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REVIEWS
Sonochemistry: Environmental Science and EngineeringApplications
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 tochemical processing. Sonochemistry enhances or promotes chemical reactions and mass transfer.It offers the potential for shorter reaction cycles, cheaper reagents, and less extreme physicalconditions, leading to less expensive and perhaps smaller plants. The amount of things thatcan be accomplished with sonochemistry is, at this stage, only limited by the minds of thoseworking in this exciting field. Existing literature on sonochemical reacting systems is chemistry-intensive, and applications of this novel means of reaction in environmental remediation andpollution prevention seem almost unlimited. For example, environmental sonochemistry is arapidly 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 fieldcomprehensively by combining the existing knowledge from chemistry with insights into thepathways and kinetic analysis of environmental sonochemical reacting systems and withchallenges for large-scale applications. The review is intended to advance our understandingand outline directions for future research.
Contents
1. Introduction 46812. Theory 4682
2.1 Fundamentals of Ultrasound 46822.2 Factors Affecting Aqueous
Sonochemical Processes4682
2.3 Fundamentals of SonochemicalReactions
4683
3. Types of Pollutants 46864. Prior Literature 4686
4.1 Aromatic Compounds 46864.2 Chlorinated Aliphatic
Hydrocarbons4700
4.3 Explosives 47024.4 Herbicides and Pesticides 47034.5 Organic Dyes 47044.6 Organic and Inorganic Gaseous
Pollutants4704
4.7 Organic Sulfur Compounds 47064.8 Oxygenates and Alcohols 47064.9 Other Organic Compounds 4706
4.10 Other EnvironmentalApplications
4707
5. Discussion 47075.1 Reaction Pathways and Kinetics 47085.2 Effects of Water Quality 47085.3 Sonication Byproducts and
Toxicity Effects4709
5.4 Efficiency and Scale-up Issues 4709
6. Concluding Remarks 47117. Literature Cited 4712
1. IntroductionWhen ultrasonic (or sonic) energy at high powers
more than 1/3 W/cm2 for water at room temperaturesisapplied to a liquid, a “cold boiling” termed cavitationtakes place. Simply put, cavitation is the formation,growth, and sudden collapse of bubbles in liquids.1,2
Ultrasonic vibration reduces the thickness of liquidfilms, enhances gas transfer, and reduces bubble coa-lescence, which increases the interfacial area for gastransfer.3-6 For example, the diffusion of liquids throughporous media is enhanced by ultrasound. Ultrasoundcan be used to separate gases because lighter moleculesin an ultrasonic field will travel further than heavierones. Ultrasonic energy is also used to remove contami-nants from air and to break down toxic compounds inwater and soil.6 Nearly half of the 189 hazardous airpollutants (“air toxics”) regulated by the Clean Air ActAmendment (CAAA) of 1990 are volatile organic com-pounds (VOCs). This diverse list includes commonsolvents or halogenated aliphatic compounds, such asmethylene chloride, chloroform, and trichloroethylene,all of which are mineralized by ultrasonic irradiation.7-14 The term mineralization implies the final productsof degradation reactions, which are carbon dioxide,short-chain organic acids, and/or inorganic ions. Ben-zene, well-known for its resistance to the action of strong
* Phone: 336-334-7564.Fax: 336-334-7904.E-mail: [email protected].
4681Ind. Eng. Chem. Res. 2001, 40, 4681-4715
10.1021/ie010096l CCC: $20.00 © 2001 American Chemical SocietyPublished on Web 10/04/2001
oxidants, succumbs under ultrasonication in aqueousmedium.15 Sonochemical oxidation techniques involvethe use of sonic or ultrasonic waves to produce anoxidative environment via cavitation that yields local-ized microbubbles and supercritical regions in theaqueous phase.16 The collapse of these bubbles leads tosurprisingly high local temperatures and pressures.Locally, the high temperature and pressure may reachup to and above 5000 K and 1000 atm, respectively.17,18
These rather extreme conditions are very short-lived buthave shown to result in the generation of highly reactivespecies including hydroxyl (OH•), hydrogen (H•) andhydroperoxyl (HO2
•) radicals, and hydrogen peroxide.19-23
These radicals are capable of initiating or promotingmany fast reduction-oxidation (REDOX) reactions.These reactions with inorganic and organic substratesare fast and often near the diffusion-controlled rate.24-25
Sonochemistry is an example of advanced oxidationprocesses (or AOPs).26,27 As shown in Table 1, AOPs owetheir enhanced reactivity, as least in part to the genera-tion of reactive free radicals, the most important ofwhich is the excited hydroxyl radical (•OH).
A number of studies have documented the role ofsonochemistry in homogeneous and heterogeneouschemistry.28-30 Although the phenomena of sonochem-istry has been recognized for many years and despiteits recent advances, the mechanisms of homogeneousand heterogeneous sonochemistry are not fully under-stood. It is only recently that applications in synthesisand pollution control have prompted interest in indus-trial scale operation.31,32 Thompson and Doraiswamy33
and Moholkar et al.34 recently reviewed the fundamen-tals and science and engineering aspects of ultrasoundand its applications to organic synthesis. Luche28 dis-cusses the use of sonochemistry in organometallicsynthesis, biphasic systems, catalytic reactions, andorganic electrochemistry and the practical consider-ations for process optimization. However, the applica-tions of this novel means of reaction in environmentalremediation and pollution prevention seem almostunlimited. Sonication improves mass transfer and chemi-cal reaction and is expected to reduce or eliminatechemical usage, resulting in minimal sludge and dis-posal problems. This paper reviews the field of envi-ronmental sonochemistry comprehensively by combin-
ing the existing knowledge from chemistry with insightsinto the pathways and kinetic analysis of environmentalsonochemical reacting systems and with challenges forlong-term reliability and economical scaleup.
2. Theory2.1. Fundamentals of Ultrasound. Ultrasound are
waves at frequencies above those within the hearingrange of the average person, i.e., at frequencies above16 kHz (16 000 cycles per second).1,2 Ultrasonic energy(high frequency sound waves) produces an alternatingadiabatic compression and rarefaction of the liquidmedia being irradiated. In the rarefaction part of theultrasonic wave (when the liquid is unduly stretchedor “torn apart”), microbubbles form because of reducedpressure (i.e., sufficiently large negative pressures).These microbubbles contain vaporized liquid or gas thatwas previously dissolved in the liquid. The microbubblescan be either stable about their average size for manycycles or transient when they grow to certain size andviolently collapse or implode during the compressionpart of the wave. The critical size depends on the liquidand the frequency of sound; at 20 kHz, for example, itis roughly 100-170 µm. The implosions are the spec-tacular part of sonochemistry. The energy put into theliquid to create the microvoids is released in this partof the wave, creating high local pressures up to 1000atm and high transitory temperatures up to 5000 K.17-21
This energy-releasing phenomena of the bubble forma-tion and collapse is simply called cavitation or (“coldboiling”), or for the case described above, acousticcavitation.1,35-37 Cavitation can also be achieved bythrottling a valve downstream from a pump. Whenpressure at an orifice or any other mechanical constric-tion falls below the vapor pressure of liquid, cavitationsare generated which then collapse downstream with arecovery of pressure, giving rise to high temperatureand pressure pulses. Cavitation achieved from thismechanism is termed hydrodynamic cavitation.38
2.2. Factors Affecting Aqueous SonochemicalProcesses. Sonochemistry is complicated by the factthat the nature or the physicochemical properties of thesolvent, solute, or gas in the bubble can have dramaticeffect on the cavitational collapse.37 Cavities are morereadily formed when using solvents with high vaporpressure (VP), low viscosity (µ), and low surface tension(σ); however, the intensity of cavitation is benefited byusing solvents of opposite characteristics (i.e., low VP;high µ, σ, and density, F). The intermolecular forces inthe liquid must be overcome in order to form thebubbles. Thus, solvents with high densities, surfacetensions, and viscosities generally have higher thresholdfor cavitation but more harsh conditions once cavitationbegins.35 There are several properties of gases that canaffect sonochemical activities.5 The heat capacity ratio(Cp/Cv) or polytropic ratio (γ) of the gas in the bubbleaffects the amount of heat released and, hence the finaltemperature produced in an adiabatic compression andthe cause of reaction. Assuming adiabatic bubble col-lapse, the maximum temperatures and pressures withinthe collapsed cavitation bubbles (eqs 1 and 2) arepredicted by Noltingk and Nepprias from approximatesolutions of Rayleigh-Plesset equations.39-40
Table 1. Some Advanced Oxidation Technologies
(i) Fenton-Type ReactionsFe2+ + H2O2 f •OH + Fe3+ + OH-
(ii) Ozone-Peroxide-UV SystemsO3 + -OH f O2
- f •OH3O3 + UV (<400 nm) f 2 •OHH2O2 + UV (<400 nm) f 2 •OHH2O2 + O3 f 2•OHH2O2 + O3 + UV f •OH
(iii) Semiconductor Oxides-UV SystemsTiO2 + 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• + H2OR• + O2 f ROO•
ROO• + RH f ROOH + R•
(vi) Sonolysis (Ultrasound)H2O f H• + •OH
Tmax ) To[Pa(γ - 1)Pv
] (1)
4682 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
where To ) ambient (experimental) temperature ortemperature of bulk solution, Pv ) pressure in thebubble at its maximum size or the vapor pressure ofthe solution, Pa ) pressure in the bubble at the momentof transient collapse (i.e., acoustic pressure), and γ )polytropic index of the cavity medium.5 As seen fromthese equations, higher temperatures and pressures aregenerated with monatomic gases with higher γ thanthose with polyatomic gases with lower γ. Anotherparameter that affects cavitational collapse is thethermal conductivity of the gas. Although compressionis adiabatic in the sonochemical process, small amountsof heat are transferred to the bulk liquid. A gas withlow thermal conductivity reduces heat dissipation fromcavitation site following adiabatic collapse and shouldfavor higher collapse temperature compared with highthermal conductivity gas. In addition, the gas with thehigher thermal conductivity reduces the temperatureachieved in a collapse. The solubility of the gas in theliquid used is also an important aspect; the more solublethe gas, the more likely it is to diffuse into the cavitationbubble. Dissolved gases form the nuclei for cavitation.Soluble gases should result in the formation of largernumber of cavitation nuclei and extensive bubble col-lapse since these gases are readily forced back to theaqueous phase.
As expected from sonochemical reactions, loweringtemperature increases the rate of reaction unlike mostchemical reacting systems. This is attributed to thelowering of the solvent vapor pressure which increasesthe intensity of cavitation. At low vapor pressure, lessvapor has an opportunity to diffuse into the bubble andthus cushion the cavitational collapse, therefore makingthe implosion more violent. Also, as liquid temperaturedecreases, the amount of gas dissolved increases andthe vapor pressure of the liquid decreases. Very volatilesolvents lead to relatively high pressures in the bubbleand also “cushion” the collapse. In some cases, theincrease of temperature may favor the reaction kineticsto a point and further increase in reaction temperatureleads 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 directlyrelated to acoustic pressure by eq 3:
where F is the density of the fluid (e.g., water) and c isthe speed of sound in the fluid (1500m/s in water). Theacoustic power (W) represents the intensity emitted bya given surface. The term Fc represents the acousticimpedance (Z) of the medium. Values of Z for air, water,benzene, and ethanol are 400, 1.5 × 106, 1.1 × 106, and0.95 × 106, kg m-2 s-1 respectively.28 The literaturepoints to the conclusion that some increase in thepressure of the system should increase the reaction ratedue to the magnified effect of cavitation implosions.However, as pressure increases, the intensity must beincreased to obtain cavitation in the first place. Toomuch pressure reduces the rate of reaction by decreas-ing the frequency or efficiency of bubble formations.37
An increase in ultrasound intensity means an increasein the acoustic amplitude (i.e., Pa). The collapse time,
the temperature, and the pressure on collapse are alldependent on acoustic amplitude; the cavitation bubblecollapse will be more violent at higher acoustic ampli-tudes. An increase in intensity will thus result ingreater sonochemical effects in the collapsing bubble.5,28
The power delivered to a system depends to some extenton the frequency level. In most cases, as the power isincreased, the reaction rate also increases. At a criticalpower level, increasing the power will decrease the rateof reaction.42 Power input to the system is dependenton the amplitude. By increasing the amplitude, thepower is also increased.42,43 Sonochemical activity riseswith increasing intensity to an optimum above whichefficiency falls. According to Raleigh, the main conditionof effective action for ultrasonic cavitation is that thetime of cavity collapse should be smaller than half theultrasonics period (τ < T/2), as shown in eq 4.44,45 For abubble under constant external pressure (hydrostatic)from an initial or maximum radius, Rmax, to some finalradius, the relation is given by
where τ ) time of cavitation bubble collapse, Rmax ) themaximum radius of cavitation bubble, T ) ultrasonicperiod, Ph ) hydrostatic pressure, and F ) density ofliquid.28,44-45
When the acoustic power increases and simulta-neously increases amplitude of vibration, the maximumradius of the cavity bubble also increases, as well as itstime of collapse, τ, and this bubble is not able to collapsewithin time equal half of the period. That is, before thesound field reverses itself, and the rarefaction phasebegins acting on the collapsing bubble.
Frequency has significant effect on the cavitationprocess because it alters the critical size of the cavita-tional bubble.46-51 At very high frequencies, the cavi-tational effect is reduced because either (i) the rarefac-tion cycle of the sound wave produces a negativepressure which is insufficient in its duration and/orintensity to initiate cavitation or (ii) the compressioncycle occurs faster than the time for the microbubble tocollapse. Lower frequency ultrasound produces moreviolent cavitation, leading to higher localized temper-atures and pressures. However, current research indi-cates that in reactions such as oxidations, higherfrequencies may lead to higher reaction rates. This isdue to the fact that higher frequency may actuallyincrease the number of free radicals in the systembecause although cavitation is less violent, there aremore cavitation events and thus more opportunities forthe free radicals to be produced. Francony and Petrierobserved the ultrasonic degradation of carbon tetra-chloride was enhanced and the yield of products fasterwhen using 500 kHz ultrasound compared with 20kHz.48 But at very high frequencies, the cavitationprocess is decreased. Entezari and Kruus studied thesonochemical reaction rate of oxidation of iodide atdifferent temperatures (0-50 °C) and with differentultrasonic horns at low frequency (20 kHz) and with ahigh-frequency (900 kHz) apparatus.50 The resultsshowed that at 900 kHz the rate of oxidation increasedup to 30 °C at lower power levels whereas at 20 kHz,the rate of oxidation decreased with increasing temper-ature.
2.3. Fundamentals of Sonochemical Reactions.The influence of ultrasonic energy on chemical activity
Pmax ) Pv{Pa(γ - 1)Pv
}[γ/γ-1]
(2)
I )Pa
2
2Fc(3)
τ ≈ 0.915Rmax( FPh
)1/2< T
2(4)
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4683
may involve any or all of the following: production ofheat, promotion of mixing (stirring) or mass transfer,promotion of intimate contact between materials, dis-persion of contaminated layers of chemicals, and pro-duction of free-chemical radicals.1-3,52-53 The physicaleffects of ultrasound can enhance the reactivity of acatalyst by enlarging the surface area or accelerate areaction by proper mixing of reagents. The chemicaleffects of ultrasound enhance reaction rates because ofthe formation of highly reactive radical species formedduring cavitation.4,21 Homogeneous sonochemistry ex-amines, mainly in the liquid phase, the activity of theradicals or excited species formed in the bubble gasphase (OH•, H•, X•, C2
•, CN/, etc.) during the violentimplosion and their possible release into the liquid.28
The cavitation event also gives rise to acoustic micros-treaming or formation of miniature eddies that en-hances the mass and heat transfer in the liquid, andalso causes velocity gradients that results in shearforces. In heterogeneous sonochemistry, the mechanicaleffects of cavitation resulting from the erosion action ofmicrojets formed during asymmetric collapse of bubblesat the vicinity of interfaces are also important.28 So far,four theories have been proposed to explain the sono-chemical events: (1) hot-spot theory; (2) “electrical”theory; (3) “plasma discharge” theory, and (4) super-critical theory. These have led to several modes ofreactivity being proposed: pyrolytic decomposition, hy-droxyl radical oxidation, plasma chemistry, and super-critical water oxidation. The “hot-spot” theory suggeststhat a pressure of thousands of atmosphere (up to 1000atm) is generated and a temperature of about 5000 Kresults during the violent collapse of the bubble.17-19
Both Margulis54 and Lepoint55 advocate that the ex-treme conditions associated with fragmentative collapseare due to the intense electrical fields. The “electrical”theory by Margulis suggests that during bubble forma-tion and collapse, enormous electrical field gradients aregenerated and these are sufficiently high to cause bondbreakage and chemical activity.22,54 The “plasma theory”by Lepoint and Mullie also suggests the extreme condi-
tions associated with the fragmentative collapse is dueto intense electrical fields and seems not to involve atrue implosion. They liken the origin of cavitationchemistry to corona-like discharges caused by a frag-mentation process and supported their views by drawingnumerous analogies between sonochemistry and coronachemistry and indicating the formation of microplasmasinside the bubbles.55 The supercritical theory recentlyproposed by Hoffmann56 suggests the existence of alayer in the bubble-solution interface where tempera-ture and pressure may be beyond the critical conditionsof water (647 K, 22.1 MPa) and which may havephysical properties intermediate between those of a gasand a liquid. They showed that supercritical water isobtained during the collapse of cavitation bubblesgenerated sonolytically.
In general, most studies in environmental sonochem-istry have adopted the “hot spot” concepts to explainexperimental results. This theory considers a sonochem-ical reaction as a highly heterogeneous reaction in whichreactive species and heat are produced from a well-defined microreactor, “the bubble of cavitation.21,57 Inthe “structured hot spot” model shown in Figure 1, threeregions for the occurrence of chemical reactions arepostulated: (1) a hot gaseous nucleus; (2) an interfacialregion with radial gradient in temperature and localradical density; and (3) the bulk solution at ambienttemperature. Reactions involving free radicals can occurwithin the collapsing bubble, at the interface of thebubble, and in the surrounding liquid. Within the centerof the bubble, harsh conditions generated on bubblecollapse cause bond breakage and/or the dissociation ofthe water and other vapors and gases, leading to theformation of free radicals or the formation of excitedstates. Solvent and/or substrates suffer homolytic bondbreakage to produce reactive species. The high temper-atures and pressures created during cavitation providethe activation energy required for the bond cleavage.The radicals generated either react with each other toform new molecules and radicals or diffuse into the bulkliquid to serve as oxidants. The second reaction site is
Figure 1. Three reaction zones in the cavitation process.
4684 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
the liquid shell immediately surrounding the implodingcavity, which has been estimated to heat up to ap-proximately 2000 K during cavity implosion. In thissolvent layer surrounding the hot bubble, both combus-tion and free-radical reactions (involving •OH derivedfrom the decomposition of H2O) occur.23 Reactions hereare comparable to pyrolysis reactions. Pyrolysis (i.e.,combustion) in the interfacial region is predominant athigh solute concentrations, while at low solute concen-trations, free-radical reactions are likely to predominate.At this interface between the bubble and bulk liquid,surface-active reagents also accumulate and speciesproduced in the bubble first react with chemicals in thebulk liquid. It has been shown that the majority ofdegradation takes place in the bubble-bulk interfaceregion.58,59 The liquid reaction zone was estimated toextend ∼200 nm from the bubble surface and had alifetime of <2 µs.17-19 In the bulk liquid, no primarysonochemical activity takes place although subsequentreactions with ultrasonically generated intermediatesmay occur. A small number of free radicals produced inthe cavities or at the interface may move into the bulk-liquid phase and react with the substrate presenttherein in secondary reactions to form new products.Depending on their physical properties and concentra-tions, 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 notoccur in the gaseous phase (where H2O2 is unstable)because of the prevailing high temperatures and pres-sures. Rather, these reactions occur at the relativelycooler interfacial region.
Some of the sonochemical reactions identified in thecavitational events are summarized in Table 2. Theformation of •H and •OH is attributed to the thermaldissociation of water vapor present in the cavities duringthe compression phase (R1). Sonolysis of water alsoproduces H2O2 and hydrogen gas, via hydroxyl radicalsand hydrogen atoms (R2-R7). The presence of oxygenimproves sonochemical activities, but it is not essentialfor water sonolysis, and sonochemical oxidation andreduction can proceed in the presence of any gas.However, in the presence of oxygen acting as a scaven-ger of hydrogen atom (and thus suppressing the recom-bination 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 occurresulting in the formation of H2O2, O2, O, and H2 asproducts (R4-R9). Thermal dissociation of oxygen mol-ecule may also occur, leading to the generation ofadditional hydroxyl radicals (R38-R39). In the absenceof •OH scavengers, the main product of the sonolysis ofwater is H2O2 (R7). H2O2 can also be produced in an“inert” atmosphere but only at the expense of •OH
Table 2. Proposed Kinetic Mechanisms
A. Water DissociationH2O f H• + •OH R1 •OH + •OH f H2O2 R7 H• + H2O2 f •OH + H2O R13H• + H• f H2 R2 •OH + •OH f H2O + O• R8 H• + H2O2 f H2 + HO2
• R14H• + O2 f HO2
• R3 •OH + •HO2 f H2O + O2 R9 •OH + H2O2 f HO2• + H2O R15
HO2• + HO2
• f H2O2 + O2 R4 O• + O• f O2 R10 •OH + H2 f H2O + H• R16HO2
• + HO2• f H2O + 3/2O2 R5 1/2O2 + 2H• f H2O R11 H2O + •OH f H2O2 + H• R17
H• + HO2• f H2O2 R6 H• + OH• f H2O R12
B. Under N2 AtmosphereAs in R1 Plus
N2 f 2N• R18 NO + NO2 f N2O3 R25 NO + NO f N2O + O R32N• + •OH f NO + H• R19 N2O3 + H2O f 2HNO2 R26 N2O + O• f 2NO (or N2 + O2) R33NO + •OH f HNO2 R20 N• + H• f :NH R27 N2O + N2 f O• R34NO + •OH f NO2 + H• R21 :NH + :NH f N2 + H2 R28 N• + O2 f NO + O• R35NO + H• f N• + •OH R22 N2 + •OH f N2O + H• R29 NO + O f NO2
• R362 NO2(aq) + H2O f HNO2 + HNO3 R23 N2 + O• f N2O (or NO + N•) R30 2NO + O2 f 2NO2 R37NO2 + •OH f HNO3 R24 N• + NO f N2 + O R31
C. Under O2 AtmosphereAs in R1 Plus
O2 f 2O• R38 O• + H2 f •OH + H• R42 O3 + O f 2O2 R46O• + H2O f 2•OH R39 O• + HO2
• f •OH + O2 R43 2HO2• f H2O2 + O2 R47
O• + O2 fO3 R40 O• + H2O2 f •OH + HO2• R44 2•OH f H2O2 R48
H• + O2 f •OH + O• (or HO2•) R41 HO2
• f •OH + O• R45 2•OH f O• + H2O R49
D. Under Ar AtmosphereAs in R1 plus
•OH + •H + M f H2O + M R50 O• + H2O + M f H2O2 + M R55 O• + H2O f 2•OH R602•OH f •O + H2O R51 O2 + H• + M f HO2
• + M R56 Ar f Ar/ R612•OH + M f H2O2 + M R52 •OH + H2O2 f HO2
• + H2O R57 Ar/ + H2O f H2O/ + Ar R622H• + M f H2 + M R53 2HO2
• f H2O2 + O2 R58 H2O/ f H• + •OH R632O• + M f O2 + M R54 HO2
• + H• f H2O + O• R59
E. Under Air AtmospehreAs in B and C Plus
N2 + O2 f 2NO R64 NO2• + O• f NO3 R66 N2O5 + H2O f 2HNO3 R68
N2O• + O• f 2NO R65 NO2 + NO3 f N2O5 R67
F. Under H2 AtmosphereAs in R1 Plus
H2 f H• + H• R69 H2 + •OH f H2O + H• R70
G. Under CO2 AtmosphereAs in R1 Plus
CO2 + •H f CO + •OH R71 O• + O• f O2 R74 CO2 + H• f HCOO• R76CO2 f CO + O• R72 CO + O• f CO2 R75 HCOO• + H• f HCHO + O• R772H• + O• f H2O R73
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4685
radical (R51). Formation of atomic nitrogen and oxygen,nitrogen fixation (with HNO2 as the major acid compo-nent formed) can occur in the cavity.60 Nitrogen mol-ecules inside the cavitation bubble may react at hightemperature with hydroxyl radicals and oxygen atomsto give nitrous oxide and nitrogen oxide by a mechanismanalogous to that in combustion chemistry (R19, R29-R30). N2O is unstable under the high-temperatureconditions of the cavitation bubble and is furtherdecomposed in the gas phase and may be transformedto NO (R33).60 NO may undergo further reactions eitherin the gas-phase of acoustic cavities or free-radicalreactions in the cooler interfacial zone, ultimatelyresulting in the formation of nitrous and nitric acids(R20-R23). Nitrogen fixation is inhibited by H2 and COunder a hydrogen atmosphere; oxidation reactions arealmost completely suppressed owing to the strongreducing ability of •H. The inhibiting effect of CO onnitrogen is as follows. The oxygen formed in the irradi-ated water is used up in the oxidation of the COintroduced into the water with subsequent formationof CO2 (R74). The presence of CO (or CO2 in smallamount) may cause formation of HCHO in the cavita-tion bubble (R75-R76). If CO is introduced with N2 andH2, then HCN and HCHO are major products. HCN,NH3, and HCHO can be formed from solutions saturatedwith N2, CO, and H2.60 Another mechanism suggestedis that in which inert gases penetrating into a cavitycan contribute to the transfer of electron excitation towater or hydrogen molecules.22,44,60 For example, theexcited argon atom (Ar*) can participate in energytransfer reactions, as shown in reactions (R60-R62). Inthe presence of H2, argon also facilitates the formationof ‚H by direct dissociation of H2 within the cavitationbubble.
3. Types of Pollutants
A number of previous studies have examined thetransformation of pollutants by ultrasonic irradiationor combined ultrasound and other advanced oxidationtechniques to organic techniques to organic intermedi-ates with mineralizaton to inorganic ions, CO2, andshort-chain organic acids as final products in somecases. The pollutants studied and other environmentalapplications 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-trimethylben-
zene (mesitylene), xylene, fluoro-, bromo-, iodo- andchlorobenzene, 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 perchloroethyl-
ene (PCE)• Carbon tetrachloride (CCl4), chloroform (CHCl3), dichlo-
romethane (CH2Cl2), and methylene chloride (CH3-Cl)
• 1,1,1-Trichloro and 1,2-dichloroethane• Chloral hydrate• Mixtures of CAHs with phenols, BTEX, or chloroben-
zenes
(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 (isopro-
pyl-3-chlorocarbanilate 3-chloraniline)• Pesticides: pentachlorophenol (PeCP) and pentachlo-
rophenate (PCP), polychlorophenyls (PCBs), par-athion (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, andcarbon 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 etha-
nol• Mixtures of alcohols and chloromethanes• Mixtures of alcohols (i.e., ethanol), polyvinlpyrrolidone
(PVP), and tetranitromethane (TNM)(9) Other Organic Compounds• Surfactantsstert-octylphenoxy polyethoxyethanol (Tri-
ton 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 pho-tosonochemical processes to treat a variety of chemicalcontaminants mostly in aqueous systems are sum-marized in Table 3, highlighting the chemical oxidationschemes and experimental conditions utilized, removaleffectiveness, and intermediates and degradation prod-ucts observed.
4.1. Aromatic Compounds. The ultrasonic degrada-tion of phenol, chlorophenols, and nitrophenols havebeen studied by a number of investigators. The inter-mediates and products of the sonochemical oxidation ofphenol usually include hydroquinone, catechol, p- ando-benzoquinone, 2,5-dioxohexen-3-dioic, muconic, ma-leic, succinic, formic, propanoic, oxalic and acetic acids,and CO2. Berlan et al.61 observed that primary degrada-tion products such as dihydroxybenzenes and quinonesare further degraded upon time into the low molecularcarboxylic acids under mild external conditions (roomtemperature, atmospheric pressure) due to local extremeconditions resulting from cavitation, without the needfor any chemical reagent. Petrier et al.62 found the rateof sonochemical phenol degradation to proceed morerapidly at higher (i.e., 487 kHz) than low (i.e., 20 kHz)frequency with concomitant better release of •OH in the
4686 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Tab
le3.
Stu
die
sU
tili
zin
gS
onoc
hem
ical
and
Ph
otos
onoc
hem
ical
Deg
rad
atio
nof
Aq
ueo
us
Org
anic
and
Inor
gan
icP
ollu
tan
ts
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Ade
wu
yian
dC
olli
ns,
2001
a,b
carb
ondi
sulf
ide
(CS
2)6.
4-7.
0×
10-
4
M,1
0.5
×10
-4
M,1
3.2-
13.6
×10
-4M
ult
raso
un
d:20
kHz,
14-
50W
tem
p,1-
50°C
;ir
radi
atin
gga
ses,
Air
,Ar,
He,
N2O
;pH
8-11
inai
r:s
ulf
ate
inth
epH
ran
ge8-
11de
grad
atio
nra
tes
inth
eir
radi
atin
gga
ses
isin
the
orde
rH
e>
Air
>N
2O>
Ar
Ali
ppie
tal
.,19
92ca
rbon
tetr
ach
lori
de(C
Cl 4
),ch
loro
form
(CH
Cl 3
)di
chlo
rom
eth
ane
(CH
2Cl 2
),(i
naq
ueo
us
solu
tion
ofK
I)
1.0-
5.0
MC
Cl 4
,1.0
MK
I
ult
raso
un
d(U
S)
(300
W)
tem
p,ro
om,
irra
diat
ion
tim
e,1-
3h
Fre
eio
din
ean
dC
l 2in
the
case
ofC
Cl 4
.HC
lin
ferr
edfr
ompH
decr
ease
.No
Cl 2
form
edw
ith
CH
Cl 2
and
CH
Cl 2
.
5.0
×10
-3
-3.
0×
10-
3(m
oldm
-3 )
free
iodi
ne
yiel
d.
Bar
bier
and
Pet
rier
,19
96
4-n
itro
phen
ol(4
-Np)
0.5
mM
ult
raso
un
d/oz
onat
ion
ult
raso
un
d,20
kHz
(30
W),
500
kHz
(30
W);
O3
con
cen
trat
ion
,1.
72×
10-
4M
at20
kHz,
1.58
×10
-4
Mat
500
kHz
tem
p,20
(2
°C;p
H2;
irra
diat
ing
gas,
O3/
O2
(2-
3%O
3)
4-n
itro
cate
chol
,CO
2u
ltra
sou
nd-
enh
ance
dO
3m
iner
iliz
atio
npo
ten
tial
pote
nti
alof
subs
trat
e
Ber
lan
etal
.,19
94ph
enol
;an
ilin
e;2-
chlo
roph
enol
100
mg/
Lu
ltra
sou
nd
(30
W),
inte
nsi
ty,1
Wcm
-2at
541
kHz
and
27W
cm-
1at
20kH
z
tem
p,27
(2
°C;p
H6;
diss
olve
dga
s,ai
r,O
2,an
dA
r;so
lid
cata
lyst
hyd
roqu
inon
e,ca
tech
ol,
p-be
nzo
quin
one,
oxal
icm
alei
c,ac
etic
,for
mic
and
prop
anoi
cac
ids,
and
CO
2
Tot
alde
grad
atio
naf
ter
100
min
at54
1kH
z,u
nch
ange
dat
20kH
z.M
iner
aliz
atio
nof
phen
olim
prov
edw
ith
Ran
eyn
icke
l.B
hat
nag
aran
dC
heu
ng,
1994
met
hyl
ene
chlo
ride
;ch
loro
form
,CH
Cl 3
;ca
rbon
tetr
ach
lori
de,
CC
l 4;1
,2-d
ich
loro
eth
ane,
DC
A;
1,1,
1-tr
ich
loro
eth
ane,
TC
A;
tric
hlo
roet
hyl
ene
(TC
E);
perc
hlo
roet
hyl
ene
(PC
E)
50-
350
mg/
Laq
ueo
us
solu
tion
sof
VO
Cs
and
mix
ture
s
ult
raso
un
d(2
0kH
z,0.
1kW
/L)
tem
p,25
°C;
pH6-
5,7,
9N
och
lori
nat
edpr
odu
cts
dete
cted
.HC
lin
ferr
edfr
ompH
decr
ease
.
72-
99.9
%de
stru
ctio
nin
20-
40m
in.F
irst
-ord
erde
grad
atio
nki
net
ics
vari
esfr
om0.
021
to0.
046
min
-1 .
Bu
ttn
eret
al.,
1991
met
han
ol-
wat
erm
ixtu
res
0-10
0vo
l.%
CH
3OH
ult
raso
un
d(U
S)
(1-M
Hz,
2W
/cm
2 )
irra
diat
ion
tim
e,10
min
H2,
CH
2O,C
O,C
H4,
C2H
4,C
2H6
un
der
argo
n;C
O2,
CO
,H
CO
OH
,CH
2O,H
2O2,
H2
un
der
oxyg
en
degr
adat
ion
effi
cien
cyn
otex
plic
itly
give
n
Cat
allo
and
Jun
k,19
95ch
lorp
yrif
os,3
,3′,4
,4′-
tetr
ach
loro
azox
yben
zen
e(T
CA
OB
),2-
chlo
robi
phen
yl,
2,4,
8-tr
ich
loro
dibe
nzo
fura
n(T
CD
F),
lin
dan
e,al
drin
,h
exac
hlo
robe
nze
ne
(HC
B),
mix
ture
ofch
lori
nat
edol
efin
s,pa
rafi
ns
and
arom
atic
s
3µg
/mL
ult
raso
un
d(1
.6kW
h)
tem
p,4-
12°C
;so
nic
atio
nti
me,
6-10
hat
5-10
0ps
i.S
parg
edu
nde
rA
rfo
r1
hpr
ior
toru
n.
dech
lori
nat
ion
prod
uct
sde
tect
edfo
ral
lco
mpo
un
ds(e
xcep
tfo
rT
CA
OB
and
TC
DF
)u
nde
rm
inim
also
noc
hem
ical
con
diti
ons
not
expl
icit
lygi
ven
Ch
enet
al.,
1971
phen
ol(P
hO
H),
sew
erw
aste
trea
tmen
tef
flu
ent
Ph
OH
,10
0-70
0m
g/L
,sol
idca
taly
sts,
20-
200
mg
ult
raso
un
d,25
,55
,an
d80
0kH
z;in
ten
sity
,0-
33.3
W/c
m2
tem
p,22
°C;
irra
diat
ing
gas,
air;
cata
lyst
s,V
2O5,
PtO
2,A
g 2O
,Mn
O2,
and
Ru
O2
hyd
roqu
inon
e,ca
tech
olox
idat
ion
proc
eede
dto
war
dco
mpl
etio
nat
the
hig
hfr
equ
ency
Ch
endk
ean
dF
ogle
r,19
83
carb
onte
trac
hlo
ride
(pu
reC
Cl 4
,tw
o-ph
ase
wat
er-
CC
l 4so
luti
on,
satu
rate
dw
ater
solu
tion
inC
Cl 4
)
0-10
0vo
l.%
CC
l 4u
ltra
sou
nd
irra
diat
ion
tim
e,15
min
;pre
ssu
re,
1-20
atm
maj
orpr
odu
cts,
HC
lan
dH
OC
l;m
inor
prod
uct
s,C
2Cl 4
orC
2Cl 6
7.5-
20.0
%C
Cl 4
con
sum
ed
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4687
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Ch
eun
get
al.,
1991
met
hyl
ene
chlo
ride
,car
bon
tetr
ach
lori
de,
1,1,
1-tr
ich
loro
eth
ane,
tric
hlo
roet
hyl
ene
100-
1000
mg/
Laq
ueo
us
solu
tion
s
ult
raso
un
d(2
50W
)te
mp,
15-
20°C
Pro
duct
sn
otex
plic
itly
repo
rted
.HC
lfo
rmat
ion
infe
rred
from
pHdr
op.
Met
hyl
ene
chlo
ride
drop
ped
from
120
to25
ppm
in40
min
.Fir
st-o
rder
rate
con
stan
tof
3.93
×10
-2
min
-1ca
lcu
late
d.C
heu
ng
and
Ku
rup,
1994
flu
orot
rich
loro
met
han
e(C
FC
11),
trif
luor
otri
chlo
roet
han
e(C
FC
113)
50m
g/L
aqu
eou
sso
luti
on
ult
raso
un
d(2
0kH
z),b
atch
reac
tor
(160
W/4
.6W
/mL
),ci
rcu
lati
ng
reac
tor
(CR
),0.
64W
/mL
tem
p,5-
10°C
CR
at5
psig
HC
l,H
For
oth
erac
idic
spec
ies
(in
ferr
edfr
ompH
drop
)
alm
ost
100%
in40
min
for
CR
and
10m
info
rba
tch
reac
tor
Cos
tet
al.,
1993
p-n
itro
phen
ol(i
nth
epr
esen
ceof
com
mon
chem
ical
sof
nat
ura
lwat
er)
2×
10-
5M
ult
raso
un
d(2
0kH
z,50
W)
tem
p,10
°C;
pH5.
5;ir
radi
atio
nm
ediu
m,a
ir
4-n
itro
cate
chol
degr
adat
ion
not
affe
cted
bych
emic
als
ofn
atu
ralw
ater
Dah
i,19
76R
hod
amin
eB
,E.c
oli
E.c
oli,
106 -
107
bact
eria
per
mL
son
ozon
atio
n;
ult
raso
un
d,20
kHz
ozon
e,3.
68-
3.94
mg
min
-1
L-
1
tem
p,18
°C;
pH7.
2;ir
radi
atio
nm
ediu
m,
O2/
O3
not
dete
rmin
edu
ltra
son
ictr
eatm
ent
ofef
flu
ent
from
biol
ogic
alse
wag
ere
duce
dth
est
eril
izat
ion
dose
ofO
3by
50%
and
deco
lora
tion
zero
-ord
erra
teco
nst
ant
ofR
hod
amin
eB
by55
%D
avid
etal
.,19
98ch
lorp
roph
am(i
sopr
opyl
-3-
chlo
roca
rban
ilat
e),
3-ch
loro
anil
ine
0.1
mM
aqu
eou
sso
luti
ons
son
olys
is,p
robe
tran
sdu
cer
(20
&48
2kH
z,40
W)
tem
p,20
°C;
irra
diat
ion
med
ium
,air
3-ch
loro
anil
ine,
HC
OO
H,C
O,C
O2,
Cl-
for
chlo
rpro
pham
;Cl-
,N
O2-
,N
O3-
,CO
,CO
2fo
r3-
chlo
roan
ilin
e
Deg
rada
tion
effi
cien
cyw
as10
0%af
ter
45m
inat
482
kHz
and
60%
at20
kHz.
Eff
icie
nt
elim
inat
ion
ofbo
thco
mpo
un
dsat
482
kHz.
De
Vis
sch
eran
dL
ange
nh
ove
1998
tric
hlo
roet
hyl
ene
(TC
E),
o-ch
loro
phen
ol(o
-CP
),1,
3-di
chlo
ro-2
-pro
pan
ol(D
CP
)
TC
E,3
.34
mM
;o-C
P,
0.36
2an
d0.
724
mM
;D
CP
,0.2
9m
M;H
2O2,
0,1,
10,1
00m
M;
Cu
SO
4,10
,41
mM
ult
raso
un
d(5
18kH
z,18
.2W
)w
ith
/wit
hou
tF
ento
nre
agen
t,h
ydro
gen
pero
xide
(H2O
2)or
orga
nic
pero
xide
s
solu
tion
tem
p,30
°C;
irra
diat
ion
med
ium
,air
not
repo
rted
H2O
2en
han
ced
o-C
Pde
grad
atio
nra
teby
20-
25%
.TC
Ean
dD
CP
un
affe
cted
byH
2O2.
Pre
sen
ceof
t-B
uO
OH
and
t-B
u-O
O-t
-Bu
inh
ibit
edde
grad
atio
n.
De
Vis
sch
eret
al.,
1997
eth
ylbe
nze
ne
(EB
)0.
5-1m
Maq
ueo
us
solu
tion
ult
raso
un
d(5
18(
1kH
z,13
Wpe
r15
0cm
3
solu
tion
)
solu
tion
tem
p,30
(1
°Cbe
nze
ne,
tolu
ene,
styr
ene,
cum
ene
prop
ylbe
nze
ne,
diph
enyl
met
han
e,1,
2-di
phen
ylet
han
e,be
nza
ldeh
yde
acet
oph
enon
e,ph
enyl
acet
ylen
e
Rat
esar
efi
rst-
orde
r.P
yrol
ysis
isan
impo
rtan
tpa
thw
ay.
De
Vis
sch
eret
al.,
1996
ben
zen
e(B
),et
hyl
ben
zen
e(E
B),
styr
ene
(S),
o-ch
loro
tolu
ene
(OC
)
B,3
.38,
1.69
,0.
9,an
d0.
45m
M;E
B,1
,0.
5,an
d0.
33m
M;O
C,
0.68
,0.3
4,0.
17m
M;S
,0.
97,0
.49,
and
0.25
mM
ult
raso
un
d(5
20kH
z)te
mp,
29(
1°C
;pH
7n
otre
port
edfi
rst-
orde
rki
net
icra
tes,
k(m
in-
1 ):
B,0
.027
1;E
B,
0.06
22;O
C,0
.043
1;S
,0.
0446
Dri
jver
set
al.,
1999
tric
hlo
roet
hyl
ene
(TC
E)
chlo
robe
nze
ne
(CB
)m
ixtu
res
ofT
CE
and
CB
TC
E,0
.84,
1.67
,an
d3.
37m
M;C
B,0
86,
1.72
,an
d3.
44m
M
ult
raso
un
d,52
0kH
z(1
4.23
(0.
73W
)te
mp,
29.5
+0.
5°C
;so
luti
onpH
7
prod
uct
sfr
omm
ixtu
reof
TC
Ean
dC
B,
C8H
4Cl 2
,C8H
6,C
l 2,
C8H
5,C
l 3,a
nd
C8H
4Cl 4
.
TC
E,3
.34
to0.
1in
90m
in.
Son
olys
isra
tes
ofT
CE
and
CB
depe
nd
onin
itia
lco
nce
ntr
atio
nof
TC
Ean
dC
B
4688 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Dri
jver
set
al.,
1998
chlo
robe
nze
ne
1.72
mM
aqu
eou
sso
luti
on.
ult
raso
un
d,52
0kH
z(1
4.23
(0.
73W
,0.0
95W
ml-
1 )
tem
p,29
.5(
0.5
°C;
solu
tion
pH,
4.7,
7,10
;ir
radi
atio
nga
s,A
r,ai
r
met
han
e,ac
etyl
ene,
bute
nyn
e,bu
tadi
yne,
phen
ylac
etyl
ene,
ben
zen
e,n
on-c
hlo
rin
ated
mon
o-an
ddi
cycl
ich
ydro
carb
ons,
chlo
roph
enol
s,C
P(i
nth
epr
esen
ceof
air
only
)
Arg
onac
cele
rate
dde
grad
atio
nra
tes
com
pare
dw
ith
air.
Dri
jver
set
al.,
1996
tric
hlo
roet
hyl
ene
(TC
E)
3.34
mM
aqu
eou
sso
luti
on
ult
raso
un
d,52
0kH
z(0
.095
W/m
L),
20kH
z(0
.43
W/m
L)
tem
p,29
-32
°C;p
H,
4.7,
7,10
;ir
radi
atio
nga
s,ai
r,A
r
C2H
Cl,
C2C
l 2,C
4Cl 2
,C
2Cl 4
,C4H
Cl 3
,C4C
l 4,
C4H
Cl 5
,C4C
l 6
Con
cen
trat
ion
decr
ease
dfr
om3.
34to
0.25
mM
at52
0kH
zan
d3.
34to
0.5
mM
at20
kHz
in60
min
.
D’S
ilvi
aet
al.,
1990
ben
zo[a
}pyr
ene
(B[a
]P,
ben
zo[g
hi]
pery
len
e,co
ron
ene
(PA
Hs)
and
2-ch
loro
dibe
nzo
-p-d
ioxi
n(i
n20
%et
han
ol-
wat
er),
tric
hlo
roet
hyl
ene
(TC
E),
1,1,
1-tr
ich
loro
eth
ane
(TC
A)
PA
Hs,
8.9-
15m
g/L
,pu
reor
nea
tT
CE
and
TC
A
ult
raso
un
d(1
8kH
z,50
W)
PA
Hso
luti
ons
typi
call
yso
nic
ated
for
over
2h
peri
od
PA
Han
dD
ioxi
n,f
ine
carb
onac
eou
sde
posi
ts;
TC
Ean
dT
CA
,bro
wn
poly
mer
icm
ater
ials
B[a
]Pin
20%
aqu
eou
set
han
ol,9
9.1%
,94.
1%an
d22
%de
grad
atio
nra
tes
afte
r60
min
for
init
ial
con
cen
trat
ion
sof
1.1,
8.9
and
21.5
ppm
resp
ecti
vely
,si
gnif
ican
tly
low
erfo
r10
0%et
han
olso
luti
onal
one.
Eil
er,
1994
VO
Cs,
tric
hlo
roth
ylen
e(T
CE
)pr
imar
ily;
BT
EX
,B
enze
ne
(Bz)
;tol
uen
e(T
L);
eth
ylbe
nze
ne
(EB
);xy
len
es(X
L)
TC
E,
1475
-20
00;
Bz,
240-
500;
TL
,8-
11;X
L,
up
to10
0(a
llin
ppb)
CA
VO
Xsy
stem
,se
quen
tial
hyd
rody
nam
icca
vita
tion
(360
W,5
&10
kW)
and
UV
phot
olys
is(l
ow-p
ress
ure
Hg
lam
p,2.
5-10
kW)
H2O
2ad
ded
befo
reth
eca
vita
tion
cham
ber
orga
nic
acid
s,h
alid
es,
CO
2an
dH
2Ore
mov
alef
fici
enci
esgr
eate
rth
an99
.9%
for
TC
Ean
dB
TE
X
En
teza
riet
al.,
1997
carb
ondi
sulf
ide
(CS
2)pu
reC
S2
ult
raso
un
d,90
0kH
z(2
9W
)an
d20
kHz
(49
W)
tem
p,-
50to
+10
°C;
irra
diat
ing
gas,
Ar,
H2,
air,
He,
O2,
CO
2
amor
phou
sca
rbon
and
mon
ocli
nic
sulf
ur
effe
ctof
irra
diat
ing
gas
onra
te,H
e>
H2
>A
ir>
O2,
neg
ligi
ble
rate
at90
0kH
z
Gon
drex
onet
al.,
1999
pen
tach
loro
phen
ol(P
CP
)10
-4
Maq
ueo
us
solu
tion
s
thre
e-st
age
son
och
emic
alre
acto
r(5
00kH
zea
ch,0
-10
0W
)
tem
p,20
(2
°C;s
olu
tion
pH7
No
prod
uct
san
din
term
edia
tes
dete
rmin
ed.
con
vers
ion
rate
su
pto
80%
for
ult
raso
nic
un
its
inse
ries
Gon
zeet
al.,
1999
Sod
ium
pen
tach
loro
phen
ate
(NaP
CP
)as
was
tew
ater
mod
elco
mpo
un
d.B
acte
ria
(vib
rio
fisc
her
i).D
aph
nid
s(D
aph
nia
mag
na)
.
0.1
mM
sequ
enti
alu
ltra
son
ic/
biol
ogic
altr
eatm
ent;
Ele
ctri
cal
gen
erat
or,5
00kH
z,0-
100W
tem
p,20
(2
°Cso
luti
on,
pH6.
8-7.
5;op
erat
ing
wat
tage
,55
-65
W
degr
adat
ion
prod
uct
sn
otre
port
edU
ltra
son
icir
radi
atio
nde
crea
sed
imm
ense
lyth
eto
xici
tyof
NaP
CP
tom
icro
orga
nis
ms
and
cou
ldbe
use
das
preo
xida
tion
step
befo
rebi
olog
ical
trea
tmen
t.G
uti
erre
zan
dH
engl
ein
,19
88
poly
(vin
ylpy
rrol
idon
e)(P
VP
),et
han
ol(E
tOH
)an
dte
tran
itro
met
han
e(T
NM
)in
aqu
eou
sso
luti
on
PV
P,0
-0.
1M
EtO
H,
0-0.
5M
TN
M,4
.5×
10-
3M
and
5×
10-
3M
(in
wat
erpl
us
glyc
ol,
glyc
erin
,or
prop
anol
-2)
ult
raso
un
d(3
00kH
z,2
Wcm
-2 )
irra
diat
ing
med
ium
,Ar
PV
Pan
dE
tOH
,CH
4,C
2H6,
CH
2O,C
H3C
HO
,C
2H6,
CO
,CO
2;T
NM
,C
(NO
2)3-
,NO
2-,
NO
3-,
N2,
CO
,CO
2
Th
ede
com
posi
tion
ofT
NM
ison
eof
the
fast
est
son
och
emic
alre
acti
ons.
Max
imu
mra
tes
ofP
VP
occu
rred
atab
out
0.04
M
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4689
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Gu
tier
rez
etal
.,19
86aq
ueo
us
acet
ate
solu
tion
s0-
0.8
mol
/dm
3
acet
ate
ult
raso
un
d,30
0kH
z(1
6W
)te
mp,
room
(2
°C;
irra
diat
ing
gas,
Ar;
irra
diat
ing
tim
e,30
min
maj
or,s
ucc
inic
,gly
coli
can
dgl
yoxy
lic
acid
,an
dsm
alle
ram
oun
tsof
HC
HO
,CO
2;m
inor
,CH
4
At
acet
ate
con
cen
trat
ion
grea
ter
than
0.4
mol
/dm
3 ,C
O2
and
CO
beca
me
the
pred
omin
ant
prod
uct
sof
son
olys
is.
Ham
lin
etal
.,19
61ca
rbon
tetr
ach
lori
de(C
Cl 4
)an
dch
loro
form
(CH
Cl 3
)in
aqu
eou
ssu
spen
sion
2m
LC
Cl 4
orC
HC
l 3in
20m
LH
2O
ult
raso
nic
gen
erat
or,
med
ical
(15
W,
1mc-
sec,
3W
/cm
2 ),e
lect
ric
(5W
,300
kc/s
)
tem
p,20
°C;
irra
diat
ing
gas,
Ar;
irra
diat
ing
tim
e,15
-18
0m
in
CC
l 4:
CO
2,O
2,C
l 2,
HC
l,C
2Cl 6
,C2C
l 4;
CH
Cl 3
:H
Cl,
C2C
l 6,
C2C
l 4
wit
hC
Cl 4
alon
e(n
ow
ater
),ca
vita
tion
occu
rred
but
ther
ew
asn
ore
acti
on
Har
ada,
1998
carb
ondi
oxid
e(C
O2)
CO
2in
dist
ille
dw
ater
(0.0
-0.
60m
ole
frac
tion
)
ult
raso
un
d(2
00kH
z,20
0W
)te
mp,
5-45
°C;
irra
diat
ion
gas,
Ar,
He,
H2,
N2
gase
ous
prod
uct
s;m
ajor
,CO
,H2;
min
or,
O2
the
decr
easi
ng
rate
for
CO
2fo
llow
edth
eor
der
Ar
>H
eH
2>
N2
Har
tet
al.,
1990
acet
ylen
ein
aqu
eou
sso
luti
ons
0-10
-2
Mu
ltra
sou
nd
quar
tzge
ner
ator
(1M
Hz,
2W/c
m2 )
irra
diat
ing
med
ium
,Ar
H2,
CO
,CH
4,H
CO
OH
,C
H3C
OO
H,H
CH
O,
CH
3CH
O,o
ther
C2-
C8
hyd
roca
rbon
s,in
solu
ble
soot
,C6H
6,st
yren
e,n
aph
thal
ene,
and
phen
ylac
etyl
ene
Pro
duct
ssi
mil
arto
thos
ein
pyro
lysi
san
dco
mbu
stio
n.
Max
imu
mra
teoc
curr
edat
2×
10-
3M
or5
vol%
ofC
2H2
Har
tet
al.,
1990
met
han
e(C
H4)
,eth
ane
(C2H
6)w
ater
con
tain
ing
2-10
0vo
l%C
H4
orC
2H6
inA
r
ult
raso
un
d,30
0kH
z,2
W/c
m2
irra
diat
ion
gas,
Ar
maj
orpr
odu
cts,
hyd
roge
n,a
cety
len
e,et
hyl
ene,
eth
ane;
min
or,C
O,p
ropa
ne,
prop
ene
max
imu
mde
com
posi
tion
occu
rsat
15%
for
met
han
ean
d10
%fo
ret
han
e
Har
tan
dH
engl
ein
,198
8fo
rmic
acid
-w
ater
mix
ture
s0-
30M
aqu
eou
sso
luti
on
ult
raso
un
d(3
00kH
z,2
W/c
m2 )
gas
med
ium
,A
rm
ajor
,H2C
O2,
CO
;m
inor
,oxa
lic
acid
At
15M
HC
OO
H,o
vera
llra
teof
deco
mpo
siti
onis
540
mM
min
-1 .
Har
tan
dH
engl
ein
,19
86
ozon
e(O
3)in
aqu
eou
sso
luti
ons
O3,
353
&62
8m
M,1
.6m
Mu
ltra
sou
nd,
300
kHz
tem
p,33
°C;
irra
diat
ion
gas,
Ar
+O
2/O
3m
ixtu
re
H2O
2fo
rmat
ion
rapi
dde
com
posi
tion
rate
ofoz
one
(3m
M/m
in)
occu
rred
at[O
3])
1m
M
Har
tan
dH
engl
ein
,19
86
nit
rou
sox
ide
(N2O
)w
ater
con
tain
ing
0-10
0vo
l%
N2O
inA
r
ult
raso
un
d,30
0kH
zir
radi
atio
nga
s,A
rN
2,O
2,N
O2-
,N
O3-
max
imu
myi
eld
obta
ined
atA
r/N
2Ovo
l%ra
tio
of85
:15
Har
tan
dH
engl
ein
,19
85
solu
tion
sof
KI
and
sodi
um
form
ate
(HC
O2N
a)in
pure
and
ozon
ized
wat
er
HC
O2-
,0.
001,
0.01
&0.
1M
KI;
0.1
M
ult
raso
un
d,30
0kH
z;O
3,31
.8&
67.4
mM
irra
diat
ing
gas,
Ar,
O2,
and
Ar/
O2
mix
ture
;so
luti
onpH
6.32
-12
.07
KI:
iodi
ne,
H2O
2;H
2al
sofo
rmed
inth
eab
sen
ceof
O2.
HC
O2N
a:H
2O2,
H2,
CO
2an
dox
alat
ein
the
abse
nce
ofO
2;H
2O2
and
CO
2(a
bsen
ceof
O2)
the
yiel
dof
H2O
2w
asab
out
4ti
mes
grea
ter
un
der
Ar/
O2
mix
ture
(70/
30%
)th
anu
nde
rth
epu
rega
ses
Hen
glei
n,
1985
carb
ondi
oxid
e(C
O2)
,n
itro
us
oxid
e(N
2O),
met
han
e(C
H4)
CO
2-N
2O-
CH
4co
nta
inin
gaq
.so
luti
ons
(0.0
.1m
ole
frac
tion
);E
than
ol(0
.05M
);K
I(0
.1M
)
ult
raso
un
d,30
0kH
z(3
.5W
/cm
2 )bu
lkso
luti
onte
mp,
20°C
;ir
radi
atin
gga
s,A
r
CO
2:m
ajor
,CO
;min
or,
HC
OO
H.N
2O:
N2,
nit
rite
,nit
rate
.CH
4:C
2H6,
C3-
and
C4-
hyd
roca
rbon
s,C
O,C
O2,
CH
2O
No
chem
ical
effe
cts
occu
rred
duri
ng
irra
diat
ion
un
der
anat
mos
pher
eof
pure
CO
2,N
2O,o
rC
H4.
Son
och
emis
try
char
acte
rize
dby
ast
ron
gli
nka
gebe
twee
nth
eso
nol
ysis
ofw
ater
and
the
gas.
Hir
aiet
al.,
1996
CF
C-1
13(F
2ClC
-C
Cl 2
F),
HC
FC
-225
ca(F
3C-
CF
2-C
Cl 2
H),
HC
FC
-225
cb(F
2ClC
-C
F2-
CC
lFH
)an
dH
FC
-134
a(F
3C-
CF
2H)
inw
ater
CF
C-1
13,
25-
1000
ppm
;H
CF
C-
225c
a,cb
,100
ppm
;H
FC
-134
a,30
0pp
m
ult
raso
un
d(2
00kH
z,6
W/c
m2 )
irra
diat
ion
med
ium
,Ar,
air
CO
,CO
2,C
l-,F
-C
FC
san
dH
CF
Cs
are
read
ily
deco
mpo
sed
∼CF
C-1
13de
grad
atio
nfa
ster
un
der
argo
nth
anai
r
4690 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Hu
aet
al.,
1995
p-n
itro
phen
ol(p
-Np)
inaq
ueo
us
solu
tion
100
µMn
ear-
fiel
dac
oust
ical
proc
esso
r(N
AP
),16
and
20kH
z,0-
1775
W
tem
p,22
-25
°C;s
olu
tion
pH4.
5-5.
0;ir
radi
atio
nm
ediu
m,A
r,O
2,m
ixtu
reof
Ar/
O2
4-n
itro
cate
chol
(4-N
C);
oth
erpr
odu
cts
not
repo
rted
her
e
firs
t-or
der
rate
con
stan
tsat
1.4W
/cm
2h
igh
erin
Ar
than
inO
2,k O
2)
5.19
×10
-4
s-1
k Ar
)7.
94×
10-
4s-
1
Hu
aet
al.,
1995
p-n
itro
phen
ylac
etat
e(P
NP
A)
inaq
ueo
us
solu
tion
100
µMim
mer
sion
-pro
beu
ltra
sou
nd
(115
W,9
6W
/cm
2 )
tem
p,25
°Cso
luti
onpH
3-8;
irra
diat
ion
gas,
Kr,
Ar,
He
maj
or,h
ydro
lysi
spr
odu
cts;
min
or,N
O2-
,NO
3-h
ydro
lysi
sra
teco
nst
ants
vari
esfr
om9.
8×
10-
5to
3.8
×10
-4
s-1 ,
depe
ndi
ng
ondi
ssol
ved
gas,
acce
lera
ted
byu
ltra
sou
nd
Hu
aan
dH
offm
ann
1996
carb
onte
trac
hlo
ride
(CC
l 4)
and
p-n
itro
phen
ol(p
-Np)
+C
Cl 4
inw
ater
wit
h/w
ith
out
satu
rati
onw
ith
Ozo
ne
(O3)
CC
l 4,1
.95
×10
-4
and
1.95
×10
-5
mol
L-
1 ;p-
Np,
100
µM
ult
raso
un
d(2
0kH
z,13
5or
112.
5W
/cm
2 )
solu
tion
pH11
.8;
irra
diat
ion
gas,
Ar
CC
l 4:
C2C
l 4,
C2C
l 6,C
l-,
HO
Cl,
p-N
p;4-
nit
roca
tech
ol(4
-NC
)
CC
l 4:
90%
and
99%
redu
ctio
naf
ter
12an
d90
min
,res
pect
ivel
y;n
otaf
fect
edby
O3.
Deg
rada
tion
ofp-
Np
enh
ance
dsi
gnif
ican
tly
byC
Cl 4
and
4-n
itro
cate
chol
accu
mu
lati
onm
inim
ized
Hu
ng
etal
.,20
00n
itro
ben
zen
e(N
B)
25µM
ult
raso
un
d(U
S)/
Fe°
.US
,20
kHz
(139
W/L
);F
e°,0
-88
g/L
tem
p,15
(2
°C;i
nit
ialp
H6
inte
rmed
iate
s:w
ith
US
,o-
,p-,
m-n
itro
phen
olan
d4-
nit
roca
tech
ol;
wit
hF
e°,n
itro
ben
zen
e,an
ilin
e
com
bin
atio
nof
ult
raso
un
dan
dF
e°h
adsy
ner
gist
icef
fect
onth
ere
duct
ion
ofn
itro
ben
zen
e
Hu
ng
and
Hof
fman
n,
1998
carb
onte
trac
hlo
ride
(CC
l 4)
0.1
mM
ult
raso
un
d(U
S)/
Fe°
.US
,20
kHz
(62
W/2
85L
);F
e°,
0-24
.49
g/L
(pow
der)
0-42
g/L
(tu
rnin
gs)
tem
p,15
°C;
irra
diat
ion
gas,
Ar;
init
ial
pH7
inte
rmed
iate
s:U
S/F
e°,
C2C
l 4an
dC
2Cl 6
;Fe°
(on
ly),
CH
Cl 3
,an
dC
H2C
l 2
com
bin
edU
S/F
e°sy
stem
had
apo
siti
vesy
ner
gist
icef
fect
onC
Cl 4
deh
alog
enat
ion
Inaz
uet
al.,
1993
tric
hlo
roet
hyl
ene
(TC
E),
tetr
ach
loro
eth
ylen
e(P
CE
),1,
1,1-
tric
hlo
roet
han
e(T
CA
),ch
loro
form
(CH
Cl 3
),ca
rbon
tetr
ach
lori
de(C
Cl 4
)
10pp
mu
ltra
sou
nd
gen
erat
or/b
ariu
mti
tan
ate
osci
llat
or(2
00kH
z,6
W/c
m2 )
irra
diat
ion
gas,
Ar,
O2,
orai
r
inai
ror
O2,
Cl-
,H2,
CO
2,n
egli
gibl
eC
O;i
nar
gon
,Cl-
,H2,
CO
,CO
2
Ove
r75
%in
itia
lam
oun
tde
com
pose
din
10m
in.
Ord
erof
degr
adat
ion
rate
s:in
argo
n,T
CE
>T
CA
>P
CE
;in
air,
CC
l 4>
CH
Cl 3
Inga
leet
al.,
1995
refr
acto
ryco
mpo
nen
tin
the
indu
stri
alw
aste
ofa
cycl
ohex
ane
oxid
atio
nu
nit
CO
D,1
000
mg
dm-
3se
quen
tial
son
icat
ion
/wet
oxid
atio
n(S
ON
IWO
);u
ltra
son
iccl
ean
ing
bath
(40
kHz,
150/
300W
)
son
icat
ion
at30
and
50°C
wit
hC
uS
O4
(6.2
6×
10-
4
cata
lyst
for
1h
;WO
,250
°C,
0.69
MP
a
form
atio
nof
acet
icac
idas
bypr
odu
cth
ybri
dsy
stem
mor
eef
fect
ive
than
son
icat
ion
-66
%in
CO
Dre
duct
ion
(147
3to
499
mg
dm-
3 )vs
36%
(153
3to
980)
for
Cu
2+
syst
em
Joh
nst
onan
dH
ocki
ng,
1993
pen
tach
loro
phen
ol(P
CP
),3-
chlo
robi
phen
yl(3
-CB
),4-
chlo
roph
enol
(4-C
P),
2,4-
dich
loro
phen
ol(D
CP
)
PC
P,2
.4×
10-
4M
;3-C
B,
4×
10-
4M
;4-
CP
,7×
10-
3
M;D
CP
,2×
10-
3M
sim
ult
aneo
us
UV
(Hg
bulb
-100
W,
7000
W/c
m2 /
ult
raso
un
d(2
0kH
z,47
5W
)/ph
otoc
atal
ytic
(TiO
2)tr
eatm
ent
tem
p,35
(2
°C;T
iO2:
surf
ace
area
,55
(10
m2 /
g;am
oun
t,0.
2%W
/Wca
taly
st/
solu
tion
PC
P:
chlo
ride
Son
icat
ion
ofU
V-i
rrad
iate
dca
taly
stso
luti
onsi
gnif
ican
tly
impr
oved
rate
s.
Kan
gan
dH
offm
ann
,19
98
met
hyl
tert
-bu
tyle
ther
(MT
BE
)0.
01-
1.0
mM
sim
ult
aneo
us
son
olys
is/
ozon
atio
n,
ult
raso
un
d(2
05kH
z,20
0W
L-
1 ),
[O3]
o)
0.26
-0.
34µM
tem
p,20
°C;
solu
tion
pH6.
6-6.
8;ir
radi
atio
nga
s,O
2/O
3;ir
rad.
tim
e,0-
60m
in
tert
-bu
tylf
orm
ate
(8%
),te
rt-b
uty
lalc
ohol
(5%
),m
eth
ylac
etat
e(3
%),
acet
one
(12%
)
the
pres
ence
ofO
3ac
cele
rate
dM
TB
Ede
grad
atio
nra
tes
subs
tan
tial
ly,e
nh
ance
men
tby
1.5-
3.9
depe
ndi
ng
on[M
TB
E] o
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4691
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Kh
enok
han
dL
apin
skay
a,19
56
ben
zen
e(B
z),t
olu
ene
(TL
),ph
enol
(Ph
OH
)(s
atu
rate
daq
ueo
us
solu
tion
)
Bx,
2.4,
5.0
&10
.2×
10-
4M
;P
hO
H,0
.01,
0.08
&0.
32M
ult
raso
un
d,43
5kc
tem
p,40
°CB
z&
TL
,HC
HO
,ph
enol
ich
ydro
xyls
HC
HO
yiel
dsin
crea
sed
wit
hin
itia
lcon
cen
trat
ion
ofB
zor
TL
and
tim
e
Koi
ke,1
992
alco
hol
-w
ater
mix
ture
s,C
H3O
H,C
2H5O
H,
n-C
3H7O
H,i
-C
3H7O
H
10-
80vo
l%so
nic
atio
nba
th(S
B),
50kH
z,12
0W
argo
nbu
bble
dfo
r15
min
befo
reso
nic
atio
n
typi
calp
rodu
cts,
CH
4,C
2H6,
C3H
6,C
2H4,
C2H
2
orde
rof
yiel
dfo
rC
H4
was
C2H
5OH
>C
H3O
H>
i-C
3H7O
H∼
n-C
3H7O
H
Kos
zalk
aet
al.,
1992
syn
thet
icca
rbon
tetr
ach
lori
deC
Cl 4
),co
nta
min
ated
wat
er
13.6
-15
.8pp
mu
ltra
son
icpr
obe
(475
W)
(son
icat
ion
at25
0W
),u
nkn
own
chem
ical
agen
t
tem
p,25
-35
°C;p
ress
ure
,15
-45
psig
;ir
radi
atio
n,a
ir,A
r,H
2;ir
rad.
tim
e,0-
5m
in
inte
rmed
iate
s,ch
loro
form
,di
chlo
rom
eth
ane,
chlo
rom
eth
ane;
fin
alpr
odu
cts,
met
han
e,ch
lori
ne/
chlo
ride
CC
l 4re
duce
dfr
om13
.6to
1.71
ppm
(g90
%de
stru
ctio
n)
byso
nol
ysis
,15
.8pp
mto
<0.
5pp
bw
ith
son
olys
ispl
us
chem
ical
agen
t.K
otro
nar
ouet
al.,
1992
para
thio
n(O
,O-d
ieth
ylO
-p-n
itro
phen
ylth
ioph
osph
ate)
satu
rate
daq
ueo
us
para
thio
nso
luti
on(8
2µM
)
ult
raso
un
d(2
0kH
z,75
Wcm
-2 )
tem
p,30
°C;
solu
tion
pH6.
0;fi
nal
pHaf
ter
2h
son
icat
ion
,3.7
p-n
itro
phen
ol(p
-Np)
,S
O42-
,PO
42-,o
xala
te(C
2O42-
),N
O2-
,NO
3-
init
ialp
arat
hio
nw
asde
grad
ede
2h
sin
ce[S
O42-
] ow
asor
igin
ally
equ
al82
µM
Kot
ron
arou
etal
.,19
91p-
nit
roph
enol
(p-N
p)in
oxyg
enat
edaq
ueo
us
solu
tion
s
100
µMu
ltra
sou
nd
(20
kHz,
84W
)te
mp,
30°C
solu
tion
pH2.
12;
irra
diat
ion
gas,
air
NO
2-,N
O3-
,p-
ben
zoqu
inon
e(p
-BQ
),h
ydro
quin
one,
4-n
itro
cate
chol
,fo
rmat
e(H
CO
2-),
oxal
ate
firs
t-or
der
rate
con
stan
tsde
crea
sed
from
3.67
×10
-4 s
-1
(pH
o5)
to2.
0×
10-
4
s-1
(pH
o8)
.
Kot
ron
arou
etal
.,19
92h
ydro
gen
sulf
ide
(H2S
)(S
[-II
])
[H2S
]+
[HS
-]
+[S
2-])
92,1
00an
d20
µMpr
obe
ult
raso
un
d(2
0kH
z,∼7
5W
/cm
2or
∼85W
)
tem
p,25
°Cso
luti
onpH
10.6
-7.
4;ir
radi
atio
nga
s,ai
r
sulf
ate
(SO
42-),
sulf
ite
(SO
32-),
thio
sulf
ate
(S2O
32-)
inth
em
ole
rati
o2.
2:2.
7:1
resp
ecti
vely
S(-
II)
com
plet
ely
oxid
ized
inle
ssth
an30
min
Kru
ger
etal
.,19
99n
atu
ralg
rou
ndw
ater
con
tain
ing
mai
nly
1,2-
dich
loro
eth
ane
(1,2
-DC
A)
350
µg/L
ult
raso
un
d[3
61kH
z(6
0W
),62
0kH
z(7
5W
),an
d10
86kH
z(1
05W
)
tem
p,22
-31
°C;s
olu
tion
pH6.
2-7;
irra
diat
ion
gas,
air
chlo
ride
com
plet
ede
stru
ctio
nw
ith
in60
min
Ku
etal
.,19
972-
chlo
roph
enol
(2-C
P)
inaq
ueo
us
solu
tion
1.26
×10
-4
Mpr
obe
ult
raso
un
d(2
0kH
z,0-
550
W);
inte
nsi
ty,
38.1
W/c
m2
tem
p,17
-60
°C;p
H3,
5,7,
9,an
d10
;ir
rad.
gas,
air,
O2,
N2
chlo
roh
ydro
quin
one,
cate
chol
,ch
lori
deat
pH11
,bu
tgl
yoxi
lic
acid
also
dete
cted
atpH
3
appa
ren
tps
eudo
firs
t-or
der
rate
con
stan
tra
nge
dfr
om3.
3to
1.5
×10
-3
inth
epH
ran
ge3-
11at
24(
°C
Lin
etal
.,19
962-
chlo
roph
enol
(2-C
P)
10an
d10
0m
g/L
ult
raso
un
d(2
0kH
z,16
0W
)/H
2O2
(200
mg/
L)
proc
ess
tem
p,25
°C;
pH3,
7,11
;ca
taly
st,
FeS
O4
not
expl
icit
lygi
ven
;to
talo
rgan
icca
rbon
(TO
C)
mon
itor
ed
2-C
Pde
com
posi
tion
was
99%
,62%
,an
d15
%at
pH3,
7,an
d11
,res
pect
ivel
y,at
350
min
,an
dw
asen
han
ced
byF
eSO
4as
cata
lyst
.L
inet
al.,
1996
2-ch
loro
phen
ol(2
-CP
)10
0m
g/L
ult
raso
un
d(2
0kH
z,0,
125,
and
160
W)/
H2O
2(0
,10
0,20
0,an
d50
0m
g/L
)pr
oces
s
tem
p,25
°C;
solu
tion
pH3,
5,7,
9,11
not
expl
icit
lygi
ven
;to
talp
rgan
icca
rbon
(TO
C)
mon
itor
ed
H2O
2im
prov
ed2-
CP
deco
mpo
siti
on,5
7%ov
erth
eco
ntr
olw
ith
500
mg/
LH
2O2
Lu
r′e,
1962
phen
ol(P
hO
H),
ben
zen
eP
hO
H,
25-
300
mg/
L;
Bz,
200-
500
mg/
L
ult
raso
un
d,80
0kc
/s,1
500
W;
Inte
nsi
ty,4
-7
W/c
m2
(350
mL
solu
tion
)
solu
tion
tem
p,40
-60
°C;
trea
tmen
tti
me,
40m
into
5h
.
Ph
OH
:ca
tech
ol,
pyro
gall
ol,o
-qu
inon
e;B
z;ph
enol
,pyr
ogal
lol,
cate
chol
,for
mal
deh
yde
Pro
lon
ged
trea
tmen
tre
sult
edin
rapi
dox
idat
ion
ofca
tech
olto
mu
con
icac
id.
Mea
det
al.,
1975
thym
ine
(aer
ated
aqu
eou
sso
luti
ons)
2×
10-
3 -2
×10
-2
Mu
ltra
sou
nd
(447
.5(
0.6
kHz,
5W
cm-
2 )
solu
tion
tem
p,25
°Cci
s-an
dtr
ans-
5,6-
dih
ydro
xy-5
,6-
dih
ydro
thym
ine,
5-h
ydro
xym
eth
ylu
raci
lan
du
rea
Ava
lue
of1.
8(
0.3
×10
-5
Mm
in-
1w
asob
tain
edfo
rth
eze
ro-
orde
rra
teco
nst
ant.
4692 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Nag
ata
etal
.,19
96m
onoh
ydro
xybe
nzo
icac
id(H
BA
);3,
4-di
hyd
roxy
ben
zoic
acid
(3,4
-DH
BA
);ga
llic
or3,
4,5-
trih
ydro
xybe
nzo
icac
id(G
A);
tan
nic
acid
(TA
);h
um
icac
id(H
A)
100
µM/L
ult
raso
un
d,20
0kH
z,20
0W
tem
p,25
°C;
irra
diat
ing
gas,
air
&A
r
only
CH
Cl 3
mea
sure
dR
ates
ofth
eH
BA
sw
ere
fast
erin
Ar
than
inai
rbu
tra
tes
ofT
Aan
dH
Aw
ere
fast
erin
air.
Nag
ata
etal
.,20
002-
,3-,
&4-
chlo
roph
enol
(CP
OH
),pe
nta
chlo
roph
enol
(PC
P)
100
µM/L
ult
raso
un
d,20
0kH
z,20
0W
irra
diat
ing
gas,
air
&A
r,pr
esen
ceof
Fe(
II)
chlo
ride
Deg
rada
tion
ofC
PO
Hs
and
PC
Pw
ere
alm
ost
100%
un
der
Ar
and
80-
90%
un
der
air
afte
r1
h.
Ols
onan
dB
arbi
er,
1994
fulv
icac
id(F
A)
and
nat
ura
lly
colo
red
grou
ndw
ater
(NC
G)
FA
,10
mg/
LT
OC
;NC
G,
con
tain
s2.
5-3.
0m
g/L
(dis
solv
edor
gan
icca
rbon
)
son
olys
is/
ozon
olys
is(s
onoz
one
proc
ess)
;u
ltra
sou
nd,
55W
,20
kHz
tem
p,25
°C;
grou
ndw
ater
pH8.
6;ir
radi
atin
gga
s,2.
1%O
3in
O2
(3.2
mg/
min
)
CO
291
%T
OC
rem
oved
asC
O2
(fro
mF
A)
afte
r60
min
.S
onoz
one
ism
ore
effe
ctiv
eth
anoz
one
oru
ltra
sou
nd
alon
e
Oko
uch
iet
al.,
1992
phen
ol(P
hO
H)
100
ppm
(mg/
dm3 )
ult
raso
nic
gen
erat
or(2
00W
;19
.5,5
0,20
0,an
d60
0kH
z)
tem
p,25
°C;
cata
lyst
,Fe2
+,
Mn
O2
irra
diat
ing
gas,
Air
,O2,
N2,
He
cate
chol
,hyd
roqu
inon
ede
grad
atio
nra
tes
incr
ease
din
the
orde
rO
2>
air
>H
e,N
2an
den
han
ced
byca
taly
stO
rzec
how
ska
etal
.,19
95ca
rbon
tetr
ach
lori
de(C
Cl 4
),ch
loro
form
(CH
Cl 3
),tr
ich
loro
eth
ylen
e(T
CE
),ch
loro
ben
zen
e(C
B),
poly
chlo
robi
phen
ols
(PC
Bs)
Tri
ton
X-1
00(T
X)
CC
l 4,4
0pp
m;
CH
Cl 3
,37
ppm
;TC
E,3
7pp
m;C
B,9
4pp
m;P
CB
,55
ppm
;TX
,1%
aq.s
oln
.
ult
raso
un
d(2
0kH
z);c
up
hor
n,
100
W;h
orn
prob
e,12
0W
solu
tion
tem
p,27
-40
°C;
deio
niz
edan
dta
pw
ater
(pH
6.3-
8.37
);so
nic
atio
nti
me,
up
to60
min
.
chlo
ride
and
form
ate
ion
s;ch
lori
de:
CC
l 4,2
.80
ppm
;CH
Cl 3
,4.5
5pp
m;
TC
E,2
.80
ppm
;CB
,0.
50pp
m;P
CB
,0.3
0pp
m;T
X,0
.30
ppm
;fo
rmat
e,<
1.00
ppm
for
each
At
20m
in,t
he
pHde
crea
sed
by2.
3u
nit
sfo
rC
Cl 4
solu
tion
s.
Pet
rier
etal
.,19
92pe
nta
chlo
roph
enat
e(P
CP
)10
-4
Mu
ltra
son
icdi
sktr
ansd
uce
r(5
30kH
z,20
W)
tem
p,24
(1
°C;p
H7;
irra
diat
ion
gas,
Ar,
air,
O2
CO
2,(C
Oin
Ar
only
),C
l-,N
O2-
,an
dN
O3-
fast
clea
vage
ofth
eC
-C
lbo
nd,
rele
asin
gC
l-an
dm
iner
aliz
atio
nof
PC
Pto
CO
2in
100
min
ute
s
Pet
rier
etal
.,19
94(a
)ph
enol
(Ph
OH
)0.
05-
10m
Mu
ltra
sou
nd
(30
W):
disk
tran
sdu
cer,
487
kHz
tita
niu
mpr
obe,
20kH
z
tem
p,25
°C;
irra
diat
ion
gas,
air
CO
2as
the
only
gase
ous
prod
uct
,h
ydro
quin
one
(HQ
),ca
tech
ol(C
C)
ben
zoqu
inon
e(B
Q)
rate
isde
pen
den
ton
[Ph
OH
] 0w
ith
max
imu
mva
lue
reac
hin
gli
mit
sof
:11
.6×
10-
6M
min
-1
(for
487
kHz)
and
1.84
×10
-6
Mm
in-
1(f
or20
kHz)
Pet
rier
etal
.,19
96at
razi
ne
(AT
Z),
pen
tach
loro
phen
ol(P
CP
)A
TZ
,0.1
mM
;P
CP
,0.1
mM
ult
raso
un
d:im
mer
sion
prob
e,20
kHz,
18.5
W;
ult
raso
nic
emit
ter,
500
kHz,
18.5
W
tem
p,20
°C;
pH7
(for
PC
P);
diss
olve
dga
s,ai
r;ir
radi
atio
nti
me,
AT
Z12
0m
in,P
CP
180
min
AT
Z:
deal
kyla
tion
prod
uct
sof
atra
zin
e,C
O2,
chlo
ride
;PC
P:
chlo
ride
the
degr
adat
ion
proc
eede
d7.
8ti
mes
(for
AT
Z)
and
4.8
tim
es(f
orP
CP
)m
ore
rapi
dly
at50
0kH
zth
anat
20kH
z
Pet
rier
etal
.,19
94(b
)ch
loro
form
(CH
Cl 3
),di
chlo
rom
eth
ane
(CH
2Cl 2
),ca
rbon
tetr
ach
lori
de(C
Cl 4
),an
ddi
lute
mix
ture
sof
CC
l 4in
met
han
ol(M
eOH
),bu
tan
ol(B
uO
H),
and
1,2-
Eth
aned
iol
[Et(
OH
) 2]
MeO
H:
1%C
Cl 4
;Bu
OH
:0.
5%C
Cl 4
;E
t(O
H) 2
:0.
02an
d0.
05%
CC
l 4
ult
raso
un
d(3
0W),
Vib
race
llem
itte
r,20
kHz;
Un
dati
mIr
radi
atio
nsy
stem
,500
kHz
irra
diat
ion
gas,
O2
and
argo
n
acid
icde
grad
atio
npr
odu
cts
dete
cted
wit
hâ-
carb
olin
e(1
0-4 -
10-
5
M)
espe
cial
lyfo
rso
luti
ons
ofC
Cl 4
inal
coh
ol
chlo
rom
eth
anes
exte
nsi
vely
deco
mpo
sed
MeO
Han
dB
uO
Hre
mai
ned
stab
le,b
ut
Et(
OH
) 2ox
idiz
ed
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4693
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Pet
rier
etal
.,19
98aq
ueo
us
solu
tion
sof
chlo
robe
nze
ne
(CIB
Z),
4-ch
loro
phen
ol(4
-CIP
h),
1,2-
dich
loro
ben
zen
e(1
,2-D
CB
),1,
3-di
chlo
robe
nze
ne
(1,3
-DC
B),
1,4-
dich
loro
ben
zen
e(1
,4-D
CB
),1,
3,5-
tric
hlo
robe
nze
ne,
(1,3
,5-T
CB
),1-
chlo
ron
aph
thal
ene
(1-C
N)
and
mix
ture
ofC
IBZ
and
CIP
h
CIB
Zan
dC
IPH
,0.5
mM
;1,
2-D
CB
,0.4
mM
1,3-
DC
B,
0.05
mM
;1,
4-D
CB
,0.2
mM
;1,
3,5-
TC
B,
0.02
mM
;1-C
N,
0.04
mM
ult
raso
un
d(3
0W
),di
sktr
ansd
uce
r(5
00kH
z),t
itan
ium
prob
e(2
0kH
z)
tem
p,20
°C;
wat
ereq
uil
ibra
ted
air
CIB
Z:
Cl-
,CO
,CO
2,C
2H2;
4-C
IPH
:C
l-,a
nd
hyd
roxl
ated
inte
rmed
iate
s(h
ydro
quin
one,
4-ch
loro
cate
chol
,etc
.)
Ch
loro
arom
atic
hyd
roca
rbon
syi
elde
dg
89%
Cl-
aspr
odu
ctin
150
min
.For
the
mix
ture
,CIB
Zde
grad
esbe
fore
4-C
IPH
(100
vs30
0m
in).
1,2-
DC
B,
1,3-
DC
B,1
,4-D
CB
,1,
3,5-
TC
B,a
nd
1-C
Nm
ore
effi
cien
tly
dest
roye
d.
Pet
rier
and
Fra
nco
ny,
1997
phen
ol(P
h),
carb
onte
trac
hlo
ride
(CC
l 4)
Ph
,10-
3
mol
/L;C
Cl 4
,4-
4.6
×10
-4
mol
/L
ult
raso
un
d(3
0W
),ti
tan
ium
hor
n,2
0kH
;pi
ezo-
elec
tric
disk
,200
,500
and
800
kH
tem
p,20
°C;
solu
tion
pH2;
O2-
satu
rate
dso
luti
ons
Ph
hyd
roqu
inon
e,ca
tech
ol;C
Cl 4
:C
l-,a
nd
CO
2
degr
adat
ion
rate
sfo
rC
Cl 4
incr
ease
wit
hfr
equ
ency
,bu
tm
axim
um
for
Ph
OH
at20
0kH
z
Ph
ull
etal
.,19
97ba
cter
ialc
ells
(E.c
oli)
676
bact
eria
lco
lon
ies
per
cm3
(1/1
0di
luti
on)
ult
raso
un
d(U
S)
and/
orch
lori
nat
ion
.US
,20
kHz
(8W
cm-
2 )&
800
kHz
(15
Wcm
-2 )
;ch
lori
nat
ion
,0.
01-
30pp
m
tem
p,20
°C;
son
icat
ion
tim
e,1,
2,5,
10,o
r20
min
dead
bact
eria
lcel
lsu
ltra
sou
nd
ampl
ifie
dth
ebi
ocid
alef
fect
san
dre
duce
dth
eam
oun
tof
chlo
rin
ere
quir
edfo
rdi
sin
fect
ion
Pop
aan
dIo
nes
u,1
992
chlo
rom
eth
anes
inaq
ueo
us
med
ia(C
H2C
l 2/H
2O,
CH
Cl 3
/H2O
,CC
l 4/H
2O)
1.5
cm3
sam
ples
(ch
loro
-m
eth
ane)
in98
.5cm
3H
2O
ult
raso
un
d,1
meg
acyc
les/
s,58
4W
tem
p,25
°C;
irra
diat
ion
gas,
argo
n;p
H6
C2H
2Cl 4
,C2H
Cl 5
,C
2Cl 6
the
son
olyt
icre
acti
onfa
irly
obey
sze
ro-o
rder
kin
etic
s.T
he
rate
con
stan
tsw
ere
1.73
×10
-7 ,
2.22
×10
-7 ,
and
2.78
×10
-7
Ms-
1fo
rC
Cl 4
,C
HC
l 3,a
nd
CH
2Cl 2
,re
spec
tive
ly.
Pri
ceet
al.,
1994
chlo
robe
nze
ne
(CB
),1,
4-di
chlo
robe
nze
ne,
(1,4
-DC
B),
nap
hth
alen
e(N
T),
anth
race
ne
(AN
),py
ren
e(P
R)
satu
rate
dso
luti
ons
(M):
CB
,4.1
×10
-3 ;
DC
B,
6.2
×10
-5 ;
NT
,2.
36×
10-
4 ;A
N,2
.30
×10
-7 ;
PR
,6.4
×10
-7
hor
nu
ltra
sou
nd
(22
kHz)
,in
ten
siti
es(W
/cm
2 ),1
3.5,
11.7
,20.
9,an
d38
.9
tem
p,20
°C(a
mbi
ent)
;so
luti
on,p
H5.
48-
4.50
(not
con
trol
led
bybu
ffer
)
not
expl
icit
lyre
port
edA
t20
.9W
/cm
2 ,th
era
teco
nst
ants
for
1,4-
DC
B,N
T,
AN
,an
dP
Rar
e0.
038,
0.01
5,0.
033,
and
0.01
6(m
in-
1 ),r
espe
ctiv
ely,
and
0.02
2fo
rC
Bat
23.9
W/c
m2
Raj
anet
al.,
1998
a,b
wat
er-K
I-C
Cl 4
syst
em(K
I-w
ater
solu
tion
satu
rate
dw
ith
CC
l 4)
1%,1
6.6%
,an
d25
%K
Iso
luti
ons
con
tain
ing
5.2
×10
-3
MC
Cl 4
hor
nu
ltra
sou
nd
(25
kHz)
tem
p,34
°C;
irra
diat
ion
gase
s,ai
r,N
2,O
2
I 2,C
l 2,C
l,H
OC
l,H
Cl,
CO
2,an
dO
2
Mod
elde
velo
ped
pred
icte
dw
ellt
he
rate
sob
serv
edfo
rC
Cl 4
susp
ensi
onin
KI
solu
tion
.
Rei
nh
art
etal
.,19
96tr
ich
loro
eth
ylen
e(T
CE
)5-
20pp
mso
nol
ysis
/ze
rova
len
tF
e.U
S:
20kH
z,90
&18
0W
;iro
n(F
e),
1&3
g/L
tem
p,25
°C;
irra
diat
ion
gas,
N2
not
expl
icit
lyre
port
edC
l-in
ferr
edco
mbi
ned
son
olys
is/F
era
tes
was
3ti
mes
that
ofF
eal
one
Sak
aiet
al.,
1977
chlo
ralh
ydra
te(C
H)
0.1
Min
aqu
eou
sso
luti
ons
ult
raso
un
d(2
9an
d40
0kH
z);
pow
erou
tpu
t,0.
5W
/cm
tem
p,30
°C;s
olu
tion
satu
rate
dw
ith
air
and
mix
ture
s(2
0/80
and
2/98
)of
O2
and
N2
chlo
ride
ion
(Cl-
)ki
net
icra
teex
pres
sion
obta
ined
for
the
form
atio
nof
HC
lagr
eed
wit
hex
peri
men
talr
esu
lt
4694 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Seh
gala
nd
Wan
g,19
81th
ymin
e(T
HY
)0.
1m
Mu
ltra
sou
nd
(1-
7W
cm2 )
tem
p,20
-70
°C;a
erat
ion
med
ium
,air
not
dete
rmin
edA
tan
aera
tion
rate
of50
mL
/min
at34
°C,[
TH
Y]
isre
duce
dto
hal
fin
30m
in.
Ser
pon
eet
al.,
1994
2-,3
-,an
d4-
chlo
roph
enol
s(2
-CP
OH
,3-C
PO
H,a
nd
4-C
PO
H)
4-C
PO
H,7
5.1
µM;3
-CP
OH
,77
.8µM
;2-
CP
OH
,83.
1µM
hor
nu
ltra
sou
nd
(20
kHz,
49.5
W,5
2.1
W/c
m2 )
tem
p,33
(2
°C;a
ireq
uil
ibra
ted;
nat
ura
lsol
uti
onpH
:4-
CP
OH
,5.1
;3-
CP
OH
,5.4
;2-
CP
OH
,5.7
4-C
PO
H,h
ydro
quin
one
(HQ
),4-
chlo
rore
sorc
inol
(4C
R),
4-ch
loro
cate
chol
(4-C
C);
3-C
PO
H,C
l-,
chlo
roh
ydro
quin
one
(CH
Q),
4-3-
CC
,ca
rbon
aceo
us
spec
ies;
C-
CP
OH
,CH
Q,3
-C
C,
cate
chol
(CC
)
tota
ldeg
rada
tion
in10
hfo
r2-
CP
OH
and
3-C
PO
Han
d15
hfo
r4-
CP
OH
wit
hra
tes
[4.8
(0.
4,4.
4(
0.5,
and
3.3
+0.
2]×
10-
3m
in-
1 ,re
spec
tive
ly.
Ser
pon
eet
al.,
1992
phen
ol(P
hO
H)
and
inte
rmed
iate
s,ca
tech
ol(C
C)
hyd
roqu
inon
e(H
Q),
p-be
nzo
quin
one
(BQ
)
Ph
OH
,75
µM(p
H12
),68
µM(p
H3)
and
51µM
(pH
5.7)
;HQ
,44
µM,p
H3;
BQ
,43
µM,
pH3;
CC
,54
µM,p
H3
imm
ersi
onh
orn
ult
raso
un
d20
0kH
z,25
,50,
or75
W/c
m2
30(
2°C
;pH
3,5.
4,5.
7,12
;op
ento
air
Ph
OH
,app
eara
nce
(in
9h)
and
subs
equ
ent
disa
ppea
ran
ceof
CC
,H
Qan
dB
QB
Q,H
Q+
un
iden
tifi
edsp
ecie
sH
Q,B
Q+
un
iden
tifi
edsp
ecie
s
At
pH12
,nei
ther
HQ
,CC
orB
Cw
ere
dete
cted
from
the
inso
nat
ion
ofP
hO
Hsi
nce
they
wer
eu
nst
able
.C
O2
cou
ldn
otbe
dete
cted
.
Sie
rka,
1985
trin
itro
tolu
ene
(TN
T)
and
cycl
otri
met
hyl
ene
-tr
init
ram
ine
(RD
X)
70/3
0m
g/L
TN
T/R
DX
;aq
ueo
us
solu
tion
s
ult
raso
un
d(5
-50
Wat
852-
863
kHz,
60.6
-1,
007
kHz)
/ozo
ne
(72.
91m
gO
3/m
in)
tem
p,25
-59
°C;p
H5.
84-
10.0
not
expl
icit
lyde
term
ined
ult
raso
un
din
hib
ited
kin
etic
sat
hig
hte
mp
and
pHdu
eto
radi
cal-
radi
cal
exti
ngu
ish
men
tre
acti
on
Sie
rka
and
Am
y,19
85h
um
icac
id(H
A)
solu
tion
un
know
nco
nce
ntr
atio
nu
ltra
viol
et/
ult
raso
un
d/O
3;U
S,4
0kH
z,30
0W
;UV
lam
p,25
3.7
nm
;O3,
4.3
mg/
L(2
min
),21
.6m
g/L
(10
min
)an
d43
.2m
g/L
(20
min
)
irra
diat
ing
gas,
O3/
O2;
pHo
4.0,
7.1,
and
10.0
trih
alom
eth
anes
form
atio
npo
ten
tial
(TH
MF
P),
and
non
vola
tile
tota
lor
gan
icca
rbon
(NV
TO
C)
mon
itor
edas
bypr
odu
cts
O3-
US
-U
Vsy
stem
prov
edto
bem
ost
effe
ctiv
ere
acti
onco
ndi
tion
s,fo
llow
edby
O3-
UV
,O3
alon
e,an
dO
3-U
S,p
rovi
din
g93
%,
86%
,75%
,an
d71
%re
duct
ion
inT
HM
FP
leve
ls,
resp
ecti
vely
,in
20m
in.
Sou
daga
ran
dS
aman
t,19
95
tolu
ene
(T),
nit
roto
luen
e(N
T),
O-c
hlo
roto
luen
e(O
-T
),O
-p-x
ylen
e,m
esit
ylen
e(1
,3,5
-tr
imet
hyl
ben
zen
e(M
TB
),an
dis
opro
pyl-
tert
-bu
tylb
enze
ne
(IP
B,T
BB
)
mix
ture
of20
mm
olof
KM
nO
4;an
d10
mm
olof
aryl
alka
ne
in50
cm3
wat
er
KM
nO
4;u
ltra
son
iccl
ean
er(2
3kH
z,12
0W)
tem
p,30
-35
°CT
,ben
zoic
acid
;O-
Tan
dM
TB
,co
rres
pon
din
gca
rbox
ylic
acid
s
Tst
udi
edin
deta
ilam
ong
the
aryl
alka
nes
.Th
eop
tim
alti
me
for
oxid
atio
nof
Tw
as3
h.K
Mn
O4
acce
lera
tes
oxid
atio
nin
the
pres
ence
ofu
ltra
sou
nd.
Spu
rloc
ket
al.,
and
Rei
fnei
der
1970
,197
1
carb
onte
trac
hlo
ride
(CC
l 4);
di-n
-bu
tyls
ulf
ide
(R2S
)
satu
rate
daq
ueo
us
solu
tion
ofC
Cl 4
ult
raso
un
d,28
0-80
0kH
zat
5-11
W/c
m2 )
tem
p,20
°C;
irra
diat
ion
gase
s,O
2an
dA
r
CC
l 4,C
O,C
O2,
HO
Cl,
and
hex
ach
loro
eth
ane
(C2C
l 6)
(in
AR
only
);M
inor
,R2S
O2,
RS
O3H
,bu
tyri
cac
id,C
O,C
2H4,
C2H
2,an
dC
H4
irra
diat
ion
ofpr
odu
cts
R2
SO
and
R2S
O2
yiel
ded
RS
O3H
aspr
inci
palp
rodu
cton
lyat
800
kHz
Sto
cket
al.,
2000
azo
dye,
nap
hth
albl
ue
blac
k(N
BB
)50
(5
µMso
nol
ysis
/ph
otoc
atal
ysis
;U
S,6
40kH
z,25
0W
;TiO
2(P
.25)
,1g/
L
tem
p,20
(5
°CN
BB
inte
rmed
iate
sm
iner
iliz
edto
inor
gan
icsp
ecie
s
Deg
rada
tion
rate
for
son
olys
isw
asab
out
2ti
mes
fast
erth
anth
atof
phot
ocat
alys
is.
Su
zuki
etal
.,20
00su
rfac
tan
t,po
lyox
yeth
ylen
e-al
kyl
eth
er(S
S-7
0)
100
ppm
in10
00m
Laq
ueo
us
sam
ple
ult
raso
un
d/ph
otoc
atal
ysis
,20
0kH
z/20
0W
,T
iO2/
Hg
lam
p(2
53.7
nm
/20W
)
tem
p,25
°C;
irra
diat
ion
gas,
air;
irra
d.ti
me,
60m
in
not
expl
icit
lyde
term
ined
TO
Cm
onit
ored
sign
ific
ant
enh
ance
men
tin
the
phot
ocat
alyt
icre
acti
onob
serv
edw
hen
com
bin
edw
ith
US
irra
diat
ion
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4695
Tab
le3.
(Con
tin
ued
)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Tak
izaw
aet
al.,
1996
phen
ol(P
h),
2-n
aph
thol
(2-N
ph),
4-m
eth
ylph
enol
(4-m
Ph
),ca
tech
ol(C
C),
reso
rcin
ol(R
S),
3-te
rt-b
uty
l-4-
hyd
roxy
anis
ole
(3-t
b-4-
HA
N),
3-m
eth
yl-4
-hyd
roxy
anis
ole
(3-m
-4-H
AN
)
8.0
×10
-3
Min
wat
eran
dw
ater
-et
han
olm
ixtu
re(1
:1V
/V,1
00m
L)
ult
raso
un
d(2
00kH
z)te
mp,
25°C
;ir
radi
atio
nti
me,
12-
18h
;ir
radi
atio
nga
s,O
2
Ph
,CC
,hyd
roqu
inon
e(H
Q);
4-m
pH4-
met
hyl
cate
chol
;4-
HA
N,h
ydro
quin
one;
2-N
pH2,
3-di
hyd
ron
aph
thol
;C
Can
dR
S,p
yrog
allo
l;3-
tb-4
-HA
N,
tert
-bu
tyl-
HQ
;3-
m-4
-HA
N,m
etyh
lHQ
Hyd
roxy
late
dpr
odu
cts
yiel
dsra
nge
dfr
om15
%to
Ph
and
4-H
AN
hav
eth
eh
igh
est
yiel
dsof
prod
uct
s,P
h30
%C
Can
d26
%H
Qin
12h
;4-
HA
N,5
0%H
Qin
12h
.
Tau
ber
etal
.,20
004-
nit
roph
enol
5×
10-
3
mol
/dm
3u
ltra
sou
nd,
321
kHz,
170
W/k
gir
radi
atio
nga
s,A
r;pH
4&
10
pH10
,4-n
itro
cate
chol
,h
ydro
quin
one
+be
nzo
quin
one,
nit
rite
;pH
4,pr
odu
cts
atpH
10+
phen
ol,n
itra
te,C
O,
CO
2,H
2
ther
mol
ysis
prod
uct
sob
serv
edat
low
pHco
mpa
red
wit
hO
H-i
ndu
ced
reac
tion
sat
hig
hpH
.
Th
eron
etal
.,19
99ph
enyl
trif
luor
omet
hyl
keto
ne
(PT
MK
)aq
.sol
uti
on16
0µm
ol/L
TiO
2ph
otoc
atal
ysis
and/
oru
ltra
sou
nd
(US
);T
iO2,
3.5
g/L
;US
,30
&51
5kH
z,16
W
tem
p,29
3K
;pH
6-6.
5(n
atu
ral
wat
er)
CF
3CO
OH
and
hyd
roxl
ated
PT
MK
the
amou
nt
ofC
F3C
OO
Hw
as8
tim
eslo
wer
inso
nic
ated
solu
tion
sth
anin
UV
-irr
adia
ted
TiO
2su
spen
sion
atbo
thfr
equ
enci
es
Th
oma
etal
.,19
98be
nze
ne
(Bz)
,tol
uen
e(T
)4,
40,a
nd
80m
g/L
;Bz,
51-
1030
µM;
T,4
3.5-
870
µM
nea
r-fi
eld
acou
stic
proc
esso
r(N
AP
),20
0-80
0W
,16
-20
kHz,
1.3
W/m
L-
1
air
orO
2sa
tura
ted
solu
tion
s
not
repo
rted
rate
con
stan
tsfo
r4
to80
mg/
Lso
luti
ons;
Bz,
0.01
-0.
003
min
-1 ;
T,0
.028
-0.
004
min
-1
Toy
etal
.,19
901,
1,1-
tric
hlo
roet
han
e(C
Cl 3
CH
3,or
TC
A)
3.6
and
2500
ppm
cup-
hor
nu
ltra
sou
nd
(20
kHz)
/Hg/
Xe
lam
p(2
00W
)[p
hot
olys
isan
d/or
son
olys
is]
tem
p,am
bien
t;pH
3ch
lori
de(C
l-)
deco
mpo
siti
onm
ore
exte
nsi
vew
ith
com
bin
edph
otol
ysis
and
son
olys
is(p
hot
oson
olys
is)
than
each
only
Toy
etal
.,19
921,
1,1-
tric
hlo
roet
han
e(C
Cl 3
CH
3or
TC
A)
1.06
and
10pp
mim
mer
sion
hor
nu
ltra
sou
nd
(475
W,2
0kH
z)
tem
p,am
bien
t;op
ento
atm
osph
ere
chlo
ride
(Cl-
)di
sapp
eara
nce
ofT
CA
was
91.4
in18
min
.
Tra
bels
iet
al.,
1996
phen
ol89
-10
0m
g/L
son
olys
is,S
S(2
0an
d50
0kH
z,20
W)/
elec
trol
ysis
,ES
(cu
rren
tde
nsi
ty)
68A
/m2 )
[son
oele
ctro
-ox
idat
ion
(SE
O)]
tem
p,27
(2
°C;s
olu
tion
satu
rate
dw
ith
air
for
20m
in
mal
eic
acid
(MA
),ox
alic
acid
(OA
),m
uco
nic
acid
(MC
A),
p-be
nzo
quin
one
(BQ
),h
ydro
quin
one
(HQ
),ca
tech
ol(C
C),
CO
2
At
20kH
z,5%
con
vers
ion
wit
hso
nol
ysis
alon
eco
mpa
red
wit
h75
5fo
rS
EO
in10
min
.SE
O’s
con
vers
ion
is95
%at
500
kHz
in10
min
and
100%
in60
min
.H
Q,C
C,a
nd
BQ
disa
ppea
raf
ter
2h
at54
0kH
zV
inod
gopa
let
al.,
1998
azo
dye,
rem
azol
blac
kB
(RB
)33
µMu
ltra
sou
nd
(640
kHz,
240
W)
tem
p,am
bien
t;u
nde
rco
nst
ant
bubb
lin
gof
O2
oxal
ate
(C2O
42-)
RB
disa
ppea
red
in90
min
wit
hra
teco
nst
ant
of2.
9×
10-
2m
in-
1an
d60
%m
iner
aliz
atio
nac
hie
ved
in6
h.
Wea
vers
and
Hof
fman
n,
1998
ozon
ein
aqu
eou
sso
luti
ons
85(
5to
245
(3
µMO
3sa
tura
ted
solu
tion
s
ult
raso
un
ddi
rect
prob
e,20
kHz;
263
WL
-1
ult
raso
nic
tran
sdu
cer,
500
kHz,
96W
L-
1
tem
p,23
(3
°C;i
rrad
iati
ng
gas,
O2/
O3
mix
ture
;pH
2
not
expl
icit
lyde
term
ined
firs
t-or
der
degr
adat
ion
rate
con
stan
tsar
e0.
84an
d0.
66m
in-
1 ,re
spec
tive
ly,a
t20
kHz
and
500
kHz
at[O
3]o
)24
5µM
4696 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
solution in the former case. Okouchi et al.63 observedthe degradation of phenol in less than 100 min toproceed with pseudo first-order rate and the additionof Fe2+ and MnO2 to increase the rates in aqueoussolutions. Serpone et al.64 noticed the disappearance ofphenol at the initial concentration range of ∼30 to 70µM to follow a zero-order kinetics with k ) 0.08 µM/min at [phenol]initial ) 51 µM and pH 5.7 and slower atan alkaline pH of 12. They also observed three principalintermediate species formed at pH 3 (catechol, hydro-quinone (HQ), p-benzoquinone (BG)), only catechol (CC)and HQ at natural pH 5.4-5.7, and no intermediatesdetected at pH 12. The mechanism is given as
For a constant phenol concentration, the rate of loss ofphenol as a function of sonocation time was foundlinearly related to the sonocation power, P, accordingto
At low concentrations of phenol, k2 [•OH] > k4[phenol],and at low constant P, the rate follows first-orderkinetics; at high concentration, where k2 [•OH] ,k4[phenol], the rate follows zero-order kinetics at con-stant P. However, contrary to the zero-order kineticbehavior observed at 25 and 50 W/cm2, the sono-oxidation of phenol was found to be only first-order at75 W/cm2. Over a pH range of 3-9, Lur’e et al.65 foundthe decomposition of phenol to be practically indepen-dent of pH. Chen et al.66 also found that at highconcentrations ([PhOH] > 300 ppm) the conversion waszero-order and become first-order at very low concentra-tion (<100 ppm). The rate was also found to be negli-gible at low frequency and the oxidation proceededtoward completion at high frequency. Trabelsi et al.67
investigated the oxidation of phenol in a screenedreactor with ultrasound alone at 20 kHz and withultrasound associated with electrolysis. They found thatthe electrochemical oxidation of phenol in NaCl mediacombined with sonication at 20 kHz resulted in 75%conversion of the initial phenol within 10 min and ledto the formation of toxic p-quinone. At a higher fre-quency of 500 kHz, a conversion of 95% was obtainedT
able
3.(C
onti
nu
ed)
auth
ors
con
tam
inan
tsde
grad
edco
nce
ntr
atio
n
chem
ical
oxid
atio
nsc
hem
eex
peri
men
tal
con
diti
ons
degr
adat
ion
inte
rmed
iate
s/pr
odu
cts
degr
adat
ion
effi
cien
cy/o
ther
resu
lts
orre
mar
ks
Wh
eat
and
Tu
meo
,199
7po
lycy
clic
arom
atic
hyd
roca
rbon
s(P
AH
S),
phen
anth
ren
e(P
A),
biph
enyl
(BP
)
PA
orB
P,0
.39
mg/
310
mL
wat
er
ult
raso
un
d(1
47W
/cm
2 )te
mp,
21-
27°C
;air
flow
reac
tor,
100
mL
/min
(ope
nto
atm
osph
ere)
PA
,ph
enan
thre
ne
diol
;B
P,o
rth
o,m
eta,
and
para
-1,1
biph
enol
s
the
toxi
city
ofre
acti
onpr
odu
cts
rem
ain
sto
bede
term
ined
.
Wu
etal
.,19
92ca
rbon
tetr
ach
lori
de(C
Cl 4
)0.
53,1
.6,8
,an
d13
0pm
prob
eu
ltra
sou
nd
(20
kHz,
27W
/cm
2 )te
mp,
20-
60°C
;pH
3-9;
pow
erin
ten
sity
,1-
22W
/cm
2 ;ir
radi
atin
gga
s,ai
r
not
expl
icit
lyre
port
edgr
eate
rth
an99
%de
stru
ctio
nef
fici
enci
esin
6m
inw
ith
anav
erag
efi
rst-
orde
rra
teco
nst
ant
of0.
7m
in-
1at
25°C
Zh
ang
and
Hu
a,20
00po
lych
lori
nat
edbi
phen
yls
(PC
Bs)
,2-c
hlo
robi
phen
yl(2
-PC
B),
4-ch
loro
biph
enyl
(4-P
CB
),2,
4,5-
chlo
robi
phen
yl(2
,4,5
-PC
B)
2-P
CB
,4.6
µM;4
-PC
B,
5.4
µM;
2,4,
5-P
CB
,7.6
×10
-2
µM
prob
eu
ltra
sou
nd
(PU
S),
20kH
zfo
ral
lPC
Bs;
also
at20
5,35
8,61
8,an
d10
71kH
zfo
r2-
PC
Bon
ly
tem
p,15
°C;
pow
erin
ten
sity
,30
.8W
/cm
2 ;ir
radi
atin
gga
s,A
r
biph
enyl
,eth
ylbe
nze
ne,
diet
hyl
biph
enyl
,di
buty
lbip
hen
yl,
phen
ol,p
ropy
lph
enol
,di
-ter
t-bu
tylp
hen
ol,
chlo
ride
pseu
do-f
irst
-ord
erra
teco
nst
ants
at20
kHz/
30.8
Wcm
-2
(s-
1 )ar
e:2-
PC
B,2
.1×
10-
3(
2.8
×10
-5 ;
4-P
CB
,1.
6×
10-
3(
3.4
×10
-5 ;
2,4,
5-P
CB
,2.6
×10
-3
(9.
4×
10-
5
H2O + ))) 798k1
k-1
•OH + H• (5)
•OH 98k2 1/2H2O2 (6)
•H 98k3 1/2H2 (7)
•OH + phenol 98k4
products (8)
Rate )k1k4[phenol]
k2[•OH] + k4[phenol]
(10)
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4697
within the same treatment time with acetic and chlo-rocrylic acids as final products of degradation. However,sonoelectrooxidation at high frequency allowed totaldegradation in 20 min with no toxic aromatic intermedi-ates. They also observed that high frequency ultrasoundenhanced mass transfer substantially up to 70-fold themass transfer by diffusion, and without ultrasounddimerization of phenol radicals yielded polymeric specieswhich 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-dichlo-rophenol (2,4-DCP) in air equilibrated media generallyinclude chlorohydroquinone (CHQs), catechols (CC),chlorocatechols, chlororesorcinols (CRSs), and chlorides.In addition, 2,4-dichlorophenol has been found to pro-duce some 2- and 4-chlorophenols as intermediates. Themajor mechanism of oxidation for these chlorophenolsis a radical attack similar to equations for phenol (eqs5-9).64 Serpone et al.68 obtained CHQ as the majorintermediate for both 2-CPOH and 3-CPOH sonolysis,with a small quantity of catechol (CC) in the case of2-CPOH. By contrast, the principal intermediate for4-CPOH was found to be HQ with a small amount of4-CRS. Ku et al.69 in the sono-oxidation of 2-CPOH withoxygen purging at various solution pH levels identifiedCHQ, CC and glycoxilic acid as intermediates at pH 3but only CHQ and CC at pH 11. Lin et al.70 found thatthe combination of ultrasound with H2O2 increased theefficiency of 2-CPOH decomposition significantly. Withthe ultrasound/H2O2 process, they observed that withpH 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 pHcontrolled at 11. The variations in decomposition ratesat the different pH values were attributed to the pKavalue of 8.49 for 2-CPOH at 25 °C. The 2-CPOH isalmost completely in the ionic form when the pH exceedsthe pKa value of 8.49 but exists in molecular form whenpH < pKa, the fraction in molecular state increasingwith pH drop. In ionic state, 2-CPOH does not vaporizeinto the cavitation bubbles, and oxidation reactionoccurs with OH radicals outside the bubble film. How-ever, in the molecular state, reactions occur by boththermal cleavage inside the bubble and oxidation withOH radicals outside, leading to more effective decom-position. Lin et al.71 also studied the effect of H2O2concentration on decomposition. It was shown that themore the H2O2 that was added, the greater the degrada-tion efficiency. In a particular study with 500 mg/L ofH2O2 and initial 2-CPOH concentration of 100 mg/L,they observed, after a reaction time of 30 min, animprovement as great as 57% over the control, i.e.,without H2O2 addition. The result was attributed toincrease in OH radicals generated on addition of H2O2.However, the increase in degradation rates was notsignificant between the additions of 200 and 500 mg/LH2O2, suggesting the concentration saturation of OHradicals at the 200 mg/L level. Petrier et al.72 studiedthe sonochemical degradation of 4CPOH and concludedthat the release of end products (i.e., Cl-) and formationof hydroxylated intermediates (HQ, CC) were evidenceof a two-step reaction involving OH radicals withsubstrate in the liquid layer surrounding the cavitationbubble. The dechlorination yields were also found to behigher at 500 kHz (using undatim ultrasonic source)than with the common 20 kHz ultrasonic probe. Thedechlorination rates for 4-chloro-3,5-dimethyl-phenol
were found to be approximately 2 orders of magnitudelower than that of 2,4-chlorophenol (2,4-DCP), and thiswas attributed to the fact that the formation of aryneintermediates is blocked by the ortho methyl groups.73
Nagata et al.74 found the first-order ultrasonic degrada-tion rates of 2-, 3-, and 4-chlorophenol to be faster underargon atmosphere compared to air atmosphere. With 0.1mM 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 µMmin-1 in air atmosphere. The rate was even faster for3-CPOH (k ) 20.0 µM min-1) with an initial concentra-tion of 1 mM in argon atmosphere. The faster degrada-tion of 3-CP compared to 2-CP and 4-CP was attributedto the fact that with 3-CP, there are three points ofsimultaneous ortho and para orientation to Cl and OHgroups where, due to the electrophilic character of OH,OH radical addition would easily occur. Johnston andHocking75 investigated the photocatalytic and photolyticdegradation of 2,4-DCP with and without sonication.They found that the use of sonication (1 × 10-3 M, 2,4-DCP, or 0.2% or 0.05% TiO2) in photolysis resulted inthe enhancement of chloride release rate by a factor of4 compared with that of UV irradiation only.
The main sonooxidation products of p-nitrophenol (p-Np) are nitrates (NO2
-), nitrates (NO3-), short-chain
carboxylic acids, formic acid (or formate, HCO2-), acetic
acid (or acetate), and oxalic acid (or oxalate) withintermediates such as 4-nitrocatechol (4-NP), hydro-quinone, and benzoquinone. Kotronarou et al.76 andHoffmann et al.58 found that in the presence of ultra-sound, p-nitrophenol solutions was denitrated to yieldNO2
- and NO3- by two independent mechanisms: (a)
via the gas-phase reaction, which takes place in purewater, and (b) from the decay of p-NP molecules. It wasshown that while NO3
- did not interfere with thesonochemical reactions of p-NP, NO2
- appeared to affectthe decay of p-nitrophenol and the formation of 4-ni-trocatechol (4-NP). Ultrasonic irradiation of a solutioncontaining 100 µM p-NP and 100 µM NaNO2 was foundto result in a slower disappearance of p-NP (k1 ) 2.0 ×10-4 s-1) when compared to a kinetic run in the absenceof NO2
- (k1 ) 3.67 × 10-4 s-1). On the basis of theproducts and kinetic observations, it was concluded thatthe degradation of p-NP involved high-temperaturereactions of p-NP in the interfacial region of cavitationbubbles and that the main pathway was carbon-nitrogen bond cleavage with hydroxyl radical reactionproviding a secondary reaction channel. Hua et al.77
studied the sonochemical degradation of p-NP usinghigh-power ultrasonic system (parallel-plate near fieldacoustical processor, NAP, with 0-1775W) in the pres-ence of argon (Ar) and oxygen (O2) in order to investigatethe effect of power per area. They found pseudo-first-order constant for p-NP degradation in the presence ofpure O2, (k2 ) 5.19 × 10-4s-1) was lower than that inthe presence of pure Ar, (kAr ) 7.94 × 10-4s-1). Thehighest degradation rate was in the presence of Ar/O2(4:1 v/v) mixtures (kAr/O2 ) 1.20 × 10-3s-1) at the sameintensity of 1.4 W/cm2. Cost et al.78 also studied thesonochemical reactions of p-NP in solutions containingparticulate matter, phosphate (total ions: HPO4
2- +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 rateconstants, kobs, for p-NP of 4.4 × 10-4 in deionized waterand 3.4-3.2 × 10-4 s-1 in water with bicarbonate
4698 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
concentration: 10-4 e [HCO3-] e 10-2 M. They found
the degradation rate was unaffected by chemicals ofnatural water. They rationalized the results by indicat-ing that for pollutants such as p-NP which decay viathermal reactions near the interface or inside of cavi-tating bubbles, the high-temperature denitration reac-tions in this case are not significantly affected bycommon chemicals of natural waters. Barbier andPetrier79 investigated the ultrasonic degradation andminerilization of 4-nitrophenol (4-NP) at two ferquencies(20 kHz and 500 kHz) and in water saturated withoxygen or an oxygen-ozone gas mixture. They found thecoupling of ultrasound and O3 increased the potentialof ozonation to mineralize 4-NP degradation products.At low pH (pH 2), where the ozone auto-decompositionradical pathway is suppressed, they observed 4-NPmineralization at 500 kHz was 1.8 times faster than at20 kHz for the same O3 consumption and attributed theenhanced rate to increased O3 utilization occurring atthe higher frequency. Tauber et al.80 found the sonolyticdegradation of 4-nitrophenol (4-NP) in argon-saturatedaqueous solution (321 kHz) was fully due to OH-radical-induced reactions at pH 10, where 4-nitrophenol isdeprotonated (pKa ) 7.1) and nonvolatile 4-nitrophe-nolate predominates. At pH 4 and with the neutral formof 4-nitrophenol, oxidative-pyrolytic degradation pre-dominated, resulting in large yields of CO, CO2, and H2.
Takizawa et al.81 sonicated various phenolic com-pounds in aqueous media (water and water-ethanol) at215 °C for 12-18 h. using 200 kHz ultrasound in thepresence of oxygen and isolated various hydroxylatedphenols as products of ultrasonic oxidation (Table 3).They also identified catechol and hydroquinone as theoxidation products of phenol. They postulated the fol-lowing mechanism for the production of hydroquinonefrom phenol: The phenolic benzene ring was attackedby the hydroxyl radical produced in water sonochemi-cally. The attack took place at the carbon attached tothe methoxy group at the p-position of phenolic hydroxygroup and the methoxy group was eliminated withhydrogen radical to give hydroquinone and methanol.Hua et al.56 observed that ultrasonic irradiation acceler-ated the rate of hydrolysis of p-nitrophenyl acetate (p-NPA) in aqueous solution by 2 orders of magnitude overthe pH range of 3-8 compared to the same hydrolysisunder ambient conditions at 25 °C. The first-order rateconstants were independent of pH and ionic strength.They explained their observations by considering theexistence of transient supercritical water (SCW) aroundthe collapsing cavitation bubbles during sonolysis as animportant factor in the acceleration of chemical reac-tions due to higher concentration of OH- at the hotbubble interface resulting from higher ion product, Kw,of supercritical water.
The main chemical degradation intermediates andproducts from benzene usually include phenol, catechol(CC), hydroquinone (HQ), p- and o-benzoquinone (p- ando- BQ), 1,2,3-trihydroxybenzene, maleic and muconicacid, formalaldehyde, and acetylene.82-85 Chlorobenzenetransforms into 4-chlorophenol, chlorophenol, 4-chloro-catechol, hydroquinone, benzene, butenyne, butadiyne,nonchlorinated mono- and dicyclic hydrocarbons, CH4,and C2H2 upon ultrasonic irradiation in air-saturatedsolutions.84 De Visscher et al.82 investigated the sono-chemical degradation of monocyclic aromatic com-pounds, benzene, ethylbenzene, styrene, and o-chloro-toluene, in aqueous solutions and found the first-order
reactions to be dependent on initial concentrations ofthe substrates and the sonication time. Pyrolysis isexpected to be the main reaction path for the degrada-tion of polar compounds. De Visscher et al.83 analyzedthe reaction products of the sonochemical degradationof ethylbenzene in aqueous solutions and found themto be typical of high-temperature pyrolysis reactions.The reaction rates were also found to be first-order.Unlike ethylbenzene, both pyrolysis and radical attackappear to be important pathways of benzene andchlorobenzene degradations. Drijvers et al.84 observedthat chlorobenzene was mainly degraded thermally andthat the first-order degradation rates for the sonolysiswas not influenced by the pH of the aqueous solutionwhile saturating the solution with argon instead of airaccelerated the degradation. It was also suggested thatchlorophenol (CP), the only oxygen-containing interme-diate detected, was not formed directly by the reactionof OH radicals with chlorobenzene but resulted from theaddition of oxygen to chlorophenyl radicals in theinterface since chlorobenzene could not break down inthe bulk solution itself. Price et al.86 obtained a first-order 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 after40-50 min. For a given intensity (i.e., 20.9 W/cm2), thereaction rate of 1,4-DCB was found to be faster com-pared with those of CB, naphthalene (NT), anthracene(AN), and pyrene (PR). Drijvers et al.85 compared thesonolysis at 520 kHz (9.4 or 0.063 Wml-1) and 25 °C offour monohalogenated benzenes, fluorobenzene (FB),chlorobenzene (CB), bromobenzene (BB), and iodoben-zene (IB), at different initial concentrations, 0.5, 1, and2 mM. They found all four to degrade by similarmechanisms and the sonolysis rates to depend on initialconcentrations. Petrier et al.72 also studied the sonochem-ical degradation of chlorobenzene (CB). On the basis ofthe chloride ion evolution, the low level of hydroxylatedintermediates, and the formation of gaseous stableproducts (CO2, CO, C2H2), they suggested a thermaldegradation inside the cavitation bubble as the prefer-ential reaction pathway, with C2H2 formed by ther-molytic ring cleavage. The formation of brown carbon-aceous particles was attributed to soot formation underpyrolytic conditions, phenyl radical combination, orC2H2 reactions at high temperatures. For a mixture ofthe hydrophobic CB and the hydrophilic 4-CPOH at 0.5mM each, CB decayed first, and the 4-CPOH transfor-mation started only when CB reached a low level (0.02mM). The results also indicated that the presence ofhydrophobic compound with high vapor pressure (Hen-ry’s law constant, H, for CB ) 3.77 × 10-3 atm.m3mol-1)hinders efficiently the degradation of hydrophilic com-pound 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 oxidativeprocesses arising from the ultrasonic irradiations ofbenzene and toluene in aqueous solutions resulted inthe destruction of the six-membered ring compounds,with the formation of formaldehyde and other simulta-neous reactions leading to the formation of phenol.Thoma et al.88 studied the sonochemical treatment ofbenzene and toluene in water using a parallel plate nearfield acoustic processor (NAP). They found the magne-tostrictive system to be capable of degrading bothcontaminants in a continuous stirred tank reactor
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4699
configuration with the rate constant inversely propor-tional to the target pollutant concentration. They alsoobserved that the apparent first-order rate constantincreased approximately linearly with the power input(measured from the wall) to the system.
Hung et al.89 found the rate of reduction of nitroben-zene by Feo enhanced in the presence of ultrasound(US). For example, the first-order rate constant for theultrasonic degradation of nitrobenzene in the absenceof Feo was found to be 1.8 × 10-3 min-1 compared to7.8 × 10-3 min-1 in the presence of 70 g/L Feo. Theydescribed the overall rate of nitrobenzene disappearanceby the linear relation:
where kus, kFeo, and k′ are, respectively, the first-orderdegradation rate constants for sonolysis, reduction byFeo, and the synergistic kinetic effect achieved bycombining the two systems. They also obtained thefollowing relation from eq 11:
where ksec, kp ) xoka, xo, and ka are the average second-order rate constant for the reaction between the Feo
surface sites or the reaction intermediates and ni-trobenzene, a zero-order rate constant that increaseswith added [Feo], the initial activated Feo surface beforethe 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 thecombined US/Feo system to several causes. These in-clude the indirect chemical effects associated with thecontinuous ultrasonic cleaning and activation of the Feo
surface, the accelerated rates of mass transport result-ing from the turbulent effects of cavitation, and theacidity produced by H+ released from HNO3, a productof sonolysis of nitrobenzene and water. The intermedi-ates such as 4-nitrocatechol and benzoquinone usuallyobserved when Feo is used for reduction were negligiblein the combined US/Feo system. They also found thesonolytic degradation of aniline to be a first-orderreaction 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 ultrasound-
catalyzed oxidation of various aryl alkanes (eg., toluene,nitro- and o-chloro-toluene, p-xylene) to the correspond-ing aryl carboxylic acids using aqueous KMnO4 underheterogeneous conditions and optimized parameterssuch as quantity of KMnO4, stirring rate and reactiontime. It was shown that heterogeneous oxidation ofunsubstituted aryl alkanes was significantly acceleratedusing aqueous KMnO4 at ambient temperatures and inthe presence of ultrasound. While under optimizedconditions o-chlorotoluene and mesitylene (1,3,5-tri-methylbenzene) were effectively converted into thecorresponding carboxylic acids, isopropyl benzene andtert-butyl benzene did not undergo oxidation and wererecovered unreacted. Nagata et al.91 investigated theaqueous sonochemical degradation of hydroxybenzoicacids such as monohydroxbenzoic acids (HBAs), 3,4-dihydroxybenzoic acid (3,4-DHBA), 3,4,5-trihydroxyben-zoic acid (gallic acid, GA), tannic acid (TA), and humic
acids (HA) under air or argon atmosphere. They foundthe pseudo-first-order constants for 2-HBA, 3-HBA, and4-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 and3.1 × 10-2 min-1 under air. However, the rate constantsof GA and TA were higher under air irradiation (5.5 ×10-2 and 16.4 × 10-2 min-1) compared with irradiationunder Ar (2.6 × 10-2 and 6.4 × 10-2 min-1). The rateconstant for 3,4-DHBA was the same under bothatmospheres (1.9 × 10-2 min-1). The decomposition of3-HBA was almost completely quenched in the presenceof t-BuOH at a concentration of 0.1 mM. They arguedthat the sonolytic degradation of the HBA under Aratmosphere proceeded mainly via reactions with OHradicals in the bulk solution. Whereas under air oxygenmolecules contributed to the decomposition of the poly-hydroxybenzoic acids at the interface, thermal decom-position in cavitation bubbles or interfacial regionsplayed a negligible role in both atmospheres. They alsosuggested that the participation of oxygen moleculesaccounted for the faster decomposition rates of GA andTA in air, the rates in air facilitated with an increasingnumber of OH groups (kTA > kGA) as expected withoxidation of phenolic compounds with oxygen. D-Silvaet al.92 observed the rapid destruction of polycyclicaromatic hydrocarbons (PAHs) and dioxin when solu-tions of benzo[a]pyrene (B[a]P), benzo[ghi]perylene,coronene, or 2-chlorodibenzo-p-dioxin in 20% ethanol-water at approximately 10-15 ppm levels were sub-jected to high-intensity sonication. It was shown thatthe sonolytic decomposition of PAH and dioxin wasaccompanied by the formation of a black carbonaceousresidues. It was also shown that the effect of ultrasoundon a 100% ethanol solution of B[a]P was not as pro-nounced as in water-ethanol mixture.
4.2. Chlorinated Aliphatic Hydrocarbons (CAHs).The sonochemical reactivity of chlorinated hydrocarbonsin aqueous solutions is attributed to their volatility andtheir low solubility in water, properties facilitating theirconcentration in the cavitation bubbles resulting inrapid decomposition by high temperature and highpressure. The sonication of 15 mL of neat chlorinatedsolvents 1,1,1-trichloroethane (TCA) and trichloroeth-ylene (TCE) produced a yellow color (presumably dueto elemental chlorine) in 10 min and in 2 h produced abrown suspension indicating the solvents have polym-erized due the treatment.92 Inazu et al.93 found thataqueous solutions of trichloroethylene (TCE), tetrachlo-roethylene (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 ultrasonicirradiation in the presence of argon, O2, or air. Theysuggested that the main reactions were thermal decom-position or combustion in cavitation bubbles and notreactions by OH radicals or H atom. It was also shownthat the main products of TCE degradation productsunder Ar were Cl-, CO, and H2. The minor products ofthe decomposition under Ar were CO2, CH4, C2H4, anda trace amount of dichloroethylene. Under the sonica-tion of O2 or air, considerable amount of CO2 evolved,and no CO was observed. Drijvers et al.94 identified thefollowing volatile products of TCE sonication at 50kHz: chloroacetylene (C2HCl), dichloroacetylene (C2Cl2),dichlorodiacetylene (C4Cl2), tetrachloroethylene (C2Cl4),two isomers of trichlorobutenyne (C4HCl3), tetrachlo-robutenyne (C4Cl4), pentachlorobutadiene (C4HCl5), and
-d[NB]
dt) kus[NB] + kFeo[NB] + k′[NB] )
(kUS + kFeo + k′)[NB] (11)
-ln[ [NB][NB]o] ) (kus + kFeo)t + 1
2kseckpt
2 (12)
4700 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
hexachlorobutadiene (C4Cl6). It was also shown that thedegradation rate of TCE was more energetically efficientat 520 kHz than at 20 kHz. Reinhart et al.95 studiedthe combined effects of sonication and zerovalent ironon the destruction rate of TCE and found ultrasound toincrease the rate significantly. For example, they ob-tained 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, respec-tively.
The main intermediates of the sonication of theaqueous carbon tetrachloride (CCl4) are tetrachloro-ethylene (C2Cl4) and hexachloroethane (C2Cl6) withtrace amounts of dichlorocarbene, dichlromethane,trichloroethylene, tetrachloroethane, pentachloroethane,hexachloropropene, hexachlorobutadiene, and chloro-form, and the final products are usually chlorine,chlorides (or HCl), chlorohydric acid, hypochlorous acid(HOCl), CO, and CO2.96-100 Jennings et al.97 investi-gated the sonochemical reactions of CCl4-H2O mixturesin an argon atmosphere and compared results withthose of CHCl3-H2O mixtures. They observed cavitationwhen CCl4 was sonicated alone, but there was noreaction. In the presence of as little as 1 mL H2O per100 mL of CCl4, reaction ensued, and the total inorganicchlorine yield was found not to be significantly sensitiveto large changes in the CCl4 to H2O ratios. The observedproducts from the CCl4 reaction included: CO2, O2, Cl2,HCl, C2Cl6, and C2Cl4 compared with products ofCHCl3: HCl, C2Cl6, and C2Cl4. Hua and Hoffman99
studied the sonolytic destruction of aqueous CCl4 in theabsence or presence of ozone (O3). The observed first-order degradation rates in argon-saturated aqueoussolutions were 3.3 × 10-3 and 3.9 × 10-3 s-1 with CCl4initial concentration of 1.95 × 10-4 and 1.95 × 10-5 molL-1, respectively. It was shown that the presence of O3did not affect the degradation rate significantly butinhibited the accumulation of C2Cl4 and C2Cl6 interme-diates. However, sonication of p-nitrophenol in Ar-saturation aqueous solution in the presence of CCl4resulted in the degradation rate enhancement of p-NPby a factor of 4.5 compared to sonication without CCl4and the reduction of 4-nitrocatechol, an aromatic deg-radation product. The improvement in rate and mini-mization of 4-nitrocatechol formation were shown to bedue to the presence of HOCl, a product of CCl4 sonolysis.
Koszalka et al.100 obtained 98% reduction of CCl4 inthe presence of H2, air, or Ar and observed the ultrasonicdestruction to be an exponential process. It was shownthat the initial reaction products: chloroform, dichlo-romethane, and chloromethane were further degradedto methane and chlorine/chloride. Total halogen analy-ses of unreacted and reacted CCl4-contaminated samplesalso confirmed Cl balance, indicating that CCl4 was notvolatilized or masked by the chemistry of the process.Wu et al.7 studied the destruction of CCl4 under dis-solved air and varied process parameters such as pH,irradiation, time, steady-state temperatures, and ultra-sonic intensity. It was shown that CCl4 destruction ratewas significantly affected by the intensity of the ultra-sonic energy, the rate increasing proportionally to theintensity. However, the addition of H2O2 as an oxidantwith or without ultrasonic irradiation was also foundto have negligible effect upon the rate. This was at-tributed to the predominance of the thermal dissociationreaction in the cavitation holes under supercriticalconditions compared to the bulk-liquid-phase reactions
involving CCl4 and oxidants such as OH radicals. Thebulk-liquid rate constants (e.g., 107 M-1 min-1) werefound to be about several times smaller than those inthe cavities (e.g., 1012 M-1 min-1). On the basis of adetailed chemical reaction mechanism and assuming abatch reactor, they described the destruction of CCl4 inwater by the following differential equations usingsecond-order rate constants and concentrations in thebulk-liquid phase and cavity respectively:
where M is any collision partner and kc is the systemadjustment coefficient, which is a function of bubbleconcentration, bubble radius concentration, mixing ex-tent of the system, etc. The results of this modelformulation via a series of elementary reactions werefound to be in general in agreement with experimentaldata. They obtained the value of kc to be constant at2.5 × 10-11 by fitting experimental data to the model,assuming constant vessel size, steady-state tempera-ture, and power. Hung and Hoffmann101 investigatedthe degradation of CCl4 by combined ultrasound (US)and iron (Feo) in Ar-saturated solutions and observed a40-fold enhancement in overall rate for the coupledsystem compared with US alone. The first-order ultra-sonic rate constant for CCl4 was 0.107 min-1 at 62 W(20 kHz) in 285 mL solution, whereas the apparent rateconstant in the presence of Feo was found to depend onthe total surface area of elemental iron as follows:
where kus ) 0.107 min-1, kFeo ) 0.105 L m-2 min-1, andAFeo is the reactive surface area of Feo in units of m2/L.Rajan et al.102,103 investigated experimentally and bymodeling of the water-KI-CCl4 system and attributedthe significant increase in oxidation rates observed ina suspension of CCl4 liquid in KI solution to the releaseof Cl2, Cl, and HOCl, which act as a separate source ofreactants to yield I2. They used a complex mathematicalmodel that included detailed kinetics of CCl4 to predictthe extent of degradation of dissolved CCl4 and the largeenhancements in the rate of oxidation of aqueous iodineion (I-) in the presence of CCl4. A similar study of theCCl4/KI (1.0 M solution) system by Alippi et al.9 foundthe yield of free iodine to increase linearly with soni-cation time.
Petrier et al.104 studied the sonolysis of chloromethaneand their dilute mixtures in alcohols under saturationwith Ar or O2 and found the zero-order rate constantsto be greater in Ar atmosphere. Since the sonochemicalcleavage of the alcohols were found to be weak, theinitial step in the transformation (faster in Ar) wasattributed to the C-Cl cleavage, in constrast to thereaction in water, where sonochemical cleavage of thesolvent plays a major role. They also explained that thesonolysis, probably in the cavitation bubble of CCl4 andwhich was more efficient in alcohols than in the purestate, could result from the preferential vaporization ofthis apolar molecule into a bubble formed in a hydrogen-bonded structured liquid. Toy et al.12,13 investigated the
-d[CCl4]dt
) kI[OH][CCl4] + kII[H][CCl4] +
kIII[HO2][CCl4] + kIV[O][CCl4] (13)
d[CCl4]dt
) kckv[CCl4][M] (14)
kobs ) (kus + kFeoAFeo) min-1 (15)
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4701
decomposition of aqueous 1,1,1-trichloroethane (1,1,1-TCA) under sonolysis, photolysis, and the synergisticcombination of the two and reported more extensivedecomposition with combined photolysis and sonolysis(photosonolysis) than UV photolysis or sonolysis alone.It was also shown that TCA (CCl3CH3) decomposed intogases, VOCs, and ionic species, and as the volume ofthe same concentration was increased, the sonolysisdigestion efficiency decreased. Cheung et al.8 examinedthe sonochemical destruction of methylene chloride(CH3Cl), TCA, TCE, and CCl4 and attributed the rapiddecrease of reactor solution pH in all cases to theprobable formation of HCl from the chlorinated reac-tants with the hydrogen source being water. Bhatnagarand Cheung10 studied the kinetic behavior of individualVOCs in the presence of other VOCs by sonicatingmixtures of (1) two chloromethanes (CH3Cl + CCl4) and(2) CH2Cl2, CCl4 (C1 compounds) + 1,1,1-TCA (C2compound). The first-order rate constants for the com-pounds were essentially unchanged by the presence ofother reacting species. Petrier and Francony105 com-pared the decomposition rates of CCl4 and phenol atdifferent frequencies (20-800 kHz) and observed thatirrespective of frequency, it was easier to decompose thehydrophobic and volatile CCl4 than the hydrophilicphenol with a low vapor pressure. For the phenol, theyfound that degradation proceeded faster at 200 kHz andthat the addition of an excess of 1-butanol (10-foldphenol initial concentration), used to scavenge hydroxylradicals totally inhibited the reaction. The rate ofdisappearance of CCl4 on the other hand was found toincrease slightly with increase in frequency, and theaddition of 1-butanol (10-fold CCl4 initial concentration)did not affect the rate of CCl4 destruction. It was alsoshown that both phenol degradation and H2O2 forma-tion involving reactions with OH radical at the interfaceincreased with frequency reaching an optimal value at200 kHz. Drijvers et al.106 investigated the mutualinfluence of TCE and chlorobenzene by sonicating diluteaqueous mixtures of the two volatile organic compoundsand found the sonolysis rates to depend strongly on theinitial concentration of TCE and CB. The presence ofCB was found to lower the sonolysis rate of TCE, whilethe sonolysis rate of CB (with initial concentration 3.44mM) was enhanced by the presence of 0.84 mM TCE.They attributed the different effects of both volatileorganics on each other’s sonolysis to the difference inreaction rate of TCE and CB with the radicals formedduring sonolysis. They suggested that the effect of TCEon the sonolysis rate of CB (i.e., the lowering of the γvalue of the gas present in the cavitation bubble andconsequently the temperature during collapse) wascompensated by an increased indirect degradation of CBby radicals formed out of TCE.
De Visscher and Van Langelove107 explored the influ-ence of Fenton-type oxidants on the aqueous sonochemi-cal degradation of trichloroethylene (TCE), o-chlorophe-nol (o-CP), and 1,3-dichloro-2-propanol (DCP). It wasshown that di-tert-butyl peroxide inhibited the sonolysisof both TCE and CP by scavenging OH radicals and bylowering the maximum temperature generated duringa cavitation event. However, 10 mM tert-butyl hydro-peroxide had no considerable effect on the sonochemicaldegradation of TCE but inhibited completely the deg-radation of o-CP. The effect of TCE was interpreted tomean that the amount of tert-butyl hydroperoxide in thecavitation bubbles (where TCE is degraded) was too low
to change the conditions of a cavitation event signifi-cantly. The effect on o-CP indicated that tert-butylhyroperoxide scavenged OH radicals without producingany radicals that entered the liquid phase where oxida-tion was expected to occur. Unlike the less polar, organicperoxides, the presence of H2O2 did not affect thedegradation of TCE and DCP but enhanced the degra-dation of o-CP on the order of 20-25%. The degradationof both o-CP and DCP (with low Henry’s constant) areexpected to occur by radical attack. Hence, they at-tributed the low reactivity of DCP toward OH to thelack of aromatic rings on which the radicals can addand to the presence of Cl atoms on DCP that could leadto electronic repulsion toward the OH radical. They alsoobserved that in the presence of H2O2 and copper ions,the sonochemical degradation of o-CP is the sum of theeffect of the ultrasound and the chemical oxidation effectand likened the overall degradation mechanisms tosonochemical switching effect. That is, sonication in-duces a specific reactivity and silent chemical, andsonochemical degradations follow separate paths. TheCAV-OX has been shown to degrade TCE and BTEXin contaminated groundwater with efficiencies betterthan 99.9% when varied principal opearting parameterssuch as H2O2 dose, pH, and flow rate are used.14 Theprocess involves the use of sequential hydrodynamiccavitation and direct UV photolysis of H2O2 to generatehydroxyl and hydroperoxyl radicals. Orzechowska etal.108 monitered and compared the chloride releasetendencies of CCl4, CHCl3, TCE, aromatic chloro com-pounds (e.g., CB, polyaromatic chloro compounds, PCBs,4,4′- and 3,3′-dichlorobiphenyl) and TritonX-100 (a sur-factant) upon sonication and found that CB and PCBsdid not release Cl- as readily as did CCl4, CHCl3, andTCE. They attributed the results to possible hydroxy-lation reactions at an open position of aromatic ring,i.e., oxidation of PCBs and CB by OH radical withoutdehalogenation. The presence of humic substances wasalso found not to affect Cl- yields. Catallo and Junk109
studied the sonochemical transformation of hazardouschlorinated chemicals and a mixture of chlorinatedolefins, paraffins, and aromatics from a Louisianasuperfund site (LSS) and observed dechlorination viaaryl C-Cl bond heterolysis and hydroxylation reactionsto be the main reaction pathways. Dechlorination and/or other transformations were observed for chlorinatedalkanes (e.g., CCl4) 2,4-dichlorophenol, chlorpyrifos, andlindane (hexachlorocyclohexane, γ-isomer) and a forrange of chlorinated aromatic and aliphatic chemicalsin the LSS mixture. The herbicide byproducts, chlorpy-rifos, 3,3′,4,4-tetrachloroazoxybenzene (TCAOB), andtrichlorodibenzofuran (TCDF), were not dechlorinated.However, TCAOB appeared to deoxygenate sonochemi-cally to the 3,3′,4,4 tetrachloroazobenzene. Sakai et al.110
studied the sonochemical degradation of aqueous solu-tions of chloral hydrate (CCl3CH(OH)2 or simply CH)and found the rate to be proportional to the square ofoxygen concentration at low concentrations, while atrelatively high concentration, the rate is independentof the oxygen concentration. They also found the reac-tion rate to be proportional to the square root of theconcentration of chloral hydrate and to the intensity ofthe ultrasound regardless of the concentration of oxygengas dissolved in the solutions.
4.3. Explosives. Sierka111 investigated the treatmentof aqueous solutions of trinitrotoluene (TNT) and cy-clotrimethylene-trinitramine (RDX) in the weight ratio
4702 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
of 70:30 (typical of munitions wastewater) at 25-59 °Cusing a combination of ozone and ultrasound in the pHrange of 5.84-10.00. They demonstrated the synergisticeffects of both ozone and ultrasound. While the maxi-mum destruction of both TOC and TNT was observedat 859 kHz and 50W, experiments performed withoutO3 at these same conditions resulted in no TNT or TOCremovals. The increased removal with O3/ultrasoundsystems was explained partially by the increased tem-perature in the reactor. While at 25 °C, the TNTdestroyed after 60 min of ozonation was 23%; 99% wasdestroyed when temperature averaged 59 °C. Removalrates of both TNT and TOC increased directly withincreases in reaction temperature and initial pH. How-ever, it was shown that at solution pH values main-tained in excess of 9.6, the combination of high pH andtemperature was not beneficial. This was attributed tothe fact that ultrasound inhibited kinetics at hightemperatures and pH by promoting radical-radicalextinguishment reaction. But ultrasound was also shownto have an additional benefit potential in that it can actas a catalyst by enhancing the autodecomposition of O3to free radicals once the ozone has been dissolved in theaqueous phase. Sonolysis was evident in the degrada-tion of residues of the military explosives TNT (2,4,6-trinitrotoluene) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), as indicated by release of nitrate(NO3
-) during treatment.73 Hoffmann and co-work-ers16,58 112 investigated the sonochemical degradation ofTNT at 20 and 500 kHz to acetate, formate, glycolate,oxalate, CO2, NO2
-, and NO3- as products with 1,3,5-
trinitrobenzene as intermediate and found the overalldegradation rate to follow an apparent first-order ratelaw. The overall reaction stoichiometry for the autooxidation of TNT was found to be:
where ))) denotes ultrasonic irradiation. They foundTNT degradation to be more efficient at 500 kHz than20 kHz, irrespective of the background gas. The first-order kinetic rates vary by a factor up to 3, dependingon the nature of dissolved gases and also on theultrasonic 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 com-pared 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 thesystemic herbicide, chlorpropham (isopropyl-3-chloro-carbanilate) in air equilibrated aqueous solutions leadto the formation of hydroxylated products and 3-chlo-roaniline as intermediates, and subsequent mineraliza-tion of intermediates to Cl-, NO2
-, NO3-, CO, and CO2.
David et al.113 observed that the ultrasonic treatmentof chlorpropham at 482 and 20 kHz led to a highlydrastic degradation with the initial rate at the highfrequency (13.0 × 10-8 M s-1), much higher than thatat the low frequency (4.2 × 10-8 M s-1). They alsonoticed a complete degradation after 45 min at 482 kHzcompared with 1/3 of chlorpropham remaining after 60min at 20 kHz. It was also shown that the yield of Cl-
was about 98% after 80 min at 482 kHz, indicating theC-Cl bond cleavage is a major primary process. Twopathways 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 thehydroxyl radical in the degradation of chloropropham.Petrier et al.114 investigated the ultrasonic degradationof atrazine, CIET [2,4-diamino-6-chloro-N-ethhyl-N′-(1-methylethyl)-1,3,5 triazine] and pentachlorophenol (PCP)at 500 kHz and 20 kHz and demonstrated in both casesthat degradations were more efficient and complete atthe higher frequency. Sonochemical degradation ofatrazine resulted in dealkylation product: CIAT [2,4-diamino-6-chloro-N-(1-methylethyl)-1,3,5 triazine],CEAT[2,4-diamino-6-chloro-N-ethyl-1,3,5 triazine], andCAAT[2,4-diamino-6-chloro-1,3,5- triazine], which aretypical oxidation products of atrazine from degradationby ozone, Fentons reagent, or TiO2 photocatalysis. Longirradiation times (3 h) resulted in the formation of CO2(13% of initial atrazine) and chloride. They observed fastreactions for PeCP (10-4 M aqueous solution at pH 7)at both frequencies, yielding 98% of the theoreticalamount of chloride and 20% theoretical carbon as CO2recovered after 180 min for the reaction conducted at500 kHz. However, the addition of n-pentanol (10 mM)inhibited completely both atrazine and PeCP degrada-tion at high and low frequency, suggesting reactionswith OH• radicals escaping from the cavitation bubble.Koshkinen et al.115 found the sonochemical degradationof 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 constantsand extrapolated half-lives were 8.01 × 10-3 min-1 and86 min and 2.1 × 10-3 min-1 and 330 min for alachlorand atrazine, respectively. They also attributed thedecomposition of both atrazine and alachlor to theattack by hydroxyl free radicals. Nagata et al.74 foundthe aqueous sonochemical degradation of 0.1 mM pen-tachlorophenol (PeCP) to be faster under argon atmo-sphere compared with air atmosphere (first-order rateconstant of 6.4 vs 4.5 µM min-1).
Gondrexon et al.116 studied the sonochemical degra-dation of PeCP in a three-stage, continuous flow,laboratory-scale ultrasonic reactor and obtained a con-version up to 80%. Johnston and Hocking75 investigatedthe photocatalytic and photolytic degradation of PeCP(2.4 × 10-4 M solution) with and without sonication.They observed the initial rate of chloride formation dueto photocatalysis using TiO2 to be about 2.7 times fasterwith sonication than without. In the absence of sonica-tion, the percent degradation measured by Cl- releasereached a value of 40% after 50 min, beyond whichcontinued irradiation up to 200 min did not significantlyaffect increase in degradation. In contrast, combinedsonication, UV, and photolysis resulted in a rapid initialdegradation, with near quantitative release of chlorideafter 120 min. They attributed their observation topossible poisoning of catalyst active sites during pho-todegradation by inert intermediates formed but whichare removed from active sites by sonication, allowingfurther degradation or decrease in pH during photolysis,leading to significant decrease in a photochemicaldegradation process. Petrier et al.117 also studied thesonochemical degradation of pentachlorophenate (PCP)at 530 kHz in aqueous solutions saturated with air,oxygen, or argon and observed the degradation and theresulting toxicity decrease to be faster under argonbubbling (PCP disappeared in 50 min) than under airor oxygen. The ultrasonic degradation of PCP was alsoattributed to a rapid cleavage of carbon-chlorine bond
C7H5(NO2)3 + 9O2 98)))))
7CO2 + H2O + 3H+ + 3NO3- (16)
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4703
to release Cl- (90% chlorine recovered as Cl- in 150min). This was followed by degradation to CO2 in air-or oxygen-saturated solutions (or CO in argon-saturatedsolutions) and nitrite and nitrate in solutions aeratedwith air or oxygen. They also postulated that becauseof the high temperature inside the cavitation bubble,the formation of CO in the presence of argon was dueto reactions of the type:
Wheat and Tumeo118 studied the aqueous sonolysis oftwo polycyclic aromatic hydrocarbons or PAH’s (phenan-threne and biphenyl), and the results obtained suggestpolychlorinated biphenyls (PCB’s) may be susceptibleto hydroxyl radical substitution reactions under thevigorous reaction conditions supplied by high-intensityultrasound. The principal products of biphenyl reactionidentified through mass spectral analysis were [1,1-biphen]-2-ol, [1,1-biphen]-3-ol and [1,1-biphen]-4-ol, andthe principal products of phenanthrene appeared to bedi-hydroxy substituted phenanthrene. The phenolicproducts produced from these hydrophobic compoundswere considered consistent with a free radical attackby hydroxyl free radicals (OH•) on the hydrocarbonskeleton. Zhang and Hua119 also observed that poly-chlorinated biphenyls (2-, 4-, and 2,4,5-chlorobiphenyls)dechlorinated rapidly to chloride ion during sonolysisat 20 kHz with 99% destruction of 2-PCB, 4-PCB, and2,4,5-PCB in 36, 47, and 29 min, respectively. Thechloride recovery ratio defined as [Cl-] /([M]i - [M]f) ×n, where n is the number of chlorine atoms per cogenerin each sample and [M]i and [M]f are initial and finalconcentrations 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 identifiedproducts such as biphenyl, toluene, ethylbenzene, phe-nol, trichlorophenol, hexenyldiphenol, and other phenols(Table 3). They investigated the role of aqueous hydroxylradical in the decomposition of 2-PCB at 205, 358, 618,and 1071 kHz. It was shown that ultrasonic irradiationwas optimal at 358 kHz but chloride recovery wasoptimal at 1071 kHz. They proposed that both ther-molysis and free radical attack are important pathwaysof PCB destruction but that free radical attack (espe-cially •OH) in the aqueous phase played a more signifi-cant role at 358 kHz than at other three frequencies.Johnston and Hocking75 investigated the photo catalytic/photolytic degradation of PCB isomer 3-chlorobiphenylat 75 ppm level (4 × 10-4 M) by monitoring the chlorideformation. TiO2 (0.2% w/w) and 30 mL aliquots weresubjected to UV irradiation and to combined UV/ultrasound irradiation. They found a linear rate ofappearance of chloride with time for both conditions,but rates with sonication were approximately 3 timesgreater that without sonication, and they attributed theenhanced rates partially to the highly hydrophobicnature (i.e., low solubility) of substrate. They alsoattributed the significant increase in degradation ratesand efficiency of the concurrent UV/ultrasonic irradia-tion to cavitational effects, bulk, and localized masstransport effects and sonochemical reactions.
Kotronarou et al.120 showed that the sonolysis ofparathion (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 proceededvia thermal decomposition in the hot interfacial regionof the collapsing bubbles with secondary reactions with•OH radicals. Theron et al.121 determined the removalrates of phenyltrifluoromethyl ketone (160 µmol L-1
PTMK) by sonolysis at 30 and 515 kHz, by UV-irradiated TiO2 (Degussa P25 and Rhodia), and bysimultaneous photocatalysis and ultrasonic irradiations.The PTMK first-order removal rate constant was foundto be 14 times greater at the higher frequency comparedwith that at the lower frequency at the same energylevel and 2.5 times higher when synthesized RhodiaTiO2 was used instead of Degussa P25. The amount oftrifluoroacetic acid (CF3COOH or TFA), an undesirableand stable product of TiO2-treated CF3-containing pol-lutants, was about 8 times lower in sonicated solutionsthan in UV-irradiated suspensions, for both frequencies.
4.5. Organic Dyes. Vinodgopal et al.122 studied thesonochemical degradation of a reactive black dye (Azodye, Remazol Black B or RB) at high frequency (640kHz) in O2-saturated aqueous solution. They foundsonolysis as a feasible method to achieve both decolori-zation and 65% degradation of the dye in 6 h asmeasured by the decrease in the in total organic content.The pseudo-first-order rate constant for the disappear-ance of the dye was determined to be 2.9 × 10-2min-1.They also observed that degradation stopped far shortof completion when performed in an aqueous solutionof about 2% tert-butyl alcohol, an excellent OH radicalscavenger, and attributed the conversion of the dye tooxalate (C2O4
2-) to •OH radical-initiated oxidative deg-radation. They also compared photocatalytic and radi-olytic methods of degradation to sonolysis. Their resultssuggested that sonochemical degradation rates of Azodyes were substantially faster and led to better destruc-tion of the dye compared to photocatalysis. Stock et al.123
investigated the degradation of an azo dye, naphtholblue black (NBB), using sequential combination of high-frequency sonolysis and photocatalysis in four differentconfigurations: (1) sonolysis only, (2) photocatalysisonly, (3) simultaneous sonolysis and photocatalysis, and(4) sequential sonolysis and photocatalysis. They ob-served that for individual techniques, sonolysis waseffective for inducing faster degradation of the parentdye, while TiO2 photocatalysis was effective for promot-ing degradation. For example, photocatalysis was re-sponsible for 68% conversion compared to 35% forsonolysis after 12 h. It was also shown that the first-order rate was enhanced by the combined systems(simultaneous and sequential). For example, pseudo-first-order rate constant was 1.83 × 10-2 min-1 for thecombined case compared with 1.04 × 10-2 min-1 forsonolysis or 0.56 × 10-2 min-1 for photocatalytic experi-ments.
4.6. Organic and Inorganic Gaseous Pollutants.Cheung and Kurup124 investigated the sonochemicaldestruction of fluorotrichloromethane (CFC11) and tri-fluorotrichloroethane (CFC113) in dilute aqueous solu-tions in a batch and circulating reactor. They found therates to be fairly rapid with less than 5% of the originalCFC’s undergoing volatilization and the bulk of materialdestroyed in the liquid phase. In these preliminarystudies, they also observed the rates to be slightly higherat 5 °C than at 10 °C and rapid drops in pH (initial valueof 7.4 to about 5.4), indicating the formation of acidicspecies. Hirai et al.125 evaluated a number of CFC’s and
CO2 + Ar f CO + O + Ar and
CO2 + •H f CO + •OH
4704 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
HFC’s and found them to be readily decomposedsonochemically with high efficiency to inorganic prod-ucts (CO, CO2, Cl-, F-). It was also shown that CFC-113 decomposed faster under argon atmosphere (withhigher CP/Cv ratio) than air atmosphere and that itsdecomposition rate was not significantly affected by theaddition of tert-butyl alcohol (tert-BuOH), a knowneffective OH radical scavenger. Hart and Henglein126
studied the sonochemistry of Ar/N2O mixtures and thepure gases in aqueous solutions at high frequency, 300kHz, and observed products of decomposition of N2O tobe N2, O2, NO2
- (or HNO2), and small amount of NO3-
(or HNO3). It was also observed that the maximum yieldof decomposition occurred at an Ar/N2O vol % ratio of85:15. However, the rate of N2O decomposition was verylow when water was sonicated under pure nitrous oxide,as expected with a triatomic gas. The yield of HNO2 wasmuch higher than observed in the irradiation of waterunder air and under mixtures of nitrogen and argon,and this was attributed to the decomposition of N2O ofthe type: 3N2O + H2O f 2HNO2 + 2N2. Henglein127
studied the ultrasonic irradiation of water under argonatmosphere containing small amounts of CO2, N2O andCH4. They observed no chemical effects in the irradia-tion under atmosphere of pure CO2, N2O, or CH4.However, they showed that sonochemistry in an argonatmosphere containing small amount of any of thepolyatomic gases was characterized by a strong linkagebetween the sonolysis of water and the added gas. Themain products of the sonolysis of CO2 were CO andsmall amount of formic acid, about 30 times smallerthan CO yield. The main products of N2O sonolysis wereN2, NO2
- and NO3- in this study.127 In the N2O-Ar
containing solutions, they observed the yields of theproducts NO2
- and NO3- were about 10 times greater
than the amounts formed in the ultrasonic irradiationof aerated water and greater than the amounts in waterirradiated in the presence of both N2O and O2. Hara-da128 investigated the sonolysis of CO2 dissolved inwater (0.0-0.6 mole fraction CO2 in water) at 5-45 °Cusing an ultrasonic generator (200 kHz, 200 W) and Ar,He, H2, and N2 as the irradiating gases. While irradia-tion of ultrasound had hardly any sonochemical effectunder CO2 atmosphere, the most favorable atmospherefor reducing was CO2-Ar mixture with 0.03 molefraction of CO2 in gas phase, a concentration consideredan equal mixture of CO2 and Ar in water because CO2is more soluble than Ar. The amount of CO2 in CO2-Ar was found to decrease to about half at 5 °C, and thedecreasing rate for CO2 followed the order Ar > He >H2 > N2. They also observed that dissolved CO2 wasdeoxidized to carbon monoxide (CO2 f CO + O) bysonolysis and showed that the hydrogen carbonate orbicarbonate ion (HCO3
-) played an important rolecompared to the carbonate ion (CO3
2-). Since hydrogenis also obtained from the solvent (water) and both COand H2 are fuel gases, which also react to produce C1compounds such as methanol, they proposed sonicationas a useful and potential technique to reduce CO2 andproduce fuel.
The main oxidation products of the methane-contain-ing 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 largeamounts of H2. Hart et al.129 also identified H2, C2H2,C2H4, and C2H6 as major products and C3-C4 hydro-carbons (e.g., propane, propene, 2-methylpropane, n-
butane, 1-butene, 2-methyl,1-butane and butadiene) andCO as other products of CH4 sonolysis under argonatmosphere. They found the products for the sonolysisof the C2H6 to be the same as CH4 except that they wereproduced in higher yields. Hart et al.130 also observedthe sonochemical degradation of acetylene to be rapid(90% consumption of C2H2 at time g 80 min) withformation of large products. In addition to small gasmolecules such as CO and CH4, various hydrocarbonswith intermediate C atom numbers, such as benzeneand other C6H6 isomers, high C atom numbers such asnaphthalene are formed.
Kotronarous et al.131 found the ultrasonic irradiationof H2S or S(-II), [[S(-II) ) [H2S] + [HS-] + [S2-]]solutions to result in rapid oxidation and the formationof sulfate (SO4
2-), sulfite (SO32-), and thiosulfate (S2O3
2-)as products at pH g 10 with the initial mole ratio of2.2:2.7:1, 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 oninitial concentration of S(-II) and increasing linearlywith [S(-II)]O up to [S(-II)]O = 450µM (where kO = 12µM min-1) and remaining constant thereafter. However,at low pH e 8.5 (e.g., pH 7.4), the rate of ultrasonicoxidation of S(-II) was first order with respect to [S(-II)], the rate of oxidation increased with a decrease inpH, and measured concentrations of SO4
2-, SO32-, and
S2O32- could not account for the total observed decrease
in S(-II), especially at the short sonication times. Thediscrepancy between the [Sox] measured and the [Sox]expected was attributed to the formation of sulfur (S8)that was not analytically measured but increased theturbidity of the solution. The apparent zero-orderdependence on [S(-II)] suggested that the rate-deter-mining step (or the main pathway) in the overallreaction was the reaction of HS- and the oxidationintermediates with •OH radical in the liquid phase asit diffused out of the cavitation bubble (HS- + •OH fHSOH-) and the observed decrease in the zero-orderrate constant at the pH > 10 was attributed partly tothe dissociation of •OH in the alkaline solutions andsince the oxide radical ion (•O-) reacts more slowly withthe same substrate than •OH according to the reaction:•OH +OH- S O•- + H2O, where the forward rate, kf )1.2 × 1010m-1s-1 and the backward rate, kb ) 9.3 ×107s-1. At pH e 8.5, thermal decomposition of H2Swithin or near collapsing cavitation bubbles was con-sidered the important reaction pathway. Kotronarouand Hoffmann132 also developed a comprehensive aque-ous-phase mechanism for the free-radical chemistry ofthe S(-II) + ‚OH + O2 system and used the mechanismto successfully model the oxidation of S(-II) at 20 kHzand 75 W/cm2 and at pH g 10, assuming a continuousand uniform ‚OH (3.5 µM/min) input into the solutionfrom the imploding cavitation bubbles.
When water was irradiated with ultrasound under anatmosphere containing O2 and O3, an extremely rapiddecomposition of O3 was found to take place, leading toa yield of H2O2 up to a factor of 6 higher than in watercontaining oxygen only.133 Weaver and Hoffmann3 in-vestigated the sonolytic degradation of O3 aqueoussolutions at pH 2 using both closed and open continuousflow systems and the mass transfer mechanisms occur-ring in sonolytic ozonation and their effects on chemicalreactivity. They found the sonochemical degradation ofO3 to follow apparent first-order kinetics with typicalrate constants of 0.84 ( 0.07 and 0.66 ( 0.08 min-1,
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4705
respectively, at 20 and 500 kHz and an initial O3concentration of 245 ( 3 µM. It was also shown thatboth the sonochemical degradation of O3 and theincrease in O2/O3 gas flow rate enhanced mass transferat 20 kHz. This effect was attributed partially toturbulence induced by acoustic streaming.
4.7. Organic Sulfur Compounds. Spurlock andReifsneider96,134 investigated the ultrasonic irradiationof diakyl sulfides at 20 °C using 280-, 610-, and 800-kHz transducers and 5-11 W/cm2 intensities underoxygen and argon atmospheres and in suspensions inpure water and in saturated aqueous solutions of CCl4.The principal products of the irradiation of 0.06 Maqueous suspension of di-n-butyl sulfide (R2S) at 800kHz and 9.4w/cm2 were di-n-butyl sulfoxide (R2SO) andn-butyl sulfonic acid (RSO3H) in the yield ratios of 85/15 and 80/10 in O2 and Ar, respectively. Minor productsobserved included di-n-butyl sulfone (R2SO2), butyricacid, carbon monoxide, ethylene, acetylene, and meth-ane.
Entezari et al.135 sonicated 50 mL of pure carbondisulfide (CS2) at 20, 50, and 900 kHz frequencies underthe atmosphere of different gases (Ar, H2, Air, He, O2,CO2) in a temperature range of -50 to +10 °C. Theyobserved that ultrasonic sonication at 900 and 50 kHz(up to 4 h) led to no visible or chemical changes due tothe sonication while irradiation of CS2 at 20 kHzfrequency resulted in the formation of heterogeneousmixture of amorphous carbon and monoclinic sulfur. Thedissociation of CS2 was proposed to be due to theextreme conditions resulting from the collapse of thecavity bubbles, the low thermal conductivity of CS2ensuring the high-temperature persisted long enoughto result in the breakage of the C-S bond to formelemental carbon and sulfur. The dissociation rate wasalso found to be rapid and efficient at lower tempera-tures and higher intensities but at the same power(49W) and under helium and hydrogen gas than underair, argon, or oxygen. The fact that the sonochemicalactivity was higher in He than in Ar (with lower heatconductivity) was attributed to other properties of gases,especially solubility. Adewuyi and Collins136,137 studiedthe kinetics of the sonochemical degradation of aqueouscarbon disulfide in a batch reactor at 20 kHz and theeffects of process parameters (e.g., concentration, tem-perature, ultrasonic intensity) on the degradation pro-cess. They found the reaction rate to be zero-order inthe temperature range 20-50 °C and first-order at lowertemperature. The zero-order rate constant for thedegradation at 20 °C, at 14 W, and in air was 2.27 ×10-5 min-1. At the same initial concentrations andtemperature of 20 °C and in the presence of air, thedegradation rate of CS2 at 50 W (39.47 W/m2) was morethan 2 times that at 14 W (11.04 W/m2). The rate ofsonochemical degradation of CS2 in the presence of thedifferent gases was on the order of He > Air > N2O >Ar; the rate with helium was found to be about 3 timesthat of argon. They found the formation of sulfate as aproduct to be enhanced at the lower pH and also atlower temperatures when solutions from experimentsin the pH range 8-11 and temperatures 5-50 °C wereanalyzed.
4.8. Oxygenates and Alcohols. Kang and Hoff-mann138 studied the sonolysis, ozonolysis and the com-bined sonolysis/ozonation of methyl tert-butyl ether(MTBE) in the concentration range of 0.01-1.0 mM anddemonstrated that the addition of O3 to the influent O2
gas ([O3]0 ) 0.26-0.34 mM in solution) accelerated thedegradation of MTBE by a factor of 1.5-3.9, dependingon the initial concentration of MTBE. The sonochemicalfirst-order degradation rate constant for the loss ofMTBE 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 to0.01 mM (90% conversion in 45 min). The O3-ultrasoundsystem was shown to effectively degrade the MTBE intoinnocuous and biodegradable products with tert-butylformate (TBF), tert-butyl alcohol (TBA), methyl acetate(MA), and acetone identified as primary intermediatesand byproducts of the degradation reaction. Acetone,which was formed from the oxidation of TBF (degrada-tion rate constant, kTBF ) 1.87 × 10-3s-1) and TBA, wasfound as the intermediate product with the highest yield(12%). Using measured values of MTBE and TBF rateconstants, a reaction mechanism involving tree parallelpathways: (1) direct pyrolytic decomposition of MTBE,(2) direct reaction of MTBE with O3, and (3) reaction ofMTBE with •OH radical were proposed and used toobtain the following kinetic rate expression:
where kpyr, kI, kII, and kIII are the pseudo-first-order rateconstant for the direct pyrolysis, thermal degradationleading to O-CH3 bond breakage, direct reaction withozone, and other direct routes to products formation,respectively.
Koike139 studied the sonolysis of alcohol-water mix-tures using CH3OH, C2H5OH, n-C3H7OH, and i-C3H7-OH 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 atalcohol composition of about 20 vol % in the watersolution and to decrease thereafter for each of thealcohol-water mixture. Buttner et al.140 studied thesonolysis of water-methanol mixtures under argon andoxygen and obtained similar results. With increasingmethanol concentration the product yields first in-creased as methanol is a reactant and decreased withincreasing concentration leading to a maximum in theyields versus composition curves. In solutions containingmore than 80% methanol, they observed almost nochemical reactions. They detected typical pyrolysis andcombustion 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 ofaqueous solutions of ethanol (as volatile and solublesolute), poly(vinylpyrrolidine) PVP (as nonvolatile sol-ute), and tetranitromethane TNM (as volatile andalmost insoluble solute) under Ar irradiation (at 300kHz and 2W/cm2). They explained the formation ofproducts 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 closeto the cavitation bubbles.
4.9. Other Organic Compounds. Suzuki et al.142
examined the effect of ultrasound (200 kHz, 200W) at298 K on the degradation rate of the surfactant, poly-oxyethylene-alkyl ether (C14H29O(CH2CH2)7H), underthree different conditions: ultrasonic irradiation only(sonoprocess), photocatalytic reaction using TiO2 (0.25
-d[MTBE]
dt) (kpyr + kI + kII + kIII)[MTBE] )
ko[MTBE] (17)
4706 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
g/mL of P25) and low-pressure mercury (20W, 253.7 nm)lamp (photoprocess), and the combination of the twoprocesses (photosonoprocess). They observed the decom-position rate in the photosonoprocess was faster thanthe photoprocess or sonoprocess alone. A significantacceleration in the degradation of TOC was also ob-served when stirring speed was increased from 100 to500 rpm. The significantly enhanced rate in the pho-tosonoprocess compared to the sonoprocess alone wasattributed to the redispersion of the agglomeratedcatalyst particles under ultrasonic irradiation. They alsosuccessfully separated solid catalyst particles agglomer-ated by ultrasonic irradiation at 28 kHz in the presenceof glass beads.
Hart and Henglein143 evaluated the sonolysis ofmixtures of formic acid and water at concentrations of0-30 M. The main reaction products under an argonatmosphere were H2, CO2, CO, and very small amountsof oxalic acid. Hart and Henglein144 also studied theirradiation of aqueous solutions of potassium iodide andsodium formate under the atmosphere of argon, oxygenand argon-oxygen mixtures of varying composition. Thesono products of formate solutions were H2O2 and CO2in the presence of O2 and H2, CO2, and oxalate in theabsence of O2. Gutierrez et al.145 found the products ofsonolysis of acetate to include succinic acid (as a productof attack of OH radicals on acetate), glyoxylic andglycolic acids, smaller amounts of HCHO, CO, and CO2,and CH4 as a minor product.
Thymine (Thy) was found to degrade sonochemicallyin aqueous solutions at 450 kHz into six products: cis-and trans-5, 6-dihydroxy-dihydrothymine (cis- and trans-thymine glycols), 5-hydroxymethyluraacid, urea, andpossibly N-formyl-N′-pyruvelurea plus an unidentifiedproduct.41 Mead et al.146 also found the degradation tovary linearly with sonication time with a zero-order rateconstant of 1.8 ( 0.3 × 10-5 M.min-1. Sehgal andWang41 found that at an aeration rate of 50 mL/min andat 34 °C, the concentration of thymine is reduced to halfits value in 30 min, implying that an average of 3 ×10-8mol or 1.8 × 1016 molecules of Thy reacted persecond per liter. The studies also indicated there was achange in kinetics of reaction of Thy from first to zeroorder as temperature of solution changed from 0 to 50°C, and they suggested that the sono reaction took placein the bubble-liquid interphase.
4.10. Other Environmental Applications. Ingaleet al.147 investigated the degradation of a refractorycomponent in the industrial waste of a cyclohexeneoxidation unit using a hybrid system, namely, sonicationfollowed by catalytic wet oxidation (SONIWO). It wasshown that sonication in the presence of CuSO4 ascatalyst resulted in an accelerated degradation of theunknown compound which was refractory to wet oxida-tion at 225 °C compared with sonication without acatalyst.
Kruger148 investigated the sonolysis of natural ground-water containing 1,2-didhloroethane (1,2-DCA) as themain organic contaminants (350 µg/L), and trichlo-romethane, cis-1,2-dichloroethane, trichloroethane andtetrachloroeathane, trichloroethene, tetrachloroethenein trace amounts (total of ∼85 µg/L), and Fe (3-17 mg/L) and Mn (0.4-0.9 mg/L) as the main inorganicproducts. They observed almost complete destruction of1,2-DCA in all cases after 60 min and pseudo-first-order
rate constants of 0.062, 0.063, and 0.044 min-1, for thesonication at 361, 620, and 1086 kHz respectively at105W.
Olson and Barbier149 studied the effectiveness of the“sonozone” process (i.e., combined ultrasound ozonolysis)in degrading refractory electrolytes such as humicmaterials using purified fulvic acid, FA (as a substrate)and naturally colored groundwater sample with pH 8.6.The combined system was found to significantly en-hance TOC removal and mineralization rates. Sierkaand Amy150 also studied the singular and combinedeffects of ultraviolet (UV) light and ultrasound (US) onthe ozone (O3) oxidation of humic substances, the mostimportant of the trihalomethane (THM) precursors.They found that the combination of O3-US-UV provedto be the most effective reaction condition, followed byO3-UV, O3 alone, and O3-US, providing 93%, 86%,75%, and 71% reduction in trihalomethane formationpotential (THMFP) levels, respectively, in the reactiontime of 20 min.
Gonze et al.151 investigated the use of the ultrasonicprocess as a preoxidation step before a classical biologi-cal treatment used for further mineralization. Theysimultaneously monitored the toxicity for sodium pen-tachlorophenate solution (NaPCP) on marine bacteria(Vibrio fischeri) and on daphnids (Daphniamagna), andthe biodegradability of the pollutant solution duringultrasonic irradiation at 500 kHz. The pollution degra-dation was found to follow an apparent first-orderkinetic:
i.e., the disappearance rate is proportional to the powerdensity applied, and the kinetic pseudo first-order rateconstant, kth (m3 J-1) is representative of ultrasonicefficiency. In general, they demonstrated that ultrasonicirradiation decreased the toxicity of NaPCP and couldbe considered a preoxidation step without which theNaPCP could not be degraded by activated sludge forup to 28 days.
Dahi152 studied the ozonation process with and with-out simultaneous sonication at 20 kHz in regard to thedisinfections of microorganism (Escherichia coli) andoxidation of organic (e.g., Rhodamine B). The ultrasonictreatment was found to intensify the action of ozone inthe oxidation of chemicals and in the inactivation ofmicroorganisms. The enhanced performance of thesimultaneous sonozonation process was attributed to (1)sonochemical degradation of O3, causing augmentationof activities of free radicals in water, and (2) increasein the aeration parameter (kLa value) and intensifiedmass transfer resulting from sonication. Phull et al.153
reported the biocidal effects of ultrasound alone (at 20-25 kHz and 800 kHz) or in conjunction with chlorineon microorganisms. Ultrasound was found to signifi-cantly amplify the biocidal effects of normal chlorinationand to reduce the amount of chlorine required fordisinfections.
5. Discussion
As reviewed above, the results of most studies seemto demonstrate that while ultrasound is effective indegrading pollutants, total mineralization is difficult toobtain with ultrasound alone, in particular, with recal-
[NaPCP] ) [NaPCP]o exp(-kth
Pth
Vt) (18)
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citrant pollutants or mixtures of pollutants. Moreover,where the final products are determined to be carbondioxide, short-chain organic acids, and/or inorganic ions,the time scale and the dissipated power necessary toobtain the complete minerilization of the hazardouspollutants are currently not economically or practicallyacceptable. Combinations of ultrasound with otheradvanced oxidation processes (AOPs) or conventionalbiological processes are found to be effective in degrad-ing some recalcitrant compounds; however, the econom-ics are poorly understood. This section discusses someof the problems related to sonochemical reaction kinet-ics, byproducts, and ecological effects and to technical-scale application and implementation of the ultrasoundfor water treatment and the need to address them.
5.1. Reaction Pathways and Kinetics. In general,three distinct pathways have been determined in thesonochemical degradation of chemical compounds. Theseinclude oxidation by hydroxyl radicals, pyrolytic decom-position, and supercritical water oxidation.58 The ultra-sonic degradation of organic compounds in dilute aque-ous solutions depends to a large extent on the natureof the organic material. The specific nature of thesaturating gas also influences the relative proportionof the pyrolytic or free radical steps. Hydrophobic andvolatile organic compounds tend to partition into thecollapsing cavitation bubbles and degrade mainly bydirect thermal decomposition leading to the formationof combustion byproducts. The sonochemical oxidationof polar organics (e.g., phenol) is slow compared to thatof nonpolar, volatile organics such as CCl4.56,58 Hydro-philic and less volatile or nonvolatile compounds de-grade to form oxidation or reduction byproducts byreacting with hydroxyl radicals or hydrogen atomsdiffusing out of the cavitation bubbles. Thermal de-struction processes are not considered important fornonvolatile substrates because they do not partitionappreciably into the bubbles. The efficiency with whicha solute reacts with hydroxyl radicals generated inultrasonic reaction mixtures is related to its hydropho-bicity; the greater the hydrophobicity is, the moreefficient the solute can act a radical scavenger.154 Thetime scales of treatment in simple batch reactorsreported in the literature are generally in the range ofminutes to hours for complete degradation. Most inves-tigators have observed the kinetics of sono degradationof pollutants to be first- or zero-order. For pollutantswith first-order degradation rates, the first-order rateconstants tend to decrease as the initial substrateconcentrations increase.112,131 In contrast, for pollutantsdisplaying zero-order substrate disappearance, the rateconstants increase as the initial substrate concentrationincreases. A change in reaction order from zero-order(at high initial substrate concentration) to first-order(at lower initial substrate concentration) has also beenobserved.66 The change in rate constants with initialsubstrate concentration is ascribed to radical scaveng-ing/recombination, i.e., a combination of a first orderthermal destruction processes and a zero order reactionwith hydroxyl radicals.112,131 Sehgal and Wang41 ob-served change in reaction order from first to zero astemperature increased. Cost et al.78 indicated that zero-order kinetics are typical of hydroxyl radical reactionssince the free radicals are generated at constant rateunder ultrasonic irradiation and suggested the slowdegradation of humic acid occurred mainly by radicalattack.
Generally, improvements in technology are needed tooxidize organics especially polar compounds more ef-ficiently. If degradation rates can be enhanced, reactiontimes will be reduced for equivalent final contaminantconcentrations, translating to reduction in reactor sizes.Consequently, the cost of application of this technologyto industrial or field-scale projects would be significantlyreduced. The manipulation of macroscopic parametershas been shown to lead to enhancement of cavitationchemistry as the number of cavitation bubbles andchemical events at each bubble are varied. Hoffmannand co-workers77 optimized the degradation rates ofaqueous-phase organic compounds with acoustical pro-cessors by adjusting the energy density, energy inten-sity, and the nature and properties of the saturatinggas in solution. They observed that the first-orderdegradation rates increase as the energy density andintensity increased to a saturation value. Continuousgassing conditions also induce a greater number ofcavitation events in homogeneous systems and assistcavitation in heterogeneous systems.28 Susuki et al.155
investigated the influence of aeration, bubble distribu-tion, and frequency on the degradation rate of surfactantSS-70 (initial concentration, 50 ppm; critical micelleconcentration, 52.2 ppm) at 298°K using cylindrical-type(26 kHz) and disk-type (100 kHz, 300 kHz) ultrasonictransducers and showed that the degradation rates wereenhanced by aeration (i.e., aeration bubbles generated)and the shape of the reactors used. It is also well-knownthat ultrasonic degradation of phenolic compoundsinvolve •OH radical attack, and hence, the use of high-frequency ultrasound (e.g., 500 kHz) has proven to bemore advantageous compared with low-frequency ul-trasound. Higher ultrasonic frequencies are more favor-able for the generation of hydroxyl radicals possibly dueto faster production rates.46,48-50 Weaver et al.156 ex-amined the first-order sono degradation kinetics of threesimilar aromatic compounds, nitrobenzene (NB), 4-ni-tophenol (4-NP), and 4-chlorophenol (4-CP), by ultra-sound at frequencies of 20 and 500 kHz. They found thatin the 20 kHz reactor, NB degraded the fastest and 4-NPthe slowest, but in the 500 kHz reactor, 4-CP was thefastest while 4-NP was the most resistant to sonolyticdestruction. They attributed the results to the greaterdegree of vapor-phase pyrolysis at the lower frequencydue to high temperatures achieved during bubble col-lapse, while higher frequency favored ‚OH production.Using 20 kHz ultrasound, large salt-induced enhance-ments in oxidation/destruction rates have been ob-served: 6-fold for chlorobenzene, 7-fold for p-ethylphe-nol, and 3-fold for phenol oxidation, in sodium chloridesolutions (0, 0.17, 0.67, and 1.38 mol/L).59 The enhance-ment was ascribed to the fact that the presence of thesalt increased the residence of the aromatice compoundsin or at the surface of the cavitation bubbles. The rateof the sonochemical degradation of 3-chlorophenol wasenhanced 2.4 times by the addition of appropriateamount of Fe(II) concentrations (e.g., 1 mM). Theenhancement was attributed to the probable regenera-tion of OH radicals from H2O2, which would otherwiseformed from recombination of OH radicals and contrib-uted to lesser degradation.74 More such inexpensiveapproaches for improving process efficiency should beinvestigated in the future to make sonochemical oxida-tion an economically viable remediation technology.
5.2. Effects of Water Quality. This section discusseshow measures of water quality (i.e., matrix components)
4708 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
such as alkalinity, particulate matter (dissolved organicand inorganic), and mixtures of compounds affect therates of sono-reactions. Sonchemical oxidation reactionslike other AOPs should generally be inhibited by highalkalinity usually resulting from the presence of largeconcentrations of radical scavengers such as bicarbon-ates and carbonates. This significantly reduces processefficiency. However, Cost et al.78 observed that addedalkalinity did not affect the rates of destruction ofp-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 pol-lutants because of the formation and stabilization of gasbubbles in the crevices of the particles. Large particlesmight be expected to decrease the rate because of soundattenuation. Fine particles containing gas- and vapor-filled crevices might enhance the rate by providingadditional nuclei for bubble formation.157 However,Kotronarou112 investigated the effect of large sandparticles (500 µm average) and fine particles (7 nmaverage) on the sonication of sulfide oxidation andobserved no significant effect for the sizes and concen-trations studied. Cost et al.78 also noted that particulatematter (sea sand or Fe2O3 particle) did not significantlyaffect the degradation rate of p-Np. Orzechowska etal.108 observed that the presence of humic substancesdid not affect Cl- yields in the sonication of aqueouschlorinated hydrocarbons. However, Taylor et al.158
found the presence of fulvic acid (FA) to significantlyinhibit the sonochemical degradation of PAHs (in theconcentration range of 0.1-0.5 µM) at 20 kHz (600W)and at 20 °C. In the presence of approximately 50 µMFA, 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 (0.006vs 0.0026), respectively.
Multicomponent waste streams are more realisticmatrices to consider for practical treatment situations.Hua and Hoffmann99 found that sonication of a mixtureof CCl4 and p-NP resulted in the enhancement of p-NPdegradation, demonstrating that ultrasound is notlimited to single-solute solutions. They also found thatthe effect of CCl4 in a mixed waste stream was particu-larly interesting in that it released a residual oxidant,which could continue to attack other refractory mol-ecules in solution after the ultrasonic irradiation washalted. Drijvers et al.106 also showed that volatileorganic compounds could strongly affect each other’ssonolysis rates. However, as indicated earlier, thepresence of hydrophobic compound CB hindered thedegradation of hydrophilic compound, 4-CPOH.72 Zhangand Hua159 found moderate decreases in degradationefficiency when a mixture containing 10µM chloropicrin(CCl3NO2 or CP), 10µM trichloroacetonitrile (C2Cl3N orTCA), and 0.5 mM bromobenzene (BB) was sonicatedat both 20 kHz (30.8 Wcm-2) and 358 kHz in a complexaqueous matrix (i.e., river water) compared to reagentgrade water. Differences in rate constants observedduring sonication of the mixture and individual com-pounds were also minimal. The first-order rate con-stants at 20 kHz for TCA, CP, and BB were, respec-tively, 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 sonicatedindividually. 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 andnonpollutant chemicals, more studies on the effects ofwater quality on sonochemical oxidation rates areneeded to advance our understanding.
5.3. Sonication Byproducts and Toxicity Effects.If ultrasound is to be used in the destruction of hazard-ous wastes on an industrial scale, the associated reac-tions must be ecologically acceptable. Hence, the toxicityof the reaction products obtained from sonochemicaltreatments is a factor that should be considered priorto subsequent utilization of treated water. In moststudies so far, the toxicity of the reaction productsproduced by treatment remains to be determined.Special attention must be paid to the types of productsformed and their toxicity and resistance to furtherultrasonic cleavage or other secondary treatments. Totalorganic carbon (TOC) analysis is one method that hasbeen used extensively to determine quantitativelywhether the sono degradation of the organic compoundresults in complete minerilization to the relativelyharmless CO2 (g) or some product that presents toxicfeatures. However, TOC analyses do not give the entirepicture with respect to toxicity because a given hazard-ous substance may also degrade to carbon deposits135
or be converted into other benign substrates that areacceptable end-products. A quick minerilization of anorganic contaminant should be the goal to minimize thesurvival time of toxic intermediates. The use of ultra-sound in combination with other advanced oxidationprocesses or conventional biological processes has beenfound to be effective in reducing toxicity. For example,Gonze et al.151 demonstrated that ultrasonic irradiationwhen used as a pretreatment step decreased the toxicityof sodium pentachlorophenol (NaPCP) solution im-mensely allowing degradation by biological activatedsludge process. Other possible ways include the couplingof H2O2, O3, or UV photocatalysis and solution chemistrymodification to enhance the efficiency of free radicalgeneration through cavitation. Hoffmann and col-leagues138 demonstrated that the combination of ultra-sound with ozonation or other AOPs effectively degradedrecalcitrant pollutants such as MTBE into innocuousand biodegradable products. Trabelsi et al.67 showedthat the combination of electrochemistry and high-frequency sonication allowed a total degradation ofphenol within 20 min with no production of toxicaromatic intermediates. It appears that using a combi-native AOP approach is certainly a positive step towardachieving quick minerilization and more such studiesshould be considered in future efforts to address andmitigate the byproduct toxicity problems.
5.4. Efficiency and Scaleup Issues. The ultrasonicsystem transforms electrical power into vibrational orultrasonic energy, i.e., mechanical energy.31-32 160-161
This mechanical energy is then transmitted into thesonicated reaction medium. The efficiency of the energytransformation depends not only on the equipment itselfbut also on the ultrasonication conditions. Therefore,the amount of acoustic energy delivered into the liquidmedium cannot be measured solely by measuring theamount of electrical energy expended to produce themechanical vibration. Sonolysis is relatively inefficientwith respect to total input energy. This is because partof the total energy input is lost to produce heat andanother part produces cavitation. But not all of thecavitational energy produces chemical and physical
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effects. Some energy is consumed in sound re-emission(i.e., harmonics and sub-harmonics).32,57 The low ef-ficiency of the conversion sequence electrical energy facoustic energy f chemical effects may preclude the useof this method as a primary treatment. As illustratedby Kotronarou et al.,131 the limitation of sonolysis forthe control of H2S and other trace contaminants inwater is that it is relatively inefficient with respect tothe energy input. Their calculations indicated only asmall portion of the total energy supplied to the systemresulted in “useful” free-radical reactions. The energytransformation losses within the system are shown inFigure 2. At each stage in the transmission of power,there is an inefficiency whereby sonochemical benefitis lost to heat or sound. Hence, it can be appreciatedthat the determination of an energy balance is not easy.The sonochemical yield (SY ) measured effect/inputpower (W)) is measured in moles of products generatedper second. Despite the difficulty in ascertaining ac-curate energy balance, “SY” is defined in general thisway similar to the definition of “photochemicalyield”.31,32,162 Hence, the acoustical power transferredto the reaction mixture is less than the overall powerinput into the power generators (i.e., the power drawnfrom the wall supply). The challenge is to harness theseenergy losses to useful purposes, e.g., pollution treat-ment. In addition to knowledge of the characteristics ofthe reaction mixtures and kinetics of reaction, effectivescaleup procedure is necessary and requires that allparameters affecting cavitation efficiency be studiedwell, properly quantified and optimized.
The majority of the reported studies to date employedlaboratory-scale piezoelectric probe (0.25-0.5 in.) batchreactors with small volumes (20-200 mL) for ultrasonicdegradation of pollutants45,160 The probe type reactoris widely used due to its ability to deliver high poweroutput and the ease of operation to give optimumperformance at different amplitudes. However, Kotron-arou and Hoffmann111,132 found that the same energyinput dispersed over a broader area resulted in asignificant enhancement in reaction rate and energyutilization efficiency. It was also shown that sonolysisreactors with larger radiating surfaces are more energyefficient the direct immersion probe reactor, which hasa small (1 cm2) radiating surface. In addition to the low-energy utilization efficiency, the probe type reactor
system is also not an optimal reactor configuration forthe efficient generation of transient cavitation for twoother reasons. First, sonication efficiency is reduced dueto blanketing effect caused by intense generation ofcavitation bubbles at the tip. Second, the probe typetransducer tips have relatively short life before thetitanium tip requires replacement. For these reasons,the application of direct probe sonolysis to specialapplications such as groundwater remediation or pre-treatment of industrial hazardous wastes is not eco-nomically feasible. In their studies of H2S ultrasonicoxidation, Hoffmann and co-workers131,132 found a directlinear relationship between the applied power at a fixedfrequency and the observed rate of loss of S(-II),indicating a continuous-flow stirred-tank probe reactorcould attain significant conversion efficiencies. Theirinitial tests indicated that the use of large high-poweredsonicators in the CSTR mode could result in viabledegradation efficiencies. Hua et al.77 found that a near-field acoustical processor or NAP (where liquid flowsbetween two high intensity vibrating plates) was moreefficient than a probe reactor for the degradation ofp-nitrophenol. They observed that NAP provided about1.5 times the power per unit volume of the probe reactorbut resulted in 19 times more p-nitrophenol destructionper kilojoule energy imparted on the system. It was alsoshown that the G value for the probe reactor is ap-proximately 1 order of magnitude less than that in theNAP, although the degradation rate constants were thesame magnitude. The G value is an efficiency measureused in radiation chemistry and for each species inneutral water is defined as the number of moleculesundergoing reaction for each 100 eV electrical energytransferred.25 The NAP parallel-plate reactor allows fora lower sound intensity but a higher acoustical powerper unit volume than those of the conventional probe-type reactor. However, limited work has been reportedon the NAP and tank reactors, and comparison of theirperformance data with the probe reactor are scarce.Gondrexon et al.116 observed a conversion rate forpentachlorophenol (PCP) up to 80% using a three-stagelaboratory-scale sonochemical reactor (each equippedwith 500 kHz piezoelectric disk) and operating in thecontinuous flow mode. Operation in continuous-flowmode also has the advantage of short residence time,especially for pollutants that are degraded quickly.
Figure 2. Energy conversion in sonochemical processes.
4710 Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001
However, as they indicated, the scaleup of such anexperimental system would not be easy, and a lot oftechnical problems remain to be overcome. But increas-ing commercial interest in sonochemical processesshould reduce cost in the very near future. Problemsrelated to technical-scale application and implementa-tion of sonochemical steps into conventional watertreatments must be considered in future investiga-tions.31,32,160
6. Concluding Remarks
Ultrasonic irradiation shows promise and has thepotential for use in environmental remediation due theproduction of high concentrations of oxidizing speciessuch as •OH and H2O2 in solution and localized transienthigh temperatures and pressures. In terms of conven-ience and simplicity of operation, sonolysis could proveto be economically competitive and far superior to manyalternative approaches. These include high-temperaturecatalytic combustion or incineration, activated carbonor zeolite adsorption, supercritical fluid extraction oroxidation, substrate-specific biodegradation, membraneseparation, electron-beam irradiation, UV-photolytic,and other chemical degradation methods.26,163-164 Someof these techniques are cost-intensive and requiretransference of the target molecule from an aqueousphase. Biodegradation provides a promising method forenvironmental remediation. However, currently avail-able processes are slow and produce unpredictableresults. Aerobic biological oxidation is also limited whenthe feed is either recalcitrant to bidegradation, orinhibitory or toxic to the bioculture.164 The sonochemicalapproach also has the advantage of being adaptable tomixed solid-liquid wastes. Ultrasonic degradation isseveral times (about 10 000-fold) faster than naturalaerobic oxidation, for example.115 Also, sonochemicaldegradation occurs over wide range concentrations(varying by order of magnitude), unlike certain biologi-cal systems that are inhibited by relatively low sub-strate concentrations. In a recent economic analysis ofa dilute p-nitrophenol aqueous waste treatment, the costof sonochemical oxidation was found to be comparableto that of incineration.77 The relative efficiency ofultrasound in terms of p-nitrophenol degraded per literof water was also shown to be far superior to conven-tional UV-photolytic degradation.58 Calculated G valueefficiencies from the literature also indicate sonochemi-cal systems are competitive with other AOPs such asUV photocatalysis and supercritical water oxida-tion.87,163,165 Sonolysis does not require the addition ofchemical additives to achieve viable degradation rates.However, some chemicals may be utilized as an effectivesonolytic catalyst for reactions involving ‚OH radical forexample.
As obvious from the degradation products in Table 3,complete mineralization of recalcitrant pollutant isdifficult to obtain at reasonable rates with ultrasoundalone. Because the efficiency of electrical-to-sound-to-thermal conversion is poor, even faster decompositionis needed to carry out the oxidation at the commerciallevel. Moreover, other barriers remain to the large-scaleimplementation of this technology for pollution degra-dation. The economics are poorly understood. Thescaleup of sonochemical reactors remains problematicbecause few studies have been done to determine whichmeasures of reactor efficiency correlates best withpollutant destruction rates. Several studies have noted
the change in biodegradability of a waste subjected toprior chemical oxidation.164 Integration of destructiveprocesses for recalcitrant or inhibiting contaminantconversion is advantageous conceptually. However,substantial research efforts are needed to developefficient processes through coupling of biological ap-proaches with chemical and physical treatments. Thedesign key for such a coupled system lies in choosingprocesses that complement each other and leads to asynergistic effect. Sonochemical oxidation technologiesare most advantageously used in combination with otheradvanced oxidation processes and/or biological treat-ment.
Ultrasound has other environmental science andengineering applications. Low permeability geomateri-als such as clay or silt are difficult to remediate becauseof low transport rates and high adsorption potential. Ithas been shown that ultrasonic vibrations enhanceremoval of volatile organic compounds (VOCs) andliquid contaminants from low permeability geologicformations that have been pneumatically or hydrauli-cally fractured.166 Mukherjee et al.167 and Liu et al.168
also showed that sonication could be used as a pretreat-ment step to improve the release of organic matterassociated with the particulate phase into the solubleaqueous phase enhancing its bioavailability during theanaerobic biodegradation of aquatic sediments. Also, inthe desorption of phenol from activated carbon andpolymeric resin, Rege et al.169 found sonication at 40kHz and 1.44 MHz to significantly increase desorptionrates, and the rates were also favored by decreasedtemperature, aerated medium, and increased ultrasonicintensity. They also found desorption rates in theabsence of ultrasound was limited by pore diffusion,whereas those under ultrasonic irradiations were lim-ited by surface reaction. The improvement in desorptionrates were attributed to an enhancement in diffusionaltransport due to the acoustic microstreaming causedwithin the pores.
The amount of things that can be accomplished withsonochemistry is, at this stage, only limited by theminds of those working in this exciting field. It is awhole new field of research that is growing very fast,with exciting prospect for any researcher. The reviewof the environmental aspects of sonochemistry high-lighting remediation processes, reaction pathways, andkinetic studies presented here indicates a large scopefor further experimental and theoretical investigationson all aspects of this important medium of reaction.More research and developmental studies, both experi-mental and theoretical and encompassing kinetics,reaction mechanisms, reactor design, and energy con-servation, are needed to improve reliability and mini-mize costs for scale-up and long-term use.170-172 De-staillat et al.173 demonstrated the effectiveness of a novelpilot-plant scale sonochemical reactor (UES 4000CPilotstation) recently developed for large-scale reme-diation applications. A budget analysis of this system(612 kHz, 3 kW) indicated that nearly one-third of theapplied power is converted into sonochemical activity.It is hoped that this review has stimulated thinkingbeyond the cases presented and should spur futureengineering-based research in the continued use ofsonochemical oxidation as an environmentally benignprocess for the removal of inorganic and organic speciesfrom industrial wastewater streams and remediation ofcontaminants in subsurface environments.
Ind. Eng. Chem. Res., Vol. 40, No. 22, 2001 4711
Acknowledgment
The author thanks Air Force Office of ScientificResearch (Grant. No. F.49620-95-1-0541) and Depart-ment of Energy (Grant No. DE-FC04-90AL66158) forfinancial support.
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Received for review January 30, 2001Revised manuscript received July 18, 2001
Accepted August 13, 2001
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