Sustain. Environ. Res., 21(2), 167-172 (2011)
167
EFFECT OF FREQUENCY ON THE SONOLYTIC DEGRADATION OF CARBON
TETRACHLORIDE
Jong-Kwon Im,1 Hyun-Seok Son,2 Seong-Keun Kim,1 Jeehyeong Khim3 and Kyung-Duk Zoh1,*
1
Department of Environmental Health
Seoul National University
Seoul 110-799, Korea
2
Department of Applied Chemistry
Konkuk University
Chungju 380-702, Korea
3
School of Civil Environmental and Architectural Engineering
Korea University
Seoul 136-701, Korea
Key Words: Sonication, frequency, argon, methanol, OH radical, chloride
ABSTRACT
The sonolytic degradation of carbon tetrachloride (CCl4) was examined under different
frequency conditions (35, 72, 100 and 170 kHz) in the presence and absence of argon purging and
radical scavenger. Also, the extent of mineralization during sonolysis of CCl4 was measured using
chloride production. At 170 kHz, the degradation CCl4 and chloride production was only 25 and 20%
within 60 min. However, the removal efficiency of CCl4 increased more than 90% at the same time
at lower frequencies (35, 72 and 100 kHz), and the production of chloride increased up to 67, 70 and
61% at 35, 72 and 100 kHz, respectively. The removal of CCl4 at 170 kHz was enhanced by argon
purging during sonication up to 87%, however, the degradation efficiency at lower frequencies
decreased by argon purging. The addition of methanol, as a radical scavenger during sonolysis, also
reduced the removal especially at higher frequency (170 kHz), indicating that the oxidation by OH
radical is the main factor in the sonolysis for the removal of CCl4 at higher frequency, and pyrolysis
is the main effect factor for degradation of CCl4 at lower frequency in the sonication. Finally, the
mineralization extent during sonolysis was estimated using the mass balance of chloride production.
INTRODUCTION
Carbon tetrachloride (CCl4) is a group of chlorinated organic compounds. CCl4 is classified as a
possible carcinogen to humans by International
Agency for Research on Cancer and US EPA [1].
CCl4 is widely used as lubricants, heat-transfer fluids,
intermediates for pharmaceuticals, herbicides, fungicides and solvents in cleaning, degreasing and extraction agent in numerous industries and hospitals. CCl4
continuously releases into ground water, soil, and
river by accidental spills, leakages and improper disposals. Once CCl4 is released into environmental media even at low concentration it does not degrade easily because of recalcitrant characteristics [2]. Therefore, the efficient method for the treatment of CCl4 is
necessary.
Sonolytic irradiation method has received in*Corresponding author
Email: [email protected]
creasing attention for the destruction of organic pollutants in waters and wastewaters [3,4]. Sonolytic degradation in aqueous phase involves several reaction
pathways and zones such as pyrolysis inside the bubble and/or at the bubble-liquid interface and hydroxyl
radical reactions at the bubble-liquid interface and/or
in the bulk liquid [5]. The formation, growth, and collapse of cavitation bubbles drive sonochemical reactions in aqueous solutions. The collapse occurs inside
a cavitation bubble (5000 K) and at the bubble-water
interface (1900 K), and H2O molecules under these
extreme conditions are thermally disintegrated to H
and OH radicals [6-9]. The extreme condition results
in not only solute pyrolysis, but also the formation of
hydroxyl radical and hydrogen peroxide [10,11].
In this study, we examined the effect of sonication frequency on the sonolytic degradation of CCl4 in
aqueous solution in the presence and absence of argon
Sustain. Environ. Res., 21(2), 167-172 (2011)
168
gas. The effect of OH radical on the degradation of
CCl4 is also examined using methanol as a radical
scavenger. Finally, the extent of mineralization during
sonolysis was estimated using chloride production.
MATERIALS AND METHODS
1. Materials
CCl4 was prepared by diluting a 1000 mg L–1
stock solution, which was prepared by dissolving reagent-grade carbon CCl4 (Aldrich) in nanopure deionization water (R = 18 MΩ cm-1; Barnstead, IA). Methanol (MeOH, HPLC grade; Baker, TX), and methylene
chloride (HPLC grade; Baker, TX) were used as received. Argon gas (99.999%) was used in the study.
2. Experimental Setup
We used a bath-type sonicator (Flexonic-1000;
Miraesonication, Korea) consisting of a reactor made
of Pyrex, a generator, a transducer, and a temperature
controller. Figure 1 shows the scheme of the sonication system. Sonolysis of CCl4 was performed at ambient condition at frequencies of 35, 72, 100 and 170
kHz with the power of 640 ± 20 W. The experiment
was conducted without controlling the solution temperature. The reactor (volume of 1 L) was closed for
the duration of the reaction. The solution was purged
with argon gas continuously for 5 min to facilitate the
formation of cavitation bubbles [12,13] before starting
the reaction. To qualitatively estimate the production
of OH radicals during sonication, a 49 mM of methanol (MeOH), a radical scavenger, was added to the solution at the start of the sonolytic reaction. The initial
pH of the CCl4 solution was around 6.4. The pH was
not controlled in any of the experiments.
3. Analysis
Before analysis, the sample was extracted by
liquid-liquid extraction using a methylene chloride
solvent (5:1, v/v). Then, the mixture was centrifuged
at 3000 rpm for 2 h. The extract was then analyzed using gas chromatograph (GC, Hewlett Packard 6890N,
Wilmington, DE) with μ-electron capture detector (μECD) and with a 5% phenyl methyl siloxane capillary
column (30 m × 0.32 mm × 0.25 µm). The analytic
conditions of GC/μ-ECD is as follows: split ratio, 10:1;
carrier gas, 2.2 mL min–1; makeup gas flow rate, 60
mL min-1; inlet temperature, 150 oC temperature in μECD, 325 oC. The programmed oven temperature; 35
o
C for 9 min, 40 oC for 3 min at 10 oC min-1, and 150
o
C for 5 min at 15 oC min-1.
For the mineralization, chloride was measured
by ion chromatography (DX-120, Dionex, Sunnyvale,
CA) with 4-mm ASRS Ultra II suppressor (Dionex).
Ion Pac AS14 column (4 × 250 mm, Dionex) and
Sonicator
(Bath type)
Reactor
Chiller
(Ternper ature controller )
Oscillator
35/72/100/
170KHz
Fig. 1. Scheme of sonication reactor.
AG14 guard column (4 × 50 mm, Dionex) were used
to separate this ion, and 3.5 mM of Na2CO3 (Sigma
Aldrich, ACS regent, St. Louis, MO) and 1 mM of
NaHCO3 (Shinyo pure chemical, special grade, Osaka,
Japan) were used as the eluent for the analysis of chloride. The pH of the solution was measured using a
model 52A pH analyzer (Orion, Reno, NV).
RESULTS AND DISCUSSION
1. Effect of Frequencies and Argon Gas
H2O in cavitation bubble is known to suffer pyrolysis to produce OH radical and HO2 radical as
shown in Eqs. 1 and 2 [14]. H2O2 also can be produced in the cavitations bubble by the recombination
of OH radical as shown in Eqs. 3 and 4 [15].
H 2 O → OH • + H •
(1)
H • + O 2 → HO 2 •
(2)
OH • + OH • ↔ H 2 O 2
(3)
2 HO 2 • → H 2 O 2 + O 2
(4)
Figure 2 shows the degradation efficiency during
CCl4 sonolysis and the chloride production in sonication in the presence and absence of argon at 35 kHz.
As shown in Fig. 2, the degradation efficiency of CCl4
was slightly reduced by adding Ar gas during sonication. Figure 2 also shows that the production of chloride was higher in the sonication condition in the absence of argon purging. It is known that the addition
of argon gas in sonication can facilitate cavitation
formation [12,13], but the result was not obtained in
this study. Instead, adding argon gas in sonication can
increase the size of bubble, which means to increase
the potential energy in bubble and to emanate greater
chemical energy into interface when bubble is collapsed. Furthermore, it is known that the generation of
OH radical may be improved in the reaction of sonication by argon purging [16,17]. Considering the results
of our study and other studies, the addition of argon in
the sonication can enhance the contribution of radicals
in the reaction, which may mainly occur in the interface between bubble and bulk solution. The results
presented in Fig. 2 implicate that most of CCl4 is de-
Im et al.: Sonolytic Degradation of CCl4
169
Ct/C0 (%)
80
60
40
20
0
0
10
20
30
40
50
60
Rate constant for CCl4 removal (k1, min-1)
0.06
US-only (CCl4)
US + Ar (CCl4)
US-only (Cl-)
US + Ar (Cl-)
100
US-only
US+Ar
0.05
0.04
0.03
0.02
0.01
0.00
35
Time (min)
Fig. 2. Degradation efficiency during CCl4 sonolysis and
the chloride production in sonication in the
presence and absence of argon (US: ultra
sonication, [CCl4]0 = 0.02 mM, sonication
frequency = 35 kHz).
graded inside the bubble. Additionally, the enhancement of OH radical reaction in sonication inhibited the
degradation efficiency of CCl4, indicating that CCl4
can be reduced by electron rich element such as H
radical. Also, since oxidation products of chlorine
atom such as ClO2-, ClO3-, or ClO4- by OH radical was
not identified, the sonolytic degradation of CCl4 is
mainly achieved by pyrolysis into chloride.
Figure 3 shows the effect of frequency on the
sonolytic degradation rate of CCl4 in the presence and
absence of argon gas. The sonolytic efficiency of CCl4
increased at 35, 72 and 100 kHz, but decreased at 170
kHz. Also, CCl4 degradation was more effective in the
sonication-only reaction than in the sonciation with
argon purging. However, the opposite results were observed at the frequency of 170 kHz.
Jiang et al. reported that the degradation efficiency increased with the increase of H2O2 concentration [18]. In addition, production rate of H2O2 increased with the increase of frequency. Equation 3
shows the generation of OH radical from H2O2 in
sonication system. Also, it was reported that the contribution of OH radical for degrading compound increased at higher frequency [19,20]. In spite of higher
production of OH radical in 170 kHz than in 35 kHz,
the degradation efficiency of CCl4 in 170 kHz was
lower as shown in Fig. 3. This result indicates that the
contribution of OH radical in CCl4 degradation was
minor in the 35, 72 and 100 kHz. Figure 3 also shows
that adding argon gas in the sonication decreased the
degradation efficiency of CCl4 compared to sonication
without the gas.
In case of 170 kHz, degradation efficiency of
CCl4 was lower in sonication with air than sonication
72
100
170
Frequency (kHz)
Fig. 3. Effect of frequency on the degradation rate in the
sonication of CCl4 with and without argon
purging.
with Ar. The reverse outcomes were made in the condition of 35, 72 and 100 kHz. The results can be explained by the decrease of dissolved oxygen in solution when purging Ar, which is also the result obtained by Entezari and Kruus [19]. That is, the scavenging effect of H radical by O2 (Eq. 2) may occur at
170 kHz, in which the effect of Ar for cavitation procedure may countervail in other frequencies. It means
that the possibility of the recombination (Eq. 5) increases with the decrease of frequency. Based on the
result, the production of radicals in 35, 72 and 100
kHz was trivial compared to that in 170 kHz, which
was strengthened by adding Ar.
• OH + H • → H 2 O
(5)
The sonolytic degradation can be attained in the gasphase interior of the bubble as well as in the interfacial region [21]. Increasing frequency can reduce the
collapsing duration and the resonance radius of the
bubble, which can enhance the formation of OH radical and the mass transfer to the interface on disintegrating the bubble [22,23]. Therefore, the degradation
of CCl4 can be obtained through the oxidation by OH
radical at the interface. In this case, the thermolysis of
the CCl4 can be compounded such as Eq. 6 emanating
from bubble collapsing.
Δ
CCl 4 ⎯
⎯→ CCl 3 • + Cl •
(6)
In case of sonication at 170 kHz with argon gas,
the degradation of CCl4 was improved compared to
that in sonication without Ar. The result can be explained by the decrease of the generation of the OH
radical by the recombination reaction (Eq. 5) due to
the decrease of dissolved oxygen. Based on the Fig. 3,
the increase of frequencies can be the index to gener-
Sustain. Environ. Res., 21(2), 167-172 (2011)
170
ate OH radical in sonication; the decomposition of
CCl4 increased with the decrease of OH radical. That
is, the sonolysis of CCl4 was achieved by the pyrolysis.
100
(a)
35 kHz
72 kHz
100 kHz
170 kHz
80
60
40
20
0
100
(b)
80
Ct/Co (%)
In order to explain lower removal of CCl4 at
higher frequency (170 kHz) by radical inhibition into
the sonolysis, methanol, as a radical scavenger, was
added to the solution during sonolysis of CCl4. Table
1 shows the effect of methanol on the removal efficiency of CCl4 during sonication. At the frequency of
35 and 72 kHz, the degradation efficiencies in the
sonication with air condition slightly decreased in the
presence of methanol compared to without methanol
from 91 and 92% to 89 and 89%, respectively. However, degradation efficiencies at 100 and 170 kHz during sonication decreased significantly in the presence
of methanol from 94 and 25% to 79 and 8%, respectively.
Methanol can react with OH radical, which generated from the reaction of CH2OH radical with H2O
with a higher rate constant (9.7 × 108 M-1 s-1) [24,25].
This indicates that while methanol plays as a scavenger for OH radical, CH2OH radical can be the acceptor of H radical. In this study, the removal rate of CCl4
decreased at 170 kHz, indicating that the increase of H
radical in bulk solution at 170 kHz, and CCl4 can be
reduced by the nucleophiles such as H radical [25].
Also, CH2OH radical, which is main factor in the oxidation of methanol by OH radical, can compete with
H radical for attacking CCl4. This result implicates
that the oxidation by H radical can be the main factor
in the sonolysis of CCl4 at frequency such as 170 kHz,
and the pyrolysis can be the main effect factor at
lower frequency.
Ct/Co (%)
2. Effect of Methanol on the Removal of CCl4
60
40
20
0
0
10
20
30
40
Time (min)
50
Fig. 4. Effect of the frequencies on the production of
chloride in the sonolysis of CCl4 at (a) without
Ar and (b) with Ar ([Cl]max = [CCl4]0 = 0.02 mM,
the ultrasonic energy = 650 W).
HOCl (8.1%), as shown in Eqs. 7 through 9.
Δ
3. Formation of Chloride
Figure 4 shows the production efficiency of
chloride in the sonication with various frequencies
with and without Ar. The production of chloride in the
sonication was not different at 35, 72 and 100 kHz.
Instead, the chloride production in the presence of Ar
was higher at 35 and 72 kHz than at 100 and 170 kHz.
Hua and Hoffmann [21] reported that the main final
product of CCl4 sonolysis was chloride (68.9%) and
60
CCl 3 • ⎯⎯→ •• CCl 2 + Cl •
(7)
•
•
(8)
CCl 2 + H 2 O → CO + 2 HCl
H O
2
2 Cl • → Cl 2 ⎯⎯
⎯
→ HOCl + HCl
(9)
This result implicates that the formation of chloride can be initiated from the pyrolysis of CCl4 and
CCl3•. It can be assumed that the reactions (Eqs. 6 and
7) occur inside the bubble because the reaction at gas-
Table 1. Effect of methanol in ultrasonication (US) system on the degradation efficiency of CCl4 under the ambient
temperature within 60 min
Frequency (kHz)
35
72
100
170
Without methanol (%)
US + air
US + Ar
91
90
92
93
94
65
25
87
With methanol (%)
US + air
US + Ar
89
82
89
85
79
44
8
71
Im et al.: Sonolytic Degradation of CCl4
phase favours the thermolysis of CCl4. This reaction
can be facilitated at lower frequency where the growth
duration of bubble is lengthened. In contrast, the resonance radius of the bubble can decrease at higher frequency where the chemical reaction among chemicals
increases. That is, the reaction between CCl4 and H
radical within bubble is readily made by increasing
frequency or adding argon gas. Therefore, the operating condition at lower frequency and without adding
argon gas is more effective in the sonication reaction.
6.
7.
8.
CONCLUSIONS
In this study, the effects of frequency and the argon purging on the sonolytic degradation using CCl4
were examined. Degradation efficiencies of CCl4
without argon purging were more than 90% at lower
frequencies such as 35, 72 and 100 kHz with the chloride production of 67, 70 and 61%, respectively. In
case of 170 kHz, degradation efficiency of CCl4 was
improved from 25 to 87% by argon purging. The results implicate that pyrolysis plays a more important
role than radical oxidation in the sonolytic degradation
of CCl4. The removal of CCl4 in the sonication may be
attained not by the oxidation of OH radical but by the
reduction of H radical. This result implicates that lowering frequency in the sonication process is more effective when the degradation of pollutants is mainly
achieved by pyrolysis, resulting in saving electricity
for the treatment.
9.
10.
11.
12.
13.
ACKNOWLEDGMENT
The authors thank the Korean Science and Engineering Foundation for supporting this work (R012007-000-20886-0(2008))
14.
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Discussions of this paper may appear in the discussion section of a future issue. All discussions should
be submitted to the Editor-in-Chief within six months
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Manuscript Received: June 15, 2009
Revision Received: December 8, 2009
and Accepted: April 26, 2010