of carbon tetrachloride was enhanced and the yield of products faster when using 500 kHz ultrasound compared with 20 kHz.48 But at very high frequencies, the cavitation process is decreased. Entezari and Kruus studied the sonochemical reaction rate of oxidation of iodide at different temperatures (0-50 °C) and with different ultrasonic horns at low frequency (20 kHz) and with a high-frequency (900 kHz) apparatus.50 The results showed that at 900 kHz the rate of oxidation increased up to 30 °C at lower power levels whereas at 20 kHz, the rate of oxidation decreased with increasing temperature. 2.3. Fundamentals of Sonochemical Reactions. The influence of ultrasonic energy on chemical activity

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Figure 1. Three reaction zones in the cavitation process.

may involve any or all of the following: production of heat, promotion of mixing (stirring) or mass transfer, promotion of intimate contact between materials, dispersion of contaminated layers of chemicals, and production of free-chemical radicals.1-3,52-53 The physical effects of ultrasound can enhance the reactivity of a catalyst by enlarging the surface area or accelerate a reaction by proper mixing of reagents. The chemical effects of ultrasound enhance reaction rates because of the formation of highly reactive radical species formed during cavitation.4,21 Homogeneous sonochemistry examines, mainly in the liquid phase, the activity of the radicals or excited species formed in the bubble gas phase (OH•, H•, X•, C2•, CN/, etc.) during the violent implosion and their possible release into the liquid.28 The cavitation event also gives rise to acoustic microstreaming or formation of miniature eddies that enhances the mass and heat transfer in the liquid, and also causes velocity gradients that results in shear forces. In heterogeneous sonochemistry, the mechanical effects of cavitation resulting from the erosion action of microjets formed during asymmetric collapse of bubbles at the vicinity of interfaces are also important.28 So far, four theories have been proposed to explain the sonochemical events: (1) hot-spot theory; (2) “electrical” theory; (3) “plasma discharge” theory, and (4) supercritical theory. These have led to several modes of reactivity being proposed: pyrolytic decomposition, hydroxyl radical oxidation, plasma chemistry, and supercritical water oxidation. The “hot-spot” theory suggests that a pressure of thousands of atmosphere (up to 1000 atm) is generated and a temperature of about 5000 K results during the violent collapse of the bubble.17-19 Both Margulis54 and Lepoint55 advocate that the extreme conditions associated with fragmentative collapse are due to the intense electrical fields. The “electrical” theory by Margulis suggests that during bubble formation and collapse, enormous electrical field gradients are generated and these are sufficiently high to cause bond breakage and chemical activity.22,54 The “plasma theory” by Lepoint and Mullie also suggests the extreme condi-

tions associated with the fragmentative collapse is due to intense electrical fields and seems not to involve a true implosion. They liken the origin of cavitation chemistry to corona-like discharges caused by a fragmentation process and supported their views by drawing numerous analogies between sonochemistry and corona chemistry and indicating the formation of microplasmas inside the bubbles.55 The supercritical theory recently proposed by Hoffmann56 suggests the existence of a layer in the bubble-solution interface where temperature and pressure may be beyond the critical conditions of water (647 K, 22.1 MPa) and which may have physical properties intermediate between those of a gas and a liquid. They showed that supercritical water is obtained during the collapse of cavitation bubbles generated sonolytically. In general, most studies in environmental sonochemistry have adopted the “hot spot” concepts to explain experimental results. This theory considers a sonochemical reaction as a highly heterogeneous reaction in which reactive species and heat are produced from a welldefined microreactor, “the bubble of cavitation.21,57 In the “structured hot spot” model shown in Figure 1, three regions for the occurrence of chemical reactions are postulated: (1) a hot gaseous nucleus; (2) an interfacial region with radial gradient in temperature and local radical density; and (3) the bulk solution at ambient temperature. Reactions involving free radicals can occur within the collapsing bubble, at the interface of the bubble, and in the surrounding liquid. Within the center of the bubble, harsh conditions generated on bubble collapse cause bond breakage and/or the dissociation of the water and other vapors and gases, leading to the formation of free radicals or the formation of excited states. Solvent and/or substrates suffer homolytic bond breakage to produce reactive species. The high temperatures and pressures created during cavitation provide the activation energy required for the bond cleavage. The radicals generated either react with each other to form new molecules and radicals or diffuse into the bulk liquid to serve as oxidants. The second reaction site is

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4685 Table 2. Proposed Kinetic Mechanisms H2O f H• + •OH H• + H• f H2 H• + O2 f HO2• HO2• + HO2• f H2O2 + O2 HO2• + HO2• f H2O + 3/2O2 H• + HO2• f H2O2

N2 f 2N• N• + •OH f NO + H• NO + •OH f HNO2 NO + •OH f NO2 + H• NO + H• f N• + •OH 2 NO2(aq) + H2O f HNO2 + HNO3 NO2 + •OH f HNO3

O2 f 2O• O• + H2O f 2•OH O• + O2 fO3 H• + O2 f •OH + O• (or HO2•)

•OH

+ •H + M f H2O + M 2•OH f •O + H2O 2•OH + M f H2O2 + M 2H• + M f H2 + M 2O• + M f O2 + M

N2 + O2 f 2NO N2O• + O• f 2NO

H2 f H• + H•

CO2 + •H f CO + •OH CO2 f CO + O• 2H• + O• f H2O

A. Water Dissociation + •OH f H2O2 •OH + •OH f H O + O• 2 •OH + •HO f H O + O 2 2 2 O• + O• f O2 1/2O2 + 2H• f H2O H• + OH• f H2O

R7 R8 R9 R10 R11 R12

H• + H2O2 f •OH + H2O H• + H2O2 f H2 + HO2• •OH + H O f HO • + H O 2 2 2 2 •OH + H f H O + H• 2 2 H2O + •OH f H2O2 + H•

R13 R14 R15 R16 R17

R18 R19 R20 R21 R22 R23 R24

B. Under N2 Atmosphere As in R1 Plus NO + NO2 f N2O3 N2O3 + H2O f 2HNO2 N• + H• f :NH :NH + :NH f N2 + H2 N2 + •OH f N2O + H• N2 + O• f N2O (or NO + N•) N• + NO f N2 + O

R25 R26 R27 R28 R29 R30 R31

NO + NO f N2O + O N2O + O• f 2NO (or N2 + O2) N2O + N2 f O• N• + O2 f NO + O• NO + O f NO2• 2NO + O2 f 2NO2

R32 R33 R34 R35 R36 R37

R38 R39 R40 R41

C. Under O2 Atmosphere As in R1 Plus O• + H2 f •OH + H• O• + HO2• f •OH + O2 O• + H2O2 f •OH + HO2• HO2• f •OH + O•

R42 R43 R44 R45

O3 + O f 2O2 2HO2• f H2O2 + O2 2•OH f H2O2 2•OH f O• + H2O

R46 R47 R48 R49

R50 R51 R52 R53 R54

D. Under Ar Atmosphere As in R1 plus O• + H2O + M f H2O2 + M O2 + H• + M f HO2• + M •OH + H O f HO • + H O 2 2 2 2 2HO2• f H2O2 + O2 HO2• + H• f H2O + O•

R55 R56 R57 R58 R59

O• + H2O f 2•OH Ar f Ar/ Ar/ + H2O f H2O/ + Ar H2O/ f H• + •OH

R60 R61 R62 R63

R64 R65

E. Under Air Atmospehre As in B and C Plus NO2• + O• f NO3 NO2 + NO3 f N2O5

R66 R67

N2O5 + H2O f 2HNO3

R68

R69

F. Under H2 Atmosphere As in R1 Plus H2 + •OH f H2O + H•

R70

G. Under CO2 Atmosphere As in R1 Plus O • + O • f O2 CO + O• f CO2

R74 R75

CO2 + H• f HCOO• HCOO• + H• f HCHO + O•

R76 R77

R1 R2 R3 R4 R5 R6

R71 R72 R73

•OH

the liquid shell immediately surrounding the imploding cavity, which has been estimated to heat up to approximately 2000 K during cavity implosion. In this solvent layer surrounding the hot bubble, both combustion and free-radical reactions (involving •OH derived from the decomposition of H2O) occur.23 Reactions here are comparable to pyrolysis reactions. Pyrolysis (i.e., combustion) in the interfacial region is predominant at high solute concentrations, while at low solute concentrations, free-radical reactions are likely to predominate. At this interface between the bubble and bulk liquid, surface-active reagents also accumulate and species produced in the bubble first react with chemicals in the bulk liquid. It has been shown that the majority of degradation takes place in the bubble-bulk interface region.58,59 The liquid reaction zone was estimated to extend ∼200 nm from the bubble surface and had a lifetime of <2 µs.17-19 In the bulk liquid, no primary sonochemical activity takes place although subsequent reactions with ultrasonically generated intermediates may occur. A small number of free radicals produced in the cavities or at the interface may move into the bulkliquid phase and react with the substrate present therein in secondary reactions to form new products. Depending on their physical properties and concentrations, molecules present in the medium will be “burned” in close to the bubble or will undergo radical reactions.

For example, reactions R4 and R7 in Table 2 do not occur in the gaseous phase (where H2O2 is unstable) because of the prevailing high temperatures and pressures. Rather, these reactions occur at the relatively cooler interfacial region. Some of the sonochemical reactions identified in the cavitational events are summarized in Table 2. The formation of •H and •OH is attributed to the thermal dissociation of water vapor present in the cavities during the compression phase (R1). Sonolysis of water also produces H2O2 and hydrogen gas, via hydroxyl radicals and hydrogen atoms (R2-R7). The presence of oxygen improves sonochemical activities, but it is not essential for water sonolysis, and sonochemical oxidation and reduction can proceed in the presence of any gas. However, in the presence of oxygen acting as a scavenger of hydrogen atom (and thus suppressing the recombination of •H and •OH), the hydroperoxyl radical (HO2•) is additionally formed which is an oxidizing agent (R3). This radical causes a number other reactions to occur resulting in the formation of H2O2, O2, O, and H2 as products (R4-R9). Thermal dissociation of oxygen molecule may also occur, leading to the generation of additional hydroxyl radicals (R38-R39). In the absence of •OH scavengers, the main product of the sonolysis of water is H2O2 (R7). H2O2 can also be produced in an “inert” atmosphere but only at the expense of •OH

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radical (R51). Formation of atomic nitrogen and oxygen, nitrogen fixation (with HNO2 as the major acid component formed) can occur in the cavity.60 Nitrogen molecules inside the cavitation bubble may react at high temperature with hydroxyl radicals and oxygen atoms to give nitrous oxide and nitrogen oxide by a mechanism analogous to that in combustion chemistry (R19, R29R30). N2O is unstable under the high-temperature conditions of the cavitation bubble and is further decomposed in the gas phase and may be transformed to NO (R33).60 NO may undergo further reactions either in the gas-phase of acoustic cavities or free-radical reactions in the cooler interfacial zone, ultimately resulting in the formation of nitrous and nitric acids (R20-R23). Nitrogen fixation is inhibited by H2 and CO under a hydrogen atmosphere; oxidation reactions are almost completely suppressed owing to the strong reducing ability of •H. The inhibiting effect of CO on nitrogen is as follows. The oxygen formed in the irradiated water is used up in the oxidation of the CO introduced into the water with subsequent formation of CO2 (R74). The presence of CO (or CO2 in small amount) may cause formation of HCHO in the cavitation bubble (R75-R76). If CO is introduced with N2 and H2, then HCN and HCHO are major products. HCN, NH3, and HCHO can be formed from solutions saturated with N2, CO, and H2.60 Another mechanism suggested is that in which inert gases penetrating into a cavity can contribute to the transfer of electron excitation to water or hydrogen molecules.22,44,60 For example, the excited argon atom (Ar*) can participate in energy transfer reactions, as shown in reactions (R60-R62). In the presence of H2, argon also facilitates the formation of ‚H by direct dissociation of H2 within the cavitation bubble. 3. Types of Pollutants A number of previous studies have examined the transformation of pollutants by ultrasonic irradiation or combined ultrasound and other advanced oxidation techniques to organic techniques to organic intermediates with mineralizaton to inorganic ions, CO2, and short-chain organic acids as final products in some cases. The pollutants studied and other environmental applications include: (1) Aromatic Compounds • Phenol, 2-, 3-, 4- and 2,4-chlorophenols, p-nitrophenol, and p-nitrophenyl acetate (PNPA) • Benzene, toluene, ethylbenzene, 1,3,5-trimethylbenzene (mesitylene), xylene, fluoro-, bromo-, iodo- and chlorobenzene, hydroxybenzoic acids, humic acids, nitrobenzene, nitro- and chloro-toluene, and styrene • Polycyclic aromatic hydrocarbons (PAHs),- anthracene, phenanthrene, pyrene, byphenyl, and dioxin • Mixtures of chlorophenol and chlorobenzenes (2) Chlorinated Aliphatic Hydrocarbons (CAHs) • Trichloroethylene (TCE) and tetra- or perchloroethylene (PCE) • Carbon tetrachloride (CCl4), chloroform (CHCl3), dichloromethane (CH2Cl2), and methylene chloride (CH3Cl) • 1,1,1-Trichloro and 1,2-dichloroethane • Chloral hydrate • Mixtures of CAHs with phenols, BTEX, or chlorobenzenes

(3) Explosives • 2,4,6-Trinitrotoluene (TNT) • Cyclotrimethylene-trinitramine (RDX) • HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) (4) Herbicides and Pesticides • Herbicides: atracine, alachlor, chlorpropham (isopropyl-3-chlorocarbanilate 3-chloraniline) • Pesticides: pentachlorophenol (PeCP) and pentachlorophenate (PCP), polychlorophenyls (PCBs), parathion (O5-O-diethylo-p-nitrophenyl-thiophosphate), and phenyltrifluoromethyl ketone (PTMK) (5) Organic Dyes • Azo dye, remazol black (RB) • Azo dye, naphthol blue black (NBB) (6) Organic and Inorganic Gaseous Pollutants • Greenhouse gasessfluorotrichlormethane (CFC 11), trifluorotrichloro ethane (CFC 113), nitrous oxide, and carbon dioxide • Hydrocarbonssacetylene, methane, ethane, propane • Hydrogen sulfide • Ozone (7) Organic Sulfur Compounds • Carbon disulfide • Di-n-butylsulfide (8) Oxygenates and Alcohols • Methyl tert-butyl ether (MTBE), methanol, and ethanol • Mixtures of alcohols and chloromethanes • Mixtures of alcohols (i.e., ethanol), polyvinlpyrrolidone (PVP), and tetranitromethane (TNM) (9) Other Organic Compounds • Surfactantsstert-octylphenoxy polyethoxyethanol (Triton X-100), polyoxyethylene alkyl ether (SS 70) • Formic acid and formates • Acetate • Thymine (10) Other Environmental Applications • Industrial wastes of a cyclohexane oxidation unit • Natural groundwater and organic matter • Biological treatment systems: toxicity reduction and disinfection 4. Prior Literature Studies involving the use of sonochemical or photosonochemical processes to treat a variety of chemical contaminants mostly in aqueous systems are summarized in Table 3, highlighting the chemical oxidation schemes and experimental conditions utilized, removal effectiveness, and intermediates and degradation products observed. 4.1. Aromatic Compounds. The ultrasonic degradation of phenol, chlorophenols, and nitrophenols have been studied by a number of investigators. The intermediates and products of the sonochemical oxidation of phenol usually include hydroquinone, catechol, p- and o-benzoquinone, 2,5-dioxohexen-3-dioic, muconic, maleic, succinic, formic, propanoic, oxalic and acetic acids, and CO2. Berlan et al.61 observed that primary degradation products such as dihydroxybenzenes and quinones are further degraded upon time into the low molecular carboxylic acids under mild external conditions (room temperature, atmospheric pressure) due to local extreme conditions resulting from cavitation, without the need for any chemical reagent. Petrier et al.62 found the rate of sonochemical phenol degradation to proceed more rapidly at higher (i.e., 487 kHz) than low (i.e., 20 kHz) frequency with concomitant better release of •OH in the

carbon tetrachloride (CCl4), chloroform (CHCl3) dichloromethane (CH2Cl2), (in aqueous solution of KI) 4-nitrophenol (4-Np)

Alippi et al., 1992

methylene chloride; chloroform, CHCl3; carbon tetrachloride, CCl4; 1,2-dichloroethane, DCA; 1,1,1-trichloroethane, TCA; trichloroethylene (TCE); perchloroethylene (PCE) methanol-water mixtures

chlorpyrifos, 3,3′,4,4′tetrachloroazoxybenzene (TCAOB), 2-chlorobiphenyl, 2,4,8-trichlorodibenzofuran (TCDF), lindane, aldrin, hexachlorobenzene (HCB), mixture of chlorinated olefins, parafins and aromatics phenol (PhOH), sewer waste treatment effluent

carbon tetrachloride (pure CCl4, two-phase water-CCl4 solution, saturated water solution in CCl4)

Bhatnagar and Cheung, 1994

Catallo and Junk, 1995

Chendke and Fogler, 1983

Chen et al., 1971

Buttner et al., 1991

phenol; aniline; 2-chlorophenol

Berlan et al., 1994

Barbier and Petrier, 1996

carbon disulfide (CS2)

contaminants degraded

Adewuyi and Collins, 2001a,b

authors

0-100 vol. % CCl4

PhOH, 100-700 mg/L, solid catalysts, 20-200 mg

3 µg/mL

0-100 vol. % CH3OH

50-350 mg/L aqueous solutions of VOCs and mixtures

100 mg/L

0.5 mM

1.0-5.0 M CCl4, 1.0 M KI

6.4-7.0 × 10-4 M, 10.5 × 10-4 M, 13.2-13.6 × 10-4 M

concentration

ultrasound

ultrasound, 25, 55, and 800 kHz; intensity, 0-33.3 W/cm2

ultrasound (1.6 kW h)

ultrasound (US) (1-MHz, 2 W/cm2)

ultrasound/ ozonation ultrasound, 20 kHz (30 W), 500 kHz (30 W); O3 concentration, 1.72 × 10-4 M at 20 kHz, 1.58 × 10-4 M at 500 kHz ultrasound (30 W), intensity, 1 W cm-2 at 541 kHz and 27 W cm-1 at 20 kHz ultrasound (20 kHz, 0.1 kW/L)

ultrasound (US) (300 W)

ultrasound: 20 kHz, 14-50 W

chemical oxidation scheme

temp, 22 °C; irradiating gas, air; catalysts, V2O5, PtO2, Ag2O, MnO2, and RuO2 irradiation time, 15 min; pressure, 1-20 atm

temp, 4-12 °C; sonication time, 6-10 h at 5-100 psi. Sparged under Ar for 1 h prior to run.

irradiation time, 10 min

temp, 27 ( 2 °C; pH 6; dissolved gas, air, O2, and Ar; solid catalyst temp, 25 °C; pH 6-5, 7, 9

temp, 20 ( 2 °C; pH 2; irradiating gas, O3/O2 (2-3% O3)

temp, 1-50 °C; irradiating gases, Air, Ar, He, N2O; pH 8-11 temp, room, irradiation time, 1-3h

experimental conditions

major products, HCl and HOCl; minor products, C2Cl4 or C2Cl6

hydroquinone, catechol

H2, CH2O, CO, CH4, C2H4, C2H6 under argon; CO2, CO, HCOOH, CH2O, H2O2, H2 under oxygen dechlorination products detected for all compounds (except for TCAOB and TCDF) under minimal sonochemical conditions

hydroquinone, catechol, p-benzoquinone, oxalic maleic, acetic, formic and propanoic acids, and CO2 No chlorinated products detected. HCl inferred from pH decrease.

Free iodine and Cl2 in the case of CCl4. HCl inferred from pH decrease. No Cl2 formed with CHCl2 and CHCl2. 4-nitrocatechol, CO2

in air :sulfate in the pH range 8-11

degradation intermediates/ products

Table 3. Studies Utilizing Sonochemical and Photosonochemical Degradation of Aqueous Organic and Inorganic Pollutants

7.5-20.0% CCl4 consumed

oxidation proceeded toward completion at the high frequency

not explicitly given

degradation efficiency not explicitly given

Total degradation after 100 min at 541 kHz, unchanged at 20 kHz. Mineralization of phenol improved with Raney nickel. 72-99.9% destruction in 20-40 min. First-order degradation kinetics varies from 0.021 to 0.046 min-1.

ultrasound-enhanced O3 minerilization potential potential of substrate

5.0 × 10-3 - 3.0 × 10-3 (mol dm-3) free iodine yield.

degradation rates in the irradiating gases is in the order He > Air > N2O > Ar

degradation efficiency/other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4687

methylene chloride, carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene

fluorotrichloromethane (CFC11), trifluorotrichloroethane (CFC113)

p-nitrophenol (in the presence of common chemicals of natural water)

Rhodamine B, E. coli

chlorpropham (isopropyl3-chlorocarbanilate), 3-chloroaniline

trichloroethylene (TCE), o-chlorophenol (o-CP), 1,3-dichloro-2-propanol (DCP)

ethylbenzene (EB)

benzene (B), ethyl benzene (EB), styrene (S), o-chlorotoluene (OC)

trichloroethylene (TCE) chlorobenzene (CB) mixtures of TCE and CB

Cheung and Kurup, 1994

Cost et al., 1993

Dahi, 1976

David et al., 1998

De Visscher and Langenhove 1998

De Visscher et al., 1997

De Visscher et al., 1996

Drijvers et al., 1999

contaminants degraded

Cheung et al., 1991

authors

Table 3. (Continued)

B, 3.38, 1.69, 0.9, and 0.45 mM; EB, 1, 0.5, and 0.33 mM; OC, 0.68, 0.34, 0.17mM; S, 0.97, 0.49, and 0.25 mM TCE, 0.84, 1.67, and 3.37 mM; CB, 086, 1.72, and 3.44 mM

TCE, 3.34 mM; o-CP, 0.362 and 0.724 mM; DCP, 0.29 mM; H2O2, 0, 1, 10, 100 mM; CuSO4, 10, 41mM 0.5-1mM aqueous solution

0.1 mM aqueous solutions

E. coli, 106-107 bacteria per mL

2 × 10-5 M

50 mg/L aqueous solution

100-1000 mg/L aqueous solutions

concentration

ultrasound, 520 kHz (14.23 ( 0.73 W)

ultrasound (520 kHz)

ultrasound (518 ( 1 kHz, 13 W per 150 cm3 solution)

ultrasound (518 kHz, 18.2W) with/without Fenton reagent, hydrogen peroxide (H2O2) or organic peroxides

sonolysis, probe transducer (20 & 482 kHz, 40 W)

sonozonation; ultrasound, 20 kHz ozone, 3.68-3.94 mg min-1 L-1

ultrasound (20 kHz), batch reactor (160 W/4.6 W/mL), circulating reactor (CR), 0.64 W/mL ultrasound (20 kHz, 50 W)

ultrasound (250 W)

chemical oxidation scheme

products from mixture of TCE and CB, C8H4Cl2, C8H6,Cl2, C8H5,Cl3, and C8H4Cl4.

temp, 29.5 + 0.5 °C; solution pH 7

temp, 29 ( 1 °C; pH 7

benzene, toluene, styrene, cumene propylbenzene, diphenylmethane, 1,2-diphenylethane, benzaldehyde acetophenone, phenylacetylene not reported

not reported

3-chloroaniline, HCOOH, CO, CO2, Clfor chlorpropham; Cl-, NO2-, NO3-, CO, CO2 for 3-chloroaniline

not determined

4-nitrocatechol

HCl, HF or other acidic species (inferred from pH drop)

Products not explicitly reported. HCl formation inferred from pH drop.

degradation intermediates/ products

solution temp, 30 ( 1 °C

solution temp, 30 °C; irradiation medium, air

temp, 20 °C; irradiation medium, air

temp, 10 °C; pH 5.5; irradiation medium, air temp, 18 °C; pH 7.2; irradiation medium, O2/O3

temp, 5-10 °C CR at 5 psi g

temp, 15-20 °C

experimental conditions

TCE, 3.34 to 0.1 in 90 min. Sonolysis rates of TCE and CB depend on initial concentration of TCE and CB

first-order kinetic rates, k (min-1): B, 0.0271; EB, 0.0622; OC, 0.0431; S, 0.0446

Rates are first-order. Pyrolysis is an important pathway.

ultrasonic treatment of effluent from biological sewage reduced the sterilization dose of O3 by 50% and decoloration zero-order rate constant of Rhodamine B by 55% Degradation efficiency was 100% after 45 min at 482 kHz and 60% at 20 kHz. Efficient elimination of both compounds at 482 kHz. H2O2 enhanced o-CP degradation rate by 20-25%. TCE and DCP unaffected by H2O2. Presence of t-BuOOH and t-Bu-OO-t-Bu inhibited degradation.

degradation not affected by chemicals of natural water

Methylene chloride dropped from 120 to 25 ppm in 40 min. First-order rate constant of 3.93 × 10-2 min-1 calculated. almost 100% in 40 min for CR and 10 min for batch reactor

degradation efficiency/other results or remarks

4688 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

chlorobenzene

trichloroethylene (TCE)

benzo [a}pyrene (B[a]P, benzo [ghi]perylene, coronene (PAHs) and 2-chlorodibenzo-p-dioxin (in 20% ethanol-water), trichloroethylene (TCE), 1,1,1-trichloroethane (TCA)

VOCs, trichlorothylene (TCE) primarily; BTEX, Benzene (Bz); toluene (TL); ethylbenzene (EB); xylenes (XL)

carbon disulfide (CS2)

pentachlorophenol (PCP)

Sodium pentachlorophenate (NaPCP) as wastewater model compound. Bacteria (vibrio fischeri). Daphnids (Daphnia magna).

poly(vinylpyrrolidone) (PVP), ethanol (EtOH) and tetranitromethane (TNM) in aqueous solution

Drijvers et al., 1996

D’Silvia et al., 1990

Eiler, 1994

Entezari et al., 1997

Gondrexon et al., 1999

Gonze et al., 1999

Gutierrez and Henglein, 1988

contaminants degraded

Drijvers et al., 1998

authors

Table 3. (Continued)

three-stage sonochemical reactor (500 kHz each, 0-100 W) sequential ultrasonic/ biological treatment; Electrical generator, 500 kHz, 0-100W ultrasound (300 kHz, 2 W cm-2)

10-4 M aqueous solutions

PVP, 0-0.1 M EtOH, 0-0.5 M TNM, 4.5 × 10-3 M and 5 × 10-3 M (in water plus glycol, glycerin, or propanol-2)

0.1 mM

pure CS2

CAVOX system, sequential hydrodynamic cavitation (360 W, 5 & 10 kW) and UV photolysis (low-pressure Hg lamp, 2.5-10 kW) ultrasound, 900 kHz (29 W) and 20 kHz (49 W)

ultrasound (18 kHz, 50 W)

ultrasound, 520 kHz (0.095 W/mL), 20 kHz (0.43 W/mL)

ultrasound, 520 kHz (14.23 ( 0.73 W, 0.095 W ml -1)

chemical oxidation scheme

TCE, 1475-2000; Bz, 240-500; TL, 8-11; XL, up to 100 (all in ppb)

PAHs, 8.9-15 mg/L, pure or neat TCE and TCA

3.34 mM aqueous solution

1.72mM aqueous solution.

concentration

irradiating medium, Ar

temp, 20 ( 2 °C solution, pH 6.8-7.5; operating wattage, 55-65 W

amorphous carbon and monoclinic sulfur

temp, -50 to + 10 °C; irradiating gas, Ar, H2, air, He, O2, CO2 temp, 20 ( 2 °C; solution pH 7

PVP and EtOH, CH4, C2H6, CH2O, CH3CHO, C2H6, CO, CO2; TNM, C(NO2)3-, NO2-, NO3-, N2, CO, CO2

degradation products not reported

No products and intermediates determined.

organic acids, halides, CO2 and H2O

PAH and Dioxin, fine carbonaceous deposits; TCE and TCA, brown polymeric materials

methane, acetylene, butenyne, butadiyne, phenylacetylene, benzene, non-chlorinated mono- and dicyclic hydrocarbons, chlorophenols, CP (in the presence of air only) C2HCl, C2Cl2, C4Cl2, C2Cl4, C4HCl3, C4Cl4, C4HCl5, C4Cl6

degradation intermediates/ products

H2O2 added before the cavitation chamber

temp, 29-32 °C; pH, 4.7, 7, 10; irradiation gas, air, Ar PAH solutions typically sonicated for over 2 h period

temp, 29.5 ( 0.5 °C; solution pH, 4.7, 7, 10; irradiation gas, Ar, air

experimental conditions

Ultrasonic irradiation decreased immensely the toxicity of NaPCP to microorganisms and could be used as preoxidation step before biological treatment. The decomposition of TNM is one of the fastest sonochemical reactions. Maximum rates of PVP occurred at about 0.04 M

conversion rates up to 80% for ultrasonic units in series

effect of irradiating gas on rate, He >H2 > Air > O2, negligible rate at 900 kHz

B[a]P in 20% aqueous ethanol, 99.1%, 94.1% and 22% degradation rates after 60 min for initial concentrations of 1.1, 8.9 and 21.5 ppm respectively, significantly lower for 100% ethanol solution alone. removal efficiencies greater than 99.9% for TCE and BTEX

Concentration decreased from 3.34 to 0.25 mM at 520 kHz and 3.34 to 0.5 mM at 20 kHz in 60 min.

Argon accelerated degradation rates compared with air.

degradation efficiency/ other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4689

aqueous acetate solutions

carbon tetrachloride (CCl4) and chloroform (CHCl3) in aqueous suspension

carbon dioxide (CO2)

acetylene in aqueous solutions

methane (CH4), ethane (C2H6)

formic acid-water mixtures

ozone (O3) in aqueous solutions

nitrous oxide (N2O)

solutions of KI and sodium formate (HCO2Na) in pure and ozonized water

carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4)

CFC-113 (F2ClC-CCl2F), HCFC-225ca (F3C-CF2-CCl2H), HCFC-225cb (F2ClC-CF2-CClFH) and HFC-134a (F3C-CF2H) in water

Hamlin et al., 1961

Harada, 1998

Hart et al., 1990

Hart et al., 1990

Hart and Henglein, 1988

Hart and Henglein, 1986

Hart and Henglein, 1986

Hart and Henglein, 1985

Henglein, 1985

Hirai et al., 1996

contaminants degraded

Gutierrez et al., 1986

authors

Table 3. (Continued)

concentration

CO2-N2OCH4 containing aq. solutions (0.0.1 mole fraction); Ethanol (0.05M); KI (0.1 M) CFC-113, 25-1000 ppm; HCFC225ca,cb, 100 ppm; HFC-134a, 300 ppm

water containing 0-100 vol % N2O in Ar HCO2-, 0.001, 0.01 & 0.1 M KI; 0.1 M

water containing 2-100 vol % CH4 or C2H6 in Ar 0-30 M aqueous solution O3, 353 & 628 mM, 1.6 mM

CO2 in distilled water (0.0-0.60 mole fraction) 0-10-2 M

2 mL CCl4 or CHCl3 in 20 mL H2O

0-0.8 mol/dm3 acetate

ultrasound (200 kHz, 6 W/cm2)

ultrasound, 300 kHz (3.5 W/cm2)

ultrasound, 300 kHz; O3, 31.8 & 67.4 mM

ultrasound, 300 kHz

ultrasound, 300 kHz

ultrasound (300 kHz, 2 W/cm2)

ultrasound, 300 kHz, 2 W/cm2

ultrasound quartz generator (1 MHz, 2W/cm2)

ultrasonic generator, medical (15 W, 1mc-sec, 3 W/cm2), electric (5 W, 300 kc/s) ultrasound (200 kHz, 200 W)

ultrasound, 300 kHz (16 W)

chemical oxidation scheme

irradiation medium, Ar, air

irradiating gas, Ar, O2, and Ar/O2 mixture; solution pH 6.32-12.07 bulk solution temp, 20 °C; irradiating gas, Ar

temp, 33 °C; irradiation gas, Ar + O2/O3 mixture irradiation gas, Ar

gas medium, Ar

irradiation gas, Ar

irradiating medium, Ar

temp, room ( 2 °C ; irradiating gas, Ar; irradiating time, 30 min temp, 20 °C; irradiating gas, Ar; irradiating time, 15-180 min temp, 5-45 °C; irradiation gas, Ar, He, H 2, N 2

experimental conditions

CO, CO2, Cl-, F-

KI: iodine, H2O2; H2 also formed in the absence of O2. HCO2Na: H2O2, H2, CO2 and oxalate in the absence of O2; H2O2 and CO2 (absence of O2) CO2: major, CO; minor, HCOOH. N2O: N2, nitrite, nitrate. CH4: C2H6, C3- and C4hydrocarbons, CO, CO2, CH2O

N2, O2, NO2-, NO3-

H2O2 formation

H2, CO, CH4, HCOOH, CH3COOH, HCHO, CH3CHO, other C2-C8 hydrocarbons, insoluble soot, C6H6, styrene, naphthalene, and phenylacetylene major products, hydrogen, acetylene, ethylene, ethane; minor, CO, propane, propene major, H2CO2, CO; minor, oxalic acid

gaseous products; major, CO, H2; minor, O2

major, succinic, glycolic and glyoxylic acid, and smaller amounts of HCHO, CO2; minor, CH4 CCl4: CO2, O2, Cl2, HCl, C2Cl6, C2Cl4; CHCl3: HCl, C2Cl6, C2Cl4

degradation intermediates/ products

No chemical effects occurred during irradiation under an atmosphere of pure CO2, N2O, or CH4. Sonochemistry characterized by a strong linkage between the sonolysis of water and the gas. CFCs and HCFCs are readily decomposed ∼CFC-113 degradation faster under argon than air

the yield of H2O2 was about 4 times greater under Ar/O2 mixture (70/30%) than under the pure gases

maximum yield obtained at Ar/N2O vol % ratio of 85:15

At 15 M HCOOH, overall rate of decomposition is 540 mM min-1. rapid decomposition rate of ozone (3 mM/min) occurred at [O3] ) 1 mM

maximum decomposition occurs at 15% for methane and 10% for ethane

Products similar to those in pyrolysis and combustion. Maximum rate occurred at 2 × 10-3 M or 5 vol % of C2H2

the decreasing rate for CO2 followed the order Ar > He H2 > N 2

At acetate concentration greater than 0.4 mol/dm3, CO2 and CO became the predominant products of sonolysis. with CCl4 alone (no water), cavitation occurred but there was no reaction

degradation efficiency/other results or remarks

4690 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

p-nitrophenyl acetate (PNPA) in aqueous solution

carbon tetrachloride (CCl4) and p-nitrophenol (p-Np) + CCl4 in water with/without saturation with Ozone (O3)

nitrobenzene (NB)

carbon tetrachloride (CCl4)

trichloroethylene (TCE), tetrachloroethylene (PCE), 1,1,1-trichloroethane (TCA), chloroform (CHCl3), carbon tetrachloride (CCl4) refractory component in the industrial waste of a cyclohexane oxidation unit

Hua et al., 1995

Hua and Hoffmann 1996

Hung et al., 2000

Hung and Hoffmann, 1998

Inazu et al., 1993

pentachlorophenol (PCP), 3-chlorobiphenyl (3-CB), 4-chlorophenol (4-CP), 2,4-dichlorophenol (DCP)

methyl tert-butyl ether (MTBE)

Johnston and Hocking, 1993

Kang and Hoffmann, 1998

Ingale et al., 1995

p-nitrophenol (p-Np) in aqueous solution

contaminants degraded

Hua et al., 1995

authors

Table 3. (Continued)

concentration

0.01-1.0 mM

PCP, 2.4 × 10-4 M; 3-CB, 4 × 10-4 M; 4-CP, 7 × 10-3 M; DCP, 2 × 10-3 M

COD, 1000 mg dm-3

10 ppm

0.1 mM

simultaneous sonolysis/ ozonation, ultrasound (205 kHz, 200 WL-1), [O3]o ) 0.26-0.34 µM

ultrasound (US)/ Fe°. US, 20 kHz (62 W/285 L); Fe°, 0-24.49 g/L (powder) 0-42 g/L (turnings) ultrasound generator/barium titanate oscillator (200 kHz, 6 W/cm2) sequential sonication/wet oxidation (SONIWO); ultrasonic cleaning bath (40 kHz, 150/300W) simultaneous UV (Hg bulb-100W, 7000W/cm2/ultra sound (20 kHz, 475 W)/photocatalytic (TiO2) treatment

ultrasound (US)/ Fe°. US, 20 kHz (139 W/L); Fe°, 0-88 g/L

ultrasound (20 kHz, 135 or 112.5 W/cm2)

CCl4, 1.95 × 10-4 and 1.95 × 10-5 mol L-1; p-Np, 100 µM

25 µM

immersion-probe ultrasound (115 W, 96 W/cm2)

near-field acoustical processor (NAP), 16 and 20 kHz, 0-1775W

100 µM

100 µM

chemical oxidation scheme

sonication at 30 and 50 °C with CuSO4 (6.26 × 10-4 catalyst for 1 h; WO, 250 °C, 0.69 MPa temp, 35 ( 2 °C; TiO2: surface area, 55 ( 10 m2/g; amount, 0.2% W/W catalyst/ solution temp, 20 °C; solution pH 6.6-6.8; irradiation gas, O2/O3; irrad. time, 0-60 min

irradiation gas, Ar, O2, or air

temp, 15 °C; irradiation gas, Ar; initial pH 7

temp, 15 ( 2 °C; initial pH 6

temp, 22-25 °C; solution pH 4.5-5.0; irradiation medium, Ar, O2, mixture of Ar/O2 temp, 25 °C solution pH 3-8; irradiation gas, Kr, Ar, He solution pH 11.8; irradiation gas, Ar

experimental conditions

tert-butyl formate (8%), tert-butyl alcohol (5%), methyl acetate (3%), acetone (12%)

PCP: chloride

formation of acetic acid as byproduct

in air or O2, Cl-, H2, CO2, negligible CO; in argon, Cl-, H2, CO, CO2

intermediates: with US, o-, p-, m-nitrophenol and 4-nitrocatechol; with Fe°, nitrobenzene, aniline intermediates: US/Fe°, C2Cl4 and C2Cl6; Fe° (only), CHCl3, and CH2Cl2

CCl4: C2Cl4, C2Cl6, Cl-, HOCl, p-Np; 4-nitrocatechol (4-NC)

major, hydrolysis products; minor, NO2-, NO3-

4-nitrocatechol (4-NC); other products not reported here

degradation intermediates/ products

the presence of O3 accelerated MTBE degradation rates substantially, enhancement by 1.5-3.9 depending on [MTBE]o

Sonication of UV-irradiated catalyst solution significantly improved rates.

Over 75% initial amount decomposed in 10 min. Order of degradation rates: in argon, TCE > TCA > PCE; in air, CCl4 > CHCl3 hybrid system more effective than sonication66% in COD reduction (1473 to 499 mg dm-3) vs 36% (1533 to 980) for Cu2+ system

combined US/Fe° system had a positive synergistic effect on CCl4 dehalogenation

hydrolysis rate constants varies from 9.8 × 10-5 to 3.8 × 10-4 s-1, depending on dissolved gas, accelerated by ultrasound CCl4: 90% and 99% reduction after 12 and 90 min, respectively; not affected by O3. Degradation of p-Np enhanced significantly by CCl4 and 4-nitrocatechol accumulation minimized combination of ultrasound and Fe° had synergistic effect on the reduction of nitrobenzene

first-order rate constants at 1.4W/cm2 higher in Ar than in O2, kO2 ) 5.19 × 10-4 s-1 kAr ) 7.94 × 10-4 s-1

degradation efficiency/other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4691

synthetic carbon tetrachloride CCl4), contaminated water

parathion (O,O-diethyl O-p-nitrophenyl thiophosphate)

p-nitrophenol (p-Np) in oxygenated aqueous solutions

hydrogen sulfide (H2S) (S[-II] ) [H2S] + [HS-] + [S2-])

natural groundwater containing mainly 1,2-dichloroethane (1,2-DCA)

2-chlorophenol (2-CP) in aqueous solution

2-chlorophenol (2-CP)

2-chlorophenol (2-CP)

phenol (PhOH), benzene

thymine (aerated aqueous solutions)

Kotronarou et al., 1992

Kotronarou et al., 1991

Kotronarou et al., 1992

Kruger et al., 1999

Ku et al., 1997

Lin et al., 1996

Lin et al., 1996

Lur′e, 1962

Mead et al., 1975

alcohol-water mixtures, CH3OH, C2H5OH, n-C3H7OH, i-C3H7OH

benzene (Bz), toluene (TL), phenol (PhOH) (saturated aqueous solution)

contaminants degraded

Koszalka et al., 1992

Khenokh and Lapinskaya, 1956 Koike, 1992

authors

Table 3. (Continued)

concentration

2 × 10-3-2 × 10-2 M

PhOH, 25-300 mg/L; Bz, 200-500 mg/L

100 mg/L

10 and 100 mg/L

1.26 × 10-4 M

350 µg/L

92,100 and 20 µM

saturated aqueous parathion solution (82 µM) 100 µM

13.6-15.8 ppm

Bx, 2.4, 5.0 & 10.2 × 10-4 M; PhOH, 0.01, 0.08 & 0.32 M 10-80 vol %

ultrasound (20 kHz, 0, 125, and 160 W)/H2O2 (0, 100, 200, and 500 mg/L) process ultrasound, 800 kc/s, 1500 W; Intensity, 4-7 W/cm2 (350 mL solution) ultrasound (447.5 ( 0.6 kHz, 5 W cm-2)

ultrasound (20 kHz, 160 W)/H2O2 (200 mg/L) process

ultrasound [361 kHz (60 W), 620 kHz (75 W), and 1086 kHz (105 W) probe ultrasound (20 kHz, 0-550 W); intensity, 38.1 W/cm2

probe ultrasound (20 kHz, ∼75 W/cm2 or ∼85W)

ultrasound (20 kHz, 84 W)

ultrasonic probe (475 W) (sonication at 250 W), unknown chemical agent ultrasound (20 kHz, 75 W cm-2)

sonication bath (SB), 50 kHz, 120 W

ultrasound, 435 kc

chemical oxidation scheme

solution temp, 40-60 °C; treatment time, 40 min to 5 h. solution temp, 25 °C

temp, 25 °C; solution pH 3, 5, 7, 9, 11

temp, 25 °C solution pH 10.6-7.4; irradiation gas, air temp, 22-31 °C; solution pH 6.2-7; irradiation gas, air temp, 17-60 °C; pH 3, 5, 7, 9, and 10; irrad. gas, air, O2, N2 temp, 25 °C; pH 3, 7, 11; catalyst, FeSO4

argon bubbled for 15 min before sonication temp, 25-35 °C; pressure, 15-45 psig; irradiation, air, Ar, H2; irrad. time, 0-5 min temp, 30 °C; solution pH 6.0; final pH after 2 h sonication, 3.7 temp, 30 °C solution pH 2.12; irradiation gas, air

temp, 40 °C

experimental conditions

cis- and trans-5, 6-dihydroxy-5,6dihydrothymine, 5-hydroxymethyluracil and urea

PhOH: catechol, pyrogallol, o-quinone; Bz; phenol, pyrogallol, catechol, formaldehyde

not explicitly given; total prganic carbon (TOC) monitored

not explicitly given; total organic carbon (TOC) monitored

chlorohydroquinone, catechol, chloride at pH 11, but glyoxilic acid also detected at pH 3

first-order rate constants decreased from 3.67 × 10-4s-1 (pHo 5) to 2.0 × 10-4 s-1 (pHo 8).

NO2-, NO3-, p-benzoquinone (p-BQ), hydroquinone, 4-nitrocatechol, formate (HCO2-), oxalate sulfate (SO42-), sulfite (SO32-), thiosulfate (S2O32-) in the mole ratio 2.2:2.7:1 respectively chloride

A value of 1.8 ( 0.3 × 10-5 M min-1 was obtained for the zero-order rate constant.

Prolonged treatment resulted in rapid oxidation of catechol to muconic acid.

2-CP decomposition was 99%, 62%, and 15% at pH 3, 7, and 11, respectively, at 350 min, and was enhanced by FeSO4 as catalyst. H2O2 improved 2-CP decomposition, 57% over the control with 500 mg/L H2O2

apparent pseudo first-order rate constant ranged from 3.3 to 1.5 × 10-3 in the pH range 3-11 at 24 ( °C

complete destruction within 60 min

S(-II) completely oxidized in less than 30 min

CCl4 reduced from 13.6 to 1.71 ppm (g90% destruction) by sonolysis, 15.8 ppm to <0.5 ppb with sonolysis plus chemical agent. initial parathion was degraded e 2 h since [SO42-]o was originally equal 82 µM

order of yield for CH4 was C2H5OH > CH3OH > i-C3H7OH ∼ n-C3H7OH

HCHO yields increased with initial concentration of Bz or TL and time

degradation efficiency/other results or remarks

intermediates, chloroform, dichloromethane, chloromethane; final products, methane, chlorine/chloride p-nitrophenol (p-Np), SO42-, PO42-, oxalate (C2O42-), NO2-, NO3-

typical products, CH4, C2H6, C3H6, C2H4, C2H2

Bz & TL, HCHO, phenolic hydroxyls

degradation intermediates/ products

4692 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

fulvic acid (FA) and naturally colored groundwater (NCG)

phenol (PhOH)

carbon tetrachloride (CCl4), chloroform (CHCl3), trichloroethylene (TCE), chlorobenzene (CB), polychlorobiphenols (PCBs) Triton X-100 (TX)

pentachlorophenate (PCP)

phenol (PhOH)

atrazine (ATZ), pentachlorophenol (PCP)

chloroform (CHCl3), dichloromethane (CH2Cl2), carbon tetrachloride (CCl4), and dilute mixtures of CCl4 in methanol (MeOH), butanol (BuOH), and 1,2-Ethanediol [Et(OH)2]

Okouchi et al., 1992

Orzechowska et al., 1995

Petrier et al., 1992

Petrier et al., 1994 (a)

Petrier et al., 1996

Petrier et al., 1994 (b)

monohydroxybenzoic acid (HBA); 3,4-dihydroxybenzoic acid (3,4-DHBA); gallic or 3,4,5-trihydroxybenzoic acid (GA); tannic acid (TA); humic acid (HA) 2-, 3-, & 4-chlorophenol (CPOH), pentachlorophenol (PCP)

contaminants degraded

Olson and Barbier, 1994

Nagata et al., 2000

Nagata et al., 1996

authors

Table 3. (Continued)

concentration

MeOH: 1% CCl4; BuOH: 0.5% CCl4; Et(OH)2: 0.02 and 0.05% CCl4

ATZ, 0.1 mM; PCP, 0.1 mM

0.05-10 mM

10-4 M

CCl4, 40 ppm; CHCl3, 37 ppm; TCE, 37 ppm; CB, 94 ppm; PCB, 55 ppm; TX, 1% aq. soln.

FA, 10 mg/L TOC; NCG, contains 2.5-3.0 mg/L (dissolved organic carbon) 100 ppm (mg/dm3)

100 µM/L

100 µM/L

ultrasound (30W), Vibracell emitter, 20 kHz; Undatim Irradiation system, 500 kHz

ultrasound: immersion probe, 20 kHz, 18.5 W; ultrasonic emitter, 500 kHz, 18.5 W

ultrasound (30 W): disk transducer, 487 kHz titanium probe, 20 kHz

ultrasonic disk transducer (530 kHz, 20 W)

ultrasonic generator (200 W; 19.5, 50, 200, and 600 kHz) ultrasound (20 kHz); cup horn, 100 W; horn probe, 120 W

sonolysis/ ozonolysis (sonozone process); ultrasound, 55 W, 20 kHz

ultrasound, 200 kHz, 200 W

ultrasound, 200 kHz, 200 W

chemical oxidation scheme

temp, 20 °C; pH 7 (for PCP); dissolved gas, air; irradiation time, ATZ 120 min, PCP 180 min irradiation gas, O2 and argon

temp, 25 °C; catalyst, Fe2+, MnO2 irradiating gas, Air, O2, N2, He solution temp, 27-40 °C; deionized and tap water (pH 6.3-8.37); sonication time, up to 60 min. temp, 24 ( 1 °C; pH 7; irradiation gas, Ar, air, O2 temp, 25 °C; irradiation gas, air

temp, 25 °C; groundwater pH 8.6; irradiating gas, 2.1% O3 in O2 (3.2 mg/min)

irradiating gas, air & Ar, presence of Fe(II)

temp, 25 °C; irradiating gas, air & Ar

experimental conditions

acidic degradation products detected with β-carboline (10-4-10-5 M) especially for solutions of CCl4 in alcohol

ATZ: dealkylation products of atrazine, CO2, chloride; PCP: chloride

CO2 as the only gaseous product, hydroquinone (HQ), catechol (CC) benzoquinone (BQ)

chloride and formate ions; chloride: CCl4, 2.80 ppm; CHCl3, 4.55 ppm; TCE, 2.80 ppm; CB, 0.50 ppm; PCB, 0.30 ppm; TX, 0.30 ppm; formate, <1.00 ppm for each CO2, (CO in Ar only), Cl-, NO2-, and NO3-

catechol, hydroquinone

CO2

chloride

only CHCl3 measured

degradation intermediates/ products

chloromethanes extensively decomposed MeOH and BuOH remained stable, but Et(OH)2 oxidized

rate is dependent on [PhOH]0 with maximum value reaching limits of: 11.6 × 10-6 M min-1 (for 487 kHz) and 1.84 × 10-6 M min-1 (for 20 kHz) the degradation proceeded 7.8 times (for ATZ) and 4.8 times (for PCP) more rapidly at 500 kHz than at 20 kHz

fast cleavage of the C-Cl bond, releasing Cl- and mineralization of PCP to CO2 in 100 minutes

degradation rates increased in the order O2 > air > He, N2 and enhanced by catalyst At 20 min, the pH decreased by 2.3 units for CCl4 solutions.

Degradation of CPOHs and PCP were almost 100% under Ar and 80-90% under air after 1 h. 91% TOC removed as CO2 (from FA) after 60 min. Sonozone is more effective than ozone or ultrasound alone

Rates of the HBAs were faster in Ar than in air but rates of TA and HA were faster in air.

degradation efficiency/other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4693

bacterial cells (E. coli)

chloromethanes in aqueous media (CH2Cl2/H2O, CHCl3/H2O, CCl4/H2O)

chlorobenzene (CB), 1,4-dichlorobenzene, (1,4-DCB), naphthalene (NT), anthracene (AN), pyrene (PR)

water-KI-CCl4 system (KI-water solution saturated with CCl4)

trichloroethylene (TCE)

chloral hydrate (CH)

Popa and Ionesu, 1992

Price et al., 1994

Rajan et al., 1998a,b

Reinhart et al., 1996

Sakai et al., 1977

aqueous solutions of chlorobenzene (CIBZ), 4-chlorophenol (4-CIPh), 1,2-dichlorobenzene (1,2-DCB), 1,3-dichlorobenzene (1,3-DCB), 1,4-dichlorobenzene (1,4-DCB), 1,3,5-trichlorobenzene, (1,3,5-TCB), 1-chloronaphthalene (1-CN) and mixture of CIBZ and CIPh phenol (Ph), carbon tetrachloride (CCl4)

contaminants degraded

Phull et al., 1997

Petrier and Francony, 1997

Petrier et al., 1998

authors

Table 3. (Continued)

0.1 M in aqueous solutions

saturated solutions (M): CB, 4.1 × 10-3; DCB, 6.2 × 10-5; NT, 2.36 × 10-4; AN, 2.30 × 10-7; PR, 6.4 × 10-7 1%, 16.6%, and 25% KI solutions containing 5.2 × 10-3 M CCl4 5-20 ppm

1.5 cm3 samples (chloromethane) in 98.5 cm3 H2O

sonolysis/ zerovalent Fe. US: 20 kHz, 90 & 180 W; iron (Fe), 1&3 g/L ultrasound (29 and 400 kHz); power output, 0.5 W/cm

horn ultrasound (25 kHz)

horn ultrasound (22 kHz), intensities (W/cm2), 13.5, 11.7, 20.9, and 38.9

ultrasound (30 W), titanium horn, 20 kH; piezo-electric disk, 200, 500 and 800 kH ultrasound (US) and/or chlorination. US, 20 kHz (8 W cm-2) & 800 kHz (15 W cm-2); chlorination, 0.01-30 ppm ultrasound, 1 megacycles/s, 584 W

Ph, 10-3 mol/L; CCl4, 4-4.6 × 10-4 mol/L 676 bacterial colonies per cm3 (1/10 dilution)

ultrasound (30 W), disk transducer (500 kHz), titanium probe (20 kHz)

chemical oxidation scheme

CIBZ and CIPH, 0.5 mM; 1,2-DCB, 0.4 mM 1,3-DCB, 0.05 mM; 1,4-DCB, 0.2 mM; 1,3,5-TCB, 0.02 mM; 1-CN, 0.04 mM

concentration

temp, 30 °C; solution saturated with air and mixtures (20/80 and 2/98) of O2 and N2

temp, 25 °C; irradiation gas, N2

temp, 34 °C; irradiation gases, air, N2, O2

temp, 20 °C (ambient); solution, pH 5.48-4.50 (not controlled by buffer)

temp, 25 °C; irradiation gas, argon; pH 6

temp, 20 °C; sonication time, 1, 2, 5, 10, or 20 min

temp, 20 °C; solution pH 2; O2-saturated solutions

temp, 20 °C; water equilibrated air

experimental conditions

combined sonolysis/Fe rates was 3 times that of Fe alone

kinetic rate expression obtained for the formation of HCl agreed with experimental result

chloride ion (Cl-)

Model developed predicted well the rates observed for CCl4 suspension in KI solution.

the sonolytic reaction fairly obeys zero-order kinetics. The rate constants were 1.73 × 10-7, 2.22 × 10-7, and 2.78 × 10-7 Ms-1 for CCl4, CHCl3, and CH2Cl2, respectively. At 20.9 W/cm2, the rate constants for 1,4-DCB, NT, AN, and PR are 0.038, 0.015, 0.033, and 0.016 (min-1), respectively, and 0.022 for CB at 23.9 W/cm2

ultrasound amplified the biocidal effects and reduced the amount of chlorine required for disinfection

not explicitly reported Cl- inferred

I2, Cl2, Cl, HOCl, HCl, CO2, and O2

not explicitly reported

C2H2Cl4, C2HCl5, C2Cl6

dead bacterial cells

degradation rates for CCl4 increase with frequency, but maximum for PhOH at 200 kHz

Chloroaromatic hydrocarbons yielded g 89% Cl- as product in 150 min. For the mixture, CIBZ degrades before 4-CIPH (100 vs 300 min). 1,2-DCB, 1,3-DCB, 1,4-DCB, 1,3,5-TCB, and 1-CN more efficiently destroyed.

CIBZ: Cl-, CO, CO2, C2H2; 4-CIPH: Cl-, and hydroxlated intermediates (hydroquinone, 4-chlorocatechol, etc.)

Ph hydroquinone, catechol; CCl4: Cl-, and CO2

degradation efficiency/other results or remarks

degradation intermediates/ products

4694 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

2-, 3-, and 4-chlorophenols (2-CPOH, 3-CPOH, and 4-CPOH)

phenol (PhOH) and intermediates, catechol (CC) hydroquinone (HQ), p-benzoquinone (BQ)

trinitrotoluene (TNT) and cyclotrimethylene -trinitramine (RDX)

humic acid (HA) solution

toluene (T), nitrotoluene (NT), O-chlorotoluene (O-T), O-p-xylene, mesitylene (1,3,5trimethylbenzene (MTB), and isopropyltert-butylbenzene (IPB,TBB) carbon tetrachloride (CCl4); di-n-butyl sulfide (R2S)

azo dye, naphthal blue black (NBB)

surfactant, polyoxyethylene-alkyl ether (SS-70)

Serpone et al., 1994

Serpone et al., 1992

Sierka, 1985

Sierka and Amy, 1985

Soudagar and Samant, 1995

Stock et al., 2000

Suzuki et al., 2000

Spurlock et al., and Reifneider 1970, 1971

thymine (THY)

contaminants degraded

Sehgal and Wang, 1981

authors

Table 3. (Continued)

sonolysis/ photocatalysis; US, 640 kHz, 250 W; TiO2 (P.25), 1 g/L ultrasound/ photocatalysis, 200 kHz/200 W, TiO2/Hg lamp (253.7 nm/20W)

50 ( 5 µM

100 ppm in 1000 mL aqueous sample

ultrasound, 280-800 kHz at 5-11 W/cm2)

ultrasound (5-50W at 852-863 kHz, 60.6-1,007 kHz)/ozone (72.91 mg O3/min) ultraviolet/ ultrasound/O3; US, 40 kHz, 300 W; UV lamp, 253.7 nm; O3, 4.3 mg/L (2 min), 21.6 mg/L (10 min) and 43.2 mg/L (20 min) KMnO4; ultrasonic cleaner (23 kHz, 120W)

immersion horn ultrasound 200 kHz, 25, 50, or 75 W/cm2

horn ultrasound (20 kHz, 49.5 W, 52.1 W/cm2)

ultrasound (1-7 W cm2)

chemical oxidation scheme

saturated aqueous solution of CCl4

mixture of 20 mmol of KMnO4; and 10 mmol of arylalkane in 50 cm3 water

unknown concentration

PhOH, 75 µM (pH 12), 68 µM (pH 3) and 51 µM (pH 5.7); HQ, 44 µM, pH 3; BQ, 43 µM, pH 3; CC, 54 µM, pH 3 70/30 mg/L TNT/RDX; aqueous solutions

4-CPOH, 75.1 µM; 3-CPOH, 77.8 µM; 2-CPOH, 83.1 µM

0.1 mM

concentration

temp, 25 °C; irradiation gas, air; irrad. time, 60 min

temp, 20 ( 5 °C

temp, 20 °C; irradiation gases, O2 and Ar

temp, 30-35 °C

irradiating gas, O3/O2; pHo 4.0, 7.1, and 10.0

temp, 25-59 °C; pH 5.84-10.0

30 ( 2 °C; pH 3, 5.4, 5.7, 12; open to air

temp, 20-70 °C; aeration medium, air temp, 33 ( 2 °C; air equilibrated; natural solution pH: 4-CPOH, 5.1; 3-CPOH, 5.4; 2-CPOH, 5.7

experimental conditions

not explicitly determined TOC monitored

CCl4, CO, CO2, HOCl, and hexachloroethane (C2Cl6) (in AR only); Minor, R2SO2, RSO3H, butyric acid, CO, C2H4, C2H2, and CH4 NBB intermediates minerilized to inorganic species

T, benzoic acid; O-T and MTB, corresponding carboxylic acids

trihalomethanes formation potential (THMFP), and nonvolatile total organic carbon (NVTOC) monitored as byproducts

not explicitly determined

4-CPOH, hydroquinone (HQ), 4-chlororesorcinol (4CR), 4-chlorocatechol (4-CC); 3-CPOH, Cl-, chlorohydroquinone (CHQ), 4-3-CC, carbonaceous species; C-CPOH, CHQ, 3-CC, catechol (CC) PhOH, appearance (in 9h) and subsequent disappearance of CC, HQ and BQ BQ, HQ + unidentified species HQ, BQ + unidentified species

not determined

degradation intermediates/ products

significant enhancement in the photocatalytic reaction observed when combined with US irradiation

Degradation rate for sonolysis was about 2 times faster than that of photocatalysis.

irradiation of products R2 SO and R2SO2 yielded RSO3H as principal product only at 800 kHz

T studied in detail among the arylalkanes. The optimal time for oxidation of T was 3 h. KMnO4 accelerates oxidation in the presence of ultrasound.

O3-US-UV system proved to be most effective reaction conditions, followed by O3-UV, O3 alone, and O3-US, providing 93%, 86%, 75%, and 71% reduction in THMFP levels, respectively, in 20 min.

ultrasound inhibited kinetics at high temp and pH due to radical-radical extinguishment reaction

At pH 12, neither HQ, CC or BC were detected from the insonation of PhOH since they were unstable. CO2 could not be detected.

At an aeration rate of 50 mL/min at 34 °C, [THY] is reduced to half in 30 min. total degradation in 10 h for 2-CPOH and 3-CPOH and 15 h for 4-CPOH with rates [4.8 ( 0.4, 4.4 ( 0.5, and 3.3 + 0.2] × 10-3 min-1, respectively.

degradation efficiency/other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4695

phenol (Ph), 2-naphthol (2-Nph), 4-methylphenol (4-mPh), catechol (CC), resorcinol (RS), 3-tert-butyl-4-hydroxyanisole (3-tb-4-HAN), 3-methyl-4-hydroxyanisole (3-m-4-HAN)

4-nitrophenol

phenyltrifluoromethyl ketone (PTMK) aq. solution

benzene (Bz), toluene (T)

1,1,1-trichloroethane (CCl3CH3, or TCA)

1,1,1-trichloroethane (CCl3CH3 or TCA)

phenol

azo dye, remazol black B (RB)

ozone in aqueous solutions

Tauber et al., 2000

Theron et al., 1999

Thoma et al., 1998

Toy et al., 1990

Toy et al., 1992

Trabelsi et al., 1996

Vinodgopal et al., 1998

Weavers and Hoffmann, 1998

contaminants degraded

Takizawa et al., 1996

authors

Table 3. (Continued)

ultrasound (640 kHz, 240 W)

ultrasound direct probe, 20 kHz; 263 W L-1 ultrasonic transducer, 500 kHz, 96 WL-1

33 µM

85 ( 5 to 245 ( 3 µM O3 saturated solutions

89-100 mg/L

1.06 and 10 ppm

3.6 and 2500 ppm

4, 40, and 80 mg/L; Bz, 51-1030 µM; T, 43.5-870 µM

TiO2 photocatalysis and/or ultrasound (US); TiO2, 3.5 g/L; US, 30 & 515 kHz, 16 W near-field acoustic processor (NAP), 200-800 W, 16-20 kHz, 1.3 W/mL-1 cup-horn ultrasound (20 kHz)/Hg/Xe lamp (200 W) [photolysis and/or sonolysis] immersion horn ultrasound (475 W, 20 kHz) sonolysis, SS (20 and 500 kHz, 20 W)/electrolysis, ES (current density ) 68A/m2) [sonoelectrooxidation (SEO)]

ultrasound, 321 kHz, 170 W/kg

5 × 10-3 mol/dm3

160 µmol/L

ultrasound (200 kHz)

concentration 8.0 × 10-3 M in water and waterethanol mixture (1:1 V/V, 100 mL)

chemical oxidation scheme

temp, ambient; under constant bubbling of O2 temp, 23 ( 3 °C; irradiating gas, O2/O3 mixture; pH 2

temp, ambient; open to atmosphere temp, 27 ( 2 °C; solution saturated with air for 20 min

temp, ambient; pH 3

air or O2 saturated solutions

temp, 293 K; pH 6-6.5 (natural water)

irradiation gas, Ar; pH 4 & 10

temp, 25 °C; irradiation time, 12-18 h; irradiation gas, O2

experimental conditions

disappearance of TCA was 91.4 in 18 min.

chloride (Cl-)

not explicitly determined

oxalate (C2O42-)

At 20 kHz, 5% conversion with sonolysis alone compared with 755 for SEO in 10 min. SEO’s conversion is 95% at 500 kHz in 10 min and 100% in 60 min. HQ, CC, and BQ disappear after 2 h at 540 kHz RB disappeared in 90 min with rate constant of 2.9 × 10-2 min-1 and 60% mineralization achieved in 6 h. first-order degradation rate constants are 0.84 and 0.66 min-1, respectively, at 20 kHz and 500 kHz at [O3]o ) 245 µM

decomposition more extensive with combined photolysis and sonolysis (photosonolysis) than each only

chloride (Cl-)

maleic acid (MA), oxalic acid (OA), muconic acid (MCA), p-benzoquinone (BQ), hydroquinone (HQ), catechol (CC), CO2

rate constants for 4 to 80 mg/L solutions; Bz, 0.01-0.003 min-1; T, 0.028-0.004 min-1

the amount of CF3COOH was 8 times lower in sonicated solutions than in UV-irradiated TiO2 suspension at both frequencies

thermolysis products observed at low pH compared with OH-induced reactions at high pH.

Hydroxylated products yields ranged from 15% to Ph and 4-HAN have the highest yields of products, Ph 30% CC and 26% HQ in 12 h; 4-HAN, 50% HQ in 12 h.

degradation efficiency/other results or remarks

not reported

Ph, CC, hydroquinone (HQ); 4-mpH 4-methylcatechol; 4-HAN, hydroquinone; 2-NpH 2,3-dihydronaphthol; CC and RS, pyrogallol; 3-tb-4-HAN, tert-butyl-HQ; 3-m-4-HAN, metyhl HQ pH 10, 4-nitrocatechol, hydroquinone + benzoquinone, nitrite; pH 4, products at pH 10 + phenol, nitrate, CO, CO2, H2 CF3COOH and hydroxlated PTMK

degradation intermediates/ products

4696 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

pseudo-first-order rate constants at 20 kHz/30.8 W cm-2 (s-1) are: 2-PCB, 2.1 × 10-3 ( 2.8 × 10-5; 4-PCB, 1.6 × 10-3 ( 3.4 × 10-5; 2,4,5-PCB, 2.6 × 10-3 ( 9.4 × 10-5

solution in the former case. Okouchi et al.63 observed the degradation of phenol in less than 100 min to proceed with pseudo first-order rate and the addition of Fe2+ and MnO2 to increase the rates in aqueous solutions. Serpone et al.64 noticed the disappearance of phenol at the initial concentration range of ∼30 to 70 µM to follow a zero-order kinetics with k ) 0.08 µM/ min at [phenol]initial ) 51 µM and pH 5.7 and slower at an alkaline pH of 12. They also observed three principal intermediate species formed at pH 3 (catechol, hydroquinone (HQ), p-benzoquinone (BG)), only catechol (CC) and HQ at natural pH 5.4-5.7, and no intermediates detected at pH 12. The mechanism is given as k1

H2O + ))) 79 8 •OH + H• k

biphenyl, ethyl benzene, diethylbiphenyl, dibutylbiphenyl, phenol, propylphenol, di-tert-butyl phenol, chloride

greater than 99% destruction efficiencies in 6 min with an average first-order rate constant of 0.7 min-1 at 25 °C not explicitly reported

the toxicity of reaction products remains to be determined. PA, phenanthrene diol; BP, ortho, meta, and para-1,1 biphenols

degradation intermediates/ products

degradation efficiency/other results or remarks

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4697

-1

k2

•

OH 98 1/2H2O2 k3

•

H 98 1/2H2 k4

•

(6) (7) (8)

probe ultrasound (PUS), 20 kHz for all PCBs; also at 205, 358, 618, and 1071 kHz for 2-PCB only polychlorinated biphenyls (PCBs), 2-chlorobiphenyl (2-PCB), 4-chlorobiphenyl (4-PCB), 2,4,5-chlorobiphenyl (2,4,5-PCB) Zhang and Hua, 2000

2-PCB, 4.6 µM; 4-PCB, 5.4 µM; 2,4,5-PCB, 7.6 × 10-2 µM

carbon tetrachloride (CCl4) Wu et al., 1992

0.53, 1.6, 8, and 130 pm

probe ultrasound (20 kHz, 27 W/cm2)

temp, 21-27 °C; air flow reactor, 100 mL/min (open to atmosphere) temp, 20-60 °C; pH 3-9; power intensity, 1-22 W/cm2; irradiating gas, air temp, 15 °C; power intensity, 30.8 W/cm2; irradiating gas, Ar ultrasound (147 W/cm2) PA or BP, 0.39 mg/310 mL water polycyclic aromatic hydrocarbons (PAHS), phenanthrene (PA), biphenyl (BP) Wheat and Tumeo, 1997

authors

Table 3. (Continued)

contaminants degraded

concentration

chemical oxidation scheme

experimental conditions

OH + phenol 98 products

(5)

For a constant phenol concentration, the rate of loss of phenol as a function of sonocation time was found linearly related to the sonocation power, P, according to

Rate )

k1k4[phenol] •

k2[ OH] + k4[phenol]

(10)

At low concentrations of phenol, k2 [•OH] > k4[phenol], and at low constant P, the rate follows first-order kinetics; at high concentration, where k2 [•OH] , k4[phenol], the rate follows zero-order kinetics at constant P. However, contrary to the zero-order kinetic behavior observed at 25 and 50 W/cm2, the sonooxidation of phenol was found to be only first-order at 75 W/cm2. Over a pH range of 3-9, Lur’e et al.65 found the decomposition of phenol to be practically independent of pH. Chen et al.66 also found that at high concentrations ([PhOH] > 300 ppm) the conversion was zero-order and become first-order at very low concentration (<100 ppm). The rate was also found to be negligible at low frequency and the oxidation proceeded toward completion at high frequency. Trabelsi et al.67 investigated the oxidation of phenol in a screened reactor with ultrasound alone at 20 kHz and with ultrasound associated with electrolysis. They found that the electrochemical oxidation of phenol in NaCl media combined with sonication at 20 kHz resulted in 75% conversion of the initial phenol within 10 min and led to the formation of toxic p-quinone. At a higher frequency of 500 kHz, a conversion of 95% was obtained

4698

Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001

within the same treatment time with acetic and chlorocrylic acids as final products of degradation. However, sonoelectrooxidation at high frequency allowed total degradation in 20 min with no toxic aromatic intermediates. They also observed that high frequency ultrasound enhanced mass transfer substantially up to 70-fold the mass transfer by diffusion, and without ultrasound dimerization of phenol radicals yielded polymeric species which built on the electrodes and passivated them. The sono-oxidation intermediates and products of 2-, 3-, and 4-chlorophenols (2-,3-,4-CPOH) and 2,4-dichlorophenol (2,4-DCP) in air equilibrated media generally include chlorohydroquinone (CHQs), catechols (CC), chlorocatechols, chlororesorcinols (CRSs), and chlorides. In addition, 2,4-dichlorophenol has been found to produce some 2- and 4-chlorophenols as intermediates. The major mechanism of oxidation for these chlorophenols is a radical attack similar to equations for phenol (eqs 5-9).64 Serpone et al.68 obtained CHQ as the major intermediate for both 2-CPOH and 3-CPOH sonolysis, with a small quantity of catechol (CC) in the case of 2-CPOH. By contrast, the principal intermediate for 4-CPOH was found to be HQ with a small amount of 4-CRS. Ku et al.69 in the sono-oxidation of 2-CPOH with oxygen purging at various solution pH levels identified CHQ, CC and glycoxilic acid as intermediates at pH 3 but only CHQ and CC at pH 11. Lin et al.70 found that the combination of ultrasound with H2O2 increased the efficiency of 2-CPOH decomposition significantly. With the ultrasound/H2O2 process, they observed that with pH controlled at 3, the rate of 2-CPOH decomposition (i.e., 99%) was enhanced up to 6.6-fold and TOC removal (i.e., 63%) up to 9.8-fold compared with values at pH controlled at 11. The variations in decomposition rates at the different pH values were attributed to the pKa value of 8.49 for 2-CPOH at 25 °C. The 2-CPOH is almost completely in the ionic form when the pH exceeds the pKa value of 8.49 but exists in molecular form when pH < pKa, the fraction in molecular state increasing with pH drop. In ionic state, 2-CPOH does not vaporize into the cavitation bubbles, and oxidation reaction occurs with OH radicals outside the bubble film. However, in the molecular state, reactions occur by both thermal cleavage inside the bubble and oxidation with OH radicals outside, leading to more effective decomposition. Lin et al.71 also studied the effect of H2O2 concentration on decomposition. It was shown that the more the H2O2 that was added, the greater the degradation efficiency. In a particular study with 500 mg/L of H2O2 and initial 2-CPOH concentration of 100 mg/L, they observed, after a reaction time of 30 min, an improvement as great as 57% over the control, i.e., without H2O2 addition. The result was attributed to increase in OH radicals generated on addition of H2O2. However, the increase in degradation rates was not significant between the additions of 200 and 500 mg/L H2O2, suggesting the concentration saturation of OH radicals at the 200 mg/L level. Petrier et al.72 studied the sonochemical degradation of 4CPOH and concluded that the release of end products (i.e., Cl-) and formation of hydroxylated intermediates (HQ, CC) were evidence of a two-step reaction involving OH radicals with substrate in the liquid layer surrounding the cavitation bubble. The dechlorination yields were also found to be higher at 500 kHz (using undatim ultrasonic source) than with the common 20 kHz ultrasonic probe. The dechlorination rates for 4-chloro-3,5-dimethyl-phenol

were found to be approximately 2 orders of magnitude lower than that of 2,4-chlorophenol (2,4-DCP), and this was attributed to the fact that the formation of aryne intermediates is blocked by the ortho methyl groups.73 Nagata et al.74 found the first-order ultrasonic degradation rates of 2-, 3-, and 4-chlorophenol to be faster under argon atmosphere compared to air atmosphere. With 0.1 mM initial concentrations, the rates for 2-CPOH, 3-CPOH, and 4-CPOH were 6.0, 7.2, and 7.0 µM min-1, respectively, in argon compared to 5.0, 6.6, and 4.5 µM min-1 in air atmosphere. The rate was even faster for 3-CPOH (k ) 20.0 µM min-1) with an initial concentration of 1 mM in argon atmosphere. The faster degradation of 3-CP compared to 2-CP and 4-CP was attributed to the fact that with 3-CP, there are three points of simultaneous ortho and para orientation to Cl and OH groups where, due to the electrophilic character of OH, OH radical addition would easily occur. Johnston and Hocking75 investigated the photocatalytic and photolytic degradation of 2,4-DCP with and without sonication. They found that the use of sonication (1 × 10-3 M, 2,4DCP, or 0.2% or 0.05% TiO2) in photolysis resulted in the enhancement of chloride release rate by a factor of 4 compared with that of UV irradiation only. The main sonooxidation products of p-nitrophenol (pNp) are nitrates (NO2-), nitrates (NO3-), short-chain carboxylic acids, formic acid (or formate, HCO2-), acetic acid (or acetate), and oxalic acid (or oxalate) with intermediates such as 4-nitrocatechol (4-NP), hydroquinone, and benzoquinone. Kotronarou et al.76 and Hoffmann et al.58 found that in the presence of ultrasound, p-nitrophenol solutions was denitrated to yield NO2- and NO3- by two independent mechanisms: (a) via the gas-phase reaction, which takes place in pure water, and (b) from the decay of p-NP molecules. It was shown that while NO3- did not interfere with the sonochemical reactions of p-NP, NO2- appeared to affect the decay of p-nitrophenol and the formation of 4-nitrocatechol (4-NP). Ultrasonic irradiation of a solution containing 100 µM p-NP and 100 µM NaNO2 was found to result in a slower disappearance of p-NP (k1 ) 2.0 × 10-4 s-1) when compared to a kinetic run in the absence of NO2- (k1 ) 3.67 × 10-4 s-1). On the basis of the products and kinetic observations, it was concluded that the degradation of p-NP involved high-temperature reactions of p-NP in the interfacial region of cavitation bubbles and that the main pathway was carbonnitrogen bond cleavage with hydroxyl radical reaction providing a secondary reaction channel. Hua et al.77 studied the sonochemical degradation of p-NP using high-power ultrasonic system (parallel-plate near field acoustical processor, NAP, with 0-1775W) in the presence of argon (Ar) and oxygen (O2) in order to investigate the effect of power per area. They found pseudo-firstorder constant for p-NP degradation in the presence of pure O2, (k2 ) 5.19 × 10-4s-1) was lower than that in the presence of pure Ar, (kAr ) 7.94 × 10-4s-1). The highest degradation rate was in the presence of Ar/O2 (4:1 v/v) mixtures (kAr/O2 ) 1.20 × 10-3s-1) at the same intensity of 1.4 W/cm2. Cost et al.78 also studied the sonochemical reactions of p-NP in solutions containing particulate matter, phosphate (total ions: HPO42- + H2PO4-, and a pH 6.8), bicarbonate (HCO3- ions, and pH 8), and humic acid and in solutions from lake water (natural water). They also observed first-order rate constants, kobs, for p-NP of 4.4 × 10-4 in deionized water and 3.4-3.2 × 10-4 s-1 in water with bicarbonate

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concentration: 10-4 e [HCO3-] e 10-2 M. They found the degradation rate was unaffected by chemicals of natural water. They rationalized the results by indicating that for pollutants such as p-NP which decay via thermal reactions near the interface or inside of cavitating bubbles, the high-temperature denitration reactions in this case are not significantly affected by common chemicals of natural waters. Barbier and Petrier79 investigated the ultrasonic degradation and minerilization of 4-nitrophenol (4-NP) at two ferquencies (20 kHz and 500 kHz) and in water saturated with oxygen or an oxygen-ozone gas mixture. They found the coupling of ultrasound and O3 increased the potential of ozonation to mineralize 4-NP degradation products. At low pH (pH 2), where the ozone auto-decomposition radical pathway is suppressed, they observed 4-NP mineralization at 500 kHz was 1.8 times faster than at 20 kHz for the same O3 consumption and attributed the enhanced rate to increased O3 utilization occurring at the higher frequency. Tauber et al.80 found the sonolytic degradation of 4-nitrophenol (4-NP) in argon-saturated aqueous solution (321 kHz) was fully due to OH-radicalinduced reactions at pH 10, where 4-nitrophenol is deprotonated (pKa ) 7.1) and nonvolatile 4-nitrophenolate predominates. At pH 4 and with the neutral form of 4-nitrophenol, oxidative-pyrolytic degradation predominated, resulting in large yields of CO, CO2, and H2. Takizawa et al.81 sonicated various phenolic compounds in aqueous media (water and water-ethanol) at 215 °C for 12-18 h. using 200 kHz ultrasound in the presence of oxygen and isolated various hydroxylated phenols as products of ultrasonic oxidation (Table 3). They also identified catechol and hydroquinone as the oxidation products of phenol. They postulated the following mechanism for the production of hydroquinone from phenol: The phenolic benzene ring was attacked by the hydroxyl radical produced in water sonochemically. The attack took place at the carbon attached to the methoxy group at the p-position of phenolic hydroxy group and the methoxy group was eliminated with hydrogen radical to give hydroquinone and methanol. Hua et al.56 observed that ultrasonic irradiation accelerated the rate of hydrolysis of p-nitrophenyl acetate (pNPA) in aqueous solution by 2 orders of magnitude over the pH range of 3-8 compared to the same hydrolysis under ambient conditions at 25 °C. The first-order rate constants were independent of pH and ionic strength. They explained their observations by considering the existence of transient supercritical water (SCW) around the collapsing cavitation bubbles during sonolysis as an important factor in the acceleration of chemical reactions due to higher concentration of OH- at the hot bubble interface resulting from higher ion product, Kw, of supercritical water. The main chemical degradation intermediates and products from benzene usually include phenol, catechol (CC), hydroquinone (HQ), p- and o-benzoquinone (p- and o- BQ), 1,2,3-trihydroxybenzene, maleic and muconic acid, formalaldehyde, and acetylene.82-85 Chlorobenzene transforms into 4-chlorophenol, chlorophenol, 4-chlorocatechol, hydroquinone, benzene, butenyne, butadiyne, nonchlorinated mono- and dicyclic hydrocarbons, CH4, and C2H2 upon ultrasonic irradiation in air-saturated solutions.84 De Visscher et al.82 investigated the sonochemical degradation of monocyclic aromatic compounds, benzene, ethylbenzene, styrene, and o-chlorotoluene, in aqueous solutions and found the first-order

reactions to be dependent on initial concentrations of the substrates and the sonication time. Pyrolysis is expected to be the main reaction path for the degradation of polar compounds. De Visscher et al.83 analyzed the reaction products of the sonochemical degradation of ethylbenzene in aqueous solutions and found them to be typical of high-temperature pyrolysis reactions. The reaction rates were also found to be first-order. Unlike ethylbenzene, both pyrolysis and radical attack appear to be important pathways of benzene and chlorobenzene degradations. Drijvers et al.84 observed that chlorobenzene was mainly degraded thermally and that the first-order degradation rates for the sonolysis was not influenced by the pH of the aqueous solution while saturating the solution with argon instead of air accelerated the degradation. It was also suggested that chlorophenol (CP), the only oxygen-containing intermediate detected, was not formed directly by the reaction of OH radicals with chlorobenzene but resulted from the addition of oxygen to chlorophenyl radicals in the interface since chlorobenzene could not break down in the bulk solution itself. Price et al.86 obtained a firstorder rate constant of 0.055 min-1 and 0.08 min-1, respectively, for the sonication of chlorobenzene (CB) and 1,4-dichlorobenzene (1,4-DCB) at about 39 Wcm-2 and observed that 1,4-DCB was totally consumed after 40-50 min. For a given intensity (i.e., 20.9 W/cm2), the reaction rate of 1,4-DCB was found to be faster compared with those of CB, naphthalene (NT), anthracene (AN), and pyrene (PR). Drijvers et al.85 compared the sonolysis at 520 kHz (9.4 or 0.063 Wml-1) and 25 °C of four monohalogenated benzenes, fluorobenzene (FB), chlorobenzene (CB), bromobenzene (BB), and iodobenzene (IB), at different initial concentrations, 0.5, 1, and 2 mM. They found all four to degrade by similar mechanisms and the sonolysis rates to depend on initial concentrations. Petrier et al.72 also studied the sonochemical degradation of chlorobenzene (CB). On the basis of the chloride ion evolution, the low level of hydroxylated intermediates, and the formation of gaseous stable products (CO2, CO, C2H2), they suggested a thermal degradation inside the cavitation bubble as the preferential reaction pathway, with C2H2 formed by thermolytic ring cleavage. The formation of brown carbonaceous particles was attributed to soot formation under pyrolytic conditions, phenyl radical combination, or C2H2 reactions at high temperatures. For a mixture of the hydrophobic CB and the hydrophilic 4-CPOH at 0.5 mM each, CB decayed first, and the 4-CPOH transformation started only when CB reached a low level (0.02 mM). The results also indicated that the presence of hydrophobic compound with high vapor pressure (Henry’s law constant, H, for CB ) 3.77 × 10-3 atm.m3mol-1) hinders efficiently the degradation of hydrophilic compound with less vapor pressure (H ) 2.4 × 10-3 atm.m3mol-1) or the less volatile solute, 4-CPOH. Khenokh and Lapinskaya87 observed that the oxidative processes arising from the ultrasonic irradiations of benzene and toluene in aqueous solutions resulted in the destruction of the six-membered ring compounds, with the formation of formaldehyde and other simultaneous reactions leading to the formation of phenol. Thoma et al.88 studied the sonochemical treatment of benzene and toluene in water using a parallel plate near field acoustic processor (NAP). They found the magnetostrictive system to be capable of degrading both contaminants in a continuous stirred tank reactor

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configuration with the rate constant inversely proportional to the target pollutant concentration. They also observed that the apparent first-order rate constant increased approximately linearly with the power input (measured from the wall) to the system. Hung et al.89 found the rate of reduction of nitrobenzene by Feo enhanced in the presence of ultrasound (US). For example, the first-order rate constant for the ultrasonic degradation of nitrobenzene in the absence of Feo was found to be 1.8 × 10-3 min-1 compared to 7.8 × 10-3 min-1 in the presence of 70 g/L Feo. They described the overall rate of nitrobenzene disappearance by the linear relation:

-

d[NB] ) kus[NB] + kFeo[NB] + k′[NB] ) dt (kUS + kFeo + k′)[NB] (11)

where kus, kFeo, and k′ are, respectively, the first-order degradation rate constants for sonolysis, reduction by Feo, and the synergistic kinetic effect achieved by combining the two systems. They also obtained the following relation from eq 11:

-ln

[ ]

[NB] 1 ) (kus + kFeo)t + kseckpt2 2 [NB]o

(12)

where ksec, kp ) xoka, xo, and ka are the average secondorder rate constant for the reaction between the Feo surface sites or the reaction intermediates and nitrobenzene, a zero-order rate constant that increases with added [Feo], the initial activated Feo surface before the application of ultrasound, and a proportionality (dx/ dt ) kax, x ) surface area of Feo at time t), respectively. They attributed the faster degradation rates in the combined US/Feo system to several causes. These include the indirect chemical effects associated with the continuous ultrasonic cleaning and activation of the Feo surface, the accelerated rates of mass transport resulting from the turbulent effects of cavitation, and the acidity produced by H+ released from HNO3, a product of sonolysis of nitrobenzene and water. The intermediates such as 4-nitrocatechol and benzoquinone usually observed when Feo is used for reduction were negligible in the combined US/Feo system. They also found the sonolytic degradation of aniline to be a first-order reaction with apparent rate constant of 2.4 × 10-3 min-1 at 20 kHz and 139 WL-1 with or without Feo. Soudagar and Samant90 evaluated the ultrasoundcatalyzed oxidation of various aryl alkanes (eg., toluene, nitro- and o-chloro-toluene, p-xylene) to the corresponding aryl carboxylic acids using aqueous KMnO4 under heterogeneous conditions and optimized parameters such as quantity of KMnO4, stirring rate and reaction time. It was shown that heterogeneous oxidation of unsubstituted aryl alkanes was significantly accelerated using aqueous KMnO4 at ambient temperatures and in the presence of ultrasound. While under optimized conditions o-chlorotoluene and mesitylene (1,3,5-trimethylbenzene) were effectively converted into the corresponding carboxylic acids, isopropyl benzene and tert-butyl benzene did not undergo oxidation and were recovered unreacted. Nagata et al.91 investigated the aqueous sonochemical degradation of hydroxybenzoic acids such as monohydroxbenzoic acids (HBAs), 3,4dihydroxybenzoic acid (3,4-DHBA), 3,4,5-trihydroxybenzoic acid (gallic acid, GA), tannic acid (TA), and humic

acids (HA) under air or argon atmosphere. They found the pseudo-first-order constants for 2-HBA, 3-HBA, and 4-HBA to be 3.0 × 10-2, 4.9 × 10-2 and 5.1 × 10-2 min-1, respectively, under Ar and 2.7 × 10-2, 3.4 × 10-,2 and 3.1 × 10-2 min-1 under air. However, the rate constants of GA and TA were higher under air irradiation (5.5 × 10-2 and 16.4 × 10-2 min-1) compared with irradiation under Ar (2.6 × 10-2 and 6.4 × 10-2 min-1). The rate constant for 3,4-DHBA was the same under both atmospheres (1.9 × 10-2 min-1). The decomposition of 3-HBA was almost completely quenched in the presence of t-BuOH at a concentration of 0.1 mM. They argued that the sonolytic degradation of the HBA under Ar atmosphere proceeded mainly via reactions with OH radicals in the bulk solution. Whereas under air oxygen molecules contributed to the decomposition of the polyhydroxybenzoic acids at the interface, thermal decomposition in cavitation bubbles or interfacial regions played a negligible role in both atmospheres. They also suggested that the participation of oxygen molecules accounted for the faster decomposition rates of GA and TA in air, the rates in air facilitated with an increasing number of OH groups (kTA > kGA) as expected with oxidation of phenolic compounds with oxygen. D-Silva et al.92 observed the rapid destruction of polycyclic aromatic hydrocarbons (PAHs) and dioxin when solutions of benzo[a]pyrene (B[a]P), benzo[ghi]perylene, coronene, or 2-chlorodibenzo-p-dioxin in 20% ethanolwater at approximately 10-15 ppm levels were subjected to high-intensity sonication. It was shown that the sonolytic decomposition of PAH and dioxin was accompanied by the formation of a black carbonaceous residues. It was also shown that the effect of ultrasound on a 100% ethanol solution of B[a]P was not as pronounced as in water-ethanol mixture. 4.2. Chlorinated Aliphatic Hydrocarbons (CAHs). The sonochemical reactivity of chlorinated hydrocarbons in aqueous solutions is attributed to their volatility and their low solubility in water, properties facilitating their concentration in the cavitation bubbles resulting in rapid decomposition by high temperature and high pressure. The sonication of 15 mL of neat chlorinated solvents 1,1,1-trichloroethane (TCA) and trichloroethylene (TCE) produced a yellow color (presumably due to elemental chlorine) in 10 min and in 2 h produced a brown suspension indicating the solvents have polymerized due the treatment.92 Inazu et al.93 found that aqueous solutions of trichloroethylene (TCE), tetrachloroethylene (PCE), 1,1,1-trichloroethane (1,1,1-TCA), chloroform (CHCl3), and carbon tetrachloride (CCl4) decomposed rapidly to chloride anions (Cl-), hydrogen, carbon monoxide, and carbon dioxide by ultrasonic irradiation in the presence of argon, O2, or air. They suggested that the main reactions were thermal decomposition or combustion in cavitation bubbles and not reactions by OH radicals or H atom. It was also shown that the main products of TCE degradation products under Ar were Cl-, CO, and H2. The minor products of the decomposition under Ar were CO2, CH4, C2H4, and a trace amount of dichloroethylene. Under the sonication of O2 or air, considerable amount of CO2 evolved, and no CO was observed. Drijvers et al.94 identified the following volatile products of TCE sonication at 50 kHz: chloroacetylene (C2HCl), dichloroacetylene (C2Cl2), dichlorodiacetylene (C4Cl2), tetrachloroethylene (C2Cl4), two isomers of trichlorobutenyne (C4HCl3), tetrachlorobutenyne (C4Cl4), pentachlorobutadiene (C4HCl5), and

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hexachlorobutadiene (C4Cl6). It was also shown that the degradation rate of TCE was more energetically efficient at 520 kHz than at 20 kHz. Reinhart et al.95 studied the combined effects of sonication and zerovalent iron on the destruction rate of TCE and found ultrasound to increase the rate significantly. For example, they obtained first-order rate constants, k (hr-1), of 0.0147, 0.0218, and 0.0426, respectively, for ultrasound alone, iron alone (3 g/L), and the combined system, respectively. The main intermediates of the sonication of the aqueous carbon tetrachloride (CCl4) are tetrachloroethylene (C2Cl4) and hexachloroethane (C2Cl6) with trace amounts of dichlorocarbene, dichlromethane, trichloroethylene, tetrachloroethane, pentachloroethane, hexachloropropene, hexachlorobutadiene, and chloroform, and the final products are usually chlorine, chlorides (or HCl), chlorohydric acid, hypochlorous acid (HOCl), CO, and CO2.96-100 Jennings et al.97 investigated the sonochemical reactions of CCl4-H2O mixtures in an argon atmosphere and compared results with those of CHCl3-H2O mixtures. They observed cavitation when CCl4 was sonicated alone, but there was no reaction. In the presence of as little as 1 mL H2O per 100 mL of CCl4, reaction ensued, and the total inorganic chlorine yield was found not to be significantly sensitive to large changes in the CCl4 to H2O ratios. The observed products from the CCl4 reaction included: CO2, O2, Cl2, HCl, C2Cl6, and C2Cl4 compared with products of CHCl3: HCl, C2Cl6, and C2Cl4. Hua and Hoffman99 studied the sonolytic destruction of aqueous CCl4 in the absence or presence of ozone (O3). The observed firstorder degradation rates in argon-saturated aqueous solutions were 3.3 × 10-3 and 3.9 × 10-3 s-1 with CCl4 initial concentration of 1.95 × 10-4 and 1.95 × 10-5 mol L-1, respectively. It was shown that the presence of O3 did not affect the degradation rate significantly but inhibited the accumulation of C2Cl4 and C2Cl6 intermediates. However, sonication of p-nitrophenol in Arsaturation aqueous solution in the presence of CCl4 resulted in the degradation rate enhancement of p-NP by a factor of 4.5 compared to sonication without CCl4 and the reduction of 4-nitrocatechol, an aromatic degradation product. The improvement in rate and minimization of 4-nitrocatechol formation were shown to be due to the presence of HOCl, a product of CCl4 sonolysis. Koszalka et al.100 obtained 98% reduction of CCl4 in the presence of H2, air, or Ar and observed the ultrasonic destruction to be an exponential process. It was shown that the initial reaction products: chloroform, dichloromethane, and chloromethane were further degraded to methane and chlorine/chloride. Total halogen analyses of unreacted and reacted CCl4-contaminated samples also confirmed Cl balance, indicating that CCl4 was not volatilized or masked by the chemistry of the process. Wu et al.7 studied the destruction of CCl4 under dissolved air and varied process parameters such as pH, irradiation, time, steady-state temperatures, and ultrasonic intensity. It was shown that CCl4 destruction rate was significantly affected by the intensity of the ultrasonic energy, the rate increasing proportionally to the intensity. However, the addition of H2O2 as an oxidant with or without ultrasonic irradiation was also found to have negligible effect upon the rate. This was attributed to the predominance of the thermal dissociation reaction in the cavitation holes under supercritical conditions compared to the bulk-liquid-phase reactions

involving CCl4 and oxidants such as OH radicals. The bulk-liquid rate constants (e.g., 107 M-1 min-1) were found to be about several times smaller than those in the cavities (e.g., 1012 M-1 min-1). On the basis of a detailed chemical reaction mechanism and assuming a batch reactor, they described the destruction of CCl4 in water by the following differential equations using second-order rate constants and concentrations in the bulk-liquid phase and cavity respectively:

-d[CCl4] ) kI[OH][CCl4] + kII[H][CCl4] + dt kIII[HO2][CCl4] + kIV[O][CCl4] (13) d[CCl4] ) kckv[CCl4][M] dt

(14)

where M is any collision partner and kc is the system adjustment coefficient, which is a function of bubble concentration, bubble radius concentration, mixing extent of the system, etc. The results of this model formulation via a series of elementary reactions were found to be in general in agreement with experimental data. They obtained the value of kc to be constant at 2.5 × 10-11 by fitting experimental data to the model, assuming constant vessel size, steady-state temperature, and power. Hung and Hoffmann101 investigated the degradation of CCl4 by combined ultrasound (US) and iron (Feo) in Ar-saturated solutions and observed a 40-fold enhancement in overall rate for the coupled system compared with US alone. The first-order ultrasonic rate constant for CCl4 was 0.107 min-1 at 62 W (20 kHz) in 285 mL solution, whereas the apparent rate constant in the presence of Feo was found to depend on the total surface area of elemental iron as follows:

kobs ) (kus + kFeoAFeo) min-1

(15)

where kus ) 0.107 min-1, kFeo ) 0.105 L m-2 min-1, and AFeo is the reactive surface area of Feo in units of m2/L. Rajan et al.102,103 investigated experimentally and by modeling of the water-KI-CCl4 system and attributed the significant increase in oxidation rates observed in a suspension of CCl4 liquid in KI solution to the release of Cl2, Cl, and HOCl, which act as a separate source of reactants to yield I2. They used a complex mathematical model that included detailed kinetics of CCl4 to predict the extent of degradation of dissolved CCl4 and the large enhancements in the rate of oxidation of aqueous iodine ion (I-) in the presence of CCl4. A similar study of the CCl4/KI (1.0 M solution) system by Alippi et al.9 found the yield of free iodine to increase linearly with sonication time. Petrier et al.104 studied the sonolysis of chloromethane and their dilute mixtures in alcohols under saturation with Ar or O2 and found the zero-order rate constants to be greater in Ar atmosphere. Since the sonochemical cleavage of the alcohols were found to be weak, the initial step in the transformation (faster in Ar) was attributed to the C-Cl cleavage, in constrast to the reaction in water, where sonochemical cleavage of the solvent plays a major role. They also explained that the sonolysis, probably in the cavitation bubble of CCl4 and which was more efficient in alcohols than in the pure state, could result from the preferential vaporization of this apolar molecule into a bubble formed in a hydrogenbonded structured liquid. Toy et al.12,13 investigated the

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decomposition of aqueous 1,1,1-trichloroethane (1,1,1TCA) under sonolysis, photolysis, and the synergistic combination of the two and reported more extensive decomposition with combined photolysis and sonolysis (photosonolysis) than UV photolysis or sonolysis alone. It was also shown that TCA (CCl3CH3) decomposed into gases, VOCs, and ionic species, and as the volume of the same concentration was increased, the sonolysis digestion efficiency decreased. Cheung et al.8 examined the sonochemical destruction of methylene chloride (CH3Cl), TCA, TCE, and CCl4 and attributed the rapid decrease of reactor solution pH in all cases to the probable formation of HCl from the chlorinated reactants with the hydrogen source being water. Bhatnagar and Cheung10 studied the kinetic behavior of individual VOCs in the presence of other VOCs by sonicating mixtures of (1) two chloromethanes (CH3Cl + CCl4) and (2) CH2Cl2, CCl4 (C1 compounds) + 1,1,1-TCA (C2 compound). The first-order rate constants for the compounds were essentially unchanged by the presence of other reacting species. Petrier and Francony105 compared the decomposition rates of CCl4 and phenol at different frequencies (20-800 kHz) and observed that irrespective of frequency, it was easier to decompose the hydrophobic and volatile CCl4 than the hydrophilic phenol with a low vapor pressure. For the phenol, they found that degradation proceeded faster at 200 kHz and that the addition of an excess of 1-butanol (10-fold phenol initial concentration), used to scavenge hydroxyl radicals totally inhibited the reaction. The rate of disappearance of CCl4 on the other hand was found to increase slightly with increase in frequency, and the addition of 1-butanol (10-fold CCl4 initial concentration) did not affect the rate of CCl4 destruction. It was also shown that both phenol degradation and H2O2 formation involving reactions with OH radical at the interface increased with frequency reaching an optimal value at 200 kHz. Drijvers et al.106 investigated the mutual influence of TCE and chlorobenzene by sonicating dilute aqueous mixtures of the two volatile organic compounds and found the sonolysis rates to depend strongly on the initial concentration of TCE and CB. The presence of CB was found to lower the sonolysis rate of TCE, while the sonolysis rate of CB (with initial concentration 3.44 mM) was enhanced by the presence of 0.84 mM TCE. They attributed the different effects of both volatile organics on each other’s sonolysis to the difference in reaction rate of TCE and CB with the radicals formed during sonolysis. They suggested that the effect of TCE on the sonolysis rate of CB (i.e., the lowering of the γ value of the gas present in the cavitation bubble and consequently the temperature during collapse) was compensated by an increased indirect degradation of CB by radicals formed out of TCE. De Visscher and Van Langelove107 explored the influence of Fenton-type oxidants on the aqueous sonochemical degradation of trichloroethylene (TCE), o-chlorophenol (o-CP), and 1,3-dichloro-2-propanol (DCP). It was shown that di-tert-butyl peroxide inhibited the sonolysis of both TCE and CP by scavenging OH radicals and by lowering the maximum temperature generated during a cavitation event. However, 10 mM tert-butyl hydroperoxide had no considerable effect on the sonochemical degradation of TCE but inhibited completely the degradation of o-CP. The effect of TCE was interpreted to mean that the amount of tert-butyl hydroperoxide in the cavitation bubbles (where TCE is degraded) was too low

to change the conditions of a cavitation event significantly. The effect on o-CP indicated that tert-butyl hyroperoxide scavenged OH radicals without producing any radicals that entered the liquid phase where oxidation was expected to occur. Unlike the less polar, organic peroxides, the presence of H2O2 did not affect the degradation of TCE and DCP but enhanced the degradation of o-CP on the order of 20-25%. The degradation of both o-CP and DCP (with low Henry’s constant) are expected to occur by radical attack. Hence, they attributed the low reactivity of DCP toward OH to the lack of aromatic rings on which the radicals can add and to the presence of Cl atoms on DCP that could lead to electronic repulsion toward the OH radical. They also observed that in the presence of H2O2 and copper ions, the sonochemical degradation of o-CP is the sum of the effect of the ultrasound and the chemical oxidation effect and likened the overall degradation mechanisms to sonochemical switching effect. That is, sonication induces a specific reactivity and silent chemical, and sonochemical degradations follow separate paths. The CAV-OX has been shown to degrade TCE and BTEX in contaminated groundwater with efficiencies better than 99.9% when varied principal opearting parameters such as H2O2 dose, pH, and flow rate are used.14 The process involves the use of sequential hydrodynamic cavitation and direct UV photolysis of H2O2 to generate hydroxyl and hydroperoxyl radicals. Orzechowska et al.108 monitered and compared the chloride release tendencies of CCl4, CHCl3, TCE, aromatic chloro compounds (e.g., CB, polyaromatic chloro compounds, PCBs, 4,4′- and 3,3′-dichlorobiphenyl) and TritonX-100 (a surfactant) upon sonication and found that CB and PCBs did not release Cl- as readily as did CCl4, CHCl3, and TCE. They attributed the results to possible hydroxylation reactions at an open position of aromatic ring, i.e., oxidation of PCBs and CB by OH radical without dehalogenation. The presence of humic substances was also found not to affect Cl- yields. Catallo and Junk109 studied the sonochemical transformation of hazardous chlorinated chemicals and a mixture of chlorinated olefins, paraffins, and aromatics from a Louisiana superfund site (LSS) and observed dechlorination via aryl C-Cl bond heterolysis and hydroxylation reactions to be the main reaction pathways. Dechlorination and/ or other transformations were observed for chlorinated alkanes (e.g., CCl4) 2,4-dichlorophenol, chlorpyrifos, and lindane (hexachlorocyclohexane, γ-isomer) and a for range of chlorinated aromatic and aliphatic chemicals in the LSS mixture. The herbicide byproducts, chlorpyrifos, 3,3′,4,4-tetrachloroazoxybenzene (TCAOB), and trichlorodibenzofuran (TCDF), were not dechlorinated. However, TCAOB appeared to deoxygenate sonochemically to the 3,3′,4,4 tetrachloroazobenzene. Sakai et al.110 studied the sonochemical degradation of aqueous solutions of chloral hydrate (CCl3CH(OH)2 or simply CH) and found the rate to be proportional to the square of oxygen concentration at low concentrations, while at relatively high concentration, the rate is independent of the oxygen concentration. They also found the reaction rate to be proportional to the square root of the concentration of chloral hydrate and to the intensity of the ultrasound regardless of the concentration of oxygen gas dissolved in the solutions. 4.3. Explosives. Sierka111 investigated the treatment of aqueous solutions of trinitrotoluene (TNT) and cyclotrimethylene-trinitramine (RDX) in the weight ratio

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of 70:30 (typical of munitions wastewater) at 25-59 °C using a combination of ozone and ultrasound in the pH range of 5.84-10.00. They demonstrated the synergistic effects of both ozone and ultrasound. While the maximum destruction of both TOC and TNT was observed at 859 kHz and 50W, experiments performed without O3 at these same conditions resulted in no TNT or TOC removals. The increased removal with O3/ultrasound systems was explained partially by the increased temperature in the reactor. While at 25 °C, the TNT destroyed after 60 min of ozonation was 23%; 99% was destroyed when temperature averaged 59 °C. Removal rates of both TNT and TOC increased directly with increases in reaction temperature and initial pH. However, it was shown that at solution pH values maintained in excess of 9.6, the combination of high pH and temperature was not beneficial. This was attributed to the fact that ultrasound inhibited kinetics at high temperatures and pH by promoting radical-radical extinguishment reaction. But ultrasound was also shown to have an additional benefit potential in that it can act as a catalyst by enhancing the autodecomposition of O3 to free radicals once the ozone has been dissolved in the aqueous phase. Sonolysis was evident in the degradation of residues of the military explosives TNT (2,4,6trinitrotoluene) and HMX (octahydro-1,3,5,7-tetranitro1,3,5,7-tetrazocine), as indicated by release of nitrate (NO3-) during treatment.73 Hoffmann and co-workers16,58 112 investigated the sonochemical degradation of TNT at 20 and 500 kHz to acetate, formate, glycolate, oxalate, CO2, NO2-, and NO3- as products with 1,3,5trinitrobenzene as intermediate and found the overall degradation rate to follow an apparent first-order rate law. The overall reaction stoichiometry for the auto oxidation of TNT was found to be: )))))

C7H5(NO2)3 + 9O2 98 7CO2 + H2O + 3H+ + 3NO3- (16) where ))) denotes ultrasonic irradiation. They found TNT degradation to be more efficient at 500 kHz than 20 kHz, irrespective of the background gas. The firstorder kinetic rates vary by a factor up to 3, depending on the nature of dissolved gases and also on the ultrasonic frequency: kO2 ) 1.67 × 10-5 s-1, kAr ) 4.50 × 10-5 s-1, and kO2/O3 ) 5.50 × 10-5 s-1 (with 99/1 vol % O2/O3 mixture as cavitating gases) at 20 kHz compared with kO2 ) 2.00 × 10-5 s-1, kAr ) 7.17 × 10-5 s-1, and kO2/O3 ) 8.50 × 10-4 s-1 at 500 kHz. 4.4. Herbicides and Pesticides. The sonolysis of the systemic herbicide, chlorpropham (isopropyl-3-chlorocarbanilate) in air equilibrated aqueous solutions lead to the formation of hydroxylated products and 3-chloroaniline as intermediates, and subsequent mineralization of intermediates to Cl-, NO2-, NO3-, CO, and CO2. David et al.113 observed that the ultrasonic treatment of chlorpropham at 482 and 20 kHz led to a highly drastic degradation with the initial rate at the high frequency (13.0 × 10-8 M s-1), much higher than that at the low frequency (4.2 × 10-8 M s-1). They also noticed a complete degradation after 45 min at 482 kHz compared with 1/3 of chlorpropham remaining after 60 min at 20 kHz. It was also shown that the yield of Clwas about 98% after 80 min at 482 kHz, indicating the C-Cl bond cleavage is a major primary process. Two pathways deduced to be involved in the transformation, the oxidation by OH• and pyrolysis near the cavitation

bubbles. The formation of hydrated products (e.g., 3-chlorohydroquinone) indicated the involvement of the hydroxyl radical in the degradation of chloropropham. Petrier et al.114 investigated the ultrasonic degradation of atrazine, CIET [2,4-diamino-6-chloro-N-ethhyl-N′-(1methylethyl)-1,3,5 triazine] and pentachlorophenol (PCP) at 500 kHz and 20 kHz and demonstrated in both cases that degradations were more efficient and complete at the higher frequency. Sonochemical degradation of atrazine resulted in dealkylation product: CIAT [2,4diamino-6-chloro-N-(1-methylethyl)-1,3,5 triazine], CEAT[2,4-diamino-6-chloro-N-ethyl-1,3,5 triazine], and CAAT[2,4-diamino-6-chloro-1,3,5- triazine], which are typical oxidation products of atrazine from degradation by ozone, Fentons reagent, or TiO2 photocatalysis. Long irradiation times (3 h) resulted in the formation of CO2 (13% of initial atrazine) and chloride. They observed fast reactions for PeCP (10-4 M aqueous solution at pH 7) at both frequencies, yielding 98% of the theoretical amount of chloride and 20% theoretical carbon as CO2 recovered after 180 min for the reaction conducted at 500 kHz. However, the addition of n-pentanol (10 mM) inhibited completely both atrazine and PeCP degradation at high and low frequency, suggesting reactions with OH• radicals escaping from the cavitation bubble. Koshkinen et al.115 found the sonochemical degradation of atrazine to be slower than that of alachlor (70-fold) at 20 kHz at the same concentration (e.g., 3.1 µmol/L) and temperature (30 °C). The first-order rate constants and extrapolated half-lives were 8.01 × 10-3 min-1 and 86 min and 2.1 × 10-3 min-1 and 330 min for alachlor and atrazine, respectively. They also attributed the decomposition of both atrazine and alachlor to the attack by hydroxyl free radicals. Nagata et al.74 found the aqueous sonochemical degradation of 0.1 mM pentachlorophenol (PeCP) to be faster under argon atmosphere compared with air atmosphere (first-order rate constant of 6.4 vs 4.5 µM min-1). Gondrexon et al.116 studied the sonochemical degradation of PeCP in a three-stage, continuous flow, laboratory-scale ultrasonic reactor and obtained a conversion up to 80%. Johnston and Hocking75 investigated the photocatalytic and photolytic degradation of PeCP (2.4 × 10-4 M solution) with and without sonication. They observed the initial rate of chloride formation due to photocatalysis using TiO2 to be about 2.7 times faster with sonication than without. In the absence of sonication, the percent degradation measured by Cl- release reached a value of 40% after 50 min, beyond which continued irradiation up to 200 min did not significantly affect increase in degradation. In contrast, combined sonication, UV, and photolysis resulted in a rapid initial degradation, with near quantitative release of chloride after 120 min. They attributed their observation to possible poisoning of catalyst active sites during photodegradation by inert intermediates formed but which are removed from active sites by sonication, allowing further degradation or decrease in pH during photolysis, leading to significant decrease in a photochemical degradation process. Petrier et al.117 also studied the sonochemical degradation of pentachlorophenate (PCP) at 530 kHz in aqueous solutions saturated with air, oxygen, or argon and observed the degradation and the resulting toxicity decrease to be faster under argon bubbling (PCP disappeared in 50 min) than under air or oxygen. The ultrasonic degradation of PCP was also attributed to a rapid cleavage of carbon-chlorine bond

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to release Cl- (90% chlorine recovered as Cl- in 150 min). This was followed by degradation to CO2 in airor oxygen-saturated solutions (or CO in argon-saturated solutions) and nitrite and nitrate in solutions aerated with air or oxygen. They also postulated that because of the high temperature inside the cavitation bubble, the formation of CO in the presence of argon was due to reactions of the type:

CO2 + Ar f CO + O + Ar

and CO2 + •H f CO + •OH

Wheat and Tumeo118 studied the aqueous sonolysis of two polycyclic aromatic hydrocarbons or PAH’s (phenanthrene and biphenyl), and the results obtained suggest polychlorinated biphenyls (PCB’s) may be susceptible to hydroxyl radical substitution reactions under the vigorous reaction conditions supplied by high-intensity ultrasound. The principal products of biphenyl reaction identified through mass spectral analysis were [1,1biphen]-2-ol, [1,1-biphen]-3-ol and [1,1-biphen]-4-ol, and the principal products of phenanthrene appeared to be di-hydroxy substituted phenanthrene. The phenolic products produced from these hydrophobic compounds were considered consistent with a free radical attack by hydroxyl free radicals (OH•) on the hydrocarbon skeleton. Zhang and Hua119 also observed that polychlorinated biphenyls (2-, 4-, and 2,4,5-chlorobiphenyls) dechlorinated rapidly to chloride ion during sonolysis at 20 kHz with 99% destruction of 2-PCB, 4-PCB, and 2,4,5-PCB in 36, 47, and 29 min, respectively. The chloride recovery ratio defined as [Cl-] /([M]i - [M]f) × n, where n is the number of chlorine atoms per cogener in each sample and [M]i and [M]f are initial and final concentrations of parent compound (i.e., mol/L PCB), were 77%, 79%, and 70%, respectively, for 2-PCB, 4-PCB, and 2,4,5-PCB, respectively. They also identified products such as biphenyl, toluene, ethylbenzene, phenol, trichlorophenol, hexenyldiphenol, and other phenols (Table 3). They investigated the role of aqueous hydroxyl radical in the decomposition of 2-PCB at 205, 358, 618, and 1071 kHz. It was shown that ultrasonic irradiation was optimal at 358 kHz but chloride recovery was optimal at 1071 kHz. They proposed that both thermolysis and free radical attack are important pathways of PCB destruction but that free radical attack (especially •OH) in the aqueous phase played a more significant role at 358 kHz than at other three frequencies. Johnston and Hocking75 investigated the photo catalytic /photolytic degradation of PCB isomer 3-chlorobiphenyl at 75 ppm level (4 × 10-4 M) by monitoring the chloride formation. TiO2 (0.2% w/w) and 30 mL aliquots were subjected to UV irradiation and to combined UV/ ultrasound irradiation. They found a linear rate of appearance of chloride with time for both conditions, but rates with sonication were approximately 3 times greater that without sonication, and they attributed the enhanced rates partially to the highly hydrophobic nature (i.e., low solubility) of substrate. They also attributed the significant increase in degradation rates and efficiency of the concurrent UV/ultrasonic irradiation to cavitational effects, bulk, and localized mass transport effects and sonochemical reactions. Kotronarou et al.120 showed that the sonolysis of parathion (o,o-diethyl o-p-nitro phenyl thiophosphate) led to its transformation to sulfate, nitrate, nitrite, phosphate, oxalate and p-nitrophenol, p-NP (parathion

hydrolysis product) with complete degradation in e2 h. They proposed that parathion transformation proceeded via thermal decomposition in the hot interfacial region of the collapsing bubbles with secondary reactions with •OH radicals. Theron et al.121 determined the removal rates of phenyltrifluoromethyl ketone (160 µmol L-1 PTMK) by sonolysis at 30 and 515 kHz, by UVirradiated TiO2 (Degussa P25 and Rhodia), and by simultaneous photocatalysis and ultrasonic irradiations. The PTMK first-order removal rate constant was found to be 14 times greater at the higher frequency compared with that at the lower frequency at the same energy level and 2.5 times higher when synthesized Rhodia TiO2 was used instead of Degussa P25. The amount of trifluoroacetic acid (CF3COOH or TFA), an undesirable and stable product of TiO2-treated CF3-containing pollutants, was about 8 times lower in sonicated solutions than in UV-irradiated suspensions, for both frequencies. 4.5. Organic Dyes. Vinodgopal et al.122 studied the sonochemical degradation of a reactive black dye (Azo dye, Remazol Black B or RB) at high frequency (640 kHz) in O2-saturated aqueous solution. They found sonolysis as a feasible method to achieve both decolorization and 65% degradation of the dye in 6 h as measured by the decrease in the in total organic content. The pseudo-first-order rate constant for the disappearance of the dye was determined to be 2.9 × 10-2min-1. They also observed that degradation stopped far short of completion when performed in an aqueous solution of about 2% tert-butyl alcohol, an excellent OH radical scavenger, and attributed the conversion of the dye to oxalate (C2O42-) to •OH radical-initiated oxidative degradation. They also compared photocatalytic and radiolytic methods of degradation to sonolysis. Their results suggested that sonochemical degradation rates of Azo dyes were substantially faster and led to better destruction of the dye compared to photocatalysis. Stock et al.123 investigated the degradation of an azo dye, naphthol blue black (NBB), using sequential combination of highfrequency sonolysis and photocatalysis in four different configurations: (1) sonolysis only, (2) photocatalysis only, (3) simultaneous sonolysis and photocatalysis, and (4) sequential sonolysis and photocatalysis. They observed that for individual techniques, sonolysis was effective for inducing faster degradation of the parent dye, while TiO2 photocatalysis was effective for promoting degradation. For example, photocatalysis was responsible for 68% conversion compared to 35% for sonolysis after 12 h. It was also shown that the firstorder rate was enhanced by the combined systems (simultaneous and sequential). For example, pseudofirst-order rate constant was 1.83 × 10-2 min-1 for the combined case compared with 1.04 × 10-2 min-1 for sonolysis or 0.56 × 10-2 min-1 for photocatalytic experiments. 4.6. Organic and Inorganic Gaseous Pollutants. Cheung and Kurup124 investigated the sonochemical destruction of fluorotrichloromethane (CFC11) and trifluorotrichloroethane (CFC113) in dilute aqueous solutions in a batch and circulating reactor. They found the rates to be fairly rapid with less than 5% of the original CFC’s undergoing volatilization and the bulk of material destroyed in the liquid phase. In these preliminary studies, they also observed the rates to be slightly higher at 5 °C than at 10 °C and rapid drops in pH (initial value of 7.4 to about 5.4), indicating the formation of acidic species. Hirai et al.125 evaluated a number of CFC’s and

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HFC’s and found them to be readily decomposed sonochemically with high efficiency to inorganic products (CO, CO2, Cl-, F-). It was also shown that CFC113 decomposed faster under argon atmosphere (with higher CP/Cv ratio) than air atmosphere and that its decomposition rate was not significantly affected by the addition of tert-butyl alcohol (tert-BuOH), a known effective OH radical scavenger. Hart and Henglein126 studied the sonochemistry of Ar/N2O mixtures and the pure gases in aqueous solutions at high frequency, 300 kHz, and observed products of decomposition of N2O to be N2, O2, NO2- (or HNO2), and small amount of NO3(or HNO3). It was also observed that the maximum yield of decomposition occurred at an Ar/N2O vol % ratio of 85:15. However, the rate of N2O decomposition was very low when water was sonicated under pure nitrous oxide, as expected with a triatomic gas. The yield of HNO2 was much higher than observed in the irradiation of water under air and under mixtures of nitrogen and argon, and this was attributed to the decomposition of N2O of the type: 3N2O + H2O f 2HNO2 + 2N2. Henglein127 studied the ultrasonic irradiation of water under argon atmosphere containing small amounts of CO2, N2O and CH4. They observed no chemical effects in the irradiation under atmosphere of pure CO2, N2O, or CH4. However, they showed that sonochemistry in an argon atmosphere containing small amount of any of the polyatomic gases was characterized by a strong linkage between the sonolysis of water and the added gas. The main products of the sonolysis of CO2 were CO and small amount of formic acid, about 30 times smaller than CO yield. The main products of N2O sonolysis were N2, NO2- and NO3- in this study.127 In the N2O-Ar containing solutions, they observed the yields of the products NO2- and NO3- were about 10 times greater than the amounts formed in the ultrasonic irradiation of aerated water and greater than the amounts in water irradiated in the presence of both N2O and O2. Harada128 investigated the sonolysis of CO2 dissolved in water (0.0-0.6 mole fraction CO2 in water) at 5-45 °C using an ultrasonic generator (200 kHz, 200 W) and Ar, He, H2, and N2 as the irradiating gases. While irradiation of ultrasound had hardly any sonochemical effect under CO2 atmosphere, the most favorable atmosphere for reducing was CO2-Ar mixture with 0.03 mole fraction of CO2 in gas phase, a concentration considered an equal mixture of CO2 and Ar in water because CO2 is more soluble than Ar. The amount of CO2 in CO2Ar was found to decrease to about half at 5 °C, and the decreasing rate for CO2 followed the order Ar > He > H2 > N2. They also observed that dissolved CO2 was deoxidized to carbon monoxide (CO2 f CO + O) by sonolysis and showed that the hydrogen carbonate or bicarbonate ion (HCO3-) played an important role compared to the carbonate ion (CO32-). Since hydrogen is also obtained from the solvent (water) and both CO and H2 are fuel gases, which also react to produce C1 compounds such as methanol, they proposed sonication as a useful and potential technique to reduce CO2 and produce fuel. The main oxidation products of the methane-containing solutions were oxygen-containing gases such as CO, CO2, and compounds with oxygen, such as C2H4, C2H6, and C3 and C4 compounds, and the production of large amounts of H2. Hart et al.129 also identified H2, C2H2, C2H4, and C2H6 as major products and C3-C4 hydrocarbons (e.g., propane, propene, 2-methylpropane, n-

butane, 1-butene, 2-methyl,1-butane and butadiene) and CO as other products of CH4 sonolysis under argon atmosphere. They found the products for the sonolysis of the C2H6 to be the same as CH4 except that they were produced in higher yields. Hart et al.130 also observed the sonochemical degradation of acetylene to be rapid (90% consumption of C2H2 at time g 80 min) with formation of large products. In addition to small gas molecules such as CO and CH4, various hydrocarbons with intermediate C atom numbers, such as benzene and other C6H6 isomers, high C atom numbers such as naphthalene are formed. Kotronarous et al.131 found the ultrasonic irradiation of H2S or S(-II), [[S(-II) ) [H2S] + [HS-] + [S2-]] solutions to result in rapid oxidation and the formation of sulfate (SO42-), sulfite (SO32-), and thiosulfate (S2O32-) as products at pH g 10 with the initial mole ratio of 2.2:2.7:1, respectively. They also found oxidation of S(II) at pH 10 to be zero order with respect to [S(-II)] with the zero-order rate constant, kO, dependent on initial concentration of S(-II) and increasing linearly with [S(-II)]O up to [S(-II)]O = 450µM (where kO = 12 µM min-1) and remaining constant thereafter. However, at low pH e 8.5 (e.g., pH 7.4), the rate of ultrasonic oxidation of S(-II) was first order with respect to [S(II)], the rate of oxidation increased with a decrease in pH, and measured concentrations of SO42-, SO32-, and S2O32- could not account for the total observed decrease in S(-II), especially at the short sonication times. The discrepancy between the [Sox] measured and the [Sox] expected was attributed to the formation of sulfur (S8) that was not analytically measured but increased the turbidity of the solution. The apparent zero-order dependence on [S(-II)] suggested that the rate-determining step (or the main pathway) in the overall reaction was the reaction of HS- and the oxidation intermediates with •OH radical in the liquid phase as it diffused out of the cavitation bubble (HS- + •OH f HSOH-) and the observed decrease in the zero-order rate constant at the pH > 10 was attributed partly to the dissociation of •OH in the alkaline solutions and since the oxide radical ion (•O-) reacts more slowly with the same substrate than •OH according to the reaction: •OH +OH- S O•- + H O, where the forward rate, k ) 2 f 1.2 × 1010m-1s-1 and the backward rate, kb ) 9.3 × 107s-1. At pH e 8.5, thermal decomposition of H2S within or near collapsing cavitation bubbles was considered the important reaction pathway. Kotronarou and Hoffmann132 also developed a comprehensive aqueous-phase mechanism for the free-radical chemistry of the S(-II) + ‚OH + O2 system and used the mechanism to successfully model the oxidation of S(-II) at 20 kHz and 75 W/cm2 and at pH g 10, assuming a continuous and uniform ‚OH (3.5 µM/min) input into the solution from the imploding cavitation bubbles. When water was irradiated with ultrasound under an atmosphere containing O2 and O3, an extremely rapid decomposition of O3 was found to take place, leading to a yield of H2O2 up to a factor of 6 higher than in water containing oxygen only.133 Weaver and Hoffmann3 investigated the sonolytic degradation of O3 aqueous solutions at pH 2 using both closed and open continuous flow systems and the mass transfer mechanisms occurring in sonolytic ozonation and their effects on chemical reactivity. They found the sonochemical degradation of O3 to follow apparent first-order kinetics with typical rate constants of 0.84 ( 0.07 and 0.66 ( 0.08 min-1,

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respectively, at 20 and 500 kHz and an initial O3 concentration of 245 ( 3 µM. It was also shown that both the sonochemical degradation of O3 and the increase in O2/O3 gas flow rate enhanced mass transfer at 20 kHz. This effect was attributed partially to turbulence induced by acoustic streaming. 4.7. Organic Sulfur Compounds. Spurlock and Reifsneider96,134 investigated the ultrasonic irradiation of diakyl sulfides at 20 °C using 280-, 610-, and 800kHz transducers and 5-11 W/cm2 intensities under oxygen and argon atmospheres and in suspensions in pure water and in saturated aqueous solutions of CCl4. The principal products of the irradiation of 0.06 M aqueous suspension of di-n-butyl sulfide (R2S) at 800 kHz and 9.4w/cm2 were di-n-butyl sulfoxide (R2SO) and n-butyl sulfonic acid (RSO3H) in the yield ratios of 85/ 15 and 80/10 in O2 and Ar, respectively. Minor products observed included di-n-butyl sulfone (R2SO2), butyric acid, carbon monoxide, ethylene, acetylene, and methane. Entezari et al.135 sonicated 50 mL of pure carbon disulfide (CS2) at 20, 50, and 900 kHz frequencies under the atmosphere of different gases (Ar, H2, Air, He, O2, CO2) in a temperature range of -50 to +10 °C. They observed that ultrasonic sonication at 900 and 50 kHz (up to 4 h) led to no visible or chemical changes due to the sonication while irradiation of CS2 at 20 kHz frequency resulted in the formation of heterogeneous mixture of amorphous carbon and monoclinic sulfur. The dissociation of CS2 was proposed to be due to the extreme conditions resulting from the collapse of the cavity bubbles, the low thermal conductivity of CS2 ensuring the high-temperature persisted long enough to result in the breakage of the C-S bond to form elemental carbon and sulfur. The dissociation rate was also found to be rapid and efficient at lower temperatures and higher intensities but at the same power (49W) and under helium and hydrogen gas than under air, argon, or oxygen. The fact that the sonochemical activity was higher in He than in Ar (with lower heat conductivity) was attributed to other properties of gases, especially solubility. Adewuyi and Collins136,137 studied the kinetics of the sonochemical degradation of aqueous carbon disulfide in a batch reactor at 20 kHz and the effects of process parameters (e.g., concentration, temperature, ultrasonic intensity) on the degradation process. They found the reaction rate to be zero-order in the temperature range 20-50 °C and first-order at lower temperature. The zero-order rate constant for the degradation at 20 °C, at 14 W, and in air was 2.27 × 10-5 min-1. At the same initial concentrations and temperature of 20 °C and in the presence of air, the degradation rate of CS2 at 50 W (39.47 W/m2) was more than 2 times that at 14 W (11.04 W/m2). The rate of sonochemical degradation of CS2 in the presence of the different gases was on the order of He > Air > N2O > Ar; the rate with helium was found to be about 3 times that of argon. They found the formation of sulfate as a product to be enhanced at the lower pH and also at lower temperatures when solutions from experiments in the pH range 8-11 and temperatures 5-50 °C were analyzed. 4.8. Oxygenates and Alcohols. Kang and Hoffmann138 studied the sonolysis, ozonolysis and the combined sonolysis/ozonation of methyl tert-butyl ether (MTBE) in the concentration range of 0.01-1.0 mM and demonstrated that the addition of O3 to the influent O2

gas ([O3]0 ) 0.26-0.34 mM in solution) accelerated the degradation of MTBE by a factor of 1.5-3.9, depending on the initial concentration of MTBE. The sonochemical first-order degradation rate constant for the loss of MTBE was found to increase from 4.1 × 10-4 at [MTBE]0 ) 1.0 mM (90% conversion in 93 min) to 8.5 × 10-4s-1 as the concentration of MTBE decreases to 0.01 mM (90% conversion in 45 min). The O3-ultrasound system was shown to effectively degrade the MTBE into innocuous and biodegradable products with tert-butyl formate (TBF), tert-butyl alcohol (TBA), methyl acetate (MA), and acetone identified as primary intermediates and byproducts of the degradation reaction. Acetone, which was formed from the oxidation of TBF (degradation rate constant, kTBF ) 1.87 × 10-3s-1) and TBA, was found as the intermediate product with the highest yield (12%). Using measured values of MTBE and TBF rate constants, a reaction mechanism involving tree parallel pathways: (1) direct pyrolytic decomposition of MTBE, (2) direct reaction of MTBE with O3, and (3) reaction of MTBE with •OH radical were proposed and used to obtain the following kinetic rate expression:

[MTBE] ) (kpyr + kI + kII + kIII)[MTBE] ) dt ko[MTBE] (17)

-d

where kpyr, kI, kII, and kIII are the pseudo-first-order rate constant for the direct pyrolysis, thermal degradation leading to O-CH3 bond breakage, direct reaction with ozone, and other direct routes to products formation, respectively. Koike139 studied the sonolysis of alcohol-water mixtures using CH3OH, C2H5OH, n-C3H7OH, and i-C3H7OH and found each mixture to form gaseous products, including CH4, C2H6, C2H4, C2H2, and C3H6. Koike139 found the product yields to increase to a maximum at alcohol composition of about 20 vol % in the water solution and to decrease thereafter for each of the alcohol-water mixture. Buttner et al.140 studied the sonolysis of water-methanol mixtures under argon and oxygen and obtained similar results. With increasing methanol concentration the product yields first increased as methanol is a reactant and decreased with increasing concentration leading to a maximum in the yields versus composition curves. In solutions containing more than 80% methanol, they observed almost no chemical reactions. They detected typical pyrolysis and combustion products of methanol (H2, CH2O, CO, CH4, and traces of C2H4 and C2H2 under argon, and CO2, CO, HCOOH, CH2O, H2O2 and traces of H2 under oxygen). Gutierrez and Henglein141 compared the sonolysis of aqueous solutions of ethanol (as volatile and soluble solute), poly(vinylpyrrolidine) PVP (as nonvolatile solute), and tetranitromethane TNM (as volatile and almost insoluble solute) under Ar irradiation (at 300 kHz and 2W/cm2). They explained the formation of products in all three cases (such as CH4, C2H4, C2H6, CO, and CO2 from ethanol and PVP; and NO2-, NO3-, N2, CO, CO2 from TNM) in terms of pyrolysis in or close to the cavitation bubbles. 4.9. Other Organic Compounds. Suzuki et al.142 examined the effect of ultrasound (200 kHz, 200W) at 298 K on the degradation rate of the surfactant, polyoxyethylene-alkyl ether (C14H29O(CH2CH2)7H), under three different conditions: ultrasonic irradiation only (sonoprocess), photocatalytic reaction using TiO2 (0.25

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g/mL of P25) and low-pressure mercury (20W, 253.7 nm) lamp (photoprocess), and the combination of the two processes (photosonoprocess). They observed the decomposition rate in the photosonoprocess was faster than the photoprocess or sonoprocess alone. A significant acceleration in the degradation of TOC was also observed when stirring speed was increased from 100 to 500 rpm. The significantly enhanced rate in the photosonoprocess compared to the sonoprocess alone was attributed to the redispersion of the agglomerated catalyst particles under ultrasonic irradiation. They also successfully separated solid catalyst particles agglomerated by ultrasonic irradiation at 28 kHz in the presence of glass beads. Hart and Henglein143 evaluated the sonolysis of mixtures of formic acid and water at concentrations of 0-30 M. The main reaction products under an argon atmosphere were H2, CO2, CO, and very small amounts of oxalic acid. Hart and Henglein144 also studied the irradiation of aqueous solutions of potassium iodide and sodium formate under the atmosphere of argon, oxygen and argon-oxygen mixtures of varying composition. The sono products of formate solutions were H2O2 and CO2 in the presence of O2 and H2, CO2, and oxalate in the absence of O2. Gutierrez et al.145 found the products of sonolysis of acetate to include succinic acid (as a product of attack of OH radicals on acetate), glyoxylic and glycolic acids, smaller amounts of HCHO, CO, and CO2, and CH4 as a minor product. Thymine (Thy) was found to degrade sonochemically in aqueous solutions at 450 kHz into six products: cisand trans-5, 6-dihydroxy-dihydrothymine (cis- and transthymine glycols), 5-hydroxymethyluraacid, urea, and possibly N-formyl-N′-pyruvelurea plus an unidentified product.41 Mead et al.146 also found the degradation to vary linearly with sonication time with a zero-order rate constant of 1.8 ( 0.3 × 10-5 M.min-1. Sehgal and Wang41 found that at an aeration rate of 50 mL/min and at 34 °C, the concentration of thymine is reduced to half its value in 30 min, implying that an average of 3 × 10-8mol or 1.8 × 1016 molecules of Thy reacted per second per liter. The studies also indicated there was a change in kinetics of reaction of Thy from first to zero order as temperature of solution changed from 0 to 50 °C, and they suggested that the sono reaction took place in the bubble-liquid interphase. 4.10. Other Environmental Applications. Ingale et al.147 investigated the degradation of a refractory component in the industrial waste of a cyclohexene oxidation unit using a hybrid system, namely, sonication followed by catalytic wet oxidation (SONIWO). It was shown that sonication in the presence of CuSO4 as catalyst resulted in an accelerated degradation of the unknown compound which was refractory to wet oxidation at 225 °C compared with sonication without a catalyst. Kruger148 investigated the sonolysis of natural groundwater containing 1,2-didhloroethane (1,2-DCA) as the main organic contaminants (350 µg/L), and trichloromethane, cis-1,2-dichloroethane, trichloroethane and tetrachloroeathane, trichloroethene, tetrachloroethene in trace amounts (total of ∼85 µg/L), and Fe (3-17 mg/ L) and Mn (0.4-0.9 mg/L) as the main inorganic products. They observed almost complete destruction of 1,2-DCA in all cases after 60 min and pseudo-first-order

rate constants of 0.062, 0.063, and 0.044 min-1, for the sonication at 361, 620, and 1086 kHz respectively at 105W. Olson and Barbier149 studied the effectiveness of the “sonozone” process (i.e., combined ultrasound ozonolysis) in degrading refractory electrolytes such as humic materials using purified fulvic acid, FA (as a substrate) and naturally colored groundwater sample with pH 8.6. The combined system was found to significantly enhance TOC removal and mineralization rates. Sierka and Amy150 also studied the singular and combined effects of ultraviolet (UV) light and ultrasound (US) on the ozone (O3) oxidation of humic substances, the most important of the trihalomethane (THM) precursors. They found that the combination of O3-US-UV proved to be the most effective reaction condition, followed by O3-UV, O3 alone, and O3-US, providing 93%, 86%, 75%, and 71% reduction in trihalomethane formation potential (THMFP) levels, respectively, in the reaction time of 20 min. Gonze et al.151 investigated the use of the ultrasonic process as a preoxidation step before a classical biological treatment used for further mineralization. They simultaneously monitored the toxicity for sodium pentachlorophenate solution (NaPCP) on marine bacteria (Vibrio fischeri) and on daphnids (Daphniamagna), and the biodegradability of the pollutant solution during ultrasonic irradiation at 500 kHz. The pollution degradation was found to follow an apparent first-order kinetic:

(

[NaPCP] ) [NaPCP]o exp -kth

Pth t V

)

(18)

i.e., the disappearance rate is proportional to the power density applied, and the kinetic pseudo first-order rate constant, kth (m3 J-1) is representative of ultrasonic efficiency. In general, they demonstrated that ultrasonic irradiation decreased the toxicity of NaPCP and could be considered a preoxidation step without which the NaPCP could not be degraded by activated sludge for up to 28 days. Dahi152 studied the ozonation process with and without simultaneous sonication at 20 kHz in regard to the disinfections of microorganism (Escherichia coli) and oxidation of organic (e.g., Rhodamine B). The ultrasonic treatment was found to intensify the action of ozone in the oxidation of chemicals and in the inactivation of microorganisms. The enhanced performance of the simultaneous sonozonation process was attributed to (1) sonochemical degradation of O3, causing augmentation of activities of free radicals in water, and (2) increase in the aeration parameter (kLa value) and intensified mass transfer resulting from sonication. Phull et al.153 reported the biocidal effects of ultrasound alone (at 2025 kHz and 800 kHz) or in conjunction with chlorine on microorganisms. Ultrasound was found to significantly amplify the biocidal effects of normal chlorination and to reduce the amount of chlorine required for disinfections. 5. Discussion As reviewed above, the results of most studies seem to demonstrate that while ultrasound is effective in degrading pollutants, total mineralization is difficult to obtain with ultrasound alone, in particular, with recal-

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citrant pollutants or mixtures of pollutants. Moreover, where the final products are determined to be carbon dioxide, short-chain organic acids, and/or inorganic ions, the time scale and the dissipated power necessary to obtain the complete minerilization of the hazardous pollutants are currently not economically or practically acceptable. Combinations of ultrasound with other advanced oxidation processes (AOPs) or conventional biological processes are found to be effective in degrading some recalcitrant compounds; however, the economics are poorly understood. This section discusses some of the problems related to sonochemical reaction kinetics, byproducts, and ecological effects and to technicalscale application and implementation of the ultrasound for water treatment and the need to address them. 5.1. Reaction Pathways and Kinetics. In general, three distinct pathways have been determined in the sonochemical degradation of chemical compounds. These include oxidation by hydroxyl radicals, pyrolytic decomposition, and supercritical water oxidation.58 The ultrasonic degradation of organic compounds in dilute aqueous solutions depends to a large extent on the nature of the organic material. The specific nature of the saturating gas also influences the relative proportion of the pyrolytic or free radical steps. Hydrophobic and volatile organic compounds tend to partition into the collapsing cavitation bubbles and degrade mainly by direct thermal decomposition leading to the formation of combustion byproducts. The sonochemical oxidation of polar organics (e.g., phenol) is slow compared to that of nonpolar, volatile organics such as CCl4.56,58 Hydrophilic and less volatile or nonvolatile compounds degrade to form oxidation or reduction byproducts by reacting with hydroxyl radicals or hydrogen atoms diffusing out of the cavitation bubbles. Thermal destruction processes are not considered important for nonvolatile substrates because they do not partition appreciably into the bubbles. The efficiency with which a solute reacts with hydroxyl radicals generated in ultrasonic reaction mixtures is related to its hydrophobicity; the greater the hydrophobicity is, the more efficient the solute can act a radical scavenger.154 The time scales of treatment in simple batch reactors reported in the literature are generally in the range of minutes to hours for complete degradation. Most investigators have observed the kinetics of sono degradation of pollutants to be first- or zero-order. For pollutants with first-order degradation rates, the first-order rate constants tend to decrease as the initial substrate concentrations increase.112,131 In contrast, for pollutants displaying zero-order substrate disappearance, the rate constants increase as the initial substrate concentration increases. A change in reaction order from zero-order (at high initial substrate concentration) to first-order (at lower initial substrate concentration) has also been observed.66 The change in rate constants with initial substrate concentration is ascribed to radical scavenging/recombination, i.e., a combination of a first order thermal destruction processes and a zero order reaction with hydroxyl radicals.112,131 Sehgal and Wang41 observed change in reaction order from first to zero as temperature increased. Cost et al.78 indicated that zeroorder kinetics are typical of hydroxyl radical reactions since the free radicals are generated at constant rate under ultrasonic irradiation and suggested the slow degradation of humic acid occurred mainly by radical attack.

Generally, improvements in technology are needed to oxidize organics especially polar compounds more efficiently. If degradation rates can be enhanced, reaction times will be reduced for equivalent final contaminant concentrations, translating to reduction in reactor sizes. Consequently, the cost of application of this technology to industrial or field-scale projects would be significantly reduced. The manipulation of macroscopic parameters has been shown to lead to enhancement of cavitation chemistry as the number of cavitation bubbles and chemical events at each bubble are varied. Hoffmann and co-workers77 optimized the degradation rates of aqueous-phase organic compounds with acoustical processors by adjusting the energy density, energy intensity, and the nature and properties of the saturating gas in solution. They observed that the first-order degradation rates increase as the energy density and intensity increased to a saturation value. Continuous gassing conditions also induce a greater number of cavitation events in homogeneous systems and assist cavitation in heterogeneous systems.28 Susuki et al.155 investigated the influence of aeration, bubble distribution, and frequency on the degradation rate of surfactant SS-70 (initial concentration, 50 ppm; critical micelle concentration, 52.2 ppm) at 298°K using cylindrical-type (26 kHz) and disk-type (100 kHz, 300 kHz) ultrasonic transducers and showed that the degradation rates were enhanced by aeration (i.e., aeration bubbles generated) and the shape of the reactors used. It is also well-known that ultrasonic degradation of phenolic compounds involve •OH radical attack, and hence, the use of highfrequency ultrasound (e.g., 500 kHz) has proven to be more advantageous compared with low-frequency ultrasound. Higher ultrasonic frequencies are more favorable for the generation of hydroxyl radicals possibly due to faster production rates.46,48-50 Weaver et al.156 examined the first-order sono degradation kinetics of three similar aromatic compounds, nitrobenzene (NB), 4-nitophenol (4-NP), and 4-chlorophenol (4-CP), by ultrasound at frequencies of 20 and 500 kHz. They found that in the 20 kHz reactor, NB degraded the fastest and 4-NP the slowest, but in the 500 kHz reactor, 4-CP was the fastest while 4-NP was the most resistant to sonolytic destruction. They attributed the results to the greater degree of vapor-phase pyrolysis at the lower frequency due to high temperatures achieved during bubble collapse, while higher frequency favored ‚OH production. Using 20 kHz ultrasound, large salt-induced enhancements in oxidation/destruction rates have been observed: 6-fold for chlorobenzene, 7-fold for p-ethylphenol, and 3-fold for phenol oxidation, in sodium chloride solutions (0, 0.17, 0.67, and 1.38 mol/L).59 The enhancement was ascribed to the fact that the presence of the salt increased the residence of the aromatice compounds in or at the surface of the cavitation bubbles. The rate of the sonochemical degradation of 3-chlorophenol was enhanced 2.4 times by the addition of appropriate amount of Fe(II) concentrations (e.g., 1 mM). The enhancement was attributed to the probable regeneration of OH radicals from H2O2, which would otherwise formed from recombination of OH radicals and contributed to lesser degradation.74 More such inexpensive approaches for improving process efficiency should be investigated in the future to make sonochemical oxidation an economically viable remediation technology. 5.2. Effects of Water Quality. This section discusses how measures of water quality (i.e., matrix components)

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such as alkalinity, particulate matter (dissolved organic and inorganic), and mixtures of compounds affect the rates of sono-reactions. Sonchemical oxidation reactions like other AOPs should generally be inhibited by high alkalinity usually resulting from the presence of large concentrations of radical scavengers such as bicarbonates and carbonates. This significantly reduces process efficiency. However, Cost et al.78 observed that added alkalinity did not affect the rates of destruction of p-nitrophenol (p-Np) or p-nitrocatechol by ultrasound. According to models on bubble nucleation in liquids, suspended particles are expected to affect cavitation, and hence, the rates of ultrasonic degradation of pollutants because of the formation and stabilization of gas bubbles in the crevices of the particles. Large particles might be expected to decrease the rate because of sound attenuation. Fine particles containing gas- and vaporfilled crevices might enhance the rate by providing additional nuclei for bubble formation.157 However, Kotronarou112 investigated the effect of large sand particles (500 µm average) and fine particles (7 nm average) on the sonication of sulfide oxidation and observed no significant effect for the sizes and concentrations studied. Cost et al.78 also noted that particulate matter (sea sand or Fe2O3 particle) did not significantly affect the degradation rate of p-Np. Orzechowska et al.108 observed that the presence of humic substances did not affect Cl- yields in the sonication of aqueous chlorinated hydrocarbons. However, Taylor et al.158 found the presence of fulvic acid (FA) to significantly inhibit the sonochemical degradation of PAHs (in the concentration range of 0.1-0.5 µM) at 20 kHz (600W) and at 20 °C. In the presence of approximately 50 µM FA, the first-order rate constants (k, s-1) of anthracene, phenanthrene, and pyrene decreased by factors of 3.7 (0.015 vs 0.0041), 3.3 (0.006 vs 0.0018), and 2.3 (0.006 vs 0.0026), respectively. Multicomponent waste streams are more realistic matrices to consider for practical treatment situations. Hua and Hoffmann99 found that sonication of a mixture of CCl4 and p-NP resulted in the enhancement of p-NP degradation, demonstrating that ultrasound is not limited to single-solute solutions. They also found that the effect of CCl4 in a mixed waste stream was particularly interesting in that it released a residual oxidant, which could continue to attack other refractory molecules in solution after the ultrasonic irradiation was halted. Drijvers et al.106 also showed that volatile organic compounds could strongly affect each other’s sonolysis rates. However, as indicated earlier, the presence of hydrophobic compound CB hindered the degradation of hydrophilic compound, 4-CPOH.72 Zhang and Hua159 found moderate decreases in degradation efficiency when a mixture containing 10µM chloropicrin (CCl3NO2 or CP), 10µM trichloroacetonitrile (C2Cl3N or TCA), and 0.5 mM bromobenzene (BB) was sonicated at both 20 kHz (30.8 Wcm-2) and 358 kHz in a complex aqueous matrix (i.e., river water) compared to reagent grade water. Differences in rate constants observed during sonication of the mixture and individual compounds were also minimal. The first-order rate constants at 20 kHz for TCA, CP, and BB were, respectively, 4.7 × 10-2 ( 6.5 × 10-4, 5.3 × 10-2 ( 4.4 × 10-4, and 4.4 × 10-2 ( 7.5 × 10-4 min-1 when sonicated individually. The rates were 4.4 × 10-2 ( 7.2 × 10-4, 5.0 × 10-2 ( 8.8 × 10-4, and 4.1 × 10-2 ( 9.1 × 10-4 min-1 in a mixture. Since both industrial waste streams

and contaminated waters (surface and groundwater) generally contain a wide array of both pollutant and nonpollutant chemicals, more studies on the effects of water quality on sonochemical oxidation rates are needed to advance our understanding. 5.3. Sonication Byproducts and Toxicity Effects. If ultrasound is to be used in the destruction of hazardous wastes on an industrial scale, the associated reactions must be ecologically acceptable. Hence, the toxicity of the reaction products obtained from sonochemical treatments is a factor that should be considered prior to subsequent utilization of treated water. In most studies so far, the toxicity of the reaction products produced by treatment remains to be determined. Special attention must be paid to the types of products formed and their toxicity and resistance to further ultrasonic cleavage or other secondary treatments. Total organic carbon (TOC) analysis is one method that has been used extensively to determine quantitatively whether the sono degradation of the organic compound results in complete minerilization to the relatively harmless CO2 (g) or some product that presents toxic features. However, TOC analyses do not give the entire picture with respect to toxicity because a given hazardous substance may also degrade to carbon deposits135 or be converted into other benign substrates that are acceptable end-products. A quick minerilization of an organic contaminant should be the goal to minimize the survival time of toxic intermediates. The use of ultrasound in combination with other advanced oxidation processes or conventional biological processes has been found to be effective in reducing toxicity. For example, Gonze et al.151 demonstrated that ultrasonic irradiation when used as a pretreatment step decreased the toxicity of sodium pentachlorophenol (NaPCP) solution immensely allowing degradation by biological activated sludge process. Other possible ways include the coupling of H2O2, O3, or UV photocatalysis and solution chemistry modification to enhance the efficiency of free radical generation through cavitation. Hoffmann and colleagues138 demonstrated that the combination of ultrasound with ozonation or other AOPs effectively degraded recalcitrant pollutants such as MTBE into innocuous and biodegradable products. Trabelsi et al.67 showed that the combination of electrochemistry and highfrequency sonication allowed a total degradation of phenol within 20 min with no production of toxic aromatic intermediates. It appears that using a combinative AOP approach is certainly a positive step toward achieving quick minerilization and more such studies should be considered in future efforts to address and mitigate the byproduct toxicity problems. 5.4. Efficiency and Scaleup Issues. The ultrasonic system transforms electrical power into vibrational or ultrasonic energy, i.e., mechanical energy.31-32 160-161 This mechanical energy is then transmitted into the sonicated reaction medium. The efficiency of the energy transformation depends not only on the equipment itself but also on the ultrasonication conditions. Therefore, the amount of acoustic energy delivered into the liquid medium cannot be measured solely by measuring the amount of electrical energy expended to produce the mechanical vibration. Sonolysis is relatively inefficient with respect to total input energy. This is because part of the total energy input is lost to produce heat and another part produces cavitation. But not all of the cavitational energy produces chemical and physical

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Figure 2. Energy conversion in sonochemical processes.

effects. Some energy is consumed in sound re-emission (i.e., harmonics and sub-harmonics).32,57 The low efficiency of the conversion sequence electrical energy f acoustic energy f chemical effects may preclude the use of this method as a primary treatment. As illustrated by Kotronarou et al.,131 the limitation of sonolysis for the control of H2S and other trace contaminants in water is that it is relatively inefficient with respect to the energy input. Their calculations indicated only a small portion of the total energy supplied to the system resulted in “useful” free-radical reactions. The energy transformation losses within the system are shown in Figure 2. At each stage in the transmission of power, there is an inefficiency whereby sonochemical benefit is lost to heat or sound. Hence, it can be appreciated that the determination of an energy balance is not easy. The sonochemical yield (SY ) measured effect/input power (W)) is measured in moles of products generated per second. Despite the difficulty in ascertaining accurate energy balance, “SY” is defined in general this way similar to the definition of “photochemical yield”.31,32,162 Hence, the acoustical power transferred to the reaction mixture is less than the overall power input into the power generators (i.e., the power drawn from the wall supply). The challenge is to harness these energy losses to useful purposes, e.g., pollution treatment. In addition to knowledge of the characteristics of the reaction mixtures and kinetics of reaction, effective scaleup procedure is necessary and requires that all parameters affecting cavitation efficiency be studied well, properly quantified and optimized. The majority of the reported studies to date employed laboratory-scale piezoelectric probe (0.25-0.5 in.) batch reactors with small volumes (20-200 mL) for ultrasonic degradation of pollutants45,160 The probe type reactor is widely used due to its ability to deliver high power output and the ease of operation to give optimum performance at different amplitudes. However, Kotronarou and Hoffmann111,132 found that the same energy input dispersed over a broader area resulted in a significant enhancement in reaction rate and energy utilization efficiency. It was also shown that sonolysis reactors with larger radiating surfaces are more energy efficient the direct immersion probe reactor, which has a small (1 cm2) radiating surface. In addition to the lowenergy utilization efficiency, the probe type reactor

system is also not an optimal reactor configuration for the efficient generation of transient cavitation for two other reasons. First, sonication efficiency is reduced due to blanketing effect caused by intense generation of cavitation bubbles at the tip. Second, the probe type transducer tips have relatively short life before the titanium tip requires replacement. For these reasons, the application of direct probe sonolysis to special applications such as groundwater remediation or pretreatment of industrial hazardous wastes is not economically feasible. In their studies of H2S ultrasonic oxidation, Hoffmann and co-workers131,132 found a direct linear relationship between the applied power at a fixed frequency and the observed rate of loss of S(-II), indicating a continuous-flow stirred-tank probe reactor could attain significant conversion efficiencies. Their initial tests indicated that the use of large high-powered sonicators in the CSTR mode could result in viable degradation efficiencies. Hua et al.77 found that a nearfield acoustical processor or NAP (where liquid flows between two high intensity vibrating plates) was more efficient than a probe reactor for the degradation of p-nitrophenol. They observed that NAP provided about 1.5 times the power per unit volume of the probe reactor but resulted in 19 times more p-nitrophenol destruction per kilojoule energy imparted on the system. It was also shown that the G value for the probe reactor is approximately 1 order of magnitude less than that in the NAP, although the degradation rate constants were the same magnitude. The G value is an efficiency measure used in radiation chemistry and for each species in neutral water is defined as the number of molecules undergoing reaction for each 100 eV electrical energy transferred.25 The NAP parallel-plate reactor allows for a lower sound intensity but a higher acoustical power per unit volume than those of the conventional probetype reactor. However, limited work has been reported on the NAP and tank reactors, and comparison of their performance data with the probe reactor are scarce. Gondrexon et al.116 observed a conversion rate for pentachlorophenol (PCP) up to 80% using a three-stage laboratory-scale sonochemical reactor (each equipped with 500 kHz piezoelectric disk) and operating in the continuous flow mode. Operation in continuous-flow mode also has the advantage of short residence time, especially for pollutants that are degraded quickly.

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However, as they indicated, the scaleup of such an experimental system would not be easy, and a lot of technical problems remain to be overcome. But increasing commercial interest in sonochemical processes should reduce cost in the very near future. Problems related to technical-scale application and implementation of sonochemical steps into conventional water treatments must be considered in future investigations.31,32,160 6. Concluding Remarks Ultrasonic irradiation shows promise and has the potential for use in environmental remediation due the production of high concentrations of oxidizing species such as •OH and H2O2 in solution and localized transient high temperatures and pressures. In terms of convenience and simplicity of operation, sonolysis could prove to be economically competitive and far superior to many alternative approaches. These include high-temperature catalytic combustion or incineration, activated carbon or zeolite adsorption, supercritical fluid extraction or oxidation, substrate-specific biodegradation, membrane separation, electron-beam irradiation, UV-photolytic, and other chemical degradation methods.26,163-164 Some of these techniques are cost-intensive and require transference of the target molecule from an aqueous phase. Biodegradation provides a promising method for environmental remediation. However, currently available processes are slow and produce unpredictable results. Aerobic biological oxidation is also limited when the feed is either recalcitrant to bidegradation, or inhibitory or toxic to the bioculture.164 The sonochemical approach also has the advantage of being adaptable to mixed solid-liquid wastes. Ultrasonic degradation is several times (about 10 000-fold) faster than natural aerobic oxidation, for example.115 Also, sonochemical degradation occurs over wide range concentrations (varying by order of magnitude), unlike certain biological systems that are inhibited by relatively low substrate concentrations. In a recent economic analysis of a dilute p-nitrophenol aqueous waste treatment, the cost of sonochemical oxidation was found to be comparable to that of incineration.77 The relative efficiency of ultrasound in terms of p-nitrophenol degraded per liter of water was also shown to be far superior to conventional UV-photolytic degradation.58 Calculated G value efficiencies from the literature also indicate sonochemical systems are competitive with other AOPs such as UV photocatalysis and supercritical water oxidation.87,163,165 Sonolysis does not require the addition of chemical additives to achieve viable degradation rates. However, some chemicals may be utilized as an effective sonolytic catalyst for reactions involving ‚OH radical for example. As obvious from the degradation products in Table 3, complete mineralization of recalcitrant pollutant is difficult to obtain at reasonable rates with ultrasound alone. Because the efficiency of electrical-to-sound-tothermal conversion is poor, even faster decomposition is needed to carry out the oxidation at the commercial level. Moreover, other barriers remain to the large-scale implementation of this technology for pollution degradation. The economics are poorly understood. The scaleup of sonochemical reactors remains problematic because few studies have been done to determine which measures of reactor efficiency correlates best with pollutant destruction rates. Several studies have noted

the change in biodegradability of a waste subjected to prior chemical oxidation.164 Integration of destructive processes for recalcitrant or inhibiting contaminant conversion is advantageous conceptually. However, substantial research efforts are needed to develop efficient processes through coupling of biological approaches with chemical and physical treatments. The design key for such a coupled system lies in choosing processes that complement each other and leads to a synergistic effect. Sonochemical oxidation technologies are most advantageously used in combination with other advanced oxidation processes and/or biological treatment. Ultrasound has other environmental science and engineering applications. Low permeability geomaterials such as clay or silt are difficult to remediate because of low transport rates and high adsorption potential. It has been shown that ultrasonic vibrations enhance removal of volatile organic compounds (VOCs) and liquid contaminants from low permeability geologic formations that have been pneumatically or hydraulically fractured.166 Mukherjee et al.167 and Liu et al.168 also showed that sonication could be used as a pretreatment step to improve the release of organic matter associated with the particulate phase into the soluble aqueous phase enhancing its bioavailability during the anaerobic biodegradation of aquatic sediments. Also, in the desorption of phenol from activated carbon and polymeric resin, Rege et al.169 found sonication at 40 kHz and 1.44 MHz to significantly increase desorption rates, and the rates were also favored by decreased temperature, aerated medium, and increased ultrasonic intensity. They also found desorption rates in the absence of ultrasound was limited by pore diffusion, whereas those under ultrasonic irradiations were limited by surface reaction. The improvement in desorption rates were attributed to an enhancement in diffusional transport due to the acoustic microstreaming caused within the pores. The amount of things that can be accomplished with sonochemistry is, at this stage, only limited by the minds of those working in this exciting field. It is a whole new field of research that is growing very fast, with exciting prospect for any researcher. The review of the environmental aspects of sonochemistry highlighting remediation processes, reaction pathways, and kinetic studies presented here indicates a large scope for further experimental and theoretical investigations on all aspects of this important medium of reaction. More research and developmental studies, both experimental and theoretical and encompassing kinetics, reaction mechanisms, reactor design, and energy conservation, are needed to improve reliability and minimize costs for scale-up and long-term use.170-172 Destaillat et al.173 demonstrated the effectiveness of a novel pilot-plant scale sonochemical reactor (UES 4000C Pilotstation) recently developed for large-scale remediation applications. A budget analysis of this system (612 kHz, 3 kW) indicated that nearly one-third of the applied power is converted into sonochemical activity. It is hoped that this review has stimulated thinking beyond the cases presented and should spur future engineering-based research in the continued use of sonochemical oxidation as an environmentally benign process for the removal of inorganic and organic species from industrial wastewater streams and remediation of contaminants in subsurface environments.

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Received for review January 30, 2001 Revised manuscript received July 18, 2001 Accepted August 13, 2001 IE010096L