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Water Research 39 (2005) 1093–1098

www.elsevier.com/locate/watres

Effect of hydrogen peroxide on the decomposition ofmonochlorophenols by sonolysis in aqueous solution

Young Ku�, Yu-Hsin Tu, Chih-Ming Ma

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road,

Section 4, Taipei 106, Taiwan

Received 19 March 2004; received in revised form 22 September 2004; accepted 16 November 2004

Abstract

The decomposition rates of several monochlorophenol by sonolysis were enhanced by the presence of hydrogen

peroxide. An optimum concentration of hydrogen peroxide was observed for achieving maximum sonolysis rate of

monochlorophenol. The decomposition rates of 3-chlorophenol by sonolysis were higher than those for 2- and 4-

chlorophenol for most experiments conducted, suggesting that the ring structure of 3-chlorophenol provides more sites

available for free radical attack. The temporal decomposition behavior of monochlorophenol in aqueous solutions is

markedly influenced by the species distribution and the volatility of specific monochlorophenol.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Ultrasound process; Chlorophenols; Sonolysis

1. Introduction

Sonolysis is an innovative process that has attracted

attention, at present, as a promising alternative for the

destruction of a wide range of compounds present in

water or wastewater under relatively mild conditions.

However, the oxidative decomposition rate of organic

compounds by sonolysis was relatively less than those by

other chemical treatment processes. Thus, the combina-

tion of sonolysis with hydrogen peroxide provides an

operational alternative to enhance the reaction rate and

is attracting extensive research activities, recently (Teo et

al., 2001; Gogate et al., 2002; Entezari et al., 2003) and is

considered to be a capable option for the removal of

refractory organics in aqueous solutions. The purpose of

this study is to investigate the effect of several process

e front matter r 2005 Elsevier Ltd. All rights reserve

atres.2004.11.036

ing author. Tel.: +886 2 27333141;

85535.

ess: Ku508@mail.ntust.edu.tw (Y. Ku).

variables on the sonochemical decomposition of several

monochlorophenols in aqueous solutions with the

presence of various amounts of hydrogen peroxide.

The rate and temporal behaviors of the decomposition

of monochlorophenols by sonolysis were studied and

characterized using an empirical kinetic model.

2. Experimental

Chemicals used in this study were reagent grade

purchased from major chemical companies and were

used without any further purification. All experimental

solutions were prepared with double distilled water.

Monochlorophenols investigated in this study include 2-

chlorophenol, 3-chlorophenol, and 4-chlorophenol.

Most of the reactions conducted were carried out in a

capped cylindrical reactor made entirely of Pyrex glass.

The effective volume of the reactor was 240mL with an

d.

ARTICLE IN PRESSY. Ku et al. / Water Research 39 (2005) 1093–10981094

inner diameter of 4.5 cm and a height of 15 cm. A well-

mixed storage tank was water-jacketed to keep solution

temperature isothermally at 23 1C. The storage tank was

used to hold 1-L aqueous solution containing various

concentrations of specific monochlorophenol (ranging

from 10 to 30mg/L) and hydrogen peroxide (ranging

from 100 to 1000mg/L), and was operated at total reflux

to the reactor. The solution pH was maintained constant

at predetermined levels, between pH 3.0 and 9.0, by the

addition of sodium hydroxide (NaOH) and/or sulfuric

acid (H2SO4) solutions using a Kyoto APB-118-20B

autotitrator.

The input of sonication to the reactor was controlled

at 50W/cm2 by adjusting the power input to a pre-

warmed Heat System XL-2020 sonicator with Ti-alloy

tip inserted in the centerline of the cylindrical reactor

with an airtight joint at top. The maximum output of the

sonicator was determined calorimetrically (Mason et al.,

1992) to be approximately 150W at the frequency of

20 kHz. The outer diameter and surface area of the tip

are 3/4 in and 2.865 cm2, respectively. Argon gas was

continuously purged to the reaction solutions in the

storage tank at a constant flow rate of 0.03 L/min. The

experimental system was designed for total reflux

operation, and the schematic diagram of experimental

setup is shown as Fig. 1. The surface of the tip was

found to be gradually corroded during the course of

experiment; therefore, the tip of sonicator was replaced

after about 40 h of operation time to warrant the

sonication output kept at relatively constant levels.

Typical experimental runs conducted in this study

lasted for about 6 h of reaction time. Aliquots of

6

1. Refrigarated cooler2. Electromagnetic stirrer3. Temperature4. Vessel5. Flowmeter6. pH meter

7. Magnet 8. Reciprocating pump 9. Sonication reactor10. Ultrasonic horn11. Tank12. Ultrasonic generator

14

3

5

12

10

119

82

7

Fig. 1. Schematic diagram of the experimental setup.

reaction solution were withdrawn from the storage tank

at desired time intervals for further analysis. The total

sample volume throughout an experiment was kept

below 5% of the initial solution volume in the storage

tank. The presence of hydrogen peroxide in aqueous

solution was determined instantaneously using a Shi-

madsu UV-160A spectrophotometer operated with a

wavelength of 385 nm. Chlorophenols present in aqu-

eous solution were analyzed by a Spectra-Physics P1000

HPLC equipped with Inertsil ODS-3 (5 mm) column and

a Spectra System UV 1000 detector. For some experi-

ments, the formation of chloride ion during the sonolysis

of chlorophenols was identified by a Dionex DX-100 ion

chromatograph.

3. Results and discussion

The application of sonolysis employs the ultrasonic

energy to generate cyclic pressure wave in aqueous

solution. The intermolecular distances between water

molecules are, therefore, altered by the pressure wave

to the creation of numerous microscopic bubbles

called cavities. The sizes of cavities are expanded until

acoustic cavitational threshold is reached; the cavities

are collapsed subsequently (Mason and Lorimer, 1988).

It has been reported that pressures of hundreds

of atmospheres and temperatures of thousands of

degrees may be generated during the violent collapse

of these cavities (Suslik and Hammerton, 1986).

Water molecules are therefore broken up to generate

highly oxidative hydroxyl free radicals through the

creation and collapse of cavities. Consequently, the

decomposition of organic compounds was contributed

by the pyrolysis within the cavities and by the attack

of free radicals occurred in the aqueous solutions.

Hence, the temporal decomposition behavior in aqueous

solutions by sonolysis is markedly influenced by

the species distribution and the volatility of specific

compound.

The disappearance of monochlorophenols in aqueous

solutions with the presence of high concentration

(1000mg/L) of hydrogen peroxide was found to be

negligible (less than 1.0%) without sonication within 6 h

of reaction time. Fig. 2.illustrates the temporal variation

of 2-chlorophenol concentrations for the sonolysis of 2-

chlorophenol in aqueous solution aerated with argon. In

this study, the log plots of monochlorophenols concen-

tration were almost linear over the entire reaction

durations suggesting first-order kinetics with respect to

the concentration of monochlorophenols. By a best

fitting procedure, the value of kA can be determined and

the correlation coefficients are more than 0.97. Similar

results were also obtained for experiments conducted at

other conditions.

ARTICLE IN PRESS

0 100 150 200 250 300 350 4000.0

0.2

0.4

0.6

0.8

1.0

[2-C

P]/[

2-C

P]0

Reaction time (min)

Ultrasonic systemReactor volume = 1.0 LpH = 3 ± 0.1Temperature = 23 ± 1°CAerated gas : ArGas flow = 0.03 L /minStirring speed = 400 rpmAcoustic intensity = 50 W/cm2

50

[2-CP]o = 15 mg/L

[2-CP]o = 30 mg/L

Fig. 2. Time dependent changes on the decomposition of 2-

chlorophenol by sonolysis in aqueous solution aerated with

argon.

0 600 800 10000

2

4

6

8

10

12

2-CP 3-CP 4-CP

H2O2 concentration (mg/L)

k A ×

103 (

1/m

in)

Ultrasonic system[2-CP]o

= 15 mg/L[3-CP]o = 15 mg/L[4-CP]o = 15 mg/L

pH = 3.0±0.1Intensity = 50 W/cm2

Reactor volume = 1.0 LTemperature = 23±1 °CStirring speed = 400 rpm

Gas flowrate = 0.03 L /min

200 400 1200

Fig. 3. Effect of hydrogen peroxide on the decomposition rate

constants for the sonolysis of monochlorophenol in acidic

solution.

Y. Ku et al. / Water Research 39 (2005) 1093–1098 1095

Fig. 3 illustrates the calculated decomposition rate

constants of monochlorophenols by sonolysis for

experiments conducted with the presence of various

concentrations of hydrogen peroxide in acidic solutions.

For experiments conducted with less than 500mg/L of

hydrogen peroxide, the sonication rate constant was

increased with the concentration of hydrogen peroxide

in acidic solutions. For instance, the sonication rate

constant for 3-chlorophenol was enhanced more than

50% with the addition of 200mg/L of hydrogen

peroxide. Various researchers reported similar experi-

mental results on the decomposition of numerous

organic compounds in aqueous solution by sonolysis

(Teo et al., 2001; Gogate et al., 2002; Entezari et al.,

2003). A possible explanation is that hydroxyl peroxide

molecules may be cleaved into hydroxyl radicals during

the collapse of the cavitation bubble and contribute to

the disappearance of monochlorophenols (Visscher and

Langenhove, 1998; Entezari et al., 2003). Furthermore,

part of the hydroxyl free radicals generated by sonolysis

in aqueous solution may be recombined to form

hydrogen peroxide molecules:

OHþOH Ð H2O2. (1)

Based on Le Chatelier’s principle, the presence of

hydrogen peroxide may hamper the combination of

hydroxyl free radicals, and increase the amount of free

radicals available for the decomposition of organic

compounds.

In this study, the formation of chloride ion was

identified by a Dionex DX-100 ion chromatograph for

several experiments; however, the cleavage of benzene

ring was not monitored for all experiments. The

formation rates of chloride ion were roughly comparable

to the decomposition rates of chlorophenols. In addi-

tion, the decomposition of these monochlorophenols by

only using hydrogen peroxide was found to be negligible

for blank experiments. For reasons mentioned above, it

could elucidate that only the OH radical reaction was

accelerated.

However, an optimum concentration of hydrogen

peroxide was observed in this study for achieving highest

decomposition rate of monochlorophenol by sonolysis.

The presence of excessive amounts of hydrogen peroxide

reduced the decomposition rate of monochlorophenol

by sonolysis. As illustrated in Fig. 3, the sonication rate

constant was decreased markedly for experiments

conducted with 15mg/L of 3-chlorophenol and

700mg/L of hydrogen peroxide, even lower than those

for experiments conducted without the presence of

hydrogen peroxide. For the decomposition of several

organics by various advanced oxidation processes

(Beltran et al., 1998a, b; Nelieu et al., 2000; Esplugas

et al., 2002), the hydroxyl free radicals that exist in

aqueous solution were reported to be scavenged by

excessive hydrogen peroxide molecules to form much

less oxidative hydroperoxyl radicals, which can be

described as

OHd þH2O2 ) HOd2 þH2O; (2)

HOd2 þOHd ) H2OþO2: (3)

ARTICLE IN PRESS

k A ×

103 (

1/m

in)

00

2

4

6

8

10

12

14

Intensity = 50 W/cm2

Reactor volume = 1.0 LTemperature = 23±1 °CStirring speed = 400 rpmGas flowrate = 0.03 L /min

[H2O2]o = 500 mg/LpH = 3.0±0.1

Ultrasonic system

2-CP

3-CP4-CP

10 20 30 40 50

Chlorophenol concentration (mg/L)

Fig. 5. Effect of initial concentrations on the decomposition

rate constants for the sonolysis of monochlorophenol in acidic

solution.

Y. Ku et al. / Water Research 39 (2005) 1093–10981096

The rate constants of above equations were deter-

mined by Christensen et al. (1982) to be 2.7� 107 and

7.5� 109M�1 s�1 for Eqs. (2) and (3), respectively. The

rate constants for the reaction of these monochlorophe-

nols with OH radicals in water at ambient temperature

would be in the order of 109M�1 s�1 (Nagata et al.,

2000). Comparing the above rate constants, it is

reasonable to assume that excessive hydrogen peroxide

might react with the hydroxyl free radicals competitively

to form hydroperoxyl radicals, especially in experiments

conducted in alkaline solutions, which were much less

oxidative and did not contribute to the degradation of

monochlorophenols.

The optimum concentration of hydrogen peroxide

was found to be varied with the initial concentration of

monochlorophenol, as shown in Fig. 4, for the sonolysis

of 3-chlorophenol. For experiment conducted with

20mg/L of 3-chlorophenol, the reaction rate constant

was kept high with the presence of 700mg/L of

hydrogen peroxide; however, the reaction rate constant

was slightly decreased for experiment conducted with

the presence of 1000mg/L of hydrogen peroxide. A

series of experiments was conducted in acidic solutions

to study the effect of hydrogen peroxide/monochlor-

ophenol molar ratio on the sonolysis rates of mono-

chlorophenols in the presence of 500mg/L of hydrogen

peroxide; the results are demonstrated in Fig. 5. The

sonolysis rate constants were noticeably decreased with

increasing concentration of monochlorophenol pre-

sented in aqueous solution suggesting the reaction rate

was limited by the concentration of hydrogen peroxide.

H2O2 concentration (mg/L)

k A ×

103 (1

/min

)

0 600 800 10000

2

4

6

8

10

12

200 400 1200

Ultrasonic systempH = 3.0±0.1Intensity = 50 W/cm2

Reactor volume = 1.0 L Temperature = 23±1 °CStirring speed = 400 rpmGas flowrate = 0.03 L /min

[3-CP]o = 10 mg/L [3-CP]o = 15 mg/L [3-CP]o = 20 mg/L [3-CP]o = 30 mg/L

Fig. 4. Decomposition rate constants for the sonolysis of 3-

chlorophenol in acidic solutions with the presence of hydrogen

peroxide.

Further studies are necessary for the understanding of

detailed mechanism caused by the presence of various

surfactants on the decomposition of various pollutants

by sonolysis. The optimum concentrations of hydrogen

peroxide obtained for experiments conducted in acidic

solution with various initial concentrations of 3-chlor-

ophenol were correlated and presented in Fig. 6. An

approximately linear relationship was observed with a

slope of 32.38, which could be employed to estimate the

optimum amounts of hydrogen peroxide required to

achieve higher levels of monochlorophenol decomposi-

tion by sonolysis.

Even though the vapor pressure of 3-chlorophenol is

less than those of 2- and 4-chlorophenols (Perry and

Green, 1997), the decomposition rate for 3-chlorophenol

by sonolysis in aqueous solutions is the highest among

the monochlorophenols for most experiments conducted

in this study. The free radical attack of ionic species in

aqueous solutions seems to make a more significant

contribution than the pyrolysis of molecular species in

cavities to the decomposition of these monochlorophe-

nols by sonolysis. Based on the results described by

several previous researchers (Okouchi et al., 1992;

Serpone and Terzian, 1994; Willberg et al., 1996), the

attack of free radicals took place more favorably at the

ortho- and para- positions of the benzene ring structure

for the sonolysis of various aromatic compounds.

Therefore, the higher decomposition rate of 3-chlor-

ophenol by sonolysis is due to the fact that the benzene

ring structure of 3-chlorophenol provides one more site

than those of 2- and 4-chlorophenols available for free

ARTICLE IN PRESS

0

200

400

600

800

1000

1200

3-Chlorophenol concentration (mg/L)

0

Intensity = 50 W/cm2

Reactor volume = 1.0 LTemperature = 23±1 °CStirring speed = 400 rpmGas flowrate = 0.03 L /min

pH = 3.0±0.1Ultrasonic system

10 20 30 40 50

H2O

2 co

ncen

trat

ion

(mg/

L)

Fig. 6. Optimum concentrations of hydrogen peroxide with

various initial concentrations of 3-chlorophenol in acidic

solution.

0

5

10

15

20

pH

[2-CP]o = 15 mg/L[3-CP]o = 15 mg/L[4-CP]o = 15 mg/L[H2O2]o= 500 mg/LIntensity = 50 W/cm2

Reactor volume = 1.0 LTemperature = 23±1 °CStirring speed = 400 rpmGas flowrate = 0.03 L /min

Ultrasonic system

0 102 4 6 8 12 14

2-CP 3-CP

4-CP

H2O

2 co

ncen

trat

ion

(mg/

L)

Fig. 7. Effect of solution pH on the formation of hydrogen

peroxide for the sonolysis of monochlorophenol.

0

pH

[2-CP]o = 15 mg/L[3-CP]o = 15 mg/L[4-CP]o = 15 mg/L[H2O2]o= 500 mg/LIntensity = 50 W/cm2

Reactor volume = 1.0 LTemperature = 23±1 °CStirring speed = 400 rpmGas flowrate = 0.03 L/min

Ultrasonic system

0 10

10

2

2

4

4

6

6

8

8

12

12

14

2-CP

3-CP 4-CP

k A ×

103 (

1/m

in)

Fig. 8. Effect of solution pH on the decomposition rate

constants for the sonolysis of monochlorophenol.

Y. Ku et al. / Water Research 39 (2005) 1093–1098 1097

radical attack. As depicted in Fig. 7, the formation of

about 5–20mg/L of hydrogen peroxide was detected

during the decomposition of monochlorophenols for

experiments conducted in aqueous solutions of various

pH levels. Least amounts of hydrogen peroxide were

formed in the sonolysis of 3-chlorophenol compared to

other monochlorophenols, suggesting that the combina-

tion of free radicals interfere less in the attack of

hydroxyl free radicals on 3-chlorophenol .

Effect of pH level of the solution on the calculated

reaction rate constant for the sonolysis of monochlor-

ophenols in aqueous solutions with the presence of

hydrogen peroxide is shown in Fig. 8. Various research-

ers reported similar experimental results that the

sonolysis rates of several phenolic compounds were

decreased with increase pH levels of the solution

(Kotronarous et al., 1991a, b; Tauber et al., 2000). The

dissociation constants (pKa) of the monochlorophenols

investigated in this study lie between pH 8.0 and 9.0

(Perry and Green, 1997). That is, the molecular species

of these monochlorophenols predominates in acidic and

neutral solutions while the much less volatile ionic

species dominates in alkaline solutions. Kotronarous et

al. (1991a, b) stated that the hydrophobic molecular

species present in acidic and neutral solutions may

diffuse more easily into the film region around the

cavities and decompose by free radical attack, part of

molecular species may even evaporate in the cavities and

decompose by pyrolysis. However, the decomposition of

ionic species present in alkaline solution was contributed

exclusively by the free radical attack in bulk solution.

4. Summary

The decomposition rates of monochlorophenol by

sonolysis were enhanced by the presence of hydrogen

peroxide in aqueous solution. Excessive amounts of

hydrogen peroxide present in aqueous solution reduced

ARTICLE IN PRESSY. Ku et al. / Water Research 39 (2005) 1093–10981098

the decomposition rate of monochlorophenol by sono-

lysis. An optimum concentration of hydrogen peroxide

was observed for achieving highest decomposition rate

of monochlorophenol by sonolysis. An approximately

linear relationship was observed for the optimum

concentrations of hydrogen peroxide and the initial

concentrations of 3-chlorophenol present in aqueous

solution. The decomposition rates of 3-chlorophenol by

sonolysis were higher than those for 2- and 4-chlor-

ophenol, suggesting that the benzene ring structure of 3-

chlorophenol provides one more site available for free

radical attack. The molecular species present in acidic

and neutral solutions were decomposed by both

pyrolysis and free radical attack, while the decomposi-

tion of ionic species presented in alkaline solution was

merely because of the free radical attack in bulk

solution.

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