Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic compounds in an aqueous solution under Ar: Positive and negative effects induced by the presence of salts

Article abstract of DOI:10.1016/j.ultsonch.2015.06.028

Abstract Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at 200 kHz in the absence and presence of Na₂SO₄ or NaCl. The rates of sonochemical degradation in the absence of salts decreased in the order 4-chlorophenol > phenol > catechol > resorcinol and this order was in good agreement with the order of log P (partition coefficient) value of each phenolic compound. The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or positive effects. We discussed these unclear results based on two viewpoints: one was based on the changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol aqueous solution in the absence and presence of Na₂SO₄ or NaCl suggested that phenolic compounds more easily accumulated at the interface region of bubbles at higher salt concentration. These results indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concentration. The yield of H₂O₂ formed in the presence of Na₂SO₄ or NaCl decreased with increasing salt concentration. These results suggested that sonochemical efficiency decreased with decreasing gas amount in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation of phenolic compounds became unclear. The products formed from sonochemical degradation of 4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and 4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence of salts.

Full text of DOI:10.1016/j.ultsonch.2015.06.028

Ultrasonics Sonochemistry 28 (2016) 144–149
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Effects of Na2SO4 or NaCl on sonochemical degradation of phenolic
compounds in an aqueous solution under Ar: Positive and negative
effects induced by the presence of salts
Md. Helal Uddin a,b, Ben Nanzai c, Kenji Okitsu a,
⇑
a Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
b Depertment of Applied Chemistry and Chemical Technology, Faculty of Applied Science and Technology, Islamic University, Kushtia 7003, Bangladesh
c Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 April 2015
Received in revised form 26 June 2015
Accepted 27 June 2015
Available online 6 July 2015
Sonochemical degradation of 4-chlorophenol, phenol, catechol and resorcinol was studied under Ar at
200 kHz in the absence and presence of Na2SO4 or NaCl. The rates of sonochemical degradation in the
absence of salts decreased in the order 4-chlorophenol > phenol > catechol > resorcinol and this order
was in good agreement with the order of log P (partition coefficient) value of each phenolic compound.
The effects of salts on the rates of sonochemical degradation consisted of no effect or slight negative or
positive effects. We discussed these unclear results based on two viewpoints: one was based on the
changes in pseudo hydrophobicity and/or diffusion behavior of phenolic compounds and the other was
based on the changes in solubility of Ar gas. The measured log P value of each phenolic compound slightly
increased with increasing salt concentration. In addition, the dynamic surface tension for 4-chlorophenol
aqueous solution in the absence and presence of Na2SO4 or NaCl suggested that phenolic compounds
more easily accumulated at the interface region of bubbles at higher salt concentration. These results
indicated that the rates of sonochemical degradation should be enhanced by the addition of salts. On
the other hand, the calculated Ar gas solubility was confirmed to decrease with increasing salt concen-
tration. The yield of H2O2 formed in the presence of Na2SO4 or NaCl decreased with increasing salt con-
centration. These results suggested that sonochemical efficiency decreased with decreasing gas amount
in aqueous solution: a negative effect of salts was observed. Because negative and positive effects were
induced simultaneously, we concluded that the effects of salts on the rates of sonochemical degradation
of phenolic compounds became unclear. The products formed from sonochemical degradation of
4-chlorophenol were also characterized by HPLC analysis. The formation of phenol and
4-chloro-1,3-dihydroxy benzene was confirmed and these concentrations were affected by the presence
of salts.
Keywords:
Sonochemical degradation
Phenolic compounds
Effect of salts
Dissolved argon
H2O2
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
Advanced oxidation processes (AOPs) have been developed to
degrade organic pollutants in water. AOPs include ozonation,
Phenol and substituted phenols are important organic interme-
diates for the products of industry and agriculture. However, they
are high toxic and create serious problems to human health and
aquatic species. The conventional methods to deal with such phe-
nolic compounds include activated carbon adsorption, biological
treatment, and chemical oxidation [1–5], however, the application
of these methods has its limitations and disadvantages. Actually,
high toxicity and stability of phenolic compounds hamper the
treatments of wastewater.
UV/photocatalysis, UV/H2O2, Fenton reaction, radiolysis, sonolysis
and their combinations [6–19]. When a solution containing organic
pollutants is irradiated by ultrasound, organic pollutants are
degraded by OH radicals and/or direct pyrolysis reactions during
acoustic cavitation. Although unique reaction conditions consisting
of extremely high temperatures and pressures, rapid cooling, shock
waves, micro-jets, etc. are generated in an aqueous solution
[20,21], the rate of sonochemical degradation of organic pollutants
is still slow for practical use.
Therefore, a new technique has to be developed to enhance the
rate of sonochemical degradation. So far, effects of various types of
salts on sonochemical degradation of organic compounds have
⇑
Corresponding author.
E-mail address: okitsu@mtr.osakafu-u.ac.jp (K. Okitsu).
http://dx.doi.org/10.1016/j.ultsonch.2015.06.028
1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

Md. H. Uddin et al. / Ultrasonics Sonochemistry 28 (2016) 144–149
145
been investigated, where NaCl or Na2SO4 are often used. However,
it seems that the experimental results observed often conflict each
other. For example, it was reported that the rates of sonochemical
degradation of chlorobenzene, p-ethylphenol and phenol were
enhanced by the addition of NaCl under 20 kHz ultrasound
irradiation [14,16,17]: a positive effect of NaCl was observed. On
the other hand, Chen and Smirniotis reported that the rates of
sonochemical degradation of phenol decreased by the addition of
NaCl or Na2SO4 under 20 kHz ultrasound irradiation [13]. The
effects of NaCl or Na2SO4 on the rates of sonochemical degradation
of Rhodamin B [22,23], malachite green [24] and crystal violet [25]
have been investigated, but all cases showed no effect of NaCl or
Na2SO4.
In this study, we investigated the effects of Na2SO4 or NaCl on
sonochemical degradation of several phenolic compounds by using
a standing wave type 200 kHz ultrasound irradiation system under
Ar atmosphere. We chose phenolic compounds as probe
compounds because they are the most common and recalcitrant
pollutants present in industrial wastewaters.
the surface tension was monitored at 1 s interval. Three or five
experimental runs were performed and the average data were used
in figure, where the standard deviation was 0.4 (no salt), 0.4
(0.50 M Na2SO4) and 1.3 (0.45 M NaCl) at 1000 s, respectively.
2.1.4. Sonolysis
An ultrasonic generator (Kaijo 4021, Lot No. 1033) and an oscil-
lator (Kaijo 4611, MFG. No. 34C3) of 65 mm diameter were used for
ultrasonic irradiation and were operated at 200 kHz with a nomi-
nal power of 200 W (calorimetric power for 60 ml sample solution:
16 W) at 20 °C. A glass vessel with 60 ml sample solution was used
for ultrasonic irradiation under Ar atmosphere, where the vessel
was mounted at a constant position (4 mm from the oscillator).
The vessel had flat bottom with 1 mm thick and a side arm with
a silicon rubber septum for Ar gas bubbling and sample extracting
by glass syringe (1 ml) without exposing the sample to air. The
schematic diagram of the experimental setup was described in
the literature [27]. The sonication was performed up to 30 min to
understand the initial degradation behavior of phenolic com-
pounds. When the degradation products were analyzed, the soni-
cation was performed up to 60 min. The sonicated solutions were
analyzed by a HPLC. The analysis conditions for resorcinol, cate-
chol, phenol and 4-chlorophenol were the same as the measure-
ment of log P. For determination of products formed from
sonochemical degradation of 4-chlorophenol, a mobile phase of
methanol and water (volume ratio = 40:60) was used at flow rate
of 0.70 ml minÀ1, where the detection wavelength of 224 nm was
used. The changes in UV–vis spectra of the sample solutions were
measured by a UV–vis spectrophotometer (Shimadzu UV-2450).
The concentration of H2O2 formed in the sonolysis of aqueous solu-
tion with and without salts under Ar was measured by a KI method
[28], where the absorbance at 352 nm was measured.
2. Experimental section
2.1. Experimental procedure
2.1.1. Chemicals
1-Octanol (purity > 99.5%) was purchased from Tokyo Chemical
Industries. Catechol (purity 99.0%), resorcinol (purity 99.0%), phe-
nol (purity 99.0%), 4-chlorophenol (purity 98%), Na2SO4 (purity
99%), NaCl (purity 99.5%), and methanol (purity 99.7%) were pur-
chased from Wako Pure Chemical Industries. All the chemicals
were reagent grade and used as received. Ar (purity 99.999%)
was purchased from Osaka Sanso. All solutions were prepared with
Milli-Q purified water (resistivity of 18.2 MO cm at 25 °C).
3. Results and discussion
2.1.2. Measurement of log P
Hydrophobicity is commonly expressed by log P, where P is the
Sonochemical degradation of 4-chlorophenol, phenol, catechol
and resorcinol was investigated in the absence and presence of
Na2SO4 or NaCl under Ar. The rate of degradation of phenolic com-
pounds obeyed a pseudo first order rate constant as seen in
Figs. S1–S3 in Supplementary data. Therefore, we analyzed the
pseudo rate constant as a rate of degradation in this study. The
result obtained in the absence and presence of Na2SO4 is shown
in Fig. 1(a). The rate constant for sonochemical degradation of
4-chlorophenol in the absence of salt was determined by three
times sonication experiments as shown in Fig. S1. The average
value was plotted in Fig. 1(a), where the error bar corresponds to
the standard deviation. Figs. 1(a) and S2 indicate that no effect or
a slight positive effect of Na2SO4 were observed. We also investi-
gated the effects of NaCl on the rate constant for sonochemical
degradation. The result is shown in Figs. 1(b) and S3. Even in the
case of NaCl addition, no effect for chlorophenol (k = 0.0214 minÀ1
at no salt, 0.0209 minÀ1 at 0.023 M NaCl, 0.0217 minÀ1 at 0.45 M
NaCl), a slight negative effect for phenol (k = 0.0180 minÀ1 at no
salt, 0.0168 minÀ1 at 0.023 M NaCl, 0.0160 minÀ1 at 0.045 M
NaCl), or slight positive effects for catechol (k = 0.0125 minÀ1 at
no salt, 0.0143 minÀ1 at 0.023 M NaCl, 0.0142 minÀ1 at 0.045 M
NaCl) and for resorcinol (k = 0.00977 minÀ1 at no salt,
0.0128 minÀ1 at 0.023 M NaCl, 0.0134 minÀ1 at 0.045 M NaCl) were
observed.
octanol–water partition coefficient.
P
is determined by
Coctanol/Cwater, where Coctanol and Cwater are the concentrations of a
target organic solute in octanol and water, respectively. The
stir-flask method [26] improved was used to measure P:
1-octanol (5 ml), aqueous solution with or without salt (4 ml),
and 0.45 mM phenolic solution (1 ml) were added to
a
flat-bottomed flask. The contents were sealed and stirred at
500 rpm for 24 hours at room temperature (22 1 °C). The con-
tents were settled on the desk for 2–3 h at room temperature.
50 ll of aqueous layer was injected in a high performance liquid
chromatograph (HPLC, Shimadzu LC-20ATvp with a VP-ODS C-18
reversed phase column (4.6 Â 250 mm)) for analysis. A mobile
phase of methanol and water (volume ratio = 30:70) was used for
resorcinol, catechol and phenol, and methanol and water (volume
ratio = 80:20) was used for 4-chlorophenol at flow rate of
0.70 ml minÀ1 where the detection wavelength of 254 nm was
used for resorcinol and catechol, and of 280 nm for phenol and
4-chlorophenol.
2.1.3. Measurement of surface tension
When the concentration of 4-chlorophenol was less than
1.0 mM, the change in surface tension was too small to monitor
the change in surface tension. Therefore, as a sample solution for
surface tension measurements, 20 mM 4-chlorophenol aqueous
solution was chosen. The changes in the surface tension of the
sample solution in the absence and presence of 0.50 M Na2SO4 or
0.45 M NaCl were measured in the time range from 0 to 1800 s
by using a Young–Laplace method for a pendant drop with an
interfacial tension meter (Kyowa Interface Science DM-501), where
In the absence of salt, the results showed that the rate
constants for sonochemical degradation decreased in the order
4-chlorophenol > phenol > catechol > resorcinol. It has been
reported that the hydrophobicity of an organic solute is one of the
most important parameters for the rate of sonochemical degrada-
tion, because highly hydrophobic compounds tend to accumulate

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Md. H. Uddin et al. / Ultrasonics Sonochemistry 28 (2016) 144–149
acoustic cavitation, we measured the change in log P value of
4-chlorophenol, phenol, catechol and resorcinol at different
Na2SO4 concentrations. The result is shown in Fig. 2. It was con-
firmed that the pseudo log P values measured slightly increased
with increasing Na2SO4 concentrations, suggesting that all pheno-
lic compounds behave as more hydrophobic compounds in a
higher Na2SO4 concentration solution. Since more hydrophobic
compounds are degraded more quickly [30], the addition of salts
should induce a salting out effect of phenolic compounds from bulk
solution to the interface region of bubbles which should enhance
the rate of degradation. However, Fig. 2 shows that the log P value
does not increase largely by the addition of Na2SO4. We considered
that the log P value was the information of equilibrium so that the
real diffusion behavior of phenolic compounds was unclear.
To confirm the effect of salt on the diffusion behavior of pheno-
lic compounds, we measured the dynamic surface tension of
4-chlorophenol aqueous solutions in the absence and presence of
0.50 M Na2SO4 or 0.45 M NaCl, where dynamic changes in the sur-
face tension of a pendant drop were measured as a function of
time. The results are shown in Fig. 3. The surface tension gradually
decreased with time. This meant that 4-chlorophenol molecules
accumulated at the gas-liquid interface with time. Fig. 3 shows
three results: (1) at the initial stage of equilibrium, the surface ten-
sion in the presence of salts is smaller than that in the absence of
salt, (2) the slope in the change in the surface tension in the pres-
ence of salts is steeper than that in the absence of salt, and (3) the
surface tension reached after equilibrium in the presence of salts is
smaller than that in the absence of salt. All results indicated that
4-chlorophenol molecules accumulate at the gas-liquid interface
faster in the presence of salt. This quick accumulation behavior
by the presence of salt should act as a positive effect for degrada-
tion of phenolic compounds.
Fig. 1. Pseudo first order rate constants for sonochemical degradation of 4-
chloropenol, phenol, catechol and resorcinol in the absence and presence of (a)
Na2SO4 and (b) NaCl. Initial concentration of phenolic compound: 0.45 mM.
However, as seen in Fig. 1, some results do not show such a
positive effect. To discuss this contradiction phenomenon, we con-
sidered a further aspect regarding the effects of salts. For example,
in the absence of organic solutes, when the concentration of salts is
increased, various changes in parameters such as changes in the
surface tension of solution, vapor pressure of water, viscosity of
solution, and gas solubility in a solution occur simultaneously
(Table 1).
at the interface region of cavitation bubbles in which the concentra-
tion of OH radicals is higher than that in the bulk solution [29].
Nanzai et al. reported that the initial rates of degradation of aromatic
compounds increased with increasing log P value, which was used as
an indicator of hydrophobicity of an aromatic compound [30]. To
confirm the relationship between the rate constant for sonochemi-
cal degradation and log P value, we measured P values of phenolic
compounds experimentally. The results showed that log P values
were 2.25 for 4-chlorophenol, 1.47 for phenol, 0.97 for catechol,
and 0.87 for resorcinol, respectively. The values measured were in
good agreement with the reported values [31]. The order of the rate
constant for sonochemical degradation was in good agreement with
the order of log P. In addition, the hydrophobicity of phenolic com-
pound was reported in the order of 4-chlorophenol > phenol >
catechol = resorcinol which was determined by the change in
surface tension as a function of phenolic compound concentration
in the range from 0 to 10 mM at 20 °C [32]. They also suggested that
the rates of sonochemical degradation of these compounds should
be this order. Our results were in good agreement with this order.
Next, we discuss the effects of salts on the rate of degradation.
As described above, the addition of Na2SO4 or NaCl did not affect
the rate of degradation clearly. Several researchers reported that
the presence of NaCl resulted in the enhanced sonochemical effi-
ciency [14,16,17,33]. However, our result was different from their
results. Before experiments, we had expected that the addition of
salts will enhance the rate of degradation by moving phenolic com-
pounds from bulk solution to the interface region of bubbles (here
becomes high temperature and high OH radical concentration after
collapsing bubbles). To guess whether the movement of phenolic
compounds is enhanced or not by the presence of salts during
Based on the literature [34], the surface tension of aqueous
solutions increases linearly with increasing salt concentration.
Fig. 2. Relationship between concentrations of Na2SO4 and log P of 4-chloropenol,
phenol, resorcinol, and catechol.

Md. H. Uddin et al. / Ultrasonics Sonochemistry 28 (2016) 144–149
147
Fig. 3. Changes in surface tension of 4-chlorophenol aqueous solutions in the
absence and presence of 0.50 M Na2SO4 or 0.45 M NaCl as a function of time.
Concentration of 4-chlorophenol: 20 mM.
However, the increment of the surface tension at 20 °C is as small
as from 72.5 mN mÀ1 (pure water) to 73.9 mN mÀ1 (Na2SO4:
0.51 M) and to 73.4 mN mÀ1 (NaCl: 0.50 M) [34,35]. As is well
known, the vapor pressure of water decreases with increasing salt
concentration. However, based on the literature [35,36], the decre-
ment of the vapor pressure of water at 20 °C is as small as from
17.5 mmHg (pure water) to 17.2 mmHg (Na2SO4: 0.50 M). In the
case of NaCl at 18 °C, 15.5 mmHg (pure water) decreases to
15.2 mmHg (NaCl: 0.62 M). In addition, based on the literature
[35,37], the viscosity of solutions at 20 °C changes from
1.00 mPa s (pure water) to 1.29 mPa s (Na2SO4: 0.70 M) and
1.07 mPa s (NaCl: 0.84 M). The change in the viscosity may look
like a relatively larger value than those in the surface tension
and vapor pressure, but the change in the viscosity could be con-
sidered as not so large change, because the viscosity of 0.70 M
Na2SO4 solution at 20 °C (1.29 mPa s) is almost the same as that
of pure water at 10 °C (1.31 mPa s). Therefore, we assumed that
the effects of changes in the surface tension, vapor pressure of
water and viscosity of solution are negligibly small for sonochem-
ical degradation.
It has also been suggested that the amount of dissolved gas in
water affects sonochemical efficiency [38], because the number
of acoustic cavitation bubbles is affected by the amount of dis-
solved gas. To discuss the amount of dissolved Ar gas in water,
we calculated the saturation solubility of Ar gas in the presence
of Na2SO4 or NaCl. The calculation method is shown in
Supplementary data. Table 1 shows the solubility of Ar gas at
20 °C decreasing with increasing salt concentrations: 2.75 Â 10À5
(no salt) > 1.87 Â 10À5 (0.25 M Na2SO4) > 1.28 Â 10À5 (0.50 M
Fig. 4. Sonochemical formation of H2O2 in the absence and presence of (a) Na2SO4
and (b) NaCl.
Na2SO4), and 2.75 Â 10À5 (no salt) > 2.55 Â 10À5 (0.23 M
NaCl) > 2.37 Â 10À5 (0.45 M NaCl). This tendency is in good agree-
ment with the literature [39].
Okitsu et al. reported that the yield of H2O2 formed in the sonol-
ysis of pure water increases linearly with increasing amount of dis-
solved gases of He, Ne, Ar, Kr and Xe [38], where the yield of H2O2
corresponds to sonochemical efficiency. Based on the solubility of
Ar gas in Table 1, sonochemical efficiency should decrease in a
higher salt concentration. To confirm whether sonochemical effi-
ciency changes or not by the addition of salts, the H2O2 yield was
measured in the absence and presence of salts without phenolic
compounds. Fig. 4(a) and (b) indicate the H2O2 yield as a function
of irradiation time. The change in concentration of H2O2 could not
be fitted linear lines between 0 and 30 min. This is because the
change in the irradiation condition occurs with irradiation time.
Taking into account the change in the irradiation condition, linear
lines were drawn between 0 and 20 min with the least squares
method through zero point. It was confirmed that the H2O2 yield
decreased with increasing concentration of salts, where the
Table 1
Surface tension of solution, vapor pressure of water, viscosity of solution and saturation solubility of Ar in pure water, Na2SO4 aqueous solution and NaCl aqueous solution.
Surface tension of solution
Vapor pressure of water/mmHg
Viscosity of solution
[37]/mPa s at 20 °C
Ar solubility in solution
at 20 °C
[34]/mN mÀ1 at 20 °C
[36] at 20 °Ca or 18 °Cb
Pure water
Na2SO4 aqueous solution (conc. of Na2SO4)
72.5
17.5a, 15.5b
1.00
2.75 Â 10À5 [38]
73.9 (0.51 Mc)
17.2a (0.50 Mc)
1.29 (0.70 Mc)
1.87 Â 10À5 (0.25 M)
1.28 Â 10À5 (0.50 M)
2.55 Â 10À5 (0.23 M)
2.37 Â 10À5 (0.45 M)
NaCl aqueous solution (conc. of NaCl)
73.4 (0.50 Mc)
15.2b (0.62 Mc)
1.07 (0.84 Mc)
a
b
c
The data at 20 °C.
The data at 18 °C.
The concentration of salt was converted from mol kgÀ1 to mol LÀ1 by using the specific gravity of solutions in [35].

148
Md. H. Uddin et al. / Ultrasonics Sonochemistry 28 (2016) 144–149
If phenolic compounds were pushed to the interfacial region from
bulk solution by the presence of salts, the degradation mechanism
of phenolic compounds would be different between in the absence
and presence of salts: the contribution ratio of pyrolysis reactions
to radical reactions may be changed. Fig. S4 shows the changes in
absorption spectra of 4-chlorophenol solutions during irradiation,
where the sonication was performed up to 60 min. From Fig. S4,
it was confirmed that the degradation of 4-chlorophenol occurred
to form any products which had absorption spectra in the UV
region. From HPLC analysis, the formation of various types of prod-
ucts with higher hydrophilic compounds than 4-chlorophenol was
confirmed. The peaks at the retention time of 15.6 and 14.2 min
were determined as phenol and 4-chloro-1,3-dihydroxy benzene.
Fig. S5 shows the changes in concentration of 4-chlorophenol as
a function of irradiation time. Fig. 5 shows the changes in concen-
tration of phenol and 4-chloro-1,3-dihydroxy benzene formed. By
the addition of salts, the concentration of phenol formed increased
and that of 4-chloro-1,3-dihydroxy benzene decreased, although
the rates of degradation of 4-chlorophenol were not affected signif-
icantly. The reason why this phenomenon was observed was
unclear at present, but this result suggested that the addition of
salts should affect the degradation mechanism. Recently, Brotchie
et al. [39] reported that the size of bubbles decreases by the pres-
ence of salts. Various changes in acoustic cavitation phenomenon
may be induced by the presence of salts, therefore, further studies
must be required to discuss in details.
4. Conclusions
The rates of sonochemical degradation of phenolic compounds in
the presence of salts were affected by the changes in (1) the pseudo
hydrophobicity and (2) diffusion behavior of phenolic compounds
and (3) the solubility of Ar gas. The rates of sonochemical degrada-
tion of phenolic compounds in the absence and presence of
Na2SO4 or NaCl were in the order 4-chlorophenol > phenol >
catechol > resorcinol. This order was explained by the order of log
P. The effects of Na2SO4 or NaCl on the rates of sonochemical degra-
dation were investigated. We suggested that a positive effect due to
increasing hydrophobicity induced by the addition of salts and
increasing diffusion behavior of phenolic compounds would be off-
set by a negative effect due to decreasing sonochemical efficiency
which was induced by the decrease of gas solubility at high concen-
tration of Na2SO4 or NaCl. As described in introduction section, many
studies for the effects of Na2SO4 or NaCl on the sonochemical degra-
dation have been reported, where the obtained results have often
conflicted each other. Our results suggested that the balance
between negative and positive effects may sensitively depend on
the sonication condition so that the effects of salts reported so far
showed unclear effects.
Fig. 5. Changes in concentration of (a) phenol and (b) 4-chloro-1,3-dihydroxy
benzene as a function of irradiation time. Initial concentration of 4-chlorophenol:
0.45 mM.
additional experiments at higher concentrations (Na2SO4; 1.0 M
and 1.5 M, NaCl; 0.94 M and 1.4 M) were carried out to confirm
the effect of salts more clearly. Wakeford et al. reported that the
H2O2 yield decreased with increasing concentration of NaCl under
Ar at 35 kHz ultrasound irradiation [40]. They suggested that the
lower yield of H2O2 in higher NaCl concentration may be due to
ClÀ acting as an OH radical scavenger. They do not consider the
occurrence of a gas solubility change when the salt concentration
increases. Based on the gas solubility at different salt concentration
(Table 1), the number of the formed cavitation bubbles with high
temperatures would decrease by the presence of Na2SO4 or NaCl
in water, resulting in the lower H2O2 yield in the higher salt con-
centration. It was also confirmed that Na2SO4 was more effective
for the decrement in the H2O2 yield than NaCl. This phenomenon
would be explained by the difference of Ar gas solubility between
Na2SO4 and NaCl solutions.
In spite of the lower H2O2 yield in the presence of Na2SO4 or
NaCl, some results in Fig. 1 show that the rate constants for degra-
dation do not change or slight increase when the concentration of
Na2SO4 or NaCl increases. To explain these results, we proposed for
the first time that the effects of salts on sonochemical degradation
of phenolic compounds consisted of the mixture of a positive and a
negative effect so that the effects of salts became unclear.
Acknowledgements
We acknowledge JSPS for financial support as postdoctoral fel-
lowship. Kenji Okitsu acknowledges the support of JSPS KAKENHI
Grant Number 25Á03048 and 25340072.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ultsonch.2015.06.
028.
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