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Ultrasonics Sonochemistry 13 (2006) 415–422

Sonolysis of 4-chlorophenol in aqueous solution: Effects ofsubstrate concentration, aqueous temperature

and ultrasonic frequency

Yi Jiang a,b, Christian Petrier b, T. David Waite a,*

a School of Civil and Environmental Engineering, The University of New South Wales, UNSW Sydney 2052 NSW, Australiab Laboratoire de Chimie Moleculaire et Environnement, ESIGEC—Universite de Savoie, 73376 Le Bourget du Lac, France

Received 26 November 2004; received in revised form 23 June 2005; accepted 1 July 2005Available online 26 September 2005

Abstract

The sonolysis of 4-chlorophenol (4-CP) in O2-saturated aqueous solutions is investigated for a variety of operating conditionswith the loss of 4-CP from solution following pseudo-first-order reaction kinetics. Hydroquinone (HQ) and 4-chlorocatechol(4-CC) are the predominant intermediates which are degraded on extended ultrasonic irradiation. The final products are identifiedas Cl�, CO2, CO, and HCO2H. The rate of 4-CP degradation is dependent on the initial 4-CP concentration with an essentiallylinear increase in degradation rate at low initial 4-CP concentrations but with a plateauing in the rate increase observed at highreactant concentrations. The results obtained indicate that degradation takes place in the solution bulk at low reactant concentra-tions while at higher concentrations degradation occurs predominantly at the gas bubble–liquid interface. The aqueous temperaturehas a significant effect on the reaction rate. At low frequency (20 kHz) a lower liquid temperature favours the sonochemical degra-dation of 4-CP while at high frequency (500 kHz) the rate of 4-CP degradation is minimally perturbed with a slight optimum ataround 40 �C. The rate of 4-CP degradation is frequency dependant with maximum rate of degradation occurring (of the frequenciesstudied) at 200 kHz.� 2005 Elsevier B.V. All rights reserved.

Keywords: Sonolysis; 4-chlorophenol; Degradation; Ultrasound; Wastewater treatment

1. Introduction

Chlorophenols have been widely detected in surfacewaters and groundwaters [1,2], and may be introducedinto these waters either during their manufacture anduse or through degradation of other chemicals (e.g.,phenoxyakanoic acids). They may also be formed dur-ing industrial operations (such as the bleaching of pulpwith chlorine, hydrolysis of chlorinated herbicides andoil refining) or formed as a result of the chlorination

1350-4177/$ - see front matter � 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.ultsonch.2005.07.003

* Corresponding author. Tel.: +61 2 9385 5060; fax: +61 2 93856139.

E-mail address: d.waite@unsw.edu.au (T.D. Waite).

of humic matter during the chlorination of municipaldrinking water. Chlorophenols possess relatively strongorganoleptic effects with a taste threshold of �0.1 lM[3]. They are pollutants of major environmental concerndue to their widespread presence and persistence. Thedecomposition of chlorophenols is therefore of impor-tance and has been examined extensively by photo-catalytic [4–6], biological [7,8] and more recently,sonochemical methods [9–11]. The latter method is ofparticular interest as it has been demonstrated to be par-ticularly effective in removing chlorinated organic com-pounds from contaminated water.

The propagation of ultrasonic waves in a liquidinduces the formation of cavitation bubbles which grow

416 Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422

and implode under the periodic variations of thepressure field. Cavitational collapse produces intenselocal temperature (several 1000 K), high pressure(several 100 atm.), electrical charges and plasma effectsand leads to enormous rates of heating and cooling(>109 K/s) [12,13]. Water molecules under such extremeconditions undergo thermal dissociation to yield H� andHO� radicals [14,15]. Organic solutes in the vicinity ofcollapsing bubbles or partitioned into the gas phase ofthe bubbles undergo thermal decomposition, and/orreact with the highly reactive radicals. Substrates, suchas phenol [16–19], chlorinated hydrocarbons [19–21],various aromatics [9–11,22–25], and PCBs [26] as wellas explosives [27] and surfactants [28] are transformedinto short-chain organic acids, inorganic ions, CO,CO2, and H2O as the final products. However, due tothe complexity of the sonochemical reactions, the effectsand the influences of the different parameters on the deg-radation reactions have not been clearly established asyet. In order to gain a better understanding of the deg-radation process and the reaction mechanisms, it is nec-essary to carry out further research before this methodwill be adopted by industry.

In this work, 4-chlrophenol (4-CP) is chosen as amodel chlorophenol and its degradation by sonicationexamined under a variety of different operating condi-tions. In particular, the rate of 4-CP degradation by son-ication is investigated as a function of substrateconcentration, aqueous temperature and ultrasonic fre-quency. Results are compared with those for the sonol-ysis of phenol and insights into the reaction mechanismfor the sonochemical degradation of 4-CP in O2-satu-rated aqueous solutions are presented.

2. Material and methods

4-Chlorophenol (4-CP) and 4-chlorocatechol (4-CC)were obtained from Aldrich, while phenol, hydroqui-none (HQ) and benzoquinone were obtained from Pro-labo. All chemicals were reagent grade (at least 99%purity) and were used as received. Aqueous solutionswere prepared by dissolving the compounds in ultra-pure Milli-Q deionized water.

Ultrasonic irradiation was performed in a cylindricalwater-jacketed glass cell equipped a Teflon holderwhich accepts transducers at different frequencies. Thehigh frequency ultrasonic transducers (200, 500 and800 kHz) were constituted by a piezo-electric disc (diam-eter 4 cm) fixed on a titanium plate. Each frequency hasa specific emitter connected to a high-frequency powersupply. The 20 kHz irradiations were carried out withcommercial equipment from Branson (Sonifier 450)equipped with a titanium probe (diameter 3.5 cm). Thereactor was hermetically sealed and connected to a gasburette to ensure a constant pressure (1 atm.). The tem-

perature of the liquid was monitored using a thermocou-ple immersed in the reacting medium. In all cases,250 mL of aqueous solution was saturated with O2 for20 min prior to commencing sonication. The tempera-ture of the media was maintained at 20 ± 1 �C unlessstated specifically. The ultrasonic power dissipated intothe reactors was adjusted and estimated by calorimetryin order to ensure comparative ultrasonic conditions atdifferent frequencies.

4-Chlorophenol, phenol and its primary intermedi-ates in the course of sonochemical degradation wereidentified using a high performance liquid chromato-graph (Waters model 600E) with an absorbance detector(Waters model 486) and equipped with a spherisorbODS2 5 lm C18 column (250 mm · 4.6 mm). The detec-tion wavelength was set at 254 nm and an acetonitrile/water (45/55) mixture containing acetic acid (1%) consti-tuted the mobile phase. Samples were injected directlyinto the chromatograph. The identity of intermediateswas confirmed by comparing retention times with thoseof known standards, and their concentration determinedfrom calibration curve.

Chloride ions were detected using an ion chromato-graph (Waters model ILC-1) with a conductimetricdetector (Waters model 430) and equipped with a Uni-versal anion column (150 · 4.6 mm). The mobile phasewas a benzoic acid aqueous solution (4 · 10�3 mol L�1)at pH 6 (adjusted with LiOH). The calibration was per-formed using an aqueous sodium chloride solution. Car-bon monoxide, carbon dioxide and formic acid(HCO2H) were analysed by gas chromatograph (Inter-smat model IGC 16). Separations were performed on aPorapak Q (2 mm · 2.5 m) column and detection wasachieved with a FID detector after hydrogenation.100 lL gaseous headspace was regularly sampled usinga gas syringe and immediately injected. Calibrationwas realised with standard gaseous mixtures (AlltethScotty II). Hydrogen peroxide was analysed iodometri-cally via its ammonium molybdate decomposition reac-tion in a 10% potassium iodide solution.

3. Results and discussion

3.1. Sonolysis of aqueous 4-chlorophenol at 500 kHz

Sonochemical degradation of 500 lM 4-chlorophenol(4-CP) and phenol aqueous solutions was carried out inthe cylindrical jacketed glass cell described above. Theexperimental results for 4-CP degradation are depictedin Fig. 1. Each point in the graphs represents the averageof at least three determinations. 4-CP was completelydestroyed after 300 min of sonication at 500 kHz withultrasonic power of 30 W and liquid temperature of20 ± 1 �C. The disappearance of 4-CP follows pseudo-first-order reaction kinetics with a rate constant of

0

25

50

75

100

0 60 120 180 240 300 360

Sonication time (min)

Co

nce

ntr

atio

n, % 4-CP

Cl-

HQ

4-CC

0

10

20

0 60 120 180 240 300 360Sonication time (min)

Co

nce

ntr

atio

n, %

CO2

CO

HCOOH

Fig. 1. Evolution of relative concentrations with time of reactant 4-chlorophenol (4-CP), intermediates hydroquinone (HQ) and 4-chlo-rocatechol (4-CC) and final products Cl�, CO, CO2 and HCOOH onsonication (500 kHz, 30 W) of a 500 lM 4-CP solution.

Table 1Comparison of initial rates of disappearance (Vd) of phenol and 4-chlorophenol (4-CP) at 500 kHz with ultrasonic power of 30 W andaqueous temperature of 20 ± 1 �C in O2-saturated solutions of 500 lMphenol and 4-CP

H2O Phenol 4-CP

Vf (lM min�1) 2.6 ± 0.2 1.40 ± 0.15 1.10 ± 0.15Vd (lM min�1) – 2.22 ± 0.20 3.00 ± 0.20

Initial rates of formation (Vf) of H2O2 both in the absence and pres-ence of phenol and 4-CP are also shown.

Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 417

0.012 ± 0.002 min�1. The primary intermediates areidentified and quantified as hydroquinone (HQ) and4-chlorocatechol (4-CC). These hydroxylated intermedi-ates are observed to disappear on extended ultrasonicirradiation. Dechlorination is nearly quantitative andoccurs soon after the disappearance of the initial sub-strate. The chlorine atoms are mineralised as chlorideions as the carbon–chlorine bonds are rapidly cleaved,and more than 95% of chlorine was recovered in theaqueous solution as chloride ions after sonication for360 min. In addition to chloride ions, CO, CO2 andHCO2H are also identified as final products. Their con-centrations rise slowly and combined, represent about21% (based on C content) of the starting 4-CP concen-tration in aqueous solution. In comparison with thesonochemical degradation of phenol, which is a morehydrophilic compound with lower vapour pressure,4-CP degradation appears to be considerably fasterunder the same sonication conditions (Table 1).

In the O2-saturated aqueous solution, ultrasonic irra-diation induces the formation of free radicals as a con-sequence of cavitation. The thermal decomposition ofthe water vapour and O2 in a cavitation bubble leadsto the formation of HO� and H� radicals [14], as wellas O atoms and HOO� radicals [29,30] (reactions (1)–(4)). Scavenging of H� radicals in the bubble or at the

interface by O atoms and O2 molecules increases theconcentration of the oxidizing radicals (HO� and HOO�)[15] (reactions (5)–(7)).

H2O! H� þHO� ð1ÞO2 ! 2O ð2ÞHO� þO! HOO� ð3ÞOþH2O! 2HO� ð4ÞH� þO! HO� ð5ÞH� þO2 ! HOO� ð6ÞH� þH2O! HO� þH2 ð7Þ

A large proportion of the radicals generated on sonoly-sis recombine inside the bubbles to form H2O, O and O2

(reactions (8)–(11)).

HO� þH� ! H2O ð8Þ2HO� () OþH2O ð9Þ2O! O2 ð10ÞHO� þHOO� ! O2 þH2O ð11Þ

Note that the extent of recombination would beexpected to be higher at low frequencies as the radicalswill have enough time to recombine inside the bubbles.Hydrogen peroxide (H2O2) will be formed outside thehot bubbles or at the cooler interface as a consequenceof hydroxyl and hydroperoxyl recombination (reactions(12) and (13)) and as a result of reaction of hydroxylradicals with oxygen atoms (reaction (14)):

2HO� ! H2O2 ð12Þ2HOO� ! H2O2 þO2 ð13Þ2HO� þ 2O! O2 þH2O2 ð14Þ

The radicals (HO� and HOO�) may also reach the liquid–bubble interface and may pass into bulk solution wherethey can react with solutes [31]. The production of H2O2

would not be expected to be enhanced at low frequency.As can be seen from Table 1, the rate of formation ofH2O2 in the absence of solutes is considerably higherthan in their presence due to the scavenging of a portionof the free radicals by the solutes.

For sonolysis of 4-CP, the identity of the intermedi-ates (i.e., hydroquinone and 4-chlorocatechol) indicatesthat HO� radicals are involved in 4-CP degradation. In

418 Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422

comparison to the sonochemical degradation of phenol,degradation of 4-CP is only partially inhibited by theaddition of excess n-butanol [11]. This suggests thatthe degradation not only takes place in solution, butalso occurs at the interface of liquid–gas bubbles whereit is oxidised by hydroxyl radicals formed within the cav-itation bubbles as a result of the sonolysis of water.

3.2. Substrate concentration and reaction rate

The effects of initial substrate concentrations on therate of sonochemical degradation of both 4-CP and phe-nol were investigated at 500 kHz with ultrasonic powerof 30 W and temperature of aqueous media of20 ± 1 �C. The initial rates of substrate degradationand H2O2 formation as a function of initial concentra-tions are shown in Fig. 2. The initial rate of 4-CP degra-dation increases almost linearly with initial 4-CPconcentration up to a concentration of �1000 lM.The initial degradation rate continues to increase withinitial 4-CP concentration as the reactants concentrationincreases further but the rate of increase plateaus in arectangular hyperbolic manner with an increasing rateobserved at initial 4-CP concentrations as high as10 mM. In contrast, the initial rate of H2O2 formationdecreases sharply with the increase of 4-CP concentra-tion. At concentrations of substrate on the order of1000 lM, H2O2 yield was too low to be detected in theaqueous solutions. These results suggest that the HO�

and HOO� radicals formed in the cavitation bubblesare completely scavenged by 4-CP (especially at higherconcentration) at the interface and are consequentlynot released into the liquid. Concomitantly, we mayconclude that the degradation of 4-CP occurs predomi-nantly at the liquid–gas bubble interface.

0

2

4

6

8

10

12

14

0 2500 5000 7500 10000Concentration (μM)

Initi

al ra

te (

μM/m

in)

V1 V2

V3 V4

Fig. 2. Effect of initial concentrations of phenol and 4-CP on initialreactant degradation and H2O2 formation rates. V1 and V2: initialrates for 4-CP degradation and H2O2 formation on sonolysis of 4-CPsolutions; V3 and V4: initial rates for phenol degradation and H2O2

formation on sonolysis of phenol solutions.

In the case of phenol, the effect of initial concentra-tions on the reaction rate appears to be quite similarto that for 4-CP, at least at lower initial phenol concen-trations. Some obvious differences, however, are ob-served at higher concentrations. There is a close linkbetween phenol degradation and H2O2 formation. Theformer reaches a limiting value of 6.0 ± 0.1 lM min�1

when the initial phenol concentration is more than2000 lM. The latter decreases with increase in phenolconcentration, but H2O2 formation is always obtainedeven when initial phenol concentration is quite high.This is most likely due to the fact that the hydroxyl rad-icals cannot be inhibited totally by phenol with the re-sult that a portion of the hydroxyl and hydroperoxylradicals produced recombine or interconvert with theO atoms in the cooler interfacial region to form H2O2.Furthermore, the maximum rate of phenol degradationin aqueous solution is just above two times the rate ofH2O2 formation observed in the absence of substrate.It is deduced that the hydroxyl radicals, which havenot recombined and have not been scavenged in theinterfacial region, degrade phenol in solution. Sono-chemical degradation of phenol hence takes place pre-dominantly in the bulk solution. These resultscorrespondent closely with those reported for phenoldegradation previously [18].

The concentration of HO� at a bubble interface insonolysis of pure water has been estimated [21,35] tobe 4 mM. Many of the chemical effects induced by ultra-sonic cavitation have been attributed to the secondaryeffects of HO� and H� production. Owing to the relativelyhigh HO� radical concentration at the site of the cavita-tional event, recombination to H2O2 is a likely fate ofthe HO� radical in the boundary layer of the bubble,even when the concentration of the reactive solute isquite high [24,32]. As the solute concentration is in-creased, HO� radical scavenging becomes more effectiveand the H2O2 yield concomitantly decreases. In fact, theinitial rate of 4-CP degradation at higher initial soluteconcentrations (>2000 lM), appears to surpass the for-mation rate of HO� and HOO� radicals by the sonolysisof pure water (according to H2O2 yields). In addition,4-CP has a lower solubility in water and larger Henry�slaw constant compared to phenol (Table 2). It is proba-ble that as the initial 4-CP concentration increases, thedegradation not only takes place (predominantly) atthe liquid–bubble interface, but also undergoes partialthermal decomposition. This is confirmed by the exper-imental results: initial rates of 4-CP degradation are farhigher than phenol�s for the same sonication conditions,especially in the range of high initial substrate concen-tration. The relative efficiencies of nonvolatile solutesto decompose thermally in the interface region dependon their hydrophobicity, which determines their abilityto accumulate in the gas–liquid interfacial region andon the activation energies for bond scission. The more

Table 2Comparison of physico-chemical properties of 4-chlorophenol (4-CP)and phenol [33]

Parameter 4-CP Phenol

Solubility in water (mg/L) 2.40E+004 8.28E+004HL�constant (atm. m3/mol) 6.27E�007 3.33E�007Pv (mmHg) 8.90E�002 3.50E�001OH (cm3/mol s) 1.03E�011 2.63E�011

HL�constant: Henry�s law constant; Pv: vapour pressure OH: rateconstant for reaction with hydroxyl radical.

Table 3Pseudo-first order rate constants (kobs) and half-times observed forsonochemical degradation of 4-chlorophenol (4-CP) for different initialconcentrations of 4-CP

[4-CP]0 (lM) kobs (min�1) t1/2 (min)

44 0.0342 20.285 0.0243 28.5178 0.0171 40.5570 0.0079 87.81110 0.0045 155.21985 0.0038 178.84930 0.0023 301.310280 0.0013 533.1

y = 0.062x + 144.06

R2 = 0.9979

y = 0.177x + 22.62

R2

= 0.99160

200

400

600

800

0 2500 5000 7500 10000[4-CP] i, μM

1/k

ob

s., m

in

Fig. 4. Variation in 1/kobs as a function of initial 4-CP concentrationwhere kobs represents the pseudo-first order rate constant observed forthe degradation of 4-CP on sonication of 4-CP solutions at 500 kHzwith ultrasonic power of 30 W and aqueous temperature of 20 ± 1 �C.

Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 419

hydrophobic the solute and the lower the activation en-ergy for bond scission, the greater the formation of ther-mal decomposition products [15].

3.3. Concentration and kinetic constant

As indicated earlier, and clearly demonstrated inFig. 3, the sonochemical degradation of aqueous 4-CPsolutions exhibits pseudo-first-order reaction kinetics.The pseudo-first-order rate constants (kobs) obtainedfor different initial 4-CP concentrations are given inTable 3. It is shown that the rate of 4-CP degradationis dependent on 4-CP initial concentration (Ci) and kobs

decreases with increasing Ci. The kobs values are de-picted in Fig. 4 as 1/kobs versus Ci and exhibit two dis-tinct regimes:

For regime 1, at low concentrations of 4-CP (61000lM),

1=kobs ¼ 22:62þ 0:177Ci R2 ¼ 0:9916

For regime 2, at higher concentration of 4-CP (P1000lM),

1=kobs ¼ 144:06þ 0:062Ci R2 ¼ 0:9979

-4

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0 30 60 90 120

Sonication time (min)

ln (

Ct/C

i)

44 μΜ 85 μΜ

178 μΜ 570 μΜ

1110 μΜ 1985 μΜ

4930 μ Μ 10280 μΜ

Fig. 3. Evolution of the relative concentration of 4-CP (shown asln(Ct/Ci) versus sonication time at 500 kHz with ultrasonic power30 W and reaction temperature 20 ± 1 �C. Ct represents the concen-tration of 4-CP at time t and Ci represents the initial concentration of4-CP.

The finding that the rate constant of 4-CP degradationby sonication decreases with increasing concentrationof 4-CP is consistent with observations by Serponeet al. [9] These authors examined ultrasonic irradiationof 4-CP under pulsed sonolytic conditions (frequency20 kHz, power 50 W) in air-equilibrated aqueous mediaat relatively low initial solute concentrations (18.2–394 lM). They showed that at the higher concentrationsof 4-CP, the sonochemical process displays the satura-tion-type kinetics reminiscent of Langmuirian behaviourin solid/gas systems. They suggest that sonochemicalreactions of chlorophenols take place in the solutionbulk at low concentrations, while at the higher concen-trations the reactions occur predominantly at the gasbubble/liquid interface.

3.4. Effects of solution temperature

The effects of aqueous temperature on sonochemicalreaction rate were investigated at two different ultra-sonic frequencies (20 and 500 kHz) with an ultrasonicpower of 30 W. The initial rates of 4-CP degradationon sonolysis of a 500 lM 4-CP solution at temperaturesof 10–45 �C are illustrated in Figs. 5 and 6, as are the

0

0.2

0.4

0.6

0.8

1

1.2

1.4

10 15 20 25 30 35 40 45Temperature, ˚C

Init

ial r

ate,

μM

/min

4-CP H2O2

Fig. 5. Variation of initial rates of 4-CP degradation and H2O2

formation with reaction temperature at 20 kHz with ultrasonic powerof 30 W. 4-CP degradation is examined in 500 lM solutions saturatedwith oxygen while H2O2 formation is examined in pure water saturatedwith oxygen.

0

1

2

3

4

10 15 20 25 30 35 40 45 50 55 60Temperature, ˚C

Init

ial r

ate,

μM

/min

4-CP H2O2

Fig. 6. Variation of initial rates of 4-CP degradation and H2O2

formation with reaction temperature at 500 kHz with ultrasonic powerof 30 W. 4-CP degradation is examined in 500 lM solutions saturatedwith oxygen while H2O2 formation is examined in pure water saturatedwith oxygen.

420 Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422

initial rates of H2O2 formation on sonolysis of oxygen-saturated pure water solution of the same temperature.As the aqueous temperature is increased, the reactionrate decreases slowly at the low frequency (20 kHz).The rate of formation of H2O2, resulting from sonolysisof water in the absence of substrate, appears to coincideclosely with that for 4-CP sonochemical degradation(Fig. 5). At this frequency, the initial rate of sonochem-ical reaction at 10 �C is about twice that at 45 �C,whether we are considering degradation of 4-CP in500 lM aqueous solution or formation of H2O2 on son-olysis of water.

In contrast, at a high frequency (500 kHz), the initialrate for 4-CP sonochemical degradation in solution aswell as for H2O2 formation is altered only slightly bytemperature change between 10 and 40 �C. As seen inFig. 6, a slight maximum is evident at a temperatureof around 40 �C. When the temperature is less than40 �C, the initial rate of 4-CP degradation increases

slightly with increase in temperature, but above 40 �C,the initial rates decline with increase in aqueoustemperature.

The differing effects of temperature at low and highfrequency may be explained by four important parame-ters affected by temperature. Increasing the temperatureof the liquid will (1) decrease the energy of cavitation,(2) lower the threshold limit of cavitation, (3) reducethe quantity of the dissolved gas, and (4) increase the va-pour pressure. At low frequency (20 kHz), due to thelarge number of cavitation bubbles formed, it is ex-pected that an increase in temperature will lead to an in-crease in the possibility of coalescence among thebubbles, resulting in some of the bubbles losing theiractivity. Additionally, the noise given off by the cavita-tion indicates that there is more likely transient (vapor-ous) cavitation occurring, which induces a decrease ofsound transmission lowering the ultrasonic effect of en-ergy in the liquid. The decrease in reaction rate with in-crease in solution temperature is supportive of such aneffect.

At high frequency (500 kHz), both the degradationrate of 4-CP and the rate of H2O2 formation increaseswith increase in solution temperature between 10 and40 �C, then decreases above 40 �C (Fig. 6). A tempera-ture optimum is observed at around 40 �C. As notedby Luche [35], an optimum reaction temperature is typ-ical in sonochemical processes with the optimal temper-ature dependent on the medium and specific reactionstudied.

With regard to the cavitational characteristics ofultrasonic irradiation, the cavitation bubbles formedby sonolysis have a more gaseous (stable) nature at highfrequency, especially at low ultrasonic intensities. Theincrease of aqueous temperature certainly increases thenumber of cavitation bubbles on sonolysis and thusthe rate of production of radicals (HO� and HOO�)though results in a lowering of the cavitation threshold.Additionally at low temperature (<40 �C), the vapourpressure of water is lower, and the solubility of gas ishigher, hence the cavitation bubbles exhibit a more gas-eous nature. As a result, the reaction rates do not de-crease with increase in solution temperature between10 and 40 �C. Indeed, the initial rate of sonochemicalreaction increased slowly as the aqueous temperatureincreased.

On the other hand, as the aqueous temperature in-creases, so does the vapour pressure of water. It is pos-tulated that at the higher temperatures (>40 �C), there isless dissolved gas present and, as a result, the cavitationbubbles formed have a more vaporous nature. Addition-ally, as a result of increasing the temperature of the li-quid, the surface tension or viscosity of the liquiddecreases and the cavitation threshold limit decreases[34,36]. As a consequence, the rate of 4-CP degradationis expected to decline above the threshold limit (>40 �C).

0

1

2

3

4

5

6

7

20 200 500 800Frequency (kHz)

Init

ial r

ate,

μM

/min

4-CP H2O2

Fig. 7. Effect of ultrasonic frequency on rate of 4-CP degradation inthe presence of 4-CP (500 lM initial concentration) and for H2O2

formation in the absence of 4-CP at ultrasonic power 30 W andtemperature 20 ± 1 �C in O2-aqueous solution.

Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 421

3.5. Effect of ultrasonic frequencies

Ultrasonic irradiations of 4-CP in saturated-O2 aque-ous solutions with initial 4-CP concentration of 500 lMwere conducted at frequencies of 20, 200, 500, and800 kHz with ultrasonic power of 30 W and aqueoustemperature of 20 ± 1 �C in each case. The initial rateof 4-CP degradation determined in each case is shownin Fig. 7. The best sonochemical destruction rate of4-CP in aqueous solution is observed to occur at200 kHz.

As previously shown for phenol and carbon tetrachlo-ride, the sonochemical destruction rate of an organiccompound is frequency dependent, and there is typicallyan optimum in the frequency which is linked with thephysical and chemical properties of the organic com-pound [19]. Reactions which involve HO� radicals (suchas hydrogen peroxide formation and 4-chlorophenoldegradation in this study) take place at the interface ofliquid–gas bubbles with a yield that reaches a maximumvalue at an optimal frequency—in this case at 200 kHz.This optimum can be explained by considering a two stepprocess [19]. In the first step, H2O and O2 are sonolysedinside the cavitation bubble to produce the radicals. Inthe second step, HO� and HOO� radicals move to theliquid–bubble interface to react with the organicsubstrate or recombine with each other to form H2O2.

The reaction rate hence depends on the number ofradicals formed within the bubble and on the extent ofradical release to the bulk liquid. As the ultrasonic fre-quency is increased, the production and intensity of cav-itation in the liquid decreases. It is therefore postulatedthat the cavitation event occurring at low frequency ismore efficient in decomposing molecules inside the bub-ble. On the other hand, most of radicals have enoughtime to recombine inside the cavity during the lifetimeof the collapse (12.5 ls at 20 kHz). As a result, the max-imum sonochemical benefit is not realised at 20 kHz.With the increase in frequency, collapse of cavitation

occurs more rapidly (1.25 ls at 200 kHz; 0.5 ls at500 kHz and 0.3 ls at 800 kHz) and more radicals es-cape from the cavitational bubble. The rate of 4-CP deg-radation is therefore expected to increase with increasein the ultrasonic frequency. However, as frequency in-creases the cavitation threshold increases due to the de-crease in the energy released by the cavity (smallerpulsating bubble). This will reduce the yield of the son-olysis (step 1) and hence the amount of radicals ejected.In other words, to achieve the maximum sonochemicalreaction rate, there should be an optimum ultrasonic fre-quency for the reactions induced by HO� radicals.

In summary, it is apparent that 4-CP can be decom-posed by sonochemical processes but with the efficiencyof the process very dependent upon reaction conditions.At low concentrations of 4-CP, the sonolysis takes placein the solution bulk while at higher concentrations thereactions occur predominantly at the gas bubble/liquidinterface. The sonochemical destruction rate of 4-CP isfrequency dependent. Of the range of frequencies studiedhere (20, 200, 500 and 800 kHz), the highest destructionrate occurs at 200 kHz. The temperature of the aqueoussolution also has an effect on efficiency of the degrada-tion process. At low frequency (20 kHz), the rate of deg-radation almost doubles on decreasing the solutiontemperature from 45 to 10 �C while at high frequency(500 kHz), the rate of 4-CP degradation is minimallyperturbed over this temperature range but with a slightoptimum at around 40 �C.

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