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Activation of Peroxymonosulfate by Benzoquinone: A Novel Non-Radical Oxidation Process Yang Zhou, Jin Jiang, Yuan Gao, Jun Ma, Su-yan Pang, Juan Li, Xue-Ting Lu, and Li-Peng Yuan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03595 • Publication Date (Web): 09 Oct 2015 Downloaded from http://pubs.acs.org on October 14, 2015
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Activation of Peroxymonosulfate by Benzoquinone: A Novel Non-Radical Oxidation Process Yang Zhou†, Jin Jiang*,†, Yuan Gao†, Jun Ma*,†, Su-Yan Pang‡, Juan Li†, Xue-Ting Lu‡, Li-Peng Yuan‡ †
State Key Laboratory of Urban Water Resource and Environment, School of Municipal and
Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡
Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang
Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China
*Corresponding Authors: Prof. Jin Jiang and Prof. Jun Ma (J.J.) Phone: 86−451−86283010; fax: 86 − 451−86283010; E-mail:
[email protected]. (J.M.) Phone: 86 −451− 86283010; fax: 86−451− 86283010; E-mail:
[email protected].
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Abstract
2
The reactions between peroxymonosulfate (PMS) and quinones were investigated for the
3
first time in this work, where benzoquinone (BQ) was selected as a model quinone. It was
4
demonstrated that BQ could efficiently activate PMS for the degradation of sulfamethoxazole
5
(SMX; a frequently detected antibiotic in the environments), and the degradation rate
6
increased with solution pH from 7 to 10. Interestingly, quenching studies suggested that
7
neither hydroxyl radical (•OH) nor sulfate radical (SO4•-) was produced therein. Instead, the
8
generation of singlet oxygen (1O2) was proved by using two chemical probes (i.e.,
9
2,2,6,6-tetramethyl-4-piperidinol and 9,10-diphenylanthracene) with the appearance of 1O2
10
indicative products detected by electron paramagnetic resonance spectrometry and liquid
11
chromatography mass spectrometry, respectively. A catalytic mechanism was proposed
12
involving the formation of a dioxirane intermediate between PMS and BQ and the subsequent
13
decomposition of this intermediate into 1O2. Accordingly, a kinetic model was developed,
14
and it well described the experimental observation that the pH-dependent decomposition rate
15
of PMS was first order with respect to BQ. These findings have important implications for
16
the development of novel non-radical oxidation processes based on PMS, because 1O2 as a
17
moderately reactive electrophile may suffer less interference from background organic
18
matters compared with non-selective •OH and SO4•-.
19
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Introduction
21
Peroxomonosulfate (PMS), hydrogen peroxide (H2O2), and peroxydisulfate (PDS) are
22
considered as inexpensive oxidants for the remediation of contaminated water or soil.1-3
23
Advanced oxidation processes (AOPs) based on these three common peroxides have received
24
much attention because of high standard redox potentials of sulfate radical (SO4•-, 2.5~3.1V) 4
25
and hydroxyl radical (•OH, 1.9~2.7V)
26
contaminants, such as pharmaceuticals, odor-causing compounds, and pesticides.6-8
27
Transition metal oxides, energy (e.g. heat, ultraviolet, and ultrasound), and base are
28
commonly used to activate PMS, H2O2, and PDS to generate SO4•- and •OH, and the
29
associated catalytic mechanisms have been well studied.9-13
5
. These radicals can destruct many organic
In addition to these catalytic methods, it has been reported that PDS and H2O2 can also
30
activated
by
organic
be
32
2,4,4'-trichlorobiphenyl (PCB28) could be efficiently degraded by PDS in the presence of
33
1,4-benzoquinone
34
2-chloro-1,4-benzoquinone (CBQ). This process can be described as a semiquinone
35
radical-dependent Fenton-like reaction (taking BQ for example, as shown in reactions 1 and
36
2): the comproportionation between BQ and its self-condensation or decomposition product
37
hydroquinone (HQ) can generate benzosemiquinone (BSQ), leading to the decomposition of
38
PDS into SO4•- .
(BQ)
quinones.
as
well
O
For
as
OH HQ
et
and
OH BSQ O +
+
+
(2)
O
OH BSQ
Zhu
that
(1)
+
Interestingly,
(MBQ)
2
O
40
reported
O
+ O BQ
Fang
2-methyl-1,4-benzoquinone
OH
39
41
instance,
et al.14
31
BQ
al.15
found
that
halogenated
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quinones
(e.g.,
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42
tetrachloro-1,4-benzoquinone (TCBQ), tetrabromo-1,4-benzoquinone (TBBQ), and tetra
43
fluoro-1,4-benzoquinone (TFBQ)) could activate H2O2 to produce •OH but those
44
nonhalogenated
45
tetramethyl-1,4-benzoquinone (TMBQ)) couldn’t. Further, these authors observed that TCBQ
46
itself rather than its corresponding semiquinone radical was essential for •OH production.16
47
So, the authors suggested a mechanism as following: the nucleophilic attack of TCBQ by
48
H2O2 formed a trichlorohydroperoxyl-1,4-benzoquinone (TrCBQ-OOH) intermediate, which
49
decomposed
50
trichlorohydroxy-1,4-benzoquinone (TrCBQ-OH).16 However, little is known about the
51
reaction between PMS and quinones so far.
quinones (e.g., BQ,
homolytically
to
2,6-dimethyl-1,4-benzoquinone
produce
OH
and
a
(DMBQ),
major
and
product
52
Quinones are ubiquitous in water, soil, and atmosphere,17-20 and they are potent redox
53
active compounds. Many studies have reported that quinones can participate in various
54
chemical and biochemical processes.21-23 For instance, Chen et al.24 found that quinone
55
intermediates could enhance Fenton oxidation, where BQ as an electron-transfer catalysts
56
greatly accelerated the conversion of Fe(III) to Fe(II). Jiang et al.25 found that semiquinone
57
radical produced during microbial or chemical reduction of a humic substance model quinone
58
(AQDS, 9,10-anthraquinone-2,6-disulfonic acid) could oxidize arsenite to arsenate, thus
59
decreasing arsenite toxicity and mobility.
60
In this work, the reactions between PMS and quinones were investigated for the first
61
time, where BQ was chosen as a model quinone. First, the feasibility of BQ activating PMS
62
to degrade a sulfonamide antibacterial, sulfamethoxazole (SMX), which has been frequently
63
detected in the environments, under various experimental conditions was examined. Then,
64
primary oxidizing species produced in such reactions were identified by chemical quenching
65
and trapping methods. Further, the involved mechanisms were tentatively proposed and the
66
kinetic model was developed accordingly.26
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Experimental Section Materials.
PMS
(available
Oxone®
as
(KHSO5·0.5KHSO4·0.5K2SO4)),
69
1,4-benzoquinone (BQ, 98%), sulfamethoxazole (SMX, 99%), atrazine (ATZ, 99%), benzoic
70
acid (BA, 99.5%), 2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic acid diammonium
71
(ABTS, 99%), sodium azide (NaN3, 99.5%), and furfuryl alcohol (FFA, 98%) were
72
purchased from
73
9,10-diphenylanthracene (DPA, 99%) were purchased from J&K Scientific Ltd and their
74
chemical structures were shown in SI Table S1. Methanol, ethanol, tert-butanol, acetonitrile,
75
and acetone of HPLC grade were purchased from Tedia and Ficher. A purified commercial
76
soil-humic acid which had been characterized previously was used in this study.27 Other
77
chemicals of analytical grade or better were purchased from Sinopharm Chemical Reagent
78
Co., Ltd. Stock solutions were always prepared in ultrapure water produced by a Milli-Q
79
Biocel ultrapure water system. Due to the limited aqueous solubility, TMP stock solutions
80
were made in acetonitrile and DPA stock solutions were made in acetonitrile:chloroform
81
mixture (1:1, v:v).28
Sigma-Aldrich.
2,2,6,6-tetramethyl-4-piperidinol
(TMP,
99%)
and
82
Experimental Procedure. All experiments were conducted in brown triangular flask on
83
a reciprocating shaker at 25±1 oC in the dark. Reactions were initiated by simultaneously
84
adding BQ (1-300 µM) and PMS (0.44 mM) into pH-buffered solutions (20mM sodium
85
borate; pH 7-10) containing a target compound [e.g., SMX (8 µM), ATZ (1 µM) or BA (8
86
µM)] with or without a quenching reagent [e.g., methanol (0.22 M), ethanol (0.22 M),
87
tert-butanol (0.22 M), NaN3 (30-400 µM), or FFA (2-4 mM)]. The exact experimental
88
conditions were also clearly shown in the figure captions. ATZ and BA were selected as
89
probe compounds for •OH and SO4•- in this work. Samples were periodically withdrawn and
90
quenched with sodium thiosulfate before analyzed by high performance liquid
91
chromatography (HPLC) and UV detection.29 It was demonstrated that sodium thiosulfate
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had no interference with the analysis of SMX by HPLC/UV in the preliminary study. The
93
concentration of PMS was measured by an ABTS colorimetric method.30 All the kinetic
94
experiments were conducted in duplicates or triplicates. The averaged data and standard
95
deviation were presented.
96
Chemical Detection of Singlet Oxygen. In order to verify the generation of 1O2 in the
97
reaction between PMS and BQ, TMP was chosen as a spin-trapping reagent for 1O2.31 The
98
pH-buffered solutions (pH 10) containing PMS (0.44 mM), BQ (25 µM), and TMP (1 mM)
99
were allowed to react for 60 min during which 1O2 formed could oxidize TMP to
100
2,2,6,6-tetramethyl-4-piperidinol-N-oxyl radical (TMPN). Then, the resulting solutions were
101
subjected to the detection by electron paramagnetic resonance (EPR) spectrometry.
102
In addition, DPA was also used as a chemical trapping reagent to confirm the generation
103
of 1O2. This approach was based on the fact that the rapid and specific reaction between DPA
104
and 1O2 (kr=1.3×106 M-1s-1) forms a stable DPA endoperoxide (DPAO2).
105
pH-buffered solutions (pH 10) containing DPA (24 µM) were treated by PMS with varying
106
doses (300-900 µM) in the absence or presence of BQ (25 µM) for 60min. The resulting
107
solutions
108
chromatography/atmospheric
109
spectrometry (HPLC/APCI−QqQMS) at multiple reaction monitoring (MRM) mode.
were
analyzed
for
DPA
pressure
and
DPAO2
chemical
by
high
ionization-triple
28, 32, 33
A series of
performance
liquid
quadrupole
mass
110
Analytical Methods. A Waters 1525 HPLC equipped with a Waters Symmetry C18
111
column (4.6×150 mm, 5µm particle size), a Waters 717 autosampler, and a Waters 2487 dual
112
λ UV-vis detector was used for the HPLC/UV analysis. A Varian Carry 300 UV-Vis
113
spectrometer was used for ABTS developed color measurements (i.e., ABTS•+). Oxygen
114
generation was measured in an airtight triangular flask by a portable hand-held dissolved
115
oxygen (DO) meter (HACH, HQ30D). A Bruker A200 spectrometer was used for EPR
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analysis under the following condition: temperature=293 K, microwave frequency=9.833
117
GHz, microwave power=2.2 mW, and modulation amplitude=0.1 mT.
118
An Agilent 1260 HPLC was directly coupled to an AB SCIEX QTrap 5500 MS with an
119
atmospheric pressure chemical ionization (APCI) source in the positive ion mode for the
120
HPLC/APCI−QqQMS analysis. A Poroshell 120 EC-C18 column (3.0×50 mm, 2.7 µm
121
particle size) was used for separation. The isocratic mobile phase consisted of
122
acetonitrile/water (v/v, 80/20) at a flow rate of 0.5 mL/min. To avoid the possible
123
contamination of mass spectrometer, a switching valve was used to divert the HPLC fluid to
124
the waste in a first few minutes as well as in a last few minutes.34 The MS parameters were
125
optimized and set as follows: ionspray voltage, +5500 V; source temperature, 450 °C; ion
126
source gas 1 and 2, 50 arbitrary units; curtain gas, 35 arbitrary units; declustering potential
127
(DP), 90 V; entrance potential (EP), 10 V; collision energy (CE), 24 V; collision cell exit
128
potential (CXP), 18 V; MRM ion pair, 363/330 28.
129
Results and Discussion
130
Degradation Efficiency of SMX in PMS/BQ Process. The oxidation kinetics of SMX
131
by PMS with and without BQ over a wide pH range of 7-10 were shown in Figure 1. As can
132
be seen, in the absence of BQ, the degradation of SMX by PMS was negligible within the
133
time scale investigated. Comparatively, SMX could be appreciably degraded by PMS in the
134
presence of BQ, and the degradation rate increased with the increase of BQ concentrations.
135
Also, the degradation rate showed a pH dependency and increased gradually from pH 7 to 10.
136
For instance, when BQ was 10 µM, the degradation of SMX in three minute increased from
137
5% to 86% with pH from 7 to 10. As for pH 10, with the increase of BQ concentration from 2
138
to 10 µM, degradation of SMX in three minute increased from 40% to 86%. Additionally, the
139
first-order rate constants derived from Figure 1 were listed in SI Table S2. These rate
140
constants also suggest that BQ has a significant effect on the degradation of SMX by PMS.
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For instance, when BQ was 10µM, with increasing pH from 8 to 10, the degradation rate
142
increased from 0.0434 to 0.6786 min-1. As for pH 10, SMX degradation rate increased from
143
0.1507 to 0.6786 min-1 with the addition of 2 to 10 µM BQ. These results above suggest that
144
BQ can significantly enhance the degradation of SMX by PMS and this process is dependent
145
on pH (see the following sections for details about discussion on this pH dependency).
146
In control experiments with BQ alone, the loss of SMX was always negligible (data
147
were not shown). This suggested that (i) the relatively strong oxidant BSQ, which appeared in
148
aqueous BQ solutions, contributed negligibly to SMX degradation, and (ii) the nucleophilic
149
addition reactions between sulfonamide antibiotics and quinone moieties widely reported in
150
the literatures were insignificant in this work due to the slow rate and limited time scale
151
investigated (the nucleophilic addition reactions usually needs several weeks to months). 25, 35,
152 153
36
In addition, no difference between carbonate buffer and borate buffer on SMX degradation
as well as on PMS decomposition under similar conditions was observed.
154
(Figure 1)
155
After the reactions, the residual contents of PMS (relative to the initial ones) were
156
determined and were shown in SI Figure S1. As can be seen, the decomposition of PMS
157
without BQ was negligible. However, with increasing concentrations of BQ, the
158
decomposition of PMS enhanced gradually. The pH dependent decomposition of PMS was
159
also observed, which was consistent with the trend of SMX degradation (Figure 1). For
160
instance, when BQ was 10 µM, the decomposition of PMS increased from 3% to 35% with
161
increasing pH from 7 to 10. As for pH 10, with increasing the concentration of BQ from 2 to
162
10 µM, PMS decomposition increased from 10% to 35%.
163
Identification of Oxidizing Species by Specific Quenchers. The results above suggest
164
that reactive oxidizing species is produced in the reaction between PMS and BQ, leading to
165
the enhanced degradation of SMX. Generally, •OH or SO4•- is considered to be the oxidizing
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species in oxidation processes involving PMS. Both •OH and SO4•- show high reactivity
167
towards SMX, and the rate constants are 7.89×109 M-1s-1 and 1.17×1010 M-1s-1, respectively.6
168
For the convenience, experiments to identify reactive oxidizing species were conducted at pH
169
10 due to their relatively fast production therein.
170
To verify the generation of •OH or SO4•-, the effects of radical quenchers (i.e., methanol,
171
ethanol, and tert-butanol) were investigated. The second-order rate constants for these radical
172
quenchers with •OH and SO4•- were shown in SI Table S3. If •OH or SO4•- was the primary
173
oxidizing species, alcohol scavengers in great excess (0.22 M) would completely
174
out-compete SMX (8 µM) and thus significantly inhibit SMX degradation. Surprisingly, they
175
had no effects on the degradation of SMX (Figure 2a), suggesting that neither •OH nor SO4•-
176
was produced in the reaction. To further confirm this, ATZ and BA, the widely used probe
177
compounds for •OH and SO4•- were tested. As can be seen (SI Figure S2), the combination of
178
PMS and BQ could not degrade ATZ or BA either even when the reaction time was
179
prolonged from 12 min to 120 min (Figure 1 vs. Figure S2), where PMS was fully
180
decomposed. This provides another supporting evidence that neither •OH nor SO4•- was
181
produced in the reaction between PMS and BQ.
182
(Figure 2)
183
It is well known that the self-decomposition of PMS can slowly generate 1O2 (reaction
184
3), and the rate constant k1 of this reaction is about 0.2 M-1s-1. 37,38 k1 HSO5− + SO 52 − → HSO 4− + SO 24− + 1 O 2
185 186 187
(3)
The presence of BQ can greatly accelerate the decomposition of PMS. So, it is likely that 1
O2 is produced in the reaction between PMS and BQ and thus results in the enhanced
188
degradation of SMX. 1O2 as a selective oxidizing species shows high reactivity towards
189
electron-rich compounds (e.g., phenols, sulfides, and amines) but negligible reactivity
190
towards saturated alcohols (e.g. methanol, ethanol, and tert-butanol).39, 40 NaN3 and FFA are 9
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191
reported to be efficient quenchers for 1O2 with the rate constants of 1×109 and 1.2×108 M-ls-l,
192
respectively.41 So their effects were examined to verify the generation of 1O2.
193
As shown in Figure 2b and SI Figure S3, the degradation rate of SMX markedly slowed
194
down by the addition of NaN3 or FFA as expected. For instance, SMX was completely
195
degraded in 30 min without NaN3, while in the presence of 30 and 100 µM NaN3, only 75%
196
and 32% was degraded (shown in Figure 2b). It should be noted that NaN3 and FFA are also
197
efficient scavengers for SO4•- and •OH (rate constants were shown in SI Table S3). For
198
instance, the scavenging capacities (i.e., kc value) for SO4•- and •OH of NaN3 at 100 µM are
199
calculated to be 2.52×105 and 1.2×106 s-1, respectively, and they are about one or two orders
200
of magnitude lower than those of 0.22 M methanol (about 5.5×106 and 2.13×108 s-1
201
respectively). If SO4•- or •OH is the dominant oxidizing species, a more pronounced
202
inhibitory effect of methanol than NaN3 would be noted. However, a contrasting effect was
203
observed (Figure 2a vs. Figure 2b). The comparison of the inhibitory effects of methanol vs.
204
NaN3 further confirms that neither SO4•- nor •OH is generated while 1O2 is likely produced
205
in the reaction between PMS and BQ.
206
Chemical Detection of 1O2. To further confirm the generation of 1O2 in BQ/PMS system,
207
EPR spectroscopy was used with TMP as a spin trap agent. TMP is generally considered as
208
a good probe for 1O2, because it can readily react with 1O2 to form a stable radical TMPN.42,
209
43
TMPN shows a typical three-line EPR spectrum with equal intensities (aN=16.9 G,
210
g=2.0054).44 By using this approach, the production of
211
suspensions of derivatized C60 has been demonstrated by Lee et al.
212
three-line EPR spectrum supporting the appearance of TMPN was observed in BQ/PMS
213
system as shown in Figure 3a. Comparatively, a weak signal of TMPN was detected in PMS
214
alone, and this might be due to the generation of 1O2 from the self-decomposition of PMS
215
(reaction 3).
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O2 in irradiated aqueous 45
In this work, a
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(Figure 3)
216 217
In addition, 1O2 was detected by an alternative HPLC/APCI−QqQMS technique, which
218
was based on the fact that 1O2 could react with a chemical probe DPA to generate the
219
indicative endoperoxide (DPAO2) (reaction 4). By using this method, Miyamoto et al.28, 46
220
successfully verified the generation of 1O2 in the reaction of lipid hydroperoxides with ceric
221
ion as well as in the reaction of linoleic acid hydroperoxide with peroxynitrite. Ph
Ph +
222
O
1O 2
O
(4)
Ph
Ph DPA
DPAO2
223
As shown in Figure 3b, the chromatographic peak of DPAO2 appeared in the reaction
224
between PMS and BQ, and the intensity of DPAO2 gradually increased with increasing the
225
concentrations of PMS (from 300 to 900 µM) in the presence of 25 µM BQ. The peak of
226
DPAO2 was also observed in blank experiment with DPA alone, and this may be due to the
227
impurity of the commercial DPA chemical. In the presence of 900 µM PMS, the peak
228
intensity of DPAO2 was slightly higher than that in DPA blank experiment. This may be
229
attributed to the slow production of 1O2 from the self-decomposition of PMS, which is
230
consistent with the result obtained by EPR. Also, the effect of SMX on the DPAO2 signals
231
was examined. It was found that SMX could greatly decreased the DPAO2 signals by
232
competing for 1O2 formed in the reaction between PMS and BQ. For instance, in the presence
233
of SMX (0.3 mM) the intensity of DPAO2 in the reaction of PMS (300 µM) with BQ (25 µM)
234
was similar to that in DPA blank experiment (data were not shown for clarity). These results
235
further confirm the involvement of 1O2 in the degradation of SMX by PMS with BQ (Figure
236
1).
237
Stoichiometric Evolution of O2. These results suggest that 1O2 is produced in the reaction
238
between PMS and BQ. Once formed, 1O2 will decay rapidly to triplet oxygen (3O2). So, the
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239
increase of solution DO level will be an indirect evidence for the generation of 1O2. The yield
240
of O2 ( YO ) in this reaction can be described as eq 5 2
YO =
241
2
[O2 ]t -[O2 ]0 [PMS]0 -[PMS]t
(5)
242
where [O2]0 is the initial concentration of O2, [O2]t is the concentration of O2 at time t,
243
[PMS]0 is the initial concentration of PMS, and [PMS]t is the concentration of PMS at time t.
244
As shown in SI Figure S4, the plot of [O2 ]t -[O2 ]0 vs. [PMS]0 -[PMS]t at pH 8, 9, and 10
245
was found to be linear, and the slope was 0.50. This suggests that the formation of one mole
246
of O2 requires two moles of PMS in its reaction with BQ.
247
Proposed Mechanism of PMS Activation by BQ for 1O2 Production. It is well known
248
that PMS can be catalyzed by ketones to produce 1O2. In a pioneer work, Montgomery47
249
found that cyclohexanone significantly enhanced the decomposition of PMS to 1O2 in
250
alkaline solutions, where the involvement of a dioxirane intermediate was proposed. Edwards
251
et al.48 further confirmed the generation of the dioxirane intermediates in the reaction
252
between PMS and ketones (e.g., acetone) by
253
and Brauer
254
ketone-catalyzed decomposition of PMS by direct measurement of monomol light emission
255
in the near-infrared region (λ=1270 nm).
49
18
O-labeling and kinetic studies. Later, Lange
provided spectroscopic evidence for the generation of
1
O2 from
256
In this work, BQ can be considered as a ketone containing two carbonyl groups, thus a
257
similar pathway for PMS activation may occur (Scheme 1). The first step is the nucleophilic
258
addition of PMS (i.e., HSO5-) to the carbonyl group of BQ, i.e., two molecules of HSO5-
259
attack the carbonyl carbon atoms to form a peroxide adduct intermediate I (reaction 7). The
260
conjugate base of I (i.e., intermediate II) further decomposes to a dioxirane intermediate III
261
with the release of the sulfate moiety (reaction 9) via intramolecular nucleophilic
262
displacement of alkoxide oxygen at the O-O bond.48 According to the study of Edwards et
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263
al.,50 the intramolecular nucleophilic displacement reaction of intermediate II (reaction 9) is
264
rate-limiting step. Then intermediate III will experience nucleophilic attack by two molecules
265
of ionized PMS ions ( SO 52− ) to produce 1O2 and reform BQ (reaction 10). In the proposed
266
mechanism, the formation of one molecule 1O2 requires one molecule of HSO5- as well as one
267
molecule of SO52-, i.e., the yield of O2 is expected to be 0.5. This is in good agreement with
268
the experimentally obtained value (shown in Figure S4).
269 270
Scheme 1. Proposed Mechanism for the Generation of 1O2 from PMS Activation by BQ
271
Ka O
(6)
OH O O SO3-
272 O O SO3OH
O
(7)
I O-
OH O O SO3-
273
O O SO3-
O O SO3OH
O-
I
II OO O SO3-
k4
O O SO3-
O
O
O
O
slow
274 O-
(9)
O O SO3-
II O
(8)
III O
O
k5
fast
275
(10) O
O
O
III 276
Kinetics for BQ-catalyzed Decomposition of PMS. By assuming that reactions 7, 8,
277
and 10 are fast while reaction 9 is the rate-determining step,49 the rate law of PMS
278
decomposition can be described as eq 11 by using the steady-state approach regarding
279
intermediates I, II, and III (see SI Text S1 for details).
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K 2d [ 1 O 2 ] d [PMS]T [BQ][PMS]T =− = 4 K 2 K 3k4 + w [H ] + K a dt dt
(11)
281
where [PMS]T (=[HSO5-]t + [SO52-]t) is the total concentration of PMS at time t, [BQ] is the
282
concentration of BQ, K2 (M-1) and K3 (M-1) are the equilibrium constants for eqs 7 and 8,
283
respectively, k4 (s-1) is the rate constant for rate-determining step (reaction 9), Kw (=10-14)
284
represents the ionic product of water (i.e., [H+][OH-]), and Ka (=3.98×10-10)
285
ionization constant of PMS. When defining F =
286
s-1) for the decomposition of PMS could be described by eq 12
is the
Kw , the apparent rate constant (kobs, [H ] + K a +
kobs = 4 K 2 K 3k4 F [BQ]
287
38
(12)
288
According to the study of Lange and Brauer,49 K2K3k4 (i.e., M-2s-1) was the rate constant (kDI)
289
for the formation of dioxirane intermediate III.
290 291
To further confirm the proposed mechanism described above, the effects of BQ concentration and pH on the decomposition of PMS by BQ were evaluated.
292
1) BQ concentration. Figure 4a showed the decomposition of PMS in the presence of
293
different concentrations of BQ at pH 10. As can be seen, the rate increased with the increase
294
of BQ concentration. However, it should be noted that the loss of PMS slightly deviated from
295
the first-order kinetics and decreased as the reaction progressed. Such kinetics were also
296
reported in cyclohexanone-catalyzed decomposition of PMS.47-49 This phenomenon may be
297
attributed to side reactions occurred in PMS/BQ system. For instance, intermediate I that
298
formed in the reaction between PMS and ketone can undergo Baeyer-Villiger oxidation to
299
produce esters (reaction 13).
300
can be destroyed by 1O2 (reaction 14).51-53 In addition, quinones are susceptible to hydrolysis
301
especially in alkaline solution (reaction 15).54-56
302
48, 49
Also, quinones (e.g., BQ, tocopherols, and hydroquinones)
BQ + 2HSO5−
k2 k-2
BV I → product
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BQ + 1 O 2 → product
303
hydrolysis BQ → product
304
(14) (15)
305
The initial rate was used to determine the rate constant kobs (i.e., dashed lines in Figure 4a).
306
As shown in Figure 4b, the plot of kobs vs. [BQ] resulted in a straight line, demonstrating that
307
the decomposition rate of PMS was first-order with respect to BQ. Similar finding was also
308
observed at pH 9 (data were not shown). These results are in good agreement with the
309
proposed kinetics (eq 12).
310
(Figure 4)
311
2) Effects of pH. The reactions of PMS with BQ were conducted at various pH with a
312
constant concentration of BQ, and apparent rate constant kobs for PMS decomposition and
313
factor F at each pH were calculated. According to eq 11, it can be clearly seen that with
314
increasing pH (i.e., decreasing the concentration of H+), the decomposition rate of PMS will
315
increase. This is consistent with the experimentally observed trend of pH-affected
316
decomposition of PMS (SI Figure S5a). The plot of kobs vs. F resulted in a straight line (SI
317
Figure S5b), suggesting that the decomposition rate of PMS was first order with respect to F
318
as predicted by eq 12. The pH-dependence of BQ-catalyzed decomposition of PMS (i.e.,
319
generation of 1O2 according to eq 11) well explained the pH-dependence of SMX degradation
320
shown in Figure 1. So, pH adjustment may be a good option to enhance reaction rates if
321
necessary.
322
Important Role of Dioxirane Intermediate III. The results obtained above suggest that
323
the reaction between PMS and BQ undergoes a dioxirane intermediate pathway rather than a
324
semiquinone radical-dependent Fenton-like mechanism. Dioxiranes are commonly used as
325
mild oxidants for organic synthesis.57,
326
substrates through oxygen transfer processes including epoxidations (e.g., alkene),
327
carbon-hydrogen bond insertions (e.g., aldhydes), and lone-pair oxidations (e.g., amines).59
58
They can appreciably oxidize many organic
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328
The involvement of dioxirane may reasonably explain the finding that 1O2 quenchers can’t
329
completely suppress the degradation of SMX (as shown in Figure 2b and SI Figure S3). For
330
example, no further inhibition was observed when the concentration of NaN3 increased from
331
100 to 400 µM (Figure 2b)
332
According to eq 12, the rate constant for the formation of dioxirane intermediate III (i.e., kobs ) for BQ was derived from Figure 4b. Then kDI values in the cases of 4 F [BQ]
333
kDI = K 2 K 3 k4 =
334
BQ as well as seven ketones (i.e., acetone, 2-acetylpyridine, di-2-pyridyl ketone,
335
fluoroacetone,
336
comparatively shown in Table 1. As can be seen, kDI for BQ (1.16×106 M-2s-1) was 1~3 orders
337
of magnitude greater than those for other ketones. This marked difference may be attributed
338
to the cyclic structure of BQ and its strong electrophilicity resulting from two C=C bonds and
339
two carbonyl groups. This is in good agreement with the findings of Lange and Brauer 49 that
340
(i) kDI increased with increasing the electrophilicity of the ketones, and (ii) kDI was strongly
341
ring-size dependent as cyclic ketones showed much higher rates.
342
Environmental implications
1,1,1-trifluoroacetone,
cycloheptanone,
and
cyclohexanone)49
are
343
This study has demonstrated for the first time that BQ can efficiently activate PMS for
344
the degradation of SMX via a novel non-radical mechanism, where reactive 1O2 was involved.
345
As a moderately reactive electrophile, 1O2 can effectively oxidize a variety of contaminants
346
even in the presence of background organic matters, where significant interference is
347
expected for non-selective •OH and SO4•-.7, 60 So far, the explicit pathway for 1O2 with SMX
348
is unclear. Further studies are needed to examine the transformation products of SMX by 1O2
349
and compare them with well documented •OH and/or SO4•--derived products.6
350
Our findings may have important implications for the development of heterogeneous
351
catalytic PMS oxidation processes by quinone-based materials (e.g., quinone loaded
352
carbons)61, 62 for selective contaminant remediation or bacterial inactivation,60, 63 as well as 16
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for the potential application of in situ chemical oxidation using PMS for the remediation of
354
soils and sediment, where quinone-like groups containing natural organic matters (NOM) are
355
ubiquitous.
356
in soils and groundwater,66 and the quinone group (C=O) content in HA is generally within
357
the range of 1-4 mmol/g HA.67,
358
environmental relevant concentration could appreciably enhance the degradation of SMX by
359
PMS at near neutral pH (as shown in Figure S6). Since the chemical structures as well as the
360
properties of HA are complex, the reactions of PMS with diverse sources of HA deserve
361
further studies.
362
Acknowledgments
363
This work was financially supported by the National Science & Technology Pillar Program,
364
China (2012BAC05B02), the National Natural Science Foundation of China (51178134 &
365
51378141), the Funds of the State Key Laboratory of Urban Water Resource and
366
Environment (HIT, 2013DX05), the Foundation for the Author of National Excellent
367
Doctoral Dissertation of China (201346), and the Fundamental Research Funds for the
368
Central Universities of China (AUGA5710056314). The authors greatly thank Dr. Jimin Shen
369
for his help with EPR operation.
370
Supporting Information
371
The additional texts, figures, and tables addressing supporting data. This material is available
372
free of charge via the Internet at http://pubs.acs.org.
373
Nomenclature
64, 65
The typical concentration of humic acid (HA) ranges from 1 to 50mg C L-1
68
PMS
peroxymonosulfate
BQ
benzoquinone
BSQ
benzosemiquinone
SMX
sulfamethoxazole
Preliminary experiments suggested that HA at
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ATZ
atrazine
BA
benzoic acid
FFA
furfuryl alcohol
TMP
2,2,6,6-tetramethyl-4-piperidinol
DPA
9,10-diphenylanthracene
HA
humic acid
374 375
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References:
377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418
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Figure and Table Captions (b) 1.0
0.8
0.8
0.6
0.6 C/C0
(a) 1.0
C/C0
541
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BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ BQ=50 µΜ BQ=100 µΜ BQ=300 µΜ
0.4 0.2 0.0 0
2
4
BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ
0.2
pH=7
0.0 6
8
10
12
0
2
Reaction time(min)
542
6
8
10
12
(d) 1.0 pH=9
0.8
pH=10
0.8
BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ
0.6 C/C0
0.6 C/C0
4
pH=8
Reaction time(min)
(c) 1.0
0.4 BQ free control BQ=2 µΜ BQ=5 µΜ BQ=10 µΜ
0.2 0.0 0
543
0.4
2
4
0.4 0.2 0.0
6
8
10
12
0
2
Reaction time(min)
4
6
8
10
12
Reaction time(min)
544
Figure 1. Effect of BQ on SMX degradation by PMS. (a) pH 7; (b) pH 8; (c) pH 9; (d) pH 10.
545
Experimental conditions: [PMS]0=0.44 mM, [SMX]0=8 µM, [BQ]0=2-300 µM at pH 7 and
546
2-10 µM at pH 8-10, 20 mM borate buffer, and T =25 °C.
547
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1.0
1.0
a
0.8
0.6 scavenger free control methanol=0.22 M ethanol=0.22 M ter-butanol=0.22 M
0.4 0.2
C/C0
C/C0
b
0.8
0.6
0.4 0.2
0.0
scavenger free control NaN3=30 µM NaN3=200 µM
0.0 0
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5
10
15
20
25
30
NaN3=100 µM
0
5
Reaction time (min)
10
NaN3=400 µM
15
20
25
30
Reaction time (min)
549
Figure 2. Effects of scavengers on SMX degradation in BQ/PMS system (a. for alcohols; b.
550
for NaN3). Experimental conditions: [PMS]0=0.44 mM, [SMX]0=8 µM, [BQ]0=2 µM, pH=10
551
(20 mM borate buffer), and T=25 °C, (a) [methanol]0=[ethanol]0=[tert-butanol]0=0.22 M, (b)
552
[NaN3]0=30-400 µM.
553 554
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555
a
Intensity(a.u.)
BQ/PMS/TMP
BQ/TMP
PMS/TMP
3480 556
3495 3510 Magneitc field(G)
100%
3525
b
DPAO2
Relative Intensity (%)
m/z 363→330
80% 60% DPA/BQ/PMS(900 µM) DPA/BQ/PMS(600 µM) DPA/BQ/PMS(300 µM) DPA/PMS(900 µM) DPA blank
40% 20% 0%
0
2
4
6 t (min)
557
8
10
558
Figure 3. EPR spectra of TMP-1O2 adduct (TMPN) formed in aqueous solution containing
559
PMS, TMP, and BQ (a), and HPLC/ESI−QqQMS chromatograms for the typical
560
endoperoxide (DPAO2) in MRM (b). Experimental conditions for (a): [TMP]0=1 mM,
561
[PMS]0=0.44 mM, [BQ]0=25 µM, pH=10 (20 mM borate buffer), T=25 °C, and reaction time
562
of 60 min; for (b): the mass transition from 363 to 330 m/z; [DPA]0=24 µM, [PMS]0=0.3-0.9
563
mM, [BQ]0=25 µM, pH=10 (20 mM borate buffer), T=25 °C, and reaction time of 60 min.
564
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0.0
a
2
R =0.998
-1
kobs(min )
ln(C/C0)
-3
kobs=(5.56±0.10)×10 [BQ]
0.12 BQ=5 µM BQ=7 µM BQ=10 µM BQ=15 µM BQ=17 µM BQ=19 µM BQ=30 µM Fit Curve
-1.0 -1.5
565
b
0.16
-0.5
-2.0
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0
3
0.08 0.04 Fit Curve
0.00
6 9 12 Reaction time(min)
15
0
5
10 15 20 [BQ](µM)
25
30
566
Figure 4. Effect of BQ at varying concentrations on PMS decomposition (a), and plot of kobs
567
vs. [BQ] (b). The dashed lines (Figure 4a) represent the first-order model fit. Experimental
568
conditions: [PMS]0=0.44 mM, [BQ]0=5-30 µM, pH=10 (20 mM borate buffer), and T
569
=25 °C.
570
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Table 1. Rate Constants for the Formation of Dioxirane Intermediate III NO.
compounds
molecular
1
BQ
C6H4O2
2
acetone
CH3COCH3
3
2-acetylpyridine
C7H7NO
structure O
O
O H3C
CH3
N
kDI (M-2 s-1) a (1.16±0.02)×106 (5.7±0.1)×103
CH3
(1.4±0.2)×104
N
(1.0±0.1)×105
O
4
di-2-pyridyl ketone
C11H8N2O
N O
5
fluoroacetone
6
1,1,1-trifluoroacetone
CH3COCH2F CH3COCF3
O H3C
CH2F O
H3C
CF3
(1.1±0.1)×105 b (1.0±0.3)×105 b
O
7
cycloheptanone
(1.4±0.2)×103
C7H12O O
8 572 573
a
cyclohexanone
C6H10O
at pH=10 and 25 oC unless stated otherwise. b at pH=10 and 10 oC.
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(5.6±0.1)×104
Environmental Science & Technology
574 575
TOC Art
2HSO5O
OH O O SO3O-
O O SO3OH
O O SO3-
OO
O
O
O O SO3-
O
O
1O
2
2SO52H2N
576
O S NH O N O
CH3
2SO42products
Sulfamethoxazole
577
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