Tce Sonolysis

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    SONOCHEYISTAYUltrasonics Sono chemistry 3 (1996) S83-S90

    Sonolysis of trichloroethylene in aqueous solution: volatile organicintermediates

    David Drijvers *, Robrecht De Baets, Alex De Visscher, Herman Van LangenhoveDepartment of OrganicChemistry, Faculty of Agricultural Applied Biological Sciences, University of Ghent. Coupure Links 653, B-9000 Gent, Belgium

    Received 15 February 1996; evised 30 April 19 96

    AbstractThe ultrasonic degradation of 3.34 mM trichloroethylene (TCE ) in aqueous solution was measured at 20 and 520 kHz. Asthe degradation was energetically more efficient at 520 kHz, sonication at this frequency was further investigated. The effect of

    the saturating gas (air or argon) and the influence of the pH of the aqueous solution was studied. The degradation was fastest inbasic solutions saturated with argon. TC E was not degraded in the bulk solution. During sonication volatile and non-volatileorganic degradation products were formed. The most important volatile compounds were identified: C,HCl, C,Cl,, CqCl,, C2C ld,C,HC l, (2), C4Cl,, C,HC l, and C4Cl,. T hose products are typical for pyrolysis of TCE and are an affirmation for the hot spottheory. Th e kinetics of five of those volatile intermediates were determined by headspace analysis for air-saturated as well as forargon-saturated solutions. The intermediates considered are formed in single cavitation events and disappeared from the aqueoussolution as well.Keywords: Sonolysis; Trichloroethylene; Aqueous solutions

    1. IntroductionUltrasonic waves in water provoke the collapse of

    cavitation bubbles and induce the formation of chemicalspecies such as H, OH, 0 and H,O, [l-3]. Twotheories have been suggested to explain their formation:the hot spot theory and the electrical disch arge theory.According to the hot spot theory the collapse of abubb le is almost ad iabatic and causes extremely h ightemperatures and pressures. By determining the first-orderrate coefficients of sonochem ical ligand subs titutionas a function of metal carbonyl vapour pressure, atemperature of 5000 K inside the collapsing cavitationand a temperature of 2000 K in the interfacial regionbetween the bulk solution and the collapsing bubble wasdetermined [4]. By using the temperature dependenceof C-N bon d pyrolysis of p-nitropheno l in oxygenatedaqueous solutions, a temperature of 800 K was deter-mined for the interfacial region [S]. A ccording tothe electrical discharge theory, the extreme conditions

    * Corresponding author. E-mail: [email protected]; fax:f 32-9-264-6243.


    leading to the formation of the radicals originate fromintense electrical fields during collapse of the cavitationbubb les [6]. The comb ination of both theories has beenused as well to explain all phenom ena of ultrasonicirradiation [ 71. The existence of electrical discha rge wa sargued by the analogies b etween the features of sono-chemistry an d corona discharge. How ever, data fromsonochem ical experimen ts are still mostly interpreted interms of the hot spot theory.

    Ultrasonic irradiation causes the degradation oforganic compounds in water. The OH radicals formedoxidize the organic com pound s. Apo lar and voltatileorganic compounds, however, are also degradedthermally in and around the cavitations. Thus, bothindirect and direct decomp osition by ultrasonics occur.The solution can be divided into three different zonesduring sonication [S]. The collapsing micro bubbles,filled with dissolved g as and solvent vapour, are thereaction zones in which solven t vapou r is pyrolysed,producing radicals. Volatile organics too are pyrolysedin the collapsing bubb les. T he interfacial region, a thinshell of liquid, is exposed to temperatures and pressur esexceeding the critical ones and the resulting supercritical

    1350-4177/96/ 15.00 Copyright 0 1996 Elsevier Science B.V. All rights reservedPII S1350-4177(96)00012-O

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    S84 D. Drijvers. et al. f Ultrasonics Sonochemistry 3 1996) S83-S90

    fluid possesse s physical pro perties other than the bulksolution. The dielectrical constan t is lowered for polarsolvents such as water and allows the accumulationof low-polarity solutes. The third reg ion is the bulksolution. Here, the solutes react with radicals that havenot yet recombined , disproportiona ted or that have notbeen scavenged.

    An impor tant param eter for the efficiency of thedegradation of organic com pound s is the average specificheat ratio y (=C ,/C,) of the gas and solvent vapourin the collapsing bubb le. The temperature inside thecollapsing bubb le is closely related to this specific heatratio. Riesz [9] calculated that the final collapse tem-perature for bubbles saturated with monoatomic gases,y = 1.67, is more than twice as high than for bubb lessaturated with N,O, y = 1.30.

    Asp ects of the 20 kHz ultrason ic degradation of mostCl and C2 chlorinated compounds and some chloro-fluorocarbons in aqueou s solutions have been investigated[ 10-141. C HCl, and CCL, supersaturated solutions havebeen sonicated. The observed organic products fromthe degradation of both CH Cl, and Ccl, were hexa-chloroethane and tetrachloroethylene [lo]. The sono-lysis of supersatu rated and dilute aqueou s solutions ofl,l,l-trichloroethane was investigated as well [ 11,12 1.Organic d egradation products such as chloroform,chloroethylene, l,Zdichloroethane, l,l-dichloroethane,chloroethane and 1,2-dichloroethylene were determined .How ever, these organic degradation produ cts w ere notquantified. The ultrasonic degradation was more com-plete for the dilute solution than for the supersatu ratedsolution. This was explained by the decrease of thecavitation intensity because of the higher vapour pressureof the supersatu rated solution of l,l,l-trichloroethane[ 111. For the chlorofluorocarbons CFC 11 and CFC 113[ 131 and eight chlorinated Cl and C 2 volatile organiccomp ounds [ 143, including trichloroethylene, diluteaqueous solutions have been sonicated and no organicbyproducts were sough t. Only the degradation rate,following first order, was measured. The Cl compoundsseemed to degrade by pyrolysis because of the highvapour pressure of these compounds. As the vapourpressure of the Cl com pound s increased, the first-orderrate coefficient seemed to decrease. Unlike Cl com-poun ds, there seemed to be no relation between thelower vapour pressure of the chlorinated C2 compoundand its destruction rate. This led to the conclusion thatC2 chlorinated compounds probably disintegrate by bothmech anism s: pyrolysis-type reactions in the cavitationand free radical attack in the bulk liquid phase.

    At the higher frequency of 200 kHz, aspects of theultrasonic decomposition of 10 ppm aqueous solutionsof trichloroethylene, tetrachloroethylene, l,l,l-trichloro-ethane, chloroform and carbon tetrachloride werestudied [ 151. After an irradiation time of 10 minu tesmore than 70 of the chlorinated hydrocarbon was

    degraded and it was concluded that the main reactionsare thermal decom position or comb ustion in cavitationbubbles and not reactions by OH radicals or H atoms.The sonication of an argon saturated solution of 10 ppmor 0.11 mM trichloroethylene gave Cl-, CO and H, asmajor products and COZ, methane and ethylene asminor produ cts. A trace amou nt of dichloroethylene wasdetected by GC -MS . The concentration versus timeprofile for the final products Cl-, H,, CO and CO Z wasalso determined.

    The ultrasonic degradation at high frequency of lessdilute solutions of chlorinated hydrocarbon s has not yetbeen investigated. As the concentration of the volatilehydrocarbon will decrease the average specific heat ratioof the gas in the collapsing bubb le, prov oking lowertemperatures and pressures, a higher amount of inter-mediate organics is expected than in the case of thesonication of the 10 ppm aqueous solutions [ 151. Ahigher extent of radical recomb ination reactions is alsoexpected, leading to different degradation products.The sonication at 1 MH z of argon-saturated aqueoussolutions of the volatile C2 compo und acetylene in themillimolar range was studied [ 161. The following pro-ducts were found: H,, CO, CH,, hydrocarbons con-taining two to about eight C atoms, formaldehyde andacetaldehyde, formic and acetic acid and insoluble so ot.The products observed are similar to the ones observedin the pyrolysis and comb ustion of acetylene. Initially,all products appeared proportional to the irradiationtime and it was concluded that all products are formedin single cavitation events and not by stepwise formationand subsequent sonolysis of intermediate compounds indifferent cavitation bubbles.

    The aim of this study is to investigate if the sonicationof trichloroethylene (TCE ) in the millimolar range leadsto similar volatile organ ic interm ediates as in the caseof acetylene [ 161. Know ledge of the volatile organicintermediates formed will lead to a better u nderstan dingof the ultrasonic degradation of TCE . The effect of thesaturating gas, air or argon, was investigated as well.

    2. Materials and methodsSonication experiments were performed with a 20 kHzBranson 250 sonicator and a 520 kHz U ndatim Ortho

    Reactor. T he high frequency reactor was equipped withan extra voltmeter to allow sonication with a constantpower transfer to the liquid. At low frequency, 120 mlsolution in a 160 ml reaction cell, and at high frequency150 ml solution in a 200 ml reaction cell, was sonicated.The steady state reaction temperature was 32 + 1C inthe 20 kHz reactor and 29.5 t 0.5 C in the 520 kHzreactor.

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    2.1. ChemicalsAll organic chem icals commercially available w ere

    of a purity 399 . Only two of the eight majororganic intermediates formed during sonication arecomm ercially available, namely tetrachloroethyleneand hexachlorob utadiene. Three intermediate pro ducts,dichloroacetylene, tetrachlorobutenyne and pentachloro-butadiene were synthesized. Tetrachlorobutenyne andpentachlorobu tadiene were synthesized out of hexachloro-butene. Hexachlorobutene wa s made by dimerisation oftrichloroethylene in the presence of dibenzoylperoxide[ 171. Dehyd rochlorination of hexachlorobutene byOH - yielded pentachlorobu tadiene [ 181. By usingthe stronger base NH ; for further dehydro chlorinationof hexachlorobutene tetrachlorobutenyne was obtained[ 191. Both produ cts, pentachlorobu tadiene and tetra-chlorobutenyne, were purified by preparative GC . Puritywas verified by H- or 13C-NMR and mass spectrometry.Dichloroacetylene is a very reactive comp ound whichforms explosive m ixtures with air. The presence ofdiethylether in the system retards the violent auto-oxidation of dichloroacetylene by formation of amolecular complex [20]. This complex was obtainedfrom a mixture of TCE and diethylether in the presenceof an aqueous solution of sodium hydroxide at 70Cand with an ammonium salt as a phase-transfer catalyst[20]. A 13C-N MR and a mass spectrum of the obtainedmixture of dichloroacetylene and ether was taken. Tracesof TCE were also present. It was not possible to separatethe mixture by distillation or preparative GC becauseof the reactivity of dichloroacetylene with air.2.2. Determination of the kinetics of sonochemicaldegradation of TCE

    The initial TCE concentration in all experiments wa s3.34 mM . The solution was buffered at pH 4.7 (40 mMacetate buffer), pH 7 (40 mM phosphate buffer) an dpH 10 (40 mM borate buffer). In the argon experimentsthe solution was saturated with argon before addingTCE and the reactor was flushed with argon beforeadding the solution.

    A syringe needle was pierced through the septum ofthe screw cap of both reaction cells for samp ling. Usin ga glass syringe 2 ml samples were taken from the 520 kHzreactor a t various tim e intervals dur ing sonication whilethe sonication was restarted in the case of the 20 kHzreactor for every analysis. This was done becau se thelow frequency reactor did not allow sampling duringsonication. The 2 ml samples w ere transferred to 2.5 mlbottles, and sealed with Mininert stoppers (Alltech Ass.).50 ul of a 1 ~01 2-hexanone aqueous solution wasadded as an internal standar d. In the case of sonicationat 20 kHz, 100 ul of toluene in dichloromethan e (volumeratio l/5) was added to the 1 20 ml sonicated solution as

    an internal standard. 1 ul of the mixture was then analysedon a Varian 37 00 gas chromatograph (FID-detector)with a DB-5 fused silica column (15 m, film thickness1.5 urn, ID 0.53 mm ). The initial GC oven temperaturewas 40C and a 3C min- temperature rise was used.

    2.3. Identification and determination of the kinetics ofintermediate productsTo identify the volatile intermediate products the

    mass spectra were determined with a Varian 270 0gas chromatograph with a RSL 150 fused silica column(60 m, film thickness 1.5 urn, ID 0.53 mm ) coupled to aMA T 112 mass spectrometer. A constant temperatureincrease of 3C min- from 30C to 200C was used asa temperature program . The intermediates were strippedfrom the diluted sonicated solution with helium, absorbedon Tenax GC and injected by the cold trap GC method.

    The Kov ats indices of the intermediate products weredetermined with the Varian 3700 gas chromatographwith a RSL 150 fused silica column (30 m, film thickness5 urn, ID 0.53 mm ) and with a FID-detector. The sameprocedure was followed as for the mass spectra but0.5 ul of the alkanes C5-Cl6 (1 ~01 ) in CS2 was spikedon the Tenax tube before desorption. The Kovats indexof the standard solution of dichloroacetylene and tetra-chlorobutenyne wa s determined by direct liquid injection.

    The kinetics of the intermediate volatile products weredetermined by headspac e analysis: after various so ni-cation times, 100 ml of the solution was transferred to a118 ml bottle. The bottle wa s closed imm ediately with aMinin ert valve (A lltech A ss.) and 50 ul of cyclohexanein methanol (0.06 ~01 ) was added as an internalstandar d. The bottle was then incubated overnight in athermostatic water bath at 25 f O.lC. Earlier studiesindicated that th is time is sufficient for attaining equi-librium partitioning of the organic compound s betweenthe water phase and the headspace [21,22]. After incu-bation 1 ml headspace (Syringe Pressure-Lok Series A,1 ml) w as injected in the Varian 3700 gas chromatographwith the column used for the Kovats index determination.The GC oven was kept at 35C for three minutes andthen heated to 200C at a rate of 10C min-.

    A standard curve for this headspace analysis wasdetermined for dichloroacetylene, tetrachloroethene,tetrachlorobutenyne, pentachlorobu tadiene and hexa-chlorobutadiene by injection of the headsp ace abovestandard solutions. A s dichloroacetylene wa s only avail-able as a mixture with ether and TCE, the standardcurve for ether and TCE was also determined by head-space injection. So the quantity of dichloroacetyleneinjected could be calculated by subtraction of thequantity of ether and TCE from the quantity of themixture injected.

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    ( 5= 2197 M- cm-) was measured spectrophoto-metrically. The rate of oxidation was obtained from thelinear regression of these concentrations as a functionof irradiation time (YZ 0.993; n = 5). The lower estimateto the hydroxyl radical yield was 0.0040 mM m in- andthe upper estimate 0.0176 mM min-. So, during the 60minu tes of sonication the hydroxyl radical productionin the bulk solution lies between 0.24 and 1.06 mM orbetween 7 and 32 of the initial TCE concen tration.This quantity of hydroxyl radicals m akes a considerabledegradation of TCE in the bulk solution possible.

    To measu re the importance of this degradation ofTCE in the bulk solution, the radical scavenger NaBrwas added to the TCE solution buffered at pH 7. Thereaction rate coefficient [2] kOH of Br- with OH is10 M- s-i. Makino showed, by spin trapping, that1 M formate, with the same kOH was enough to scavengeall OH in the bulk solution when air-saturated waterwas sonicated [2]. For the experiment in which 1 MNaB r wa s added to the solution a rate coefficientkl = 0.03909 + 0.00369 min- (r = 0.993; n = 11) wasobtained. As the degration rate of TCE was notinfluenced by the addition of NaB r, TCE is not degradedin the bulk solution at 520 kHz. The ap olar volatilecompound TCE is degraded in or at least near thecollapsing microbubbles.3.2. Intermediate volatile organic products

    Stripping of the sonicated solutions saturated with airor with argon and analysis by GCM S of the Tenax tubesresulted in nine major peaks of which two had the samemass spectrum. Based on the mass spectra, the followingvolatile products were tentatively identified: chloro-acetylene (C,H Cl), dichloroacetylene (CzC 1,), dichloro-diacetylene (C,Cl,), tetrachloroethylene (CzC 14), twoisomers of trichlorobutenyne (CqH C13) , tetrachloro-butenyne (&CL,), pentachlorobutadiene (CqHC1 5) andhexachlorobu tadiene (C,Cl,). To confirm the results ofinterpretation of the ma ss spectra of these products, theKovats index for five products was determined . Table 3shows the Kovats indices obtained from the headspaceanalysis of the sonicated solution of TCE (Isonic) andTable 3The Kovatz indices Isonic and Istand, respectively determined byinjecting headspace above the sonicated solution and the standardsolution, of the intermediate products


    GCCzC14C4C4C,HCI,Cl CL* Coincided with octane.

    ~aonic I stand509 509800* 800*

    1033 10331160 11611219 1218

    from the headspa ce analysis of an aqueou s solution ofthe purchas ed or synthesized product (Istand). The peakof tetrachloroethylene coincided in both cases withthe peak of octane. The Kov ats indices confirmed thestructure of the five intermediates proposed by themass spectra.

    Taylor studied the high-tem perature, oxygen-freepyrolysis of TCE in the 573-1273 K temperature range[26]. Initial decomposition was observed at 1000 K withformation of HCl and C,Cl,. Pronounced moleculargrowth was observed at higher temperatures with theformation of C,C& , C,Cl, and C,C l, (cyclic (cy)) as major(25 mole ) products and C,Cl,, CqCls, CgH Cls (cy),CgC& (cY), C&~S (cY), C10Cls (CY)and C12C18 (CY)asminor (< 5 mole ) products. T he mole of the productsdepended strongly on the temperature of pyrolysis. Apyrolysis mechanism of TCE w as proposed and testedusing compu ter simu lations based on the rate coefficientsand the activation energies E for all possible reactions.Generally, there was good agreement between predictedand experimen tal species profiles. The unimo lecular C-C lbond scission is the dom inant initiation step and theCl produced then abstracts the H atom of TCE yieldingC,Cl; and HCl (Eqs. (1) and (2))CzHC1, + C,HCl; + Cl, (1)C2HCl, + Cl + &Cl; + HCl. (2)The &Cl; radical then either loses a Cl radical toform CzClz or reacts with Cl2 to give CzCld and a Clradical. Although C,HCl was not detected, m odel resultsindicate th at C,H Cl is formed as a trace product (< 0.5of the initial C,HCl, concentration).

    In a study of the thermal or CO,-laser induceddecomposition of TCE, &Cl,, C,CL, C,HC l,, CqC1 6,C,HCl,, C&l,, CsHC l,, C8HC 17 and C8C1 , weredetected as degradation produc ts [27]. The reaction wasproposed to proceed by elimination of HC l (Eq. (3)) fromTCE , followed by radical formation and oligomerizationC,HCl, -+ &Cl, + HCl. (3)The similarity of products found in the sonolysis experi-ment with the most volatile ones from the two aforecitedstudies indicates that TC E is at least partially degradedthermally. In the study of the OH radical in itiatedoxidation of TCE [ 281 no similar products were detectedby GC-analysis. In a recent study on the plasma-assisteddecomposition of TCE in a pulsed corona dischargereactor [29] the volatile organic intermediates were notlooked for. It wou ld be interesting to do so. The absenceof the above m entioned nine volatile organ ic inter-mediates would confirm the thermal degradation of TCEduring sonication.

    In both pyrolysis studies [26,2 7] the formation of thehigher molecular weigh t produ cts is found to proceedvia CzC1, and neither C,HC l nor C,HCl, was detected.

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    The formation of for example C,Cl, was explained bythe following reactions [27] (Eqs. (4) and (5)):clc~ccl-+clc~c+c1, (4)ClC~C+C1C~CCl+C1C~C-CCl=CCl (A).

    (5)By picking up a free chlorine atom or by abstractingone from another molecule C,Cl, is formed out of theresulting radical A. By applying the same mechanism,CqH C13 must be formed out of &Cl, and C,HCl. Thismeans that both CZH C1 as CzClz are formed duringsonolysis of TCE. Formation of &Cl4 on the other handconfirms that the reaction me chanism proposed byTaylor proceeds [ 261.

    Recently, some studies were done on the mechanismof unimo lecular dissociation of trichloroethylene [ 30,3 11.The enthalpy diagram of the possible chan nels for theTCE dissociation process is given in Fig. 2. The C l,elimination reactions, which are expected to be veryhigh in energy, are not considered [30]. TSa and TSbare the transition states of the three- an d four-centeredelimination of HC l respectively. As can be seen fromFig. 2, the difference in enthalpy for reactions la, lband 2 a is sma ll and those reactions will compete. Itcan be concluded that both C-Cl bond scission (Eq. (1))and HCl elimination (Eq. (3)) are thermodynamicallypossible as initial reactions in the degrada tion of TCE .

    The formation of C,H Cl can be explained as theresult of two stepwise Cl atom eliminations from TCE[31] (Eqs. (1) and (6)).C,HCl; + C,HCl+ Cl. (6)The dissociation energy [ 311 for Eq. (6) is only16.1 kcal mol-. The reason why C,HCl and C,HC l,were not found by Taylor and by Earl as pyrolysisproduc ts, could be the difference in conditions [26,27 ].Und er less extreme con ditions the small differences inenthalpy between reactions la, lb and 2a from Fig. 2could make the reaction thermodyn amically exclusive.The HCl elimination is then energetically preferred.

    Ano ther exp lanation for the formation of C,H Clcould be that the conditions during b ubble co llapse

    (2b)CH=CC12 + Cl101.9 \

    are so extreme that even the Cl, elimination of TCEcould compete w ith the Cl abstraction and the HClelimination.The fact that the same volatile organic intermediatesare found for the solutions saturated with air as withargon is explained by the low oxygen concentration inthe aqueous solution. Normally combustion of TCE[ 321 gives partially different degradation produ cts thanoxygen-free pyrolysis. B ut in the aqueou s air-saturatedsolution, initially only 0.25 mM oxygen is present andduring the sonication the oxygen content d ecreases:0.15 mM , 0.1 mM and 0.05 mM after 10, 30 and 60minu tes so nication respectively. This decrease is explainedby degass ing of the solution and by Eq. (7):02+H +HO,. (7)In a second p hase the kinetics of the intermediate volatileproducts were determined. The standar d curves for thefive produc ts considered are given in Table 4. Thestandard deviation on the concentration of these pro-ducts was quite high. The analysis of the concentrationsafter 30 minutes sonication was performed twice andstandard deviations as high as 20 were found. Figs. 3and 4 show the kinetics for those intermediate produ cts.

    There is no basic difference betw een the kinetics underair and under argon. This means that the temperaturenear and insid e the collapsing bubb le is, in both cases,so high that the difference has no importance.

    Although no smooth curves are obtained, a linearconcentration increase during the first 10 minu tes ofsonication under air and argon can be deduced for mostTable 4The standard curves for the headspace analysis of the intermediateproducts


    C*Cl,C,C&GCl,C, HCI,C,Cl,

    Slope Intercept(relative area count, mM_) (relative area count)

    15.99 -0.104023.81 0.003617.25 0.007211.19 0.005239.93 0.0010

    \CHCl=CC12 /

    Fig. 2. Enthalpy diagram for the TCE dissociation reaction. Energies are written in units of kcal mol- (after Yokoyama et al. [26])

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    D. Drijvers, et al. / Ultrasonics Sonochemist ry 3 1996) S83-S90 S89

    0.16 T --1- C2Cl2 - air 10.12

    sc 0.08.1


    00 50 100 150 200

    time min)Fig. 3. Concentration versus time profiles of dichloroacetylene (C,C12) and tetrachloroethylene (C,C&)


    -m*--- C4HCl5 - argon----t C4Cl6 - air--*--- C4Cl6 - argon

    5 1 15 200time min)

    Fig. 4. Concentration versus time profiles of tetrachlorobutenyne (C,C&), pentachlorobutadiene (C,HCl,) and hexachlorobutadiene (C,Cl,).

    , -- C4Cl4 - air, ~-*-- C4C14 - argon--t C4HCl5 - air

    compounds. Furthermore, all maxima in Figs. 3 and 4lie around 30 minu tes. These two observations indicatethat the five volatile intermediates are formed in singlecavitation events, as was found previously for the pro-ducts formed during sonication of acetylene [ 161.

    The concentration of C,CI ,, a primary product, isat least 10 times higher th an the concentration of thefour other products. C,Cl, is formed d uring the firstminutes at a rate of approximately 0.01 mM min-.At that moment, the rate at which the 3.34mM TCEbreaks down is approximately 0.12 mM min-. So, theformation of &C l, represents ab out 10 of the totaldegradation, Considering that during those first minutesat least C,Cl,, C,HCl, and C,Cl, are formed out ofC,Cl, in single cavitation events, this percentage ishigher. Since the response factor of CPH Cl is not kno wn,the amount of CZH Cl could not be quantified. But theareas obtained for CLH Cl were in the same order ofmag nitude as for C,Cl,. So the thermal d egradation ofTCE during sonication at 520 kHz represents a signifi-cant degradation pathway.

    Also, non-volatile products are formed during thesonication. Wo rk is currently being done to identify andquantify those non-volatile products.

    AcknowledgementsDD w ishes to thank the Vlaams Instituut voor de

    Bevordering van het Wetensch appelijk-Techno logischOnd erzoek in de Industrie (IWT) for financial supp ortand ADV wishes to thank the Vlaams ImpulsprogrammaMilieutechnolog ie (VLIM ) for financial suppo rt.

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