Sonolysis of 4-chlorophenol in aqueous solution: Effects of substrate concentration, aqueous temperature and ultrasonic frequency

Ultrasonics Sonochemistry 13 (2006) 415–422 www.elsevier.com/locate/ultsonch

Sonolysis of 4-chlorophenol in aqueous solution: Effects of substrate concentration, aqueous temperature and ultrasonic frequency Yi Jiang a

a,b

, Christian Petrier b, T. David Waite

a,*

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

b

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

Abstract The sonolysis of 4-chlorophenol (4-CP) in O2-saturated aqueous solutions is investigated for a variety of operating conditions with 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 identified as Cl, CO2, CO, and HCO2H. The rate of 4-CP degradation is dependent on the initial 4-CP concentration with an essentially linear increase in degradation rate at low initial 4-CP concentrations but with a plateauing in the rate increase observed at high reactant concentrations. The results obtained indicate that degradation takes place in the solution bulk at low reactant concentrations while at higher concentrations degradation occurs predominantly at the gas bubble–liquid interface. The aqueous temperature has a significant effect on the reaction rate. At low frequency (20 kHz) a lower liquid temperature favours the sonochemical degradation of 4-CP while at high frequency (500 kHz) the rate of 4-CP degradation is minimally perturbed with a slight optimum at around 40 C. The rate of 4-CP degradation is frequency dependant with maximum rate of degradation occurring (of the frequencies studied) 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 surface waters and groundwaters [1,2], and may be introduced into these waters either during their manufacture and use or through degradation of other chemicals (e.g., phenoxyakanoic acids). They may also be formed during industrial operations (such as the bleaching of pulp with chlorine, hydrolysis of chlorinated herbicides and oil refining) or formed as a result of the chlorination *

Corresponding author. Tel.: +61 2 9385 5060; fax: +61 2 9385 6139. E-mail address: [email protected] (T.D. Waite). 1350-4177/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2005.07.003

of humic matter during the chlorination of municipal drinking water. Chlorophenols possess relatively strong organoleptic effects with a taste threshold of 0.1 lM [3]. They are pollutants of major environmental concern due to their widespread presence and persistence. The decomposition of chlorophenols is therefore of importance and has been examined extensively by photocatalytic [4–6], biological [7,8] and more recently, sonochemical methods [9–11]. The latter method is of particular interest as it has been demonstrated to be particularly effective in removing chlorinated organic compounds from contaminated water. The propagation of ultrasonic waves in a liquid induces the formation of cavitation bubbles which grow

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

and implode under the periodic variations of the pressure field. Cavitational collapse produces intense local temperature (several 1000 K), high pressure (several 100 atm.), electrical charges and plasma effects and leads to enormous rates of heating and cooling (>109 K/s) [12,13]. Water molecules under such extreme conditions undergo thermal dissociation to yield H and HO radicals [14,15]. Organic solutes in the vicinity of collapsing bubbles or partitioned into the gas phase of the bubbles undergo thermal decomposition, and/or react with the highly reactive radicals. Substrates, such as phenol [16–19], chlorinated hydrocarbons [19–21], various aromatics [9–11,22–25], and PCBs [26] as well as explosives [27] and surfactants [28] are transformed into short-chain organic acids, inorganic ions, CO, CO2, and H2O as the final products. However, due to the complexity of the sonochemical reactions, the effects and the influences of the different parameters on the degradation reactions have not been clearly established as yet. In order to gain a better understanding of the degradation process and the reaction mechanisms, it is necessary to carry out further research before this method will be adopted by industry. In this work, 4-chlrophenol (4-CP) is chosen as a model chlorophenol and its degradation by sonication examined under a variety of different operating conditions. In particular, the rate of 4-CP degradation by sonication is investigated as a function of substrate concentration, aqueous temperature and ultrasonic frequency. Results are compared with those for the sonolysis of phenol and insights into the reaction mechanism for the sonochemical degradation of 4-CP in O2-saturated aqueous solutions are presented.

2. Material and methods 4-Chlorophenol (4-CP) and 4-chlorocatechol (4-CC) were obtained from Aldrich, while phenol, hydroquinone (HQ) and benzoquinone were obtained from Prolabo. All chemicals were reagent grade (at least 99% purity) and were used as received. Aqueous solutions were prepared by dissolving the compounds in ultrapure Milli-Q deionized water. Ultrasonic irradiation was performed in a cylindrical water-jacketed glass cell equipped a Teflon holder which accepts transducers at different frequencies. The high frequency ultrasonic transducers (200, 500 and 800 kHz) were constituted by a piezo-electric disc (diameter 4 cm) fixed on a titanium plate. Each frequency has a specific emitter connected to a high-frequency power supply. The 20 kHz irradiations were carried out with commercial equipment from Branson (Sonifier 450) equipped with a titanium probe (diameter 3.5 cm). The reactor was hermetically sealed and connected to a gas burette to ensure a constant pressure (1 atm.). The tem-

perature of the liquid was monitored using a thermocouple immersed in the reacting medium. In all cases, 250 mL of aqueous solution was saturated with O2 for 20 min prior to commencing sonication. The temperature of the media was maintained at 20 ± 1 C unless stated specifically. The ultrasonic power dissipated into the reactors was adjusted and estimated by calorimetry in order to ensure comparative ultrasonic conditions at different frequencies. 4-Chlorophenol, phenol and its primary intermediates in the course of sonochemical degradation were identified using a high performance liquid chromatograph (Waters model 600E) with an absorbance detector (Waters model 486) and equipped with a spherisorb ODS2 5 lm C18 column (250 mm · 4.6 mm). The detection wavelength was set at 254 nm and an acetonitrile/ water (45/55) mixture containing acetic acid (1%) constituted the mobile phase. Samples were injected directly into the chromatograph. The identity of intermediates was confirmed by comparing retention times with those of known standards, and their concentration determined from calibration curve. Chloride ions were detected using an ion chromatograph (Waters model ILC-1) with a conductimetric detector (Waters model 430) and equipped with a Universal anion column (150 · 4.6 mm). The mobile phase was a benzoic acid aqueous solution (4 · 103 mol L1) at pH 6 (adjusted with LiOH). The calibration was performed using an aqueous sodium chloride solution. Carbon monoxide, carbon dioxide and formic acid (HCO2H) were analysed by gas chromatograph (Intersmat model IGC 16). Separations were performed on a Porapak Q (2 mm · 2.5 m) column and detection was achieved with a FID detector after hydrogenation. 100 lL gaseous headspace was regularly sampled using a gas syringe and immediately injected. Calibration was realised with standard gaseous mixtures (Allteth Scotty II). Hydrogen peroxide was analysed iodometrically via its ammonium molybdate decomposition reaction 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 in the cylindrical jacketed glass cell described above. The experimental results for 4-CP degradation are depicted in Fig. 1. Each point in the graphs represents the average of at least three determinations. 4-CP was completely destroyed after 300 min of sonication at 500 kHz with ultrasonic power of 30 W and liquid temperature of 20 ± 1 C. The disappearance of 4-CP follows pseudofirst-order reaction kinetics with a rate constant of

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

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

Concentration, %

100

75 4-CP Cl-

50

417

HQ

Vf (lM min1) Vd (lM min1)

4-CC

25

H2O

Phenol

4-CP

2.6 ± 0.2 –

1.40 ± 0.15 2.22 ± 0.20

1.10 ± 0.15 3.00 ± 0.20

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

0 0

60

120

180

240

300

360

Sonication time (min)

interface by O atoms and O2 molecules increases the concentration of the oxidizing radicals (HO and HOO) [15] (reactions (5)–(7)). H O ! H þ HO ð1Þ

20 CO2

Concentration, %

CO

2

HCOOH

10

O2 ! 2O HO þ O ! HOO

ð2Þ

O þ H2 O ! 2HO H þ O ! HO H þ O ! HOO

ð4Þ ð5Þ

ð3Þ

ð6Þ ð7Þ

2

0 0

60

120

180

240

300

360

Sonication time (min) Fig. 1. Evolution of relative concentrations with time of reactant 4chlorophenol (4-CP), intermediates hydroquinone (HQ) and 4-chlorocatechol (4-CC) and final products Cl, CO, CO2 and HCOOH on sonication (500 kHz, 30 W) of a 500 lM 4-CP solution.

H þ H2 O ! HO þ H2

A large proportion of the radicals generated on sonolysis recombine inside the bubbles to form H2O, O and O2 (reactions (8)–(11)). ð8Þ HO þ H ! H2 O  2HO () O þ H O ð9Þ 2

1

0.012 ± 0.002 min . The primary intermediates are identified and quantified as hydroquinone (HQ) and 4-chlorocatechol (4-CC). These hydroxylated intermediates are observed to disappear on extended ultrasonic irradiation. Dechlorination is nearly quantitative and occurs soon after the disappearance of the initial substrate. The chlorine atoms are mineralised as chloride ions as the carbon–chlorine bonds are rapidly cleaved, and more than 95% of chlorine was recovered in the aqueous solution as chloride ions after sonication for 360 min. In addition to chloride ions, CO, CO2 and HCO2H are also identified as final products. Their concentrations rise slowly and combined, represent about 21% (based on C content) of the starting 4-CP concentration in aqueous solution. In comparison with the sonochemical degradation of phenol, which is a more hydrophilic compound with lower vapour pressure, 4-CP degradation appears to be considerably faster under the same sonication conditions (Table 1). In the O2-saturated aqueous solution, ultrasonic irradiation induces the formation of free radicals as a consequence of cavitation. The thermal decomposition of the water vapour and O2 in a cavitation bubble leads to the formation of HO and H radicals [14], as well as O atoms and HOO radicals [29,30] (reactions (1)– (4)). Scavenging of H radicals in the bubble or at the

2O ! O2 HO þ HOO ! O2 þ H2 O

ð10Þ ð11Þ

Note that the extent of recombination would be expected to be higher at low frequencies as the radicals will have enough time to recombine inside the bubbles. Hydrogen peroxide (H2O2) will be formed outside the hot bubbles or at the cooler interface as a consequence of hydroxyl and hydroperoxyl recombination (reactions (12) and (13)) and as a result of reaction of hydroxyl radicals with oxygen atoms (reaction (14)): 2HO ! H2 O2 ð12Þ  2HOO ! H O þ O ð13Þ 2

2

2

2HO þ 2O ! O2 þ H2 O2 

ð14Þ 

The radicals (HO and HOO ) may also reach the liquid– bubble interface and may pass into bulk solution where they 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 of H2O2 in the absence of solutes is considerably higher than in their presence due to the scavenging of a portion of the free radicals by the solutes. For sonolysis of 4-CP, the identity of the intermediates (i.e., hydroquinone and 4-chlorocatechol) indicates that HO radicals are involved in 4-CP degradation. In

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comparison to the sonochemical degradation of phenol, degradation of 4-CP is only partially inhibited by the addition of excess n-butanol [11]. This suggests that the degradation not only takes place in solution, but also occurs at the interface of liquid–gas bubbles where it is oxidised by hydroxyl radicals formed within the cavitation bubbles as a result of the sonolysis of water. 3.2. Substrate concentration and reaction rate The effects of initial substrate concentrations on the rate of sonochemical degradation of both 4-CP and phenol were investigated at 500 kHz with ultrasonic power of 30 W and temperature of aqueous media of 20 ± 1 C. The initial rates of substrate degradation and H2O2 formation as a function of initial concentrations are shown in Fig. 2. The initial rate of 4-CP degradation increases almost linearly with initial 4-CP concentration up to a concentration of 1000 lM. The initial degradation rate continues to increase with initial 4-CP concentration as the reactants concentration increases further but the rate of increase plateaus in a rectangular hyperbolic manner with an increasing rate observed at initial 4-CP concentrations as high as 10 mM. In contrast, the initial rate of H2O2 formation decreases sharply with the increase of 4-CP concentration. At concentrations of substrate on the order of 1000 lM, H2O2 yield was too low to be detected in the aqueous solutions. These results suggest that the HO and HOO radicals formed in the cavitation bubbles are completely scavenged by 4-CP (especially at higher concentration) at the interface and are consequently not released into the liquid. Concomitantly, we may conclude that the degradation of 4-CP occurs predominantly at the liquid–gas bubble interface.

14 V1

V2

V3

V4

Initial rate ( μM/min)

12 10 8 6 4 2 0 0

2500

5000

7500

10000

Concentration (μM) Fig. 2. Effect of initial concentrations of phenol and 4-CP on initial reactant degradation and H2O2 formation rates. V1 and V2: initial rates for 4-CP degradation and H2O2 formation on sonolysis of 4-CP solutions; 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 concentrations on the reaction rate appears to be quite similar to that for 4-CP, at least at lower initial phenol concentrations. Some obvious differences, however, are observed at higher concentrations. There is a close link between phenol degradation and H2O2 formation. The former reaches a limiting value of 6.0 ± 0.1 lM min1 when the initial phenol concentration is more than 2000 lM. The latter decreases with increase in phenol concentration, but H2O2 formation is always obtained even when initial phenol concentration is quite high. This is most likely due to the fact that the hydroxyl radicals cannot be inhibited totally by phenol with the result that a portion of the hydroxyl and hydroperoxyl radicals produced recombine or interconvert with the O atoms in the cooler interfacial region to form H2O2. Furthermore, the maximum rate of phenol degradation in aqueous solution is just above two times the rate of H2O2 formation observed in the absence of substrate. It is deduced that the hydroxyl radicals, which have not recombined and have not been scavenged in the interfacial region, degrade phenol in solution. Sonochemical degradation of phenol hence takes place predominantly in the bulk solution. These results correspondent closely with those reported for phenol degradation previously [18]. The concentration of HO at a bubble interface in sonolysis of pure water has been estimated [21,35] to be 4 mM. Many of the chemical effects induced by ultrasonic cavitation have been attributed to the secondary effects of HO and H production. Owing to the relatively high HO radical concentration at the site of the cavitational event, recombination to H2O2 is a likely fate of the HO radical in the boundary layer of the bubble, even when the concentration of the reactive solute is quite high [24,32]. As the solute concentration is increased, HO radical scavenging becomes more effective and the H2O2 yield concomitantly decreases. In fact, the initial rate of 4-CP degradation at higher initial solute concentrations (>2000 lM), appears to surpass the formation rate of HO and HOO radicals by the sonolysis of pure water (according to H2O2 yields). In addition, 4-CP has a lower solubility in water and larger HenryÕs law constant compared to phenol (Table 2). It is probable that as the initial 4-CP concentration increases, the degradation not only takes place (predominantly) at the liquid–bubble interface, but also undergoes partial thermal decomposition. This is confirmed by the experimental results: initial rates of 4-CP degradation are far higher than phenolÕs for the same sonication conditions, especially in the range of high initial substrate concentration. The relative efficiencies of nonvolatile solutes to decompose thermally in the interface region depend on their hydrophobicity, which determines their ability to accumulate in the gas–liquid interfacial region and on the activation energies for bond scission. The more

Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 Table 2 Comparison of physico-chemical properties of 4-chlorophenol (4-CP) and phenol [33] Parameter

4-CP

Phenol

Solubility in water (mg/L) HLÕconstant (atm. m3/mol) Pv (mmHg) OH (cm3/mol s)

2.40E+004 6.27E007 8.90E002 1.03E011

8.28E+004 3.33E007 3.50E001 2.63E011

HLÕconstant: HenryÕs law constant; Pv: vapour pressure OH: rate constant for reaction with hydroxyl radical.

hydrophobic the solute and the lower the activation energy for bond scission, the greater the formation of thermal decomposition products [15].

419

Table 3 Pseudo-first order rate constants (kobs) and half-times observed for sonochemical degradation of 4-chlorophenol (4-CP) for different initial concentrations of 4-CP [4-CP]0 (lM)

kobs (min1)

t1/2 (min)

44 85 178 570 1110 1985 4930 10 280

0.0342 0.0243 0.0171 0.0079 0.0045 0.0038 0.0023 0.0013

20.2 28.5 40.5 87.8 155.2 178.8 301.3 533.1

800

3.3. Concentration and kinetic constant

y = 0.062x + 144.06 2

R = 0.9979

1=k obs ¼ 22:62 þ 0:177C i

R2 ¼ 0:9916

For regime 2, at higher concentration of 4-CP (P1000 lM), 1=k obs ¼ 144:06 þ 0:062C i

2

R ¼ 0:9979

Sonication time (min) 0

30

60

90

120

0 -0.5

ln (Ct/Ci)

-1 -1.5 -2 -2.5 -3

44 μΜ

85 μΜ

178 μΜ

570 μΜ

1110 μΜ

1985 μΜ

4930 μ Μ

10280 μΜ

1/k obs., min

As indicated earlier, and clearly demonstrated in Fig. 3, the sonochemical degradation of aqueous 4-CP solutions exhibits pseudo-first-order reaction kinetics. The pseudo-first-order rate constants (kobs) obtained for different initial 4-CP concentrations are given in Table 3. It is shown that the rate of 4-CP degradation is dependent on 4-CP initial concentration (Ci) and kobs decreases with increasing Ci. The kobs values are depicted in Fig. 4 as 1/kobs versus Ci and exhibit two distinct regimes: For regime 1, at low concentrations of 4-CP (61000 lM),

600

400

200 y = 0.177x + 22.62 2

R = 0.9916 0 0

2500

5000

[4-CP] i, μM

7500

10000

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

The finding that the rate constant of 4-CP degradation by sonication decreases with increasing concentration of 4-CP is consistent with observations by Serpone et al. [9] These authors examined ultrasonic irradiation of 4-CP under pulsed sonolytic conditions (frequency 20 kHz, power 50 W) in air-equilibrated aqueous media at relatively low initial solute concentrations (18.2– 394 lM). They showed that at the higher concentrations of 4-CP, the sonochemical process displays the saturation-type kinetics reminiscent of Langmuirian behaviour in solid/gas systems. They suggest that sonochemical reactions of chlorophenols take place in the solution bulk at low concentrations, while at the higher concentrations the reactions occur predominantly at the gas bubble/liquid interface. 3.4. Effects of solution temperature

-3.5 -4

Fig. 3. Evolution of the relative concentration of 4-CP (shown as ln(Ct/Ci) versus sonication time at 500 kHz with ultrasonic power 30 W and reaction temperature 20 ± 1 C. Ct represents the concentration of 4-CP at time t and Ci represents the initial concentration of 4-CP.

The effects of aqueous temperature on sonochemical reaction rate were investigated at two different ultrasonic frequencies (20 and 500 kHz) with an ultrasonic power of 30 W. The initial rates of 4-CP degradation on sonolysis of a 500 lM 4-CP solution at temperatures of 10–45 C are illustrated in Figs. 5 and 6, as are the

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Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 1.4

Initial rate, μM/min

1.2

4-CP

H2O2

35

40

1 0.8 0.6 0.4 0.2 0

10

15

20

25

30

45

Temperature, ˚C Fig. 5. Variation of initial rates of 4-CP degradation and H2O2 formation with reaction temperature at 20 kHz with ultrasonic power of 30 W. 4-CP degradation is examined in 500 lM solutions saturated with oxygen while H2O2 formation is examined in pure water saturated with oxygen.

4

Initial rate, μM/min

4-CP

H2O2

3

2

1

0

10

15

20

25

30

35

40

45

50

55

60

Temperature, ˚C Fig. 6. Variation of initial rates of 4-CP degradation and H2O2 formation with reaction temperature at 500 kHz with ultrasonic power of 30 W. 4-CP degradation is examined in 500 lM solutions saturated with oxygen while H2O2 formation is examined in pure water saturated with oxygen.

initial rates of H2O2 formation on sonolysis of oxygensaturated pure water solution of the same temperature. As the aqueous temperature is increased, the reaction rate decreases slowly at the low frequency (20 kHz). The rate of formation of H2O2, resulting from sonolysis of water in the absence of substrate, appears to coincide closely with that for 4-CP sonochemical degradation (Fig. 5). At this frequency, the initial rate of sonochemical reaction at 10 C is about twice that at 45 C, whether we are considering degradation of 4-CP in 500 lM aqueous solution or formation of H2O2 on sonolysis of water. In contrast, at a high frequency (500 kHz), the initial rate for 4-CP sonochemical degradation in solution as well as for H2O2 formation is altered only slightly by temperature change between 10 and 40 C. As seen in Fig. 6, a slight maximum is evident at a temperature of around 40 C. When the temperature is less than 40 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 aqueous temperature. The differing effects of temperature at low and high frequency may be explained by four important parameters affected by temperature. Increasing the temperature of the liquid will (1) decrease the energy of cavitation, (2) lower the threshold limit of cavitation, (3) reduce the quantity of the dissolved gas, and (4) increase the vapour pressure. At low frequency (20 kHz), due to the large number of cavitation bubbles formed, it is expected that an increase in temperature will lead to an increase in the possibility of coalescence among the bubbles, resulting in some of the bubbles losing their activity. Additionally, the noise given off by the cavitation indicates that there is more likely transient (vaporous) cavitation occurring, which induces a decrease of sound transmission lowering the ultrasonic effect of energy in the liquid. The decrease in reaction rate with increase in solution temperature is supportive of such an effect. At high frequency (500 kHz), both the degradation rate of 4-CP and the rate of H2O2 formation increases with increase in solution temperature between 10 and 40 C, then decreases above 40 C (Fig. 6). A temperature optimum is observed at around 40 C. As noted by Luche [35], an optimum reaction temperature is typical in sonochemical processes with the optimal temperature dependent on the medium and specific reaction studied. With regard to the cavitational characteristics of ultrasonic irradiation, the cavitation bubbles formed by sonolysis have a more gaseous (stable) nature at high frequency, especially at low ultrasonic intensities. The increase of aqueous temperature certainly increases the number of cavitation bubbles on sonolysis and thus the rate of production of radicals (HO and HOO) though results in a lowering of the cavitation threshold. Additionally at low temperature (<40 C), the vapour pressure of water is lower, and the solubility of gas is higher, hence the cavitation bubbles exhibit a more gaseous nature. As a result, the reaction rates do not decrease with increase in solution temperature between 10 and 40 C. Indeed, the initial rate of sonochemical reaction increased slowly as the aqueous temperature increased. On the other hand, as the aqueous temperature increases, so does the vapour pressure of water. It is postulated that at the higher temperatures (>40 C), there is less dissolved gas present and, as a result, the cavitation bubbles formed have a more vaporous nature. Additionally, as a result of increasing the temperature of the liquid, the surface tension or viscosity of the liquid decreases and the cavitation threshold limit decreases [34,36]. As a consequence, the rate of 4-CP degradation is expected to decline above the threshold limit (>40 C).

Y. Jiang et al. / Ultrasonics Sonochemistry 13 (2006) 415–422 7

Initial rate, μM/min

6

4-CP

H2O2

5 4 3 2 1 0

20

200

500

800

Frequency (kHz) Fig. 7. Effect of ultrasonic frequency on rate of 4-CP degradation in the presence of 4-CP (500 lM initial concentration) and for H2O2 formation in the absence of 4-CP at ultrasonic power 30 W and temperature 20 ± 1 C in O2-aqueous solution.

3.5. Effect of ultrasonic frequencies Ultrasonic irradiations of 4-CP in saturated-O2 aqueous solutions with initial 4-CP concentration of 500 lM were conducted at frequencies of 20, 200, 500, and 800 kHz with ultrasonic power of 30 W and aqueous temperature of 20 ± 1 C in each case. The initial rate of 4-CP degradation determined in each case is shown in Fig. 7. The best sonochemical destruction rate of 4-CP in aqueous solution is observed to occur at 200 kHz. As previously shown for phenol and carbon tetrachloride, the sonochemical destruction rate of an organic compound is frequency dependent, and there is typically an optimum in the frequency which is linked with the physical and chemical properties of the organic compound [19]. Reactions which involve HO radicals (such as hydrogen peroxide formation and 4-chlorophenol degradation in this study) take place at the interface of liquid–gas bubbles with a yield that reaches a maximum value at an optimal frequency—in this case at 200 kHz. This optimum can be explained by considering a two step process [19]. In the first step, H2O and O2 are sonolysed inside the cavitation bubble to produce the radicals. In the second step, HO and HOO radicals move to the liquid–bubble interface to react with the organic substrate or recombine with each other to form H2O2. The reaction rate hence depends on the number of radicals formed within the bubble and on the extent of radical release to the bulk liquid. As the ultrasonic frequency is increased, the production and intensity of cavitation in the liquid decreases. It is therefore postulated that the cavitation event occurring at low frequency is more efficient in decomposing molecules inside the bubble. On the other hand, most of radicals have enough time to recombine inside the cavity during the lifetime of the collapse (12.5 ls at 20 kHz). As a result, the maximum sonochemical benefit is not realised at 20 kHz. With the increase in frequency, collapse of cavitation

421

occurs more rapidly (1.25 ls at 200 kHz; 0.5 ls at 500 kHz and 0.3 ls at 800 kHz) and more radicals escape from the cavitational bubble. The rate of 4-CP degradation is therefore expected to increase with increase in the ultrasonic frequency. However, as frequency increases the cavitation threshold increases due to the decrease in the energy released by the cavity (smaller pulsating bubble). This will reduce the yield of the sonolysis (step 1) and hence the amount of radicals ejected. In other words, to achieve the maximum sonochemical reaction rate, there should be an optimum ultrasonic frequency for the reactions induced by HO radicals. In summary, it is apparent that 4-CP can be decomposed by sonochemical processes but with the efficiency of the process very dependent upon reaction conditions. At low concentrations of 4-CP, the sonolysis takes place in the solution bulk while at higher concentrations the reactions occur predominantly at the gas bubble/liquid interface. The sonochemical destruction rate of 4-CP is frequency dependent. Of the range of frequencies studied here (20, 200, 500 and 800 kHz), the highest destruction rate occurs at 200 kHz. The temperature of the aqueous solution also has an effect on efficiency of the degradation process. At low frequency (20 kHz), the rate of degradation almost doubles on decreasing the solution temperature from 45 to 10 C while at high frequency (500 kHz), the rate of 4-CP degradation is minimally perturbed over this temperature range but with a slight optimum at around 40 C.

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