sonochemical decomposition of volatile and non-volatile organic compounds—a comparative study

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Water Research 38 (2004) 4247–4261

www.elsevier.com/locate/watres

Sonochemical decomposition of volatile and non-volatileorganic compounds—a comparative study

Mukesh Goela, Hu Hongqianga, Arun S. Mujumdarb,Madhumita Bhowmick Raya,�

aDepartment of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4,

Singapore 117576, SingaporebDepartment of Mechanical Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576, Singapore

Received 3 February 2004; received in revised form 23 July 2004; accepted 4 August 2004

Abstract

Sonochemical degradation which combines destruction of the target compounds by free radical reaction and thermal

cleavage is one of the recent advanced oxidation processes (AOP) and proven to be effective for removing low

concentration organic pollutants from aqueous streams. This work describes the degradation of several organic

compounds of varying volatility in aqueous solution in two types of ultrasonic reactors. The process variables studied

include initial concentration of the organics, temperature, and type of saturated gas. The effects of additional oxidant

and electrolyte were also examined. A kinetic model was tested to determine its ability to predict the degradation rate

constant of different volatile organic compounds at different initial conditions. A figure of merit for the electrical energy

consumption for the two types of ultrasonic reactors is also presented.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Advanced oxidation process; Sonolysis; Cavitation; Volatile compounds; Dye; EE/O

1. Introduction

Over the last two decades, advanced oxidation

processes (AOP) have been used considerably to remove

low to trace amounts of organic compounds from both

aqueous and gaseous waste streams. Free radicals

involved in AOP can be generated using several

radiation methods including UV, g-radiation, electron-beam and ultrasonic waves. Among the above, ultra-

sonication is probably one of the less studied methods

despite its very unique and extreme conditions generated

e front matter r 2004 Elsevier Ltd. All rights reserve

atres.2004.08.008

ing author. Tel.: +65-6779-1936; fax: +65-

ess: [email protected] (M.B. Ray).

without using complicated and expensive apparatus.

Ultrasonication not only promotes oxidative degrada-

tion of the target compound by hydroxyl radicals, but

also provides a possible route for thermal decomposition

in the gas phase (Ince et al., 2001).

The chemical effect of ultrasound is produced through

the phenomenon of cavitation, which is caused by the

expansion and contraction of cavitation nuclei due to

the compression and rarefaction cycles of the ultrasonic

waves. Cavitation causes the formation, rapid g

rowth and finally implosive collapse of the bubbles,

resulting in unusual reaction environment in the

vicinity of the bubbles (Joseph et al., 2000). Compres-

sion of gas and vapor in the bubbles generates intense

heat and can generate local hot spots. Suslick et al.

d.

www.elsevier.com/locate/watres

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Nomenclature

EE/O electric energy per order of pollutant re-

moval in 1m3 wastewater, (KWh per m3 per

order)

k first order rate constant (1/min)

T temperature (1C)

Pdiss power dissipated (W)

m mass of water (kg)

Cp specific heat (J/(kg� 1C))

t operation time (s)

P0 ambient pressure

pa ambient pressure (bar)

pv pressure in the bubble at its maximum size

(bar)

Tmax maximum temperature generated inside the

bubble (bar)

tt treatment time (min)

V volume of the water (l)

Ci initial concentration (mol/l)

Cf final concentration (mol/l)

P rated power (kW)

r0 resonant radius of the bubble (m)

Greek letters

g Specific heat ratio

t Collapse time of the bubble (s)

M. Goel et al. / Water Research 38 (2004) 4247–42614248

(1986) theoretically have shown that the temperature

inside the cavity could reach about 5200K in the

collapsing bubbles and 1900K in the interfacial region

between the solution and the collapsing bubbles.

Sonochemical effect takes place either due to the

pyrolytic decomposition inside the bubbles, or by the

reduction and oxidation due to the generation of Hd

and OHd radicals at the gas–liquid interface, and to

lesser extent in bulk solution (De Visscher et al., 1996).

Hitherto, the bulk of studies conducted on sonochemical

degradation of various organic compounds in aqueous

medium concentrated mainly on the determination of

the kinetics of the degradation process with respect to

different process parameters (Kotronarou et al., 1992;

Cheung and Kurup, 1994; Kruus et al., 1998; Appaw

and Adewuyi, 2000). In addition, some studies reported

speculative mechanisms of sonochemical degradation

(Jiang et al., 2002; Zhang and Hua, 2000; Serpone et al.,

1994). However, depending on the types of compounds,

there are conflicting results on the effects of process

parameters such as temperature, type of dissolved gases,

and additional oxidants on their rates of sonochemical

degradation. In addition, very few studies addressed the

issues pertinent to the large-scale application of this

process, particularly with respect to electrical energy

consumption.

The objectives of this study are: (i) to provide a

comparative analysis of sonochemical degradation of

several organic compounds with varying physical

properties, especially varying volatility, (ii) provide an

approach for scaling up, and (iii) compare the electrical

efficiency of two types of ultrasonic reactors. The

chemical compounds studied were selected from simple

aromatics, chlorinated alkenes, and dyes. They are:

benzene, toluene, styrene, ethylbenzene and trichlor-

oethylene (TCE), and eosin B. Effects of different

process variables such as temperature, initial concentra-

tion, addition of electrolyte and H2O2, and type of

dissolved gas, and two different types of sonication

systems (probe and bath) on the degradation kinetics

were evaluated.

2. Experimental

2.1. Materials

Reagent-grade benzene and styrene (Aldrich Chemi-

cals, USA), toluene, TCE and H2O2 (Merck, Germany),

ethylbenzene (Fluka Chemika), eosin B (Sigma Chemi-

cal Company, USA) and hexane (Ashland Chemical

Italiana) were used as received. The physical properties

of the compounds are listed in Table 1. Aqueous

solutions were prepared by dissolving the compounds

in de-ionized water. Purified air, nitrogen and argon

were obtained from Soxal, Singapore.

2.2. Analytical methods

Quantitative analysis of benzene, toluene, styrene and

ethylbenzene were determined with HP 6890 purge and

trap GC equipped with an auto sampler, flame ioniza-

tion detector and a column (HP-624, 30m� 0.53mm�

3mm). The analyses were conducted under the following

GC temperatures: injector-250 1C; detector-250 1C;

oven-110 1C. For TCE, liquid–liquid extraction was

used to separate TCE from water using n-hexane as

solvent. The concentration of TCE was measured by gas

chromatograph (HP 6890, HP Ultra 2) with an electron

capture detector (ECD).

Eosin B analysis was carried out using UV Spectro-

photometer (Shimadzu UV-VIS Spectrophotometer,

Model UV Mini 1240). In order to find the degree of

degradation, some analyses were conducted using TOC

analyzer (Shimadzu, Model 5000A); the pH of the

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Table 1

Physical properties of the compounds tested

Chemicals Density (gm/cc) Diffusivity in water

at 25 1C (m2/s)

Solubility (ppm) Boiling point (1C) Henry’s Law

constant at 25 1C

(dimensionless)

TCE 1.467 7.3� 10�10 682 87.2 0.324

Benzene 0.879 1.1� 10�9 796 80.1 0.250

Toluene 0.87 8.0� 10�10 605 110 0.250

Ethylbenzene 0.867 7.7� 10�10 237 136 0.270

Styrene 0.91 7.5� 10�10 353 145 0.135

Eosin B 390 g/l at

20 1C

M. Goel et al. / Water Research 38 (2004) 4247–4261 4249

solution was measured by Oaklon pH meter (Ion 510

series).

2.3. Apparatus

The sonication reactions were carried out in two types

of ultrasonic equipment; a probe and a bath. The probe

experiments were conducted using an ultrasonic source

(VC-750, Sonics and Materials, 750W). The probe tip

was 19mm in diameter and the ultrasonic source was

employed at 50% amplitude. A water-jacketed glass

vessel with Teflon cover was used as a reaction vessel.

The volume of the solution was 200ml and the head-

space in the reactor was almost zero (Fig. 1.). The

temperature was monitored with a thermocouple im-

mersed in the reacting medium. For the experiments

conducted in the probe, frequency of the sound wave

was kept constant at 20 kHz.

In the bath experiments, 100ml Erlenmeyer flask was

used as the reactor. Three ultrasound frequencies 28, 45

and 100 kHz of the bath (Honda, W-113 SANPA,

100W) were used. The flask was fixed in the bath as

shown in Fig. 2. The efficiency of a reaction vessel

placed in an ultrasonic bath depends strongly on the

distance of the bottom of the reaction vessel to the

bottom of water bath. The distance h (shown in Fig. 2)

was carefully measured so that ultrasonic intensity

reached maximum at the bottom of the flask. The

maximum intensity occurs at half-wavelength, which is a

function of the frequency used in the ultrasound bath.

For ultrasonic frequencies 28, 45 and 100 kHz, h values

were 2.7, 1.7 and 0.8 cm, respectively. Water level inside

the bath was maintained by continuous circulation of

water, and subsequently the temperature was main-

tained constantly at 30 1C.

The reactor filled with the solution was kept closed

overnight to measure the loss of volatile compounds

from the solution due to evaporation (less than 2%). In

addition, care was taken not to introduce large head-

space during sampling to reduce the evaporation loss of

the volatiles.

3. Kinetic model

Sonochemical decomposition suffers from the disad-

vantage that the reaction rate for a compound varies

with the system. In addition to various physicochemical

properties, reaction rate is also affected by acoustic

properties such as intensity and frequency of the sound

waves. The products formed by the decomposition of

organics affect the physicochemical properties of the

solution, which are difficult to evaluate and establish. De

Visscher et al. (1996) modeled the dependence of

degradation rate constant on initial concentration for

volatile compounds. In this work, we adopt their model

and modify it to apply for all the volatile compounds

tested in this work. De Visscher et al. (1996) introduced

a, a kinetic parameter describing the inhibiting effect of

an organic compound on its own sonolysis as

k ¼ k0 expð�aClÞ; (1)

where Cl is the concentration of the compound in liquid.

a is given as

a ¼EPminK

RTPmaxðg0 � 1Þ2

!(2)

k0 is the rate constant at infinite dilution, E is the

activation energy, Pmin is the minimum pressure in the

vapor phase (vapor pressure of water), Pmax is the

maximum pressure in the liquid phase (hydrostatic

pressure) and T is the bulk liquid temperature. K is the

proportionality constant (mM�1) and it is related to Eq.

(1) as

�KC‘ ¼ x‘ðg0 � 1Þðg‘ � g0Þ

g‘ � 1; (3)

where g0 and g‘ are the specific heat ratios for pure gas

water mixture and organic compound in the cavitation

bubbles, respectively. Since the term ðg‘ � g0Þ is negative,the specific heat ratio decreases with the increase in

initial concentration. The decreasing g would decrease

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Fig. 1. Experiment setup for ultrasonic probe.


the cavitation temperature, consequently reducing the

reaction rates.

The activation energy E is obtained from the slope of

ln k vs. 1/RTmax according to Arrhenius’ equation

k ¼ A expð�E=RTmaxÞ (4)

Tmax is the maximum cavitation temperature. Under

adiabatic compression, Tmax is given by (Noltingk and

Neppiras, 1950).

Tmax ¼ T ðg� 1Þpapv

� �; (5)

where, pa is the ambient pressure, pv is the pressure in the

bubble at its maximum size, and T is the bulk

temperature. The model parameter a varies with the

bubble radius and collapse time since concentration of

the organic vapor in the gas phase (Cg) is a function of

bubble radius and collapse time. The variation in Cg will

cause the variation in x‘, which will change the value of

K according to Eq. (3). Cg is given by De Visscher et al.

(1996) as

Cg ¼ ð6=r0ÞðDlt=pÞ1=2Cl; (6)

where r0 is the bubble radius (resonant size) in meters,

D1 is the diffusion coefficient (m2/s) and t is the collapsetime.

The bubble radius is influenced by the frequency

of the ultrasonic waves. For air bubbles in water

less than one atmosphere the relationship between

frequency and the resonant radius is given by (Margulis,

1993)

r0 f � 3 Hz m; (7)

where f is the frequency of ultrasonic waves.

This is the maximum radius at which bubble loses its

stability and breaks up into the smaller fragments. At

20 kHz, r0 is estimated to be 0.015 cm. The resultant

collapse time is given by (Margulis, 1993)

t ¼ 0:915r0

ffiffiffiffiffirpa

r; (8)

where r is the density of the liquid and pa is the ambient

pressure. For r0 ¼ 0:015 cm and pa ¼ 1 atm; the collapsetime is 1.35� 10�5 s.

ARTICLE IN PRESS

Fig. 2. Experiment setup for ultrasonic bath.


4. Results and discussion

4.1. Effect of initial concentration

The initial concentrations of all the organics tested

were within their solubility limits. The reaction kinetics

for all the compounds tested followed first order rate

laws in all initial concentrations. However, the apparent

first order rate constants decreased with the increasing

initial concentration of the organics indicating

non-elementary nature of the sonochemical reactions.

This dependence of reaction rate constants on

initial concentration was found for both the bath and

probe systems, and compared well with the existing

literature (De Visscher et al., 1996; Jiang et al.,

2002; Hoffmann et al., 1996; Zhang and Hua, 2000).

A typical lnC/C0 vs. time for benzene as a representative

compound is shown in Fig. 3a, whereas the variations

of reaction rate constant with initial concentration

for different organic compounds are presented in

Fig. 3b.

At low concentration of the volatiles, free-radical

reactions in the bubble–liquid interfacial region are

likely to predominate (Kotronarou et al., 1991). The

reported rate constants of the reactions involving

benzene, toluene, ethylbenzene, styrene and TCE with

hydroxyl radicals in water are quite comparable and

vary in a narrow range of 3.0� 109–7.8� 109 lmol�1 s�1

(NDRL Radiation Chemistry Data, 2004). Thus, at

50 ppm, the degradation rate constants of all the VOCs

tested were quite similar with benzene exhibiting the

highest rate of degradation (incidentally the reported

rate constant of reaction of benzene with hydroxyl

radicals was the highest at 7.8� 109 lmol�1 s�1). At

higher initial concentration, the major route for

degradation for the volatile is by pyrolytic reactions in

the gas (bubble) phase due to greater partition of the

volatiles in the gas phase.

The rate of degradation of all the aromatics showed

greater dependence on initial concentration than TCE.

The Henry’s law constant of TCE is the highest among

all the VOCs tested causing greater amount of TCE

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Time (min)0 20 40 60 80 100 120 140 160

ln C

/C0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

50 ppm100 ppm150 ppm200 ppm

Conc (ppm)40 60 80 100 120 140 160 180 200 220

k(m

in-1

)

0.004

0.006

0.008

0.010

0.012

0.014

0.016BenzeneTolueneStyreneTCEEthyl benzene

(a)

(b)

Fig. 3. (a) Effect of initial concentration on the decomposition of benzene (T=20 1C). (b) Rate constant vs. initial concentration for

different organic compounds (T=20 1C).


vapor in the gas phase. This is probably the reason of

minimal effect of increased initial concentration on TCE

degradation unlike the other compounds tested. On the

other hand, with the increasing initial concentration of

the volatiles, the pyrolysis temperature (Tmax) decreased

(calculated Tmax values are shown in Table 2), while

TCE displayed the highest Tmax among all the VOCs

tested. However, the differences in Tmax for all the

compounds tested are very small; such small differences

at high temperature ranges may not cause any significant

change in the rate constants and need to be carefully

considered. Similar effect of initial concentration on

eosin B decomposition rate can be seen in Fig. 4 where

rate decreased due to the increased competition for

the hydroxyl radicals at high initial concentration of

eosin B.

Comparing the reaction rate constants of all the

compounds, one can observe that except for the dye

eosin B, the reaction rate constants of all the volatile

compounds tested were of similar order (0.0128–

0.0146min�1) at 50 ppm and 20 kHz. Similar results

were observed in the experiments of De Visscher et al.

(1996), where the first order degradation rate constants

for various alkylbenzenes varied in a narrow range of

0.023–0.029min�1, and also in the experiments of Jiang

et al. (2002) where the first order degradation rate

constants for chlorobenzenes varied again from 0.026 to

0.028min�1. It can be seen from Table 2 that the

maximum cavitation temperature varies in a narrow

range for all the volatile compounds tested. The decrease

in g will cause the decrease in cavitation temperature

(Eq. (5)) (Table 2), consequently reducing the reaction

rates.

The kinetic model discussed in the earlier section

was used to predict the rate constants of all the

volatile compounds tested in this work. The above

model is semi-empirical in nature because of the

various assumptions and approximations involved and

inclusion of the parameters, which affect the cavitation

such as specific heat ratio, ambient temperature and

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Time (min)

0 20 40 60 80 100 120 140

ln C

/C0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

15 µΜ 30 µΜ µΜ 45 µΜ µΜ 60 µΜ µΜ

Fig. 4. Effect of initial concentration on the degradation of eosin B (T=20 1C; H2O2=2mol/l).

Table 2

Cavitation temperatures (Tmax) for the volatiles at different initial concentrations

g0 T�max (K)

50 ppm 100 ppm 150 ppm 200ppm

Benzene 1.1 2369 2364 2359 2353

Toluene 1.08 2367 2363 2359 2353

Ethylbenzene 1.07 2366 2362 2358 2353

Styrene 1.06 2366 2361 2357 2352

TCE 1.115 2371 2366 2364 2361

*Tmax was calculated using Eq. (6); pa=1.0 bar; pv=0.042 bar (water vapor pressure at 30 1C).


pressure, makes the model applicable to a specific

system. The model parameter a and the rate

constant calculated for all the test volatile compounds

are listed in Table 3. The performance of the

model is quite satisfactory as the predicted rate

constants, especially at low concentrations, agreed very

well with the experimental data. The maximum differ-

ence of 50% between the observed and predicted rate

constants occurred at high concentration of toluene,

ethylbenzene and styrene. The model under-predicted

the reaction rate constants in many cases. This is

possible since the model takes into account only the

pyrolytic decomposition of the volatile compounds.

However, some degradation of these compounds also

occurs in the aqueous phase in the presence of reactive

radicals.

The bubble radius, and the collapse time are the two

most uncertain parameters involved in the above model.

Thus, the bubble radius was varied in some runs to test

the sensitivity of the model, and a 50% variation in

bubble radius causes a maximum of 20% variation in

the calculated rate constants.

4.2. Effect of dissolved gas

In addition to the vapors of the volatile compounds

the cavitation bubbles mostly contain the dissolved

gases present in water. To determine the effect of

dissolved gas, experiments were carried out in both air

and argon-saturated solutions. In some limited

experiments, oxygen and nitrogen were also used.

For these experiments, the test gas was bubbled

through water for 1 h before the organic compounds

were introduced into the reactor. Fig. 5a depicts

the degradation of trichloroethylene in air and

argon-saturated solutions. An increase in degradation

rate can be seen for argon-saturated solution for

TCE. On the other hand, there was no effect of

dissolved gases on the rate of benzene and toluene

degradation (Fig. 5b). The average specific heat

ratio g of the gas is an important parameter as it

increases the collapse temperature of the bubbles.

In general, monatomic gases like helium, argon,

krypton, etc. has the highest specific heat ratio

(g ¼ 1:67), and final collapse temperature for a

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Table 3

Experimental and model first order rate constants (k) (min�1) for sonochemical degradation of the volatiles at different initial

concentration

a (mM�1) 50 ppm 100ppm 150 ppm 200 ppm

kobs kmodel kobs kmodel kobs kmodel kobs kmodel

Benzene 0.60152 0.0146 0.0142 0.0076 0.0101 0.0072 0.0073 0.0056 0.0052

Toluene 0.69717 0.0128 0.0122 0.0088 0.0115 0.0101 0.0064 0.0091 0.0045

Ethylbenzene 0.90238 0.0137 0.0141 0.0115 0.0085 — 0.006 0.0082 0.0043

Styrene 0.80506 0.0138 0.0138 0.0135 0.0081 0.0085 0.0056 0.007 0.0039

TCE 0.40029 0.0133 0.016 0.0125 0.011 0.0122 0.0087 — —

Time (min)0 20 40 60 80 100 120 140 160

ln C

/C0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

AirArgon

Conc (ppm)40 60 80 100 120 140 160 180 200 220

k(m

in-1

)

0.004

0.006

0.008

0.010

0.012

0.014

0.016

ArgonAir

Toluene

Benzene

(a)

(b)

Fig. 5. (a) Effect of dissolved gas on the decomposition of TCE (T=20 1C; Ci=50 ppm). (b) Effect of dissolved gas on the

decomposition of benzene and toluene (T=20 1C).


monatomic gas could be two times higher than that of a

tri-atomic gas (Reisz and Takashi, 1992). However,

since the collapse temperature ranges from 2000 to

4000K, it is already high enough for the pyrolysis of the

volatile compounds, which can be degraded in the

cavitation bubble. Consequently, the dissolved gases

exert little influence on the degradation of volatile

compounds.

In contrast to the above results, eosin B showed

almost a six-fold increase in degradation rate when

ARTICLE IN PRESSM. Goel et al. / Water Research 38 (2004) 4247–4261 4255

argon was used instead of air (Table 4). Similar increase

in rate was observed for the degradation of alachlor and

PCP in argon-saturated solution (Wayment and Casa-

donte, 2002; Petrier et al., 1992). This is possibly due to

the greater rate of OH� production in argon-saturated

water (Hua and Hoffmann, 1997) as argon atmosphere

leads to higher temperature inside cavitation bubbles. It

is estimated that 10% of the Hd and OHd radicals

generated in the bubble can diffuse in to the bulk

Table 4

Sonolysis of eosin B at different dissolved gases (Cl=20mM�;T=20 1C)

Type of gas Rate constant

(min�1)

Bath experiments Argon 0.0023

Oxygen 0.0017

Air 0.0004

Nitrogen 0.0002

Probe experiments Argon 0.0026

Oxygen 0.0009

Nitrogen 0.0004

*The concentration of eosin B was expressed in mole according

to the standard practice.

Table 5

First order degradation rate of various systems (T=20 1C) (effect of

System

20 mM* eosin B

20 mM eosin B+

20 mM eosin B+

150 ppm Tn**

Bath experiments 150 ppm Tn+5

150 ppm Tn+2

150 ppm Tn+6

150 ppm Tn+1

50 ppm Benzen

50 ppm Bn***+

50ppm Bn+40

50 ppm Bn+10

50 ppm Toluen

50 ppm Tn+20

Probe experiments 50 ppm Tn+40

50 ppm Tn+10

50 ppm Trichlo

50 ppm TCE+

50 ppm TCE+

30 mM eosin B

30 mM eosin B+

30 mM eosin B+

*The concentration of eosin B was expressed in mole according to th

**: Toluene; ***: Benzene

solution. Eosin B being non-volatile is mostly degraded

by the radical reactions in this region, whereas some

reactions may also occur at the bubble–water interface.

However, interfacial degradation is a function of

hydrophobicity and eosin B is highly hydrophilic

because of its high solubility.

4.3. Effect of hydrogen peroxide

In general, slow sonochemical decomposition of non-

volatile is a challenging problem to overcome for the

process to be commercially viable (Seymour and Gupta,

1997). This is due to the fact that the decomposition of

non-volatile compound takes place mainly by reaction

with hydroxyl radicals in the bulk solution (Joseph et al.,

2000). Hydroxyl radicals generated in water by ultra-

sonication can produce hydrogen peroxide in the system.

Whether additional hydrogen peroxide has a synergistic

effect on the overall degradation of the non-volatiles,

some experiments were conducted at various concentra-

tions of added H2O2. The addition of hydrogen peroxide

did not alter the degradation rates of all the volatiles

tested (Table 5 and Fig. 6). The decomposition rates of

toluene at different initial concentrations of hydrogen

peroxide are shown in Fig. 6. The volatile compounds

are mostly degraded either in the interior of the bubbles

or in the bubble–water interfacial region, and thus not

H2O2)

k1 (min�1)

0.0006

100ppm H2O2 0.0002

200ppm H2O2 0.0005

0.0020

0ppm H2O2 0.0022

00 ppm H2O2 0.0038

00 ppmH2O2 0.0052

000ppm H2O2 0.0027

e 0.0146

200 ppm H2O2 0.0147

0ppm H2O2 0.0147

00ppm H2O2 0.0146

e 0.0128

0ppm H2O2 0.0128

0ppm H2O2 0.0129

00ppm H2O2 0.0131

roethylene 0.0137

115ppm H2O2 0.0138

230 ppm H2O2 0.0135

0.0065

500ppm H2O2 0.0107

1000ppm H2O2 0.0127

e standard practice.

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Time (min)0 20 40 60 80 100 120 140 160 180 200

ln C

/C0

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0 ppm H2O2

200 ppm H2O2

400 ppm H2O2

1000 ppm H2O2

Fig. 6. Effect of H2O2 on the decomposition of toluene (T=20 1C; Ci=50 ppm).


affected by the external oxidants. For the non-volatile

eosin B, the effect of H2O2 was not significant for the

bath system. In fact slight drop in the degradation rate

was observed with the addition of H2O2 probably due to

reaction between hydroxyl radical and H2O2. However,

the rate of degradation of eosin B was increased in

presence of H2O2 in the probe system (Table 5) while

large excess of H2O2 (500–1000 ppm) was used. There

was 60% increase in the rate constant of eosin B by

adding 500 ppm of H2O2 concentration in the probe

system. Similar dosage of H2O2 in absence of ultrasound

did not produce any positive effect on the degradation

rate of eosin B. Under the influence of ultrasound, H2O2

decomposes as shown below (Elkanzi and Kheng, 2000):

H2O2 þ�!ÞÞÞ

2HOd;

HOd þH2O2 ) HOd2 þH2O;

2HOd23H2O2 þO2:

There is possibly a maximum concentration of H2O2

beyond which the improvement in the rate is diminished,

although that high concentration was never achieved in

this work

4.4. Effect of temperature

In an ultrasonic reactor, the temperature increases

rapidly with sonication if it is not controlled. Thus, it is

possible to make use of the advantage of temperature

rise in a sonochemical reactor if the reaction simply

follows Arrhenius rate law. However, for sonochemical

reactions, rate depends on many factors, which causes

inconsistency in reaction rates with respect to tempera-

ture dependence. The temperature of the bulk phase

affects the viscosity, gas solubility, vapor pressure and

surface tension. For example, an increase in temperature

increases the vapor pressure of the solute. Consequently,

the cavitation bubbles are filled with the vapor of the

target compound and water readily. The increased vapor

and gas content increases the resistance to the inward

motion of a bubble during the collapse resulting in the

reduced intensity of the collapse. This causes the

reduction in the collapse temperature decreasing the

degradation rates (Mason, 1991). On the other hand, the

increased temperature will result in the reduction of

viscosity and/or surface tension lowering the threshold

intensity required to produce cavitation.

Combining all of the above, the effect of temperature

on sonochemical degradation rate is complicated. Thus,

there is no consistent report on the impact of

temperature on the decomposition of organic com-

pounds in literature. Bhatnagar and Cheung (1994) and

Wu et al. (1992) reported that the decomposition of

trichloroethylene and carbon tetrachloride remained

constant between �7–20 1C and 20–60 1C, respectively.

Whereas Ondruschka and Hoffmann (1999) and De-

staillats et al. (2001) indicated that the sonochemical

degradation of chlorobenzene and TCE, respectively

increased with increasing temperature. In this work an

increase in degradation rate for benzene and toluene was

observed whereas styrene and ethylbenzene showed

negligible change with respect to temperature (Fig. 7).

However, the changes are very marginal and less than

25% for a 301 rise in temperature from 10 to 40 1C.

4.5. Effect of electrolyte

Seymour and Gupta (1997) observed enhancement in

degradation rates for chlorobenzene, p-ethylphenol and

ARTICLE IN PRESS

Temperature (°C)5 10 15

k (m

in-1

)

0.011

0.012

0.013

0.014

0.015

0.016

0.017

BenzeneTolueneStyreneEthylbenzene

20 25 30 35 40 45

Fig. 7. Effect of temperature on the decomposition of aromatics (Ci=50 ppm).

Table 6

Effect of NaCl on the decomposition of the volatiles

(Ci=50 ppm; T=20 1C)

NaCl concentration (M) Rate constant (min�1)

Benzene Ethylbenzene TCE

0.0 0.0146 0.0135 0.0133

0.5 0.0152 0.0144 0.0147

1.0 0.0158 0.0151 0.0142

2.0 0.016 0.0153 0.0138


phenol by adding electrolyte like common salt in water.

The addition of salt increases ionic strength of the

aqueous phase, which drives the organic compounds to

the bulk–bubble interface. However, ionic strength is

not the only effect brought about by dissolved electro-

lytes. Other properties of solution such as viscosity,

vapor pressure, and heat capacity will also change

accordingly. The combined effect of these parameters is

difficult to estimate if not impossible. We observed

about 10–12% increase in the decomposition rates for

benzene, ethylbenzene and less than 10% for TCE

(Table 6). It seems, addition of an electrolyte does not

influence the rate of degradation of the volatiles

significantly. On the other hand, eosin B showed

substantial increase with the increase in NaCl concen-

tration (Fig. 8). These results indicate that the reaction

of the non-volatiles in the bulk–bubble interface may

have merits for the application of sonolysis for these

chemicals.

In addition to the above experiments, some experi-

ments were also conducted in the presence of different

concentrations of suspended silica particulates and

eosin B. The rate constant decreased with the increasing

concentration of silica due to the attenuation of energy

by the scattering of the particulates. This may have

serious implication for the sonochemical treatment of

waster water with high turbidity.

4.6. Effect of frequency

It is expected and also reported that the rate of

degradation of organic compounds increases with the

increase in frequency of sonication, although the effect

of frequency is somewhat system specific. The frequency

of the probe systems could not be changed. The

ultrasonic bath had three frequency options: 28, 45

and 100 kHz. Cavitation occurring at low frequency is

most effective to decompose molecules inside the bubble

(Petrier and Francony, 1997). Thus in this work, TCE

showed decreased rate with increasing frequency (Fig. 9)

as TCE degrades mostly in the bubble phase. On the

other hand, frequency of ultrasound has two counter-

acting effects on the generation of hydroxyl radicals. At

very low frequency, although more hydroxyl radicals are

generated inside the bubble, chances of recombination

of the OHd radicals inside the bubble are higher due to

the higher temperature inside the bubble. As the

frequency increases, the pulsation and collapse of the

bubble occur more rapidly causing more radicals to

escape from the bubble. However, at very high

frequency, the acoustic period is much shorter, thus

decreasing the size of the cavitatation bubbles. As a

result, the cavitation intensity decreases, subsequently

decreasing the amount of OHd radicals in the solution.

The existence of an optimum frequency with respect to

generation of OHd radicals is reported earlier in the

literature (Petrier and Francony (1997) and Kang et al.

(1999)). In this work, eosin B indicated the presence of

ARTICLE IN PRESS

Time (min)100 120 140

ln C

/C0

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.0 M NaCl0.5 M NaCl1.0 M NaCl2.0 M NaCl

0 20 40 60 80

Fig. 8. Effect of NaCl on the decomposition of eosin B (T=20 1C; Ci=20mM).

Time (min)0 20 40 60 80 100 120 140 160

ln C

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

28 kHz45 kHz100 kHz

Fig. 9. Sonolysis of TCE at different ultrasonic frequencies (Ci=100 ppm, T=25 1C, saturated with Ar).

Table 7

Effect of frequency of the ultrasonic bath on the degradation of

eosin B

f (kHz) k1 (min�1)

28 0.0009

45 0.0027

100 0.0002


an optimum frequency at 45 kHz where the rate constant

of degradation of eosin B was the maximum (Table 7).

At 100 kHz the intensity of the ultrasonic bath was very

low causing almost negligible cavitation. As seen earlier,

eosin B being non-volatile, degrades mostly by the

reactions with hydroxyl radicals in the bulk solution.

4.7. Cost analysis and the effect of type of ultrasonic

equipment

The economic issues of sonochemical decontamina-

tion of waste streams are not yet addressed in a

comprehensive manner, although a recent study indi-

cates that the cost of sonochemical oxidation of p-nitro-

phenol to be comparable to that of incineration

(Seymour and Gupta, 1997). The volatile compounds

treated by this method perform better than the non-

volatiles and about a ten-fold increase in the existing

rate would bring the sonochemical rates on par with

AOP processes involving ultraviolet radiation (UV).


The relatively high cost of sonochemical process is due

to the low efficiency of electrical–sound–thermal energy

conversion. Calorimetric method can be used to

determine the power dissipated into the reaction media

in a probe system (Mason et al., 1992). The power of the

probe system used in this work was calculated as

Pdiss ¼dT

dt

� �t¼0

mCp; (9)

where Cp is the heat capacity of the water, m is the mass

of water, and ðdT=dtÞt¼0 represents initial slope of the

temperature rise versus the time.

The temperature vs. time data from our experiments

was fitted into the following function:

T ¼ �3� 10�5t2 þ 0:0851t þ 23:958 (10)

and the power dissipated is calculated as

Pdiss ¼dT

dt

� �t¼0

mCp

¼ 0:0851� 0:15� 4182:83 ¼ 53:39 W: ð11Þ

Since a 375 W power was utilized, only 14.3% of the

rated power is transmitted into the reactor.

In addition to the electrical cost mentioned above, the

capital cost of an ultrasound system varies significantly.

Usually, ultrasound is generated by immersing the

reactor in a sonicating liquid (a reacting vessel in an

ultrasonic bath) or by introducing the source directly in

the reactor (an ultrasonic probe in the reactor).

Ultrasonic cleaning bath is the most widely used and

the cheapest source of ultrasound in laboratory. It

provides even distribution of energy in the immersed

reaction vessel, and efficient transfer of energy is

obtained in case of flat bottomed glass vessel instead

of round bottomed glass vessel. Compared to a bath

system, the probe can be directly immersed in the

solution for better sonochemical effect. In this study,

experiments were conducted in both systems to compare

the effects of type of equipment used and a figure of

merit (EE/O) (Bolton et al., 1996) for the consumption

of electrical energy was calculated according to the

Table 8

EE/O values for different systems. (Ci=50 ppm)

EE/O (KWh per m3 per order)

Bath Probe

TCE 4382 6136

Toluene 6643 6372

Styrene 7538 6596

Eosin B 7532 9210

Benzene — 4732

Ethylbenzene — 6720

�The initial concentration reported for the UV oxidation is 0.2mM

following equation, and the results are presented in

Table 8.

EE=O ðin KWh per m3 per orderÞ

¼P � 1000� tf

V � 60� logðCi=Cf Þð12Þ

where P is the rated power (kW), V is the volume (L) of

water treated in the time tf (in min), Ci, Cf are the initial

and final concentrations (mol l�1) of contaminant in the

water, respectively. The EE/O value was used to

compare the energy efficiency of the two systems.

Higher EE/O values would correspond to lower energy

efficiency of a system.

Experimental results indicate that somewhat higher

degradation rate was achieved for the probe system for

all the compounds whereas the effect was more

significant for eosin B.

It is evident from Table 8 that although higher rate is

observed in probe system as compared to ultrasonic

bath, the energy efficiency of both the systems is

comparable. In the probe system, the erosion of titanium

tip with use contributes to the higher operating cost for

the probe than the bath systems. The EE/O values

obtained in this work were compared with those from a

UV (254 nm)+H2O2 operation of Sundstorm et al.

(1989) (Table 8). It can be seen that in order to be energy

efficient, the present ultrasonic degradation rates need to

be improved by at least 10–100 times, especially for the

non-volatiles.

4.8. Final product analysis

Initial and final pH values of the solution were

measured for all the experimental runs. It was observed

that solution pH decreases for all cases indicating

liberation of H+ ions. For aromatics pH decreases

from 6.0 to 4.0 whereas for TCE, pH decreases to as low

as 2.7. The reduction in TOC values during the

degradation of the aromatics was also observed. An

average of 80% reduction in TOC values occur during

the experiments of the aromatics (Table 9).

Rate constant (min�1)

UV� Bath Probe

— 0.0102 0.0133

15.0 0.0079 0.0126

— 0.005 0.0135

— 0.0015 0.0065

9.8 — 0.0146

— — 0.0137

(Sundstorm et al., 1989).

ARTICLE IN PRESS

Table 9

Mineralization of aromatics (T=20 1C; t=180min)

Toluene Styrene

Initial TOC

(ppm)

Final TOC

(ppm)

Initial TOC

(ppm)

Final TOC

(ppm)

50 7.5 50 —

100 19.0 100 12.76

150 22.3 150 15.5

200 30.6 200 32.5


5. Conclusions

A comparative study of sonochemical degradation of

volatile and non-volatile compounds under different

process parameters was conducted. Effects of different

process variables such as initial concentration, tempera-

ture, addition of electrolyte and H2O2, and type of

dissolved gas on the degradation kinetics were tested.

Two different types of sonication systems, probe and

bath were also evaluated. The following important

conclusions can be drawn from the present work:

�
Sonochemical degradation of the volatiles can be a
viable process by itself. The kinetic model tested

predicts the degradation rate of the volatiles success-

fully and can be used for scaling up of the process at

low concentration of the volatiles. The reaction rates

of the volatiles have not improved significantly by the

addition of external oxidant, electrolyte and argon as

dissolved gas.

�
The reaction rate for dye eosin B is much lower than
that of the volatile compounds, however, it benefits

from the rate augmentation using external oxidants

such as hydrogen peroxide, argon as dissolved gas

and addition of an electrolyte.

�
Reaction rates of both volatile and non-volatile
compounds decreased with the increase in initial

concentration.

�
Although, the reaction rates are generally higher in
the ultrasonication system with probe than those with

bath, the energy efficiencies in both the systems are

comparable.

�
Sonochemical processes are easy to operate: however,
a preliminary energy analysis indicates that the

existing sonochemical reaction rates need to be

improved by 10–100 times, especially for the non-

volatiles in order to make the process economically

viable for large-scale application.

Further research should be directed in the optimum

design of the sonochemical reactors combining ultravio-

let radiation and external oxidants such as ozone and

hydrogen peroxide.

Acknowledgements

The authors wish to acknowledge Zhang Peiqing,

Anthony for some of the experiments conducted in this

work.

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