Factors affecting sonolytic degradation of sulfamethazine in water

Article abstract of DOI:10.1016/j.ultsonch.2013.04.007

In this study, the major factors affecting sonolytic degradation of sulfamethazine (SMT), a typical pharmaceutically active compound, in water were evaluated. The factors tested included two operational parameters (i.e. initial SMT concentration and ultrasonic power), three dissolved gases (i.e. Ar, O ₂ and N₂), five most frequently found anions in water (NO3-,Cl^-,SO42-,HCO3-andBr^-), ferrous ion (Fe ²⁺), and four alcohols (methanol, ethanol, isopropyl alcohol, tert-butyl alcohol). Typically, the degradation rate was increased with the increasing initial SMT concentration and power. The degradation rate was accelerated in the presence of argon or oxygen, but inhibited by nitrogen. Effects of anions on the ultrasonic treatment were species-dependent. The SMT degradation rate was slightly inhibited by NO3-,Cl^-,and,SO42- but significantly improved by HCO3-andBr^-. The negative effects of alcohols acted as hydroxyl radicals scavengers with the following order: tert-butyl alcohol > isopropyl alcohol > ethanol > methanol. The synergetic effect of ferrous ion was mainly due to production of additional hydroxyl radicals (·OH) through Fenton chemistry. LC/MS/MS analysis indicated that the degradation of SMT by ultrasonic irradiation is mainly ascribed to ·OH oxidation. Of interest, although the SMT could be rapidly degraded by ultrasonic irradiation, the degradation products were rarely mineralized. For example, ～100% of 180 μM SMT was decomposed, but only 8.31% TOC was reduced, within 2 h at an irradiation frequency of 800 kHz and a power of 100 W. However, the products became much biodegradable (BOD ₅/COD was increased from 0.04 to 0.45). Therefore, an aerobic biological treatment may be an appropriate post-treatment to further decompose the SMT degradation products.

Full text of DOI:10.1016/j.ultsonch.2013.04.007

Ultrasonics Sonochemistry 20 (2013) 1401–1407
Contents lists available at SciVerse ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Factor affecting sonolytic degradation of sulfamethazine in water
b
c
c
c
Yu-qiong Gao a, Nai-yun Gao a, , Yang Deng , Jin-shan Gu , Yu-liang Gu , Dong Zhang
⇑
a State Key Laboratory of Pollution Control and Resources Reuse, Ministry of Education, College of Environmental Science and Engineering, Tongji University, Shanghai 200092,
PR China
b Department of Earth and Environmental Studies, Montclair State University, Montclair, NJ 07043, USA
c Shanghai Urban Construction Investment Development Corporation, Shanghai 200020, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the major factors affecting sonolytic degradation of sulfamethazine (SMT), a typical phar-
maceutically active compound, in water were evaluated. The factors tested included two operational
parameters (i.e. initial SMT concentration and ultrasonic power), three dissolved gases (i.e. Ar, O2 and
N2), five most frequently found anions in water (NOꢀ; Clꢀ; SO2ꢀ; HCO3ꢀ andBrꢀ), ferrous ion (Fe2+), and four
Received 24 November 2012
Received in revised form 7 April 2013
Accepted 17 April 2013
3
4
Available online 28 April 2013
alcohols (methanol, ethanol, isopropyl alcohol, tert-butyl alcohol). Typically, the degradation rate was
increased with the increasing initial SMT concentration and power. The degradation rate was accelerated
in the presence of argon or oxygen, but inhibited by nitrogen. Effects of anions on the ultraꢀsonic treat-
ment were species-dependent. The SMT degradation rate was slightly inhibited by NOꢀ3 ; Cl ; and;SO42ꢀ
but significantly improved by HCOꢀ3 andBrꢀ. The negative effects of alcohols acted as hydroxyl radicals
scavengers with the following order: tert-butyl alcohol > isopropyl alcohol > ethanol > methanol. The
synergetic effect of ferrous ion was mainly due to production of additional hydroxyl radicals (ꢁOH)
through Fenton chemistry. LC/MS/MS analysis indicated that the degradation of SMT by ultrasonic irra-
diation is mainly ascribed to ꢁOH oxidation. Of interest, although the SMT could be rapidly degraded by
Keywords:
Sulfamethazine
Sonolytic degradation
Operational parameters
Additives
Products
ultrasonic irradiation, the degradation products were rarely mineralized. For example, ꢂ100% of 180
lM
SMT was decomposed, but only 8.31% TOC was reduced, within 2 h at an irradiation frequency of 800 kHz
and a power of 100 W. However, the products became much biodegradable (BOD5/COD was increased
from 0.04 to 0.45). Therefore, an aerobic biological treatment may be an appropriate post-treatment to
further decompose the SMT degradation products.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
comprised of the formation, growth, and collapse of micro-bubbles
in water. Particularly while the cavitation bubbles violently col-
Antibiotics are intensively used both for human being and in
agricultural activities [1,2]. Most of them have been detected in
different types of water bodies and even in certain solid matrices
[3,4]. As a result of their potential risks in abnormal physiological
processes, reproductive impairment, increased incidences of
cancer, development of antibiotic-resistant bacteria and potential
increased toxicity of chemical mixtures [5], they should be
effectively removed from drinking water. However, because most
of antibiotics are recalcitrant to conventional water treatment
technologies, advanced treatment methods should be considered,
such as advanced oxidation processes (AOPs) [6]. Among different
AOPs, ultrasonic irradiation is relatively new and has received
much less attention than other AOPs. However, previous efforts
have demonstrated that a wide spectrum of organic pollutants
can be successfully removed by ultrasonic irradiation [7–13]. The
major mechanism of ultrasonic irradiation is cavitation that is
lapse, extremely high temperatures (>5000 °C) and pressures
(>1000 atm) can be locally achieved inside the bubbles [14]. Be-
sides the high temperature and pressure, water molecules can be
also dissociated and release highly active oxidizing agent ꢁOH
and reducing agent Hꢁ. Both of them also contribute to removal
of target pollutants via degradation.
Sulfamethazine, a representative antibiotic, has been commonly
used for treatment and prevention of infections such as urinary
tract infections, chlamydia, rheumatic fever, toxoplasmosis and
malaria. To the best of our knowledge, literatures on the fate of
SMT under sonolytic irradiation are highly limited. The objective
of this study was to: (1) evaluate the effects of operational param-
eters, including initial concentration and power, on the sonolytic
degradation of SMT; (2) determine roles of different co-exisiting
chemical species, including dissolved gases, common anions, fer-
rous ion and alcohols, on sonolytic degradation of SMT; (3) analyze
the degradation intermediate products by HPLC and LC/MS/MS;
and (4) evaluate mineralization and biodegradability of SMT under
ultrasonic irradiation.
⇑
Corresponding author. Tel.: +86 21 65982691; fax: +86 21 65986313.
E-mail address: gaonaiyun2011@yahoo.com.cn (N.-y. Gao).
1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ultsonch.2013.04.007

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Y.-q. Gao et al. / Ultrasonics Sonochemistry 20 (2013) 1401–1407
rate of 0.3 mL minꢀ1. The elution started at 1% B for 2 min, it was
2. Materials and methods
then linearly increased to 5% B in 5 min and kept at 5% B for
5 min. After that, the elution was further increased to 95% B in
the following 10 min and kept at 95% B for 3 min. Subsequently,
the elution returned to the initial condition. The HPLC system
was connected with a triple stage quadrupole mass spectrometer
(Thermo Scientific⁄ TSQ Quantum Access MAX, USA) with ion
sources under the following conditions: spray voltage = 3500 V,
vaporizer temperature = 300 °C, sheath gas pressure = 40 au, aux
gas pressure = 10 au, capillary temperature = 320 °C. The degree
of sulfamethazine mineralization was quantified by measurement
of reduction in total organic carbon (TOC) in water. (TOC) analysis
was measured using a TOC analyzer (Shimadzu TOC 5050A)
equipped with an ASI5000 auto-sampler. Chemical oxygen demand
(COD) and 5-day biochemical oxygen demand (BOD5) were mea-
sured according to the standard methods [15].
2.1. Materials
All the chemicals used were of analytical grade, at least.
Solution was prepared using Milli-Q water (18.2 M
X
ꢁcm). SMT
(P99%) was purchased from Sigma–Aldrich (Oakville, ON, Canada).
HPLC-grade methanol, ethanol, isopropyl alcohol and tert-butyl
alcohol were obtained from Sigma–Aldrich. Sodium nitrate,
sodium chloride, sodium sulfite, sodium bicarbonate, potassium
bromide and ferrous sulfate were purchased from Shanghai
Jingchun industry Co., Ltd. (>99% purity).
2.2. Experimental procedure
All the tests were performed in a custom stainless steel cylindri-
cal vessel (80 ꢃ 120 mm), equipped with
a transducer. The
frequency was achieved by an emitter connected to a high-fre-
quency (800 kHz) supply (Shanghai Acoustics Laboratory, Chinese
Academy of Sciences, China). Fig. 1 presents a schematic of the
experiment apparatus. As seen, the vessel was completely
immersed in a water bath controlling the reaction temperature at
20 1 °C. In the tests to investigate the effects of different dis-
solved gases, the vessel was sealed with a custom stainless steel
lid via a flange, and the tested gases were introduced into the solu-
tion through a Teflon tube. In all the other tests, the vessel was
open to the air. In all the tests, 200 mL SMT-containing solution
was dispended into the reaction vessel. Reaction was initiated once
the emitter was turned on. At each designated sampling time, 1 mL
sample was collected from the vessel for further analysis. All the
experiments were conducted in triplicate. And the standard devia-
tions of the data were below 5%.
3. Results and discussions
3.1. Effect of initial SMT concentration
The effect of initial SMT concentration is shown in Fig. 2. In this
study, different runs were compared using the initial degradation
rate (computed as
the removal efficiency of SMT decreased from 98.2% to 89.3% with
increasing the initial concentration from 1.8 M to 36 M. How-
DC/Dt over the first 10 min, l
M minꢀ1). As seen,
l
l
ever, the initial degradation rate was found to be increased with
initial concentration increased.
Our findings can be expected to be dependent on the concentra-
tion of ꢁOH radicals produced and the concentration of the SMT
molecules at the interface of cavitation bubble. To understand
the reaction mechanism occurring in the interfacial region of cav-
itation bubbles, a heterogeneous kinetics model based on Lang-
muir–Hinshelwood (L–H) model was used to fit the experimental
data, as below [16].
2.3. Analytical
Sulfamethazine was measured using a Shimadzu 2010 AHT high
performance liquid chromatogram (HPLC) system (Kyoto, Japan)
kKc
1 þ Kc
r ¼
ð1Þ
with an auto-sampler and
(250 mm ꢃ 4.6 mm). The mobile phase was
acetonitrile and Milli-Q water (40%/60%, v/v) at a flow rate of
0.8 mL/min. The sample injection volume was 20 L. Sulfametha-
a
Shimadzu VP-ODS column
a
mixture of
where r is the initial degradation rate (
l
M minꢀ1); k is an observed
lM) is SMT concentration; and K
rate constant (
l
M minꢀ1); c (
l
(l
Mꢀ1) is the equilibrium adsorption constant of SMT toward the
zine was measured by a UV detector at a wavelength of 266 nm.
The products of SMT were identified by high-performance liquid
chromatography/tandem mass spectrometry. The HPLC (Waters
2010, USA) was equipped with a 2.1 ꢃ 150 mm Hypersil Gold
effective reaction sites prior to bubble collapse.
The sonolytic degradation data was analyzed by non-linear
curve fitting analysis method, using Origin 8.1 software, to fit the
kinetic model (as shown in the insert of Fig. 2). The value of
C18 column, with a particle size of 5
l
m (Thermo, Bellefonte,
L. A and B mobile
phases were Milli-Q water and acetonitrile, respectively, at a flow
the model parameters are 0.015 l l
Mꢀ1 and 3.167 M minꢀ1 for
PA). The injection volume of sample was 10
l
the equilibrium constant (K) and the observed rate constant (k),
respectively. The excellent fitting of experiential data with the
Fig. 1. Schematic diagram of the sonochemical reactor.

Y.-q. Gao et al. / Ultrasonics Sonochemistry 20 (2013) 1401–1407
1403
Fig. 2. Effect of initial SMT concentration on the degradation of SMT (frequency:
800 kHz; power: 100 W).
Fig. 4. Effects of different dissolved gases on the SMT degradation (frequency:
800 kHz; power: 100 W; SMT: 9 M).
l
L–H based model demonstrated that the degradation of SMT pri-
marily occurred at the interfacial regions by hydroxyl radical at-
tack. So the SMT degradation rate may relied upon the
concentrations of ꢁOH produced and SMT at the interface of the
cavitation bubble. At a low SMT concentration, because the OH
radical concentration at the surface of the collapsed bubble is
remarkably high [17], a high fraction of ꢁOH generated would
recombine to yield less active H2O2, which conduct to lower degra-
dation rates [18]. In contrast, at a higher SMT concentration, the
probability of ꢁOH radical attack on SMT molecules increases, thus
leading to an increase in the degradation rate of SMT.
The effect of ultrasonic power is likely because the number of
active cavitation bubbles significantly increased with the increase
in the irradiation power. As a result, the amount of ꢁOH generated
was dramatically increased [19]. Furthermore, a high ultrasound
power could increase the acoustic amplitude that led to more vio-
let cavitation bubble collapse and locally created high temperature
and pressure [20].
3.3. Effects of different dissolved gases
The effects of three dissolved gases, including Ar, N2 and O2, on
the SMT degradation are shown in Fig. 4. Under identical experi-
mental conditions, their initial degradation rate followed an order
of argon (0.727
l
M minꢀ1) > oxygen (0.695
of any gas) (0.369
l
M minꢀ1). In general, dissolved gases influence sonolysis
l
l
M minꢀ1) > control
3.2. Effect of ultrasonic power
(no
addition
M minꢀ1) > nitrogen
The effect of input power on the SMT sonolytic degradation rate
is shown in Fig. 3. The input electric power to transducer we set at
40, 60, 80 and 100 W, respectively.
(0.313
through two aspects. First, monatomic gases typically have greater
polytropic ratios than polyatomic gases, and the higher polytropic
ratios can create higher temperatures and pressures achieved bub-
ble collapse [21]. Second, gases with low thermal conductivities
can reduce the heat dissipation, thus facilitating the increase in
collapse temperatures and enhancing sonochemical activity. Third,
gases with higher solubility can create more nucleation sites, and
improve the cavitation events.
In this study, argon has the greater polytropic ratio (i.e., 1.667)
and solubility (i.e., 5.6 mL in 100 mL H2O), as well as the lower
thermal conductivity (i.e., 0.0173 Wꢁ(m K)ꢀ1) than O2 (polytropic
ratio = 1.400; solubility = 4.9 mL in 100 mL H2O; thermal conduc-
tivity = 0.0240 Wꢁ(m K)ꢀ1) and N2 (polytropic ratio = 1.401; solu-
With the increasing power from 40 to 100 W, the initial degra-
dation rate increased from 0.121 to 0.369 l
M minꢀ1. Furthermore,
within 60 min ultrasonic irradiation, the SMT removal efficiencies
were 41.2%, 68.2%, 83.1% and 94.9% under the ultrasonic powers
of 40, 60, 80 and 100 W, respectively. These findings demonstrated
an increase in the ultrasonic intensity induces a promotion in the
degradation.
bility = 2.4 mL
in
100 mL
H2O;
thermal
conductivity = 0.0228 Wꢁ(mꢁK)ꢀ1) [22]. Therefore, it is not surpris-
ing that Ar achieved the highest initial degradation rate of
0.727
l
M minꢀ1. Between N2 and O2, the former has ꢂ1/2 of solu-
bility of the latter in water, though both of them have almost same
polytropic ratios and thermal conductivities. In addition, the addi-
tion of oxygen could have enhanced the formation of additional
reactive oxygen species through the following reactions Eqs. (2)–
(8), thus promoting of the SMT degradation rate [21].
O2 ! 2ÅOÅ
ÅOÅ þ H2O ! 2ÅOH
2ÅOH ! ÅOÅ þ H2O
ð2Þ
ð3Þ
ð4Þ
Fig. 3. Effect of ultrasonic power on the degradation of SMT (frequency: 800 kHz;
SMT: 9 lM).

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Y.-q. Gao et al. / Ultrasonics Sonochemistry 20 (2013) 1401–1407
O2 þ HÅ ! ÅOH þ ÅOÅ
ð5Þ
during the sonolytic degradation of SMT (reaction (15),
k = 5.5 ꢃ 10ꢀ9).
2 ꢁ OH ! H2O2
O2 þ HÅ ! ÅOOH
2ÅOH ! H2O2
ð6Þ
ð7Þ
ð8Þ
ð15Þ
However, in the presence of HCOꢀ3 and Brꢀ, these anions may re-
act with ꢁOH to produce COꢁ3ꢀ and dibromine radicals, respectively,
as shown in the following reactions.
2ÅOOH ! H2O2 þ O2
HCOꢀ3 þ ꢁOH ! CO3ꢁꢀ þ H2O
Brꢀ þ ꢁOH ! Brꢁ þ OHꢀ
Brꢀ þ Brꢁ ! Brꢁ2ꢀ
ð16Þ
ð17Þ
ð18Þ
In contrast, the presence of nitrogen could scavenge free radi-
cals through reactions Eqs. (9)–(14), thereby inhibiting the degra-
dation rate of SMT [23].
N2 þ ꢁOH ! NO2 þ Hꢁ
N2 þ ꢁOꢁ ! NO ꢁ þ Nꢁ
O2 þ Nꢁ ! NO ꢁ þ ꢁ Oꢁ
NO ꢁ þ ꢁ Oꢁ ! ꢁNO2
ð9Þ
ð10Þ
ð11Þ
ð12Þ
ð13Þ
ð14Þ
Although both COꢁ3ꢀ and dibromine radical were less reactive
than ꢁOH [26,27], the COꢁ3ꢀ and dibromine radical appeared to ade-
quately decompose SMT, and thus enhanced the overall SMT deg-
radation rate. In contrast, the scavenging effect obviously
predominated with NO3ꢀ, Clꢀ, and SO24ꢀ , thus reducing the overall
degradation rate.
NO ꢁ þ ꢁ OH ! HNO2
ꢁNO2 þ ꢁOH ! HNO3
3.5. Effect of hydroxyl radical scavengers
Fig. 6 presents effects of four alcohols, including methanol, eth-
anol, isopropyl alcohol, and tert-butyl alcohol on the SMT degrada-
tion. All of them inhibited the sonolytic degradation, to different
extents.
3.4. Effects of different anions
Many components in water, particularly the anions, are effec-
tive free radical scavengers, which can weaken the sonochemical
effect. The effects of different common anions, including nitrate
(NOꢀ3 ), chloride (Clꢀ), sulfate (SO42ꢀ), bicarbonate (HCOꢀ3 ) and bro-
mide (Brꢀ) on the SMT sonolysis are shown in Fig. 5.
These alcohols inhibited the sonolytic degradation of SMT as a
result of competition for ꢁOH with SMT [28]. Their inhabitation
effect followed the order of tert-butyl alcohol (0.090
l
l
M minꢀ1) >
M minꢀ1) >
isopropyl alcohol (0.130
methanol (0.217
l
M minꢀ1) > ethanol (0.152
l
M minꢀ1). Of interest, this order was in agree
As seen, their effects in the SMT decomposition were different.
The SMT degradation was inhibited in the presence of NO3ꢀ
with the order of their specific heat capacities. Tert-butyl alcohol,
isopropyl alcohol, ethanol, and methanol have their specific heat
values of 113.6, 90, 65.4, and 45.2 J/(mol K) at 298 K, respectively
[22]. It appears that an alcohol with larger specific heat exhibited
a greater inhibition effect. This finding may be ascribed to the im-
pact of specific heat capacities on the cavitation collapse. Cheng
and colleagues [29] found that large specific heat capacities would
reduce the sizes of maximum micro-bubble as well as interfacial
temperatures achieved from bubble collapse. Furthermore, this
thermal response produced fewer ꢁOH radical. On the other hand,
the scavenging activity can be attributed to the possible reaction
of ꢁOH radicals boned to carbons in alcohol molecules [30]. There
are nine, seven, five and three hydrogens, exist in tert-butyl alco-
hol > isopropyl alcohol > ethanol > methanol, respectively.
(0.340 l l lM
M minꢀ1), Clꢀ (0.351 M minꢀ1), and SO24ꢀ (0.368
minꢀ1), and the inhibition degree followed the order of
NOꢀ3 > Clꢀ > SO24ꢀ. In contrast, the degradation rate dramatically
increased with the addition of HCO23ꢀ (0.570 M minꢀ1) and Brꢀ
l
(0.859 l
M minꢀ1).
Generally, two aspects, in the presence of anions, can impact
the sonolysis rate. First, ions in water may cause a so-called ‘‘salt-
ing out effect’’, in which soluble organic pollutants more readily
move toward the bubble-bulk solution interface, and the degrada-
tion is accelerated [24]. Second, certain anions can rapidly react
with reactive oxygen species produced, and thus significantly com-
pete with target compounds for the oxidizing agents [25]. In the
absence of the tested anions, the combination of ꢁOH was dominant
Fig. 5. Effects of different anions on the degradation of SMT (frequency: 800 kHz;
Fig. 6. Effects of different hydroxyl radical scavengers on the degradation of SMT
power: 100 W; SMT: 9
lM).
(frequency: 800 kHz; power: 100 W; SMT: 9 lM).

Y.-q. Gao et al. / Ultrasonics Sonochemistry 20 (2013) 1401–1407
Fe2þ þ ꢁOH ! Fe3þ þ OHꢀ
1405
ð23Þ
ð24Þ
H2O2 þ ꢁOH ! H2O þ HO2ꢁ
3.7. Analyses of SMT sonolytic degradation products
As can be seen in Fig. 8, the chromatograms show clearly the
appearance of peaks corresponding to the products from the deg-
radation of SMT. In addition, products have more hydrophilic
structure than SMT since it was eluted out from the column prior
to SMT. The intermediate products and their probably structures
identified by LC/MS/MS were summarized in Table 1. The SMT
products consistent with data on mass spectrum from previous
studies based on ꢁOH-radical attack of sulfonamides [33–35], con-
Fig. 7. Effect of ferrous ion on the degradation of SMT (frequency: 800 kHz; power:
100 W; SMT: 9 M).
Table 1
l
Products of SMT identified by LC/MS/MS analysis.
Products
Retention time
(min)
Main fragment(m/z)
Molecular structure
3.6. Effect of ferrous ion (Fe2+
)
SMT
17.75
279
CH3
N
Iron is a natural component of water, especially groundwater,
and for water that may be contaminated with SMT and treated
with ultrasound, the Fenton process augments the production of
ꢁOH created during acoustic cavitation [31]. The effect of ferrous
ion on the SMT sonolytic degradation is shown in Fig. 7. Generally,
the increase of ferrous concentration improved the decomposition
rate. The initial degradation rate was increased by 166%, 146%, and
128% with the addition of 1, 5, and 10 mM of Fe2+, respectively. The
enhancement effect of Fe2+ was principally due to production of
additional ꢁOH through Fenton chemistry. Sonochemically gener-
ated hydrogen peroxide could be effectively utilized once Fe2+
was added to the system, through Eqs. (19)–(22) [32].
N
CH3
CH3
CH3
HN
S
O
O
NH2
1
17.16
295
CH3
N
N
Fe2þ þ H2O2 ! Fe3þ þ ꢁOH þ OHꢀ
ð19Þ
ð20Þ
ð21Þ
ð22Þ
HN
S
O
O
2þ
Fe3þ þ H2O2 ! FeðOOHÞ þ Hþ
OH
2þ
FeðOOHÞ ! Fe2þ þ HOOꢁ
NH2
Fe3þ þ HOOꢁ ! Fe2þ þ O2 þ Hþ
2
3
14.10
5.33
124
174
CH3
N
N
It would be noted that the highest SMT degradation rate was
accomplished at 1 mM Fe2+, instead of the maximum Fe2+ level of
10 mM. This finding is because both Fe2+ and H2O2 are scavengers
of ꢁOH as shown in Eqs. (23), (24). Therefore, the highest degrada-
tion was always observed at an optimal ratio of Fe2+ and H2O2.
H2N
OH
O
S O
NH2
OH
4
13.58
190
O
S
O
OH
NH2
NH2
OH
5
6
1.34
1.35
94
95
Fig. 8. HPLC Chromatographs of sonolytic degradation of SMT (frequency: 800 kHz;
power: 100 W; SMT: 18
60 min).
lM; sample obtained time = 0, 2, 5, 10, 20, 30, 45 and

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Y.-q. Gao et al. / Ultrasonics Sonochemistry 20 (2013) 1401–1407
tive oxygen species (e.g. hydroxyl radicals) produced. Furthermore,
dissolved gases and common anions in water can enhance or inhi-
bit the SMT degradation, to different degrees. The SMT mineraliza-
tion seems to be very difficult for ultrasonic irradiation alone.
However, the SMT degradation products are very biodegradable.
Therefore, a traditional biological treatment may be considered
subsequent the ultrasonic irradiation in practice.
Acknowledgments
This work was financially supported by the National Natural
Science Foundation of China (No. 51108327), the National Major
Project of Science
& Technology Ministry of China (No.
2012ZX07403-001), and the research and development Project of
Ministry of Housing and Urban-Rural Development (No. 2009-
K7-4). We are also thankful to the reviewers for their valuable ad-
vices to improve this manuscript.
Fig. 9. BOD5/COD, SMT and TOC evolution versus irradiation time (frequency:
800 kHz; power: 100 W; SMT: 180 lM).
firmed that the degradation of SMT is through ꢁOH radical attack-
ing. It was observed that attack of hydroxyl radicals on the SMT
structure results in the formation of (OH)SMT. Singla et al. [10]
have also reported that hydrogen abstraction is a likely step and
may lead to the sonolytic degradation of the martius yellow dye.
The cleavage of the N–S bond was supposed to be the main
pathway for SMT degradation [3]. So, the 4,6-dimethylpyrimidin-
2-amine, sulfanilic acid and mono-hydroxyl derivative of sulfanilic
acid were probably produced by the cleavage of N–S bond of SMT
or (OH)SMT. Of note, the mono-hydroxyl derivative of sulfanilic
acid may also formed by ꢁOH radical directly attacked on sulfanilic
acid. Aniline was an intermediate product in SMT degradation
through the break of C–S bond in SMT or sulfanilic acid, which
can be oxidized to phenol. In conclusion, we may ascribe the main
degradation mechanism of SMT during ultrasonic irradiation to
ꢁOH radical oxidation.
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a
120 min ultrasonic irradiation (irradiation
l
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