Subscriber access provided by University of Winnipeg Library
Remediation and Control Technologies
Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation Process: Singlet Oxygenation Versus Mediated Electron Transfer Eun-Tae Yun, Jeong Hoon Lee, Jaesung Kim, Hee-Deung Park, and Jaesang Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00959 • Publication Date (Web): 23 May 2018 Downloaded from http://pubs.acs.org on May 23, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36
Environmental Science & Technology
1
Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation
2
Process: Singlet Oxygenation Versus Mediated Electron Transfer
3
Eun-Tae Yun1†, Jeong Hoon Lee1†, Jaesung Kim1, Hee-Deung Park1, and Jaesang Lee1*
4 5 6
1
7
Abstract. Select persulfate activation processes were demonstrated to initiate oxidation not
8
reliant on sulfate radicals, though the underlying mechanism has yet to be identified. This study
9
explored singlet oxygenation and mediated electron transfer as plausible nonradical mechanisms
10
for organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate (PMS). The
11
degradation of furfuryl alcohol (FFA) as a singlet oxygen (1O2) indicator and the kinetic
12
retardation of FFA oxidation in the presence of L-histidine and azide as 1O2 quenchers apparently
13
supported a role of 1O2 in the CNT/PMS system. However, the 1O2 scavenging effect was
14
ascribed to a rapid PMS depletion by L-histidine and azide. A comparison of CNT/PMS and
15
photoexcited Rose Bengal (RB) excluded the possibility of singlet oxygenation during
16
heterogeneous persulfate activation. In contrast to the case of excited RB, solvent exchange (H2O
17
to D2O) did not enhance FFA degradation by CNT/PMS and the pH- and substrate-dependent
18
reactivity of CNT/PMS did not reflect the selective nature of 1O2. Alternatively, concomitant
19
PMS reduction and trichlorophenol oxidation were achieved when PMS and trichlorophenol
20
were physically separated in two chambers using a conductive vertically aligned CNT membrane.
21
This result suggested that CNT-mediated electron transfer from organics to persulfate was
22
primarily responsible for the nonradical degradative route.
23
Keywords: persulfate activation, nonradical mechanism, singlet oxygenation, mediated electron
24
transfer, carbon nanotubes
School of Civil, Environmental, and Architectural Engineering, Korea University, Seoul 136-701, Korea
*Corresponding author: E-mail:
[email protected]; phone: +82-2-3290-4864; fax: +82-2-928-7656 †
These authors contributed equally to this work.
25 26 27 28
ACS Paragon Plus Environment
Environmental Science & Technology
Page 2 of 36
29
INTRODUCTION
30
The electron donating potentials of transition metals (e.g., cobalt and iron) can initiate the one-
31
electron reduction of peroxymonosulfate (PMS) and the associated production of sulfate radicals
32
(SO4•−), thus achieving radical-induced oxidation of organics.1-3 This process is in accordance
33
with various physicochemical activation strategies (e.g., photocatalysis,4,
34
radiolysis7) in which the generated (conduction band) electrons reductively cleave the peroxide
35
bond of PMS to form SO4•−. In contrast, recent studies8-15 have raised the likelihood of persulfate
36
activation via a degradative reaction pathway involving no radical attack based on the following
37
universal experimental results: no quenching by methanol and chloride as SO4•− scavengers and
38
the absence of electron paramagnetic resonance (EPR) spectra assigned to SO4•− adducts.
39
Persulfate activation that does not rely on the oxidizing power of SO4•− has been demonstrated to
40
proceed with metal- and carbon-based activators, and two hypotheses, i.e., singlet oxygenation
41
and mediated electron transfer, have been postulated for the underlying mechanism.8-10, 12-18
5
electrolysis,6 and
42
Singlet oxygen (1O2) has been hypothesized to have a role in persulfate activation
43
processes based on the following results: oxidation of organics by activated PMS is decelerated
44
in the presence of excess azide and L-histidine as 1O2 quenchers, and PMS activation in the
45
presence of 2,2,6,6-tetramethylpiperidine (TEMP) as a spin-trapping agent produces the EPR
46
spectrum assigned to the corresponding 1O2 adduct.12,
47
alkaline aqueous PMS solutions when phenols are selected as target substrates or benzoquinone
48
is added.22, 23 As ketones (characterized by the presence of a carbonyl moiety (C=O)) have been
49
found to accelerate 1O2 production associated with PMS decay under alkaline conditions (i.e.,
50
HSO5− + SO52− → HSO4− + SO42− + 1O2),24 quinones formed as intermediates during phenol
51
oxidation or supplied externally could provide carbonyl groups to activate PMS and produce 1O2
13, 16-22
ACS Paragon Plus Environment
Singlet oxygenation occurs in
Page 3 of 36
Environmental Science & Technology
52
at basic pH.22, 23 The addition of benzoquinone, which serves as a O2•− scavenger, reduces the
53
efficiency of PMS activation by base, suggesting an alternative 1O2 formation route that involves
54
the superoxide radical anion (O2•−) as an intermediate (i.e., 2O2•− + 2H+ → H2O2 + 1O2).21
55
Carbonaceous nanomaterials, such as N-doped graphene and carbon nanotubes (CNTs), could
56
catalyze the self-decomposition of persulfate, even at acidic or neutral pH (note that alkaline
57
conditions favor 1O2 generation through homogeneous persulfate activation21-23), mediating the
58
nonphotochemical production of 1O2.13, 16, 17, 20 Surface modification with glutaraldehyde, which
59
remarkably increases the surface density of carbonyl groups on CNTs, enhances 1O2 formation
60
from peroxydisulfate (PDS),13 suggesting that nonradical persulfate activation occurs via
61
carbonyl moieties intrinsically present on nanocarbon surfaces.13, 20 Perovskite oxide- and metal-
62
derived activators also enable singlet oxygenation upon PMS addition, though the relevant
63
mechanism has not been empirically verified.12, 18
64
Electron transfer from organic compounds to persulfate, facilitated by select activators,
65
could be responsible for the nonradical mechanism.8-10, 15, 25 PMS activation systems utilizing
66
CNTs and graphitized nanodiamonds have been clearly distinguished from Co2+/PMS as a
67
reference system2 based on the negligible hydroxylation characteristics of SO4•−-mediated
68
oxidation and substrate-dependent degradation efficacies that contradicted the reactivity of
69
SO4•−.8, 9, 25 PMS decay and current generation (in a three-electrode cell) is markedly pronounced
70
when the organic substrate, PMS, and activator co-exist, revealing that the electron-transfer
71
mediating action of the activator allows PMS to abstract electrons from organics.10, 25 Duan et
72
al26 suggested that surface-complexed PMS on N-doped CNTs could trigger the mediated
73
electron transfer from phenol to PMS involved in the complexation reaction. Our recent work25
74
also supports a degradative route based on mediated electron transfer, as any oxyanion that could
ACS Paragon Plus Environment
Environmental Science & Technology
75
serve as an electron acceptor (e.g., periodate, peracetate, PMS, and PDS) was found to facilitate
76
the oxidative degradation of organics in the presence of CNTs. The similarities in substrate-
77
specificity and product distribution confirmed that the reaction pathway induced on CNTs was
78
not unique to the type of oxyanion.25 In contrast, zerovalent nanosized iron (nFe0) as an activator
79
causes the reductive conversion of oxyanions into the corresponding radicals; thus, the
80
degradative reaction pathway is dependent on the nature of the oxyanion-derived radical.25 As
81
aforementioned, two hypothetical mechanisms have been established for persulfate activation not
82
involving SO4•−, but there still remains a fundamental gap in the understanding of the nonradical
83
mechanism.
84
To improve our understanding of the PMS activation mechanism involving no radical
85
attack, in this study, we examined the possibility of oxidative degradation of organics via singlet
86
oxygenation and mediated electron transfer during PMS activation by CNTs (CNT-activated
87
persulfate was demonstrated to cause the nonradical degradative pathway9, 13, 14). In an effort to
88
identify a role of 1O2 in PMS activation, we compared CNT-activated PMS (i.e., CNT/PMS)
89
with photoexcited Rose Bengal (RB), which is well-known to photosensitize singlet
90
oxygenation.27 For these systems, the effects of chemical reagents able to scavenge 1O2 (L-
91
histidine and azide) and extend the lifetime of 1O2 (D2O),27 the dependence of oxidizing capacity
92
on substrate type and pH, and EPR spectral features were compared. Furthermore, to explore the
93
electron-transfer mediating action of CNTs during PMS activation, we tested a vertically aligned
94
CNT membrane (VA-CNT membrane) that physically separates the reaction system into two
95
zones but allows electron transport through the CNT arrays for interzone electron delivery from
96
organics to PMS across the membrane.
97
MATERIALS AND METHODS
ACS Paragon Plus Environment
Page 4 of 36
Page 5 of 36
Environmental Science & Technology
98
Chemicals and Materials. Single-walled CNT (> 95%) was purchased from NanoLab Inc.
99
Other chemicals were of reagent grade (see Supporting Information), and used without further
100
purification or treatment. Ultrapure deionized water (>18 MΩ•cm), produced with a Millipore
101
system, was used to prepare all experimental suspensions and solutions
102
Preparation and Characterization of VA-CNT Membrane. VA-CNTs were synthesized by a
103
water-vapor-assisted chemical vapor deposition (CVD) technique according to the previously
104
reported procedure.28 Briefly, VA-CNTs were grown on a SiO2/Si wafer coated with aluminum
105
and iron as a catalyst layer. The coated SiO2/Si wafer (width: 1 cm, length: 1 cm) was placed in a
106
CVD reactor. The growth of millimeter-scale VA-CNT arrays (thickness: ca. 900 µm) was
107
carried out at 750 °C for 2 h under a constant flow of ethylene as a carbon source and hydrogen
108
and argon as carrier gases at rates of 100, 200, and 300 sccm (standard cubic centimeters per
109
minute), respectively. Water vapor, a known agent for enhancing and preserving the performance
110
of metal catalysts, was supplied at a feed rate of 30 sccm. Scanning electron microscopy (SEM;
111
Quanta 250 FEG, Thermo Scientific) revealed a densely packed forest of aligned CNTs on the
112
silicon wafer (Figure S1). The Raman spectrum of the CNTs (500–3000 cm−1; LabRam
113
ARAMIS, Horiba Jobin-Yvon; argon ion laser excitation (514.5 nm)) included a G-band
114
corresponding to graphite structures and a D-band sensitive to defects at 1582 and 1350 cm−1,
115
respectively29 (Figure S2). Scheme S1 illustrates the procedure for VA-CNT membrane
116
fabrication. An epoxy resin (Epon Resin 828, Miller-Stephenson Inc.) was mixed with a curing
117
agent (Jeffamine D-230, Huntsman Corporation) at a 3:1 weight ratio.30 The resin mixture was
118
infiltrated into the interstitial spaces of the as-grown CNTs. Then, the VA-CNT/epoxy composite
119
was incubated under vacuum for 3 h on a mold. The resultant composite was allowed to cure at
120
room temperature for 24 h. Finally, the residual resin and catalysts were removed, the CNT-
ACS Paragon Plus Environment
Environmental Science & Technology
121
based membrane was detached from the silicon substrate, and the VA-CNTs were uncapped by
122
cutting the top surface and bottom of the composite using a microtome (HM 340 E, MICROM
123
Lab.) to produce the VA-CNT membrane (with open CNT tips). X-ray photoelectron
124
spectroscopy (XPS; PHI X-tool, ULVAC-PHI) confirmed that no detectable amounts of metal
125
species (e.g., iron) that may activate persulfate remained on the surface of the VA-CNT
126
membrane (Figure S3).
127
To explore the possibility of interchamber electron transport across the VA-CNT
128
membrane, we monitored concurrent oxidation of organics and reduction of PMS in a crossflow
129
filtration system in which the feed water migrated tangentially across the VA-CNT membrane
130
surface. The VA-CNT membrane (effective surface area: 4 cm2, thickness: 0.9 cm) was
131
vertically mounted in a module to partition the reaction system into two chambers. Water flow in
132
the two physically separated compartments was circulated by a gear pump (REGLO-Z,
133
ISMATEC) at a feed rate of 500 mL min−1. To guarantee the complete removal of metallic
134
species that may activate persulfate, the pristine VA-CNT membrane was subjected to UV-C
135
treatment for 5 h (UV-C irradiation could photochemically cleave some organic linkers that
136
might attach metals to the membrane, which allowed for the release of loosely bound metallic
137
species) and washed thrice with deionized water prior to its application in the filtration process.
138
Experimental Procedure and Analytical Methods. Oxidative degradation of organic substrates
139
by activated PMS was performed in a magnetically stirred 40 mL reactor under air-equilibrated
140
conditions. A typical experimental suspension contained 0.1 g L−1 CNTs, 1 mM PMS, and 0.05
141
mM target pollutant. Photosensitized singlet oxygenation of organics proceeded in a 40 mL
142
cylindrical quartz reactor with six fluorescent lamps (output power: 4 W; Philips Co.). The
143
significant overlap between the emission spectrum of the light source and the absorption
ACS Paragon Plus Environment
Page 6 of 36
Page 7 of 36
Environmental Science & Technology
144
spectrum of RB (1O2 photosensitizer; applied at an initial concentration of 0.05 mM) indicated
145
that the photoexcitation of RB readily occurred under fluorescent lamp irradiation (Figure S4).
146
The incident light intensity, measured using a pyranometer (Apogee, PYR-P), was determined to
147
be 1.105 mW cm−2. The suspensions (or solutions) were initially adjusted to pH 7 and buffered
148
using 1 mM phosphate buffer in most cases. No significant pH change was observed over the
149
course of PMS activation. To investigate pH effects, the initial pH of the aqueous suspensions
150
(or solutions) was adjusted to the desired value using concentrated HClO4 and NaOH, and 1 mM
151
phosphate and carbonate buffers were used for maintaining neutral and alkaline pH, respectively.
152
Sample aliquots were withdrawn from the reactor at fixed time intervals using a 1 mL syringe,
153
filtered through a 0.45 µm PTFE filter (Millipore), and injected into a 2 mL amber glass vial. In
154
PMS activation experiments, excess methanol (0.5 M) was added to quench any remaining
155
radicals. The residual concentrations of organic pollutants were measured using an HPLC
156
(Agilent Infinity 1260) system equipped with a C-18 column (ZORBAX Eclipse XDB-C18) and
157
a UV/vis detector (G1314F 1260VWD). The mobile phase comprised 0.1% (v/v) aqueous
158
phosphoric acid solution and acetonitrile at a volume ratio of 45:55. According to the method
159
proposed by Liang et al., PMS was colorimetrically determined based on the amount of iodine
160
(λmax = 352 nm) formed via the oxidation of iodide by PMS.31 For EPR analysis, 5,5-dimethyl-
161
pyrroline N-oxide (DMPO) and TEMP were used as spin-trapping agents for SO4•−, azidyl
162
radical (N3•), and 1O2, respectively. EPR spectra of the aqueous CNT/PMS suspensions (aqueous
163
binary mixture of azide and PMS or fluorescent-light-irradiated RB solutions) were recorded
164
using a JES-TE 300 spectrometer (JEOL) under the following conditions: microwave power = 1
165
mW, microwave frequency = 9.4136 GHz, center field = 3350 G, modulation width = 0.1 mT,
166
and modulation frequency = 100 kHz. Raman spectra of fresh and used CNTs were acquired on a
ACS Paragon Plus Environment
Environmental Science & Technology
Page 8 of 36
167
LabRam ARAMIS Raman spectrometer (Horiba Jobin-Yvon) using an argon ion laser
168
(excitation at 514.5 nm). Sulfur-containing chemical moieties on CNTs (after exposure to PMS
169
solution) were identified by Fourier transform infrared spectroscopy (FT-IR, Thermo Scientific
170
Nicolet 6700) performed in ATR (attenuated total reflectance) mode. The zeta potential of the
171
aqueous-suspended CNTs was recorded as a function of pH using a Zetasizer Nano ZS
172
(Malvern).
173
RESULTS AND DISCUSSION
174
Oxidation of furfuryl alcohol (FFA) by Activated PMS. To explore whether 1O2 was produced
175
during PMS activation, oxidative degradation of FFA, as a 1O2 indicator,27 by the CNT/PMS
176
system was examined (Figure 1a). Sorption on CNTs caused no noticeable FFA decay, but PMS
177
alone directly oxidized FFA to a certain extent. However, CNTs in the presence of PMS
178
decomposed FFA more rapidly, with k(FFA) = 0.0054 ± 0.0014 min−1 for PMS alone versus
179
k(FFA) = 0.3750 ± 0.0207 min−1 for CNT/PMS. To further confirm that 1O2 had a role in CNT-
180
induced PMS activation, the effects of excess azide and L-histidine as 1O2 scavengers were
181
investigated (Figure 1a). FFA oxidation was completely quenched upon addition of L-histidine,
182
and kinetic retardation of FFA decay was significant upon addition of azide. The results
183
appeared to corroborate the previous findings12, 16-18 that singlet oxygenation was responsible for
184
the nonradical reaction pathway in PMS activation processes. However, solvent exchange (H2O
185
to D2O) did not accelerate FFA degradation at all, which contradicted the usual behavior of 1O2
186
in D2O, as the lifetime of 1O2 is extended up to 10 times when H2O is replaced with D2O.32 In
187
particular, the solvent exchange did not kinetically affect the PMS decay in the aqueous CNT
188
suspension (Figure S5). Since the highly accelerated self-decomposition of PMS was presumed
189
to result in 1O2 production during PMS activation by carbocatalysts,13,
ACS Paragon Plus Environment
16, 17, 20
the results
Page 9 of 36
Environmental Science & Technology
190
indicated that the alternative use of D2O had no influence on the kinetics of the reaction that
191
might allow the CNTs to transform PMS into 1O2.
192
Figure 1b shows the change in FFA degradation efficiency with quencher addition and
193
solvent exchange for photoexcited RB as a benchmark 1O2 producer.27 As observed in the
194
CNT/PMS system, FFA decomposition in the fluorescent-light-irradiated solution of RB was
195
drastically decelerated in the presence of azide and L-histidine. However, the use of D2O as a
196
solvent kinetically enhanced FFA degradation by RB under photoillumination. This behavior
197
was in marked contrast to the effect of D2O on the FFA degradation efficiency of CNT/PMS but
198
reflected the known properties of 1O2. Thus, this result suggested that singlet oxygenation may
199
not occur during persulfate activation. We also monitored H2O2 production as an alternative
200
indicator of singlet oxygenation during the oxidation of ascorbate by CNT/PMS and
201
photoexcited RB. The singlet oxygenation of ascorbate is accompanied by significant H2O2
202
formation.33 Though we found no difference in FFA oxidation efficiency between CNT/PMS and
203
photoexcited RB (Figures 1a and 1b), a much higher H2O2 formation yield was achieved with the
204
fluorescent-light-irradiated RB solution (Figure S6). The result also revealed that singlet
205
oxygenation likely contributed insignificantly to the oxidizing capacity of CNT/PMS.
206
The observed PMS concentrations in the PMS/quencher systems (Figure 2) revealed that and azide as 1O2 scavengers may cause misinterpretation of the
207
the choice of
208
experimental results on PMS activation processes, even though these compounds have been
209
widely employed to evidence the role of 1O2 as an oxidant in environmental processes.27, 34 The
210
inhibition of FFA degradation by CNT/PMS in the presence of excess L-histidine (Figure 1a)
211
appeared to be consistent with the retarding effect of L-histidine reported in the literature.12, 19
212
However, this drastic reduction in FFA oxidation efficiency resulted not from the scavenging
L-histidine
ACS Paragon Plus Environment
Environmental Science & Technology
213
activity of L-histidine toward 1O2 but from rapid PMS depletion by excess L-histidine (Figure 2a).
214
Note that direct reaction with L-histidine at an initial concentration of 100 mM reduced the PMS
215
concentration to the undetectable level within 5 min, and the reaction accelerated as L-histidine
216
concentration increased (Figure 2a). The quenching effect of azide (Figure 1a) also seemed to
217
support previous studies16-18 that suggest singlet oxygenation as the nonradical mechanism, but
218
we found that a binary mixture of PMS (100 mM) and azide exhibited significant FFA
219
degradation efficiency, which was not affected by the addition of CNTs (Figure 2b, inset). In fact,
220
FFA decay by CNT/PMS in the presence of excess azide, which was initially believed to be
221
kinetically hindered by the 1O2 scavenging activity of azide (Figure 1a), could instead be mainly
222
ascribed to the oxidizing capacity of azide/PMS (direct PMS reduction by L-histidine was not
223
accompanied by FFA oxidation (Figure S7)). In this case, the majority of PMS initially added
224
was rapidly consumed through direct reaction with azide as a reducing anion35 (Figure 2b),
225
rendering PMS unavailable for further reaction with residual azide (or CNTs).
226
To explore the reactivity of azide/PMS, we examined the binary mixture of azide and
227
PMS for the oxidative degradation of diverse organic substrates including 4-chlorophenol (4-CP),
228
2,4,6-trichlorophenol (TCP), 4-nitrophenol, and carbamazepine (Figure S8). Unlike FFA, which
229
was rapidly decomposed by azide/PMS, the other organic compounds were barely degraded in
230
aqueous azide/PMS solutions, which implied that direct azide oxidation by PMS led to the
231
production of a highly selective oxidant. The previous finding that N3• exhibited a very low
232
reactivity toward aromatic compounds substituted with electron-withdrawing groups36 likely
233
reveals the possibility that PMS could oxidatively convert azide into N3•. Further, the EPR
234
spectrum obtained for the aqueous azide/PMS mixture showed features that are assignable to the
235
formation of N3• (Figure S9).37 Although the kinetic rate of PMS degradation increased
ACS Paragon Plus Environment
Page 10 of 36
Page 11 of 36
Environmental Science & Technology
236
proportionally with the initial azide concentration (Figure 2b), the FFA removal efficiency
237
decreased with increasing azide concentration (inset of Figure 2b). This concentration-dependent
238
efficiency may be because excess azide likely favored the reaction routes that can deactivate N3•
239
(e.g., 2N3• → 3N2 (k = 4.5 × 109 M−1 s−1)38; N3• + N3− → N6•− (k = 1.0 × 106 M−1 s−1)38), but a
240
further study is required for an in-depth investigation into the mechanism underlying the PMS-
241
mediated production of N3• from PMS. Overall, the apparent inhibitory effects of 1O2 quenchers
242
that have been considered as evidence for 1O2 formation in persulfate activation systems are
243
attributable to the reactivity of L-histidine and azide toward PMS, which was consistent with the
244
previous report39 on the effective PMS consumption by L-histidine and azide.
245
FFA degradation was insignificant in aqueous suspensions of CNTs when PDS was
246
applied instead of PMS (Figure S10), which is consistent with the previous finding14 that
247
CNT/PMS was more effective for decomposing FFA than CNT/PDS. In contrast, a comparison
248
of the 4-CP degradation efficiencies for PMS and PDS activated with CNTs indicated that the
249
oxidizing powers of these two systems were similar (Figure S11). To examine the possibility of
250
1
251
4-CP oxidation efficiency was investigated (Figure S11). These reagents, as a 1O2 scavenger and
252
a singlet oxygenation enhancer, respectively, had no effect on the kinetic rate of 4-CP oxidation;
253
L-histidine
254
alternative solvent did not accelerate the 4-CP decay (Figure S11). The results eliminated the
255
possible contribution of 1O2 to the decomposition of organics by activated PDS. In contrast to
256
CNT/PDS, 4-CP was barely oxidized when excess
257
suspension of CNT/PMS (Figure S11). Although PMS was reductively decomposed by L-
258
histidine (Figure 2a), no loss of PDS was observed in the presence of 100 mM L-histidine
O2 formation during CNT-induced PDS activation, the effect of L-histidine and D2O addition on
negligibly quenched the degradation of 4-CP by CNT/PDS, and the use of D2O as an
L-histidine
ACS Paragon Plus Environment
was added to an aqueous
Environmental Science & Technology
259
(Figure S12). This result confirmed that the significantly inhibited oxidation of organics by
260
CNT/PMS (Figures 1a and S11) was due to rapid PMS degradation by excess L-histidine rather
261
than 1O2 scavenging. No acceleration in 4-CP degradation occurred in aqueous CNT/PMS
262
suspension when H2O was replaced with D2O (applied as an enhancer for singlet oxygenation)
263
(Figure S11).
264
While CNT/PMS caused much more rapid FFA oxidation than CNT/PDS (Figures 1a and
265
S10), similar 4-CP degradation efficiencies were observed, irrespective of whether PMS or PDS
266
was used (Figure S11). This reveals that the production of the reactive oxygen species (e.g., 1O2
267
and SO4•−) through persulfate activation may not be primarily responsible for the oxidizing
268
capacities of CNT/PMS and CNT/PDS. If the CNT-induced activation of persulfate involved 1O2
269
formation, the experiments using FFA as a 1O2 probe (Figures 1a and S10) would suggest that
270
CNT/PMS was much superior to CNT/PDS with respect to singlet oxygenation, which
271
contradicted with the lack of difference observed in the 4-CP treatment efficiency between
272
CNT/PMS and CNT/PDS (Figure S11). On the other hand, the nonradical mechanism in which
273
the CNTs effectively facilitated the transfer of electrons from organics to persulfates may
274
provide a plausible explanation for the substrate-specific reactivity of CNT/persulfate. The
275
mediated electron transfer should occur depending on how the electron flow from the organic
276
substrate to persulfate is favored. The observation that PMS achieved more rapid direct FFA
277
oxidation than PDS (Figure S13) clearly indicated that electrons were more preferentially
278
transferred from FFA to PMS than to PDS. This likely led to a much higher efficiency of
279
CNT/PMS for FFA degradation. In contrast, since 4-CP is susceptible to oxidative degradation
280
via direct electron transfer40, the electron delivery from 4-CP to either PMS or PDS appears to be
ACS Paragon Plus Environment
Page 12 of 36
Page 13 of 36
Environmental Science & Technology
281
thermodynamically plausible, which could render CNT/PMS and CNT/PDS with comparable 4-
282
CP degradation efficiencies.
283
Dependence of Reactivity on pH and Substrate Type. The substrate specificity of CNT/PMS
284
(in the dark) versus RB (under visible light irradiation) was compared (Figure 3). The systems
285
that produce identical reactive species are expected to exhibit similar substrate specificity during
286
oxidative degradation. The reactivity of the photoexcited RB varied considerably depending on
287
the type of organic compound (Figure 3b), which is consistent with the selective nature of 1O2.34,
288
41
289
degradation, whereas the other compounds, namely, benzoic acid, bisphenol A, and
290
carbamazepine, were highly persistent (Figure 3b). The oxidizing capacity of CNT/PMS was
291
also dependent on the type of substrate, but CNT/PMS caused significant degradation of all the
292
organics tested in this study, except benzoic acid (Figure 3a). In particular, organic compounds
293
that exhibited negligible or slow decomposition by photoexcited RB (e.g., bisphenol A,
294
carbamazepine, and propranolol) were effectively removed by aqueous CNT/PMS suspensions.
295
The substrate-specific reactivities of the CNT/PMS and photoexcited RB systems were clearly
296
distinguishable, suggesting that singlet oxygenation was minor (if present at all) in the
297
CNT/PMS system.
For example, cimetidine rapidly decomposed and propranolol underwent relatively slow
298
Three model substrates (4-CP, TCP, and pentachlorophenol (PCP)) were selected to
299
explore the effect of pH on the kinetic rates of chlorophenol oxidation by the CNT/PMS and
300
photoexcited RB systems (Figure 4). In general, alkaline conditions favor oxidative degradation
301
of phenolic compounds by 1O2 because phenolates, which are more electron-rich forms of
302
phenols that become predominant as the pH increases, are more susceptible to singlet
303
oxygenation than neutral phenols.42 The rate of phenol oxidation by 1O2 increases by two orders
ACS Paragon Plus Environment
Environmental Science & Technology
304
of magnitude at pH values above the pKa, with k(phenolate + 1O2) = 1.8 × 108 M−1 s−1 and
305
k(neutral phenol + 1O2) = 3.0 × 106 M−1 s−1.43 Consistent with the intrinsic reactivity of 1O2,34, 42,
306
43
307
solutions accelerated significantly with increasing pH up to or above the pKa (pKa(4-CP) =
308
9.41;44 pKa(TCP) = 6.23;44 pKa(PCP) = 4.7045). Fast 4-CP degradation proceeded in the visible-
309
light-irradiated RB solution at basic pH (pH = 11), whereas no removal of 4-CP was observed at
310
pH 7 or 4.5 (Figures 4b and S14b). However, photoexcited RB still mediated the rapid oxidation
311
of TCP at neutral pH, and the PCP decomposition efficiency was relatively constant, regardless
312
of the pH, i.e., k(PCP) = 0.0469 ± 0.0015 min−1 at pH 4.5, k(PCP) = 0.0671 ± 0.0056 min−1 at pH
313
7, and k(PCP) = 0.0711 ± 0.0025 min−1 at pH 11 (Figures 4b and S14b). In contrast, the pH
314
dependence of chlorophenol oxidation by activated PMS (Figures 4a and S14a) was different
315
from that of photosensitized chlorophenol oxidation by RB. All the tested chlorophenols were
316
significantly degraded in aqueous CNT/PMS suspensions under acidic and neutral pH conditions
317
(Figures 4a and S14a) (note that the scale of the y-axis in Figure 4a is 10 times greater than that
318
of Figure 4b). The rate of chlorophenol degradation by CNT/PMS at acidic pH was comparable
319
to or higher than the maximal rate observed in photoirradiated RB solution (k(4-CP at pH 4.5) =
320
0.159 ± 0.009 min−1 for CNT/PMS versus k(4-CP at pH 11) = 0.165 ± 0.006 min−1 for
321
photoexcited RB; k(TCP at pH 4.5) = 0.380 ± 0.017 min−1 for CNT/PMS versus k(TCP at pH 7)
322
= 0.111 ± 0.009 min−1 for photoexcited RB). Further, alkaline conditions in which phenolate
323
anions preferentially exist inhibited oxidative degradation of TCP and PCP by activated PMS.
324
These results also contradicted the possibility that heterogeneous PMS activation was
325
accompanied by singlet oxygenation.
Figure 4b demonstrates that the oxidation of chlorophenols in visible-light-irradiated RB
ACS Paragon Plus Environment
Page 14 of 36
Page 15 of 36
Environmental Science & Technology
326
Considering the possibility that the phenolate anion may have a stronger tendency to lose
327
electrons than neutral phenol, the oxidative degradation of chlorophenols could also be
328
accelerated in alkaline suspensions of CNT/PMS. However, the pH of the heterogeneous
329
persulfate activation system affects not only the interconversion between the phenolate anion and
330
the un-ionized phenol but also the surface charge of the carbocatalysts. The zeta potential
331
measured as a function of pH demonstrated that the CNT surface was negatively charged when
332
pH increased above ca. 6.2 (Figure S15), which implied that the alkaline conditions in which the
333
phenolate anion dominates the speciation caused electrostatic repulsions between the CNTs and
334
chlorophenol or PMS, kinetically hindering the electron transfer process in the ternary system.
335
EPR Study. PMS activation by CNTs in the presence of DMPO as a spin trapping agent led to
336
the formation of 5,5-dimethylpyrrolidone-2-(oxy)-(1) (DMPOX), a known product of direct
337
DMPO oxidation,46 which confirmed that SO4•− and hydroxyl radicals (•OH) were not involved
338
in the degradative mechanism (Figure S16). No EPR signals corresponding to DMPO adducts of
339
free radicals were observed for the photoilluminated RB solution (Figure S16). Signals
340
corresponding to 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), which is assignable to the
341
formation of a TEMP-1O2 adduct, have been demonstrated in the EPR spectra of persulfates in
342
the presence of activators (e.g., N-doped graphene,16,
343
provided a basis for suggesting singlet oxygenation as an alternative degradative route. We also
344
observed EPR spectral features corresponding to TEMPO generation in aqueous CNT/PMS
345
suspensions and photoirradiated RB solutions (Figure 5). However, D2O as a singlet oxygenation
346
enhancer affected the EPR spectral patterns in a different way. The peaks assigned to TEMPO
347
for CNT/PMS slightly decreased in the presence of D2O (Figure 5a), but the solvent exchange
348
caused a ca. 50% increase in the intensity of the EPR signal for photoexcited RB (Figure 5b).
17
CNTs,13 and Pd/g-C3N412), which
ACS Paragon Plus Environment
Environmental Science & Technology
349
This result implies that the TEMPO signal observed during PMS activation in the presence of
350
TEMP as a spin trap may not indicate the formation of 1O2. Nardi et al.47 suggested that EPR
351
detection of TEMPO may not be associated with 1O2 production; abstraction of one electron
352
from TEMP by the excited sensitizer results in the formation of the TEMP radical cation
353
(TEMP•+), which undergoes deprotonation and combination with dissolved oxygen to form
354
TEMPO. Therefore, TEMPO signals considered as evidence for singlet oxygenation during
355
persulfate activation could instead correspond to an electron-transfer mechanism, in which the
356
CNTs mediate electron transfer from TEMP to persulfate, leading to TEMP•+ generation.
357
Intermediate Distribution. In order to further clarify the difference in the degradative
358
mechanism between CNT/PMS (mediated electron transfer) and photoexcited RB (singlet
359
oxygenation), we compared the intermediate distribution from TCP oxidation by CNT/PMS and
360
fluorescent-light-irradiated RB. LC/MS analysis demonstrated a clear distinction in the
361
intermediate distribution between the two systems (Table S1). For instance, the main products
362
formed during the photosensitized singlet oxygenation of TCP included 1,2,3-trihydroxybenzene
363
2,6-dichloro-3-hydroxy-1,4-benzoquinone, which were not detectable over the course of TCP
364
decomposition by CNT/PMS. On the other hand, TCP oxidation in the aqueous CNT/PMS
365
suspension led to the formation of dihydroxydiphenyl ether and trichlorobenzene, which barely
366
formed when applying the photoirradiated RB for photochemical TCP degradation. The result
367
further confirmed that the degradative mechanism induced by CNT/PMS could be distinguished
368
from singlet oxygenation.
369
PMS Reduction and TCP Oxidation in Two Chambers Separated by a CNT Membrane. As
370
presented above, the empirical results supporting a role of 1O2 in persulfate activation, including
371
kinetic retardation in the presence of 1O2 quenchers and EPR spectral features characteristic of
ACS Paragon Plus Environment
Page 16 of 36
Page 17 of 36
Environmental Science & Technology
372
1
373
pathway. Further, the lack of a D2O enhancing effect and the incompatibility of the reactivity of
374
CNT-activated PMS with the pH-dependent and substrate-specific oxidizing capacity of 1O2
375
collectively implied that oxidative degradation during the heterogeneous activation of persulfate
376
was unlikely to involve 1O2. Thus, as an alternative nonradical degradative route, we examined
377
CNT-mediated electron transfer from organic substrates to PMS using a VA-CNT membrane
378
that physically partitioned the reaction system into two chambers containing PMS and TCP
379
(Figure 6a). Pilgrim et al.48 demonstrated electron exchange between photoexcited CdSe
380
quantum dots and methyl viologen located on opposite sides of a VA-CNT membrane. In this
381
system, photogenerated electrons were transported over hundreds of micrometers across the
382
CNT-based membrane. In contrast, as an extreme pressure (>120 bar) is required to allow water
383
entry into the inner pores of superhydrophobic virgin CNT membranes (with unmodified
384
entrances and exits),49 we found that VA-CNT membranes blocked the passage of water
385
molecules under ambient pressure (even with an external pressure of 5 bar, water did not pass
386
through the nanotube channels). As a VA-CNT membrane with high electric conductivity48, 50
387
and water impermeability is likely to reject organic/inorganic impurities and reactive oxygen
388
species (e.g., SO4•− and 1O2), concomitant TCP oxidation and PMS reduction in the physically
389
separated chambers (i.e., chambers A and B in Figure 6a) would be strong proof for an electron-
390
transfer mechanism, in which CNTs mediate electron transfer from organics to persulfates.
O2 formation, were not convincing evidence for singlet oxygenation as the nonradical reaction
391
When an aqueous chloride solution (1 M) and pure water were placed in the separated
392
compartments, no movement of Cl− from one side of the membrane (chamber A) to the other
393
side (chamber B) occurred as the conductivity of the pure water side did not change at all (Figure
394
S17). This result indicated that hydrophilic ions are unable to pass through the nonwettable pores
ACS Paragon Plus Environment
Environmental Science & Technology
395
of the VA-CNT membrane. Further, the PMS concentration decreased negligibly over 8 h when
396
Co2+ and PMS were placed on opposite sides of membrane, which confirmed that there was no
397
simultaneous transport of the ionic species across the CNT membrane (Figure S18). PMS
398
transport across the VA-CNT membrane would lead to a substantial increase in the PMS
399
concentration in chamber A, and the delivery of Co2+ to the other side would also cause
400
significant PMS reduction in chamber B. In contrast, when PMS and TCP were placed in the
401
separated chambers, noticeable decomposition was observed (Figure 6b). This result suggested
402
that electrons released from TCP on one side of the membrane (leading to TCP oxidation in
403
chamber A) crossed the conductive VA-CNT membrane (serving as an electron-transfer
404
mediator), eventually being accepted by PMS on the other side of the membrane (leading to PMS
405
reduction in chamber B). PMS was overconsumed for the observed TCP degradation efficiency
406
(~60%). When we repeated the experiment without the phosphate buffer, the performance with
407
respect to TCP treatment was almost unchanged, though PMS consumption was drastically
408
reduced (Figure S19). Considering ca. 10% of PMS was removable via sorption (Figure S18),
409
the observed PMS decay was insignificant. Therefore, the exposure to the phosphate buffer over
410
a relatively long reaction time (8 h) likely resulted in PMS being consumed in excessive
411
quantities (anions such as bicarbonate and phosphate added in high concentrations are capable of
412
direct PMS reduction51). On the other hand, PDS decay was minor when the TCP oxidation
413
associated with PDS activation (presented later) was performed even in the case of the buffered
414
solution (Figure S20), since PDS was unreactive toward anions present in excess
415
concentrations.51 Neither PMS nor TCP was detected on the opposite side of the membrane
416
(Figure 6b), which implied that interchamber transport did not contribute to the significant loss
417
of PMS or TCP in each chamber. TCP removal by sorption on the VA-CNT membrane was
ACS Paragon Plus Environment
Page 18 of 36
Page 19 of 36
Environmental Science & Technology
418
marginal (Figure S21). TCP degradation would not occur with PMS-derived oxidants (e.g.,
419
SO4•−, 1O2) (if any formed) owing to the impermeability of the VA-CNT membrane to water-
420
soluble species.
421
The distribution of the intermediates resulting from TCP oxidation in one chamber
422
physically separated from the other chamber in which PMS reduction concurrently occurred was
423
fairly similar to that observed when TCP was subjected to oxidation in the aqueous CNT/PMS
424
suspension (Table S1). The result confirmed that the organic oxidation associated with PMS
425
activation was achieved in the same manner irrespective of the type of CNTs used (i.e., an
426
aqueous CNT suspension versus VA-CNT membrane). Minor differences in the intermediate
427
distribution may rule out the possible role of the radical-induced reaction pathway in the CNT-
428
induced PMS activation process. If the surface-bound or free SO4•− derived from the PMS
429
molecules (if any) contributed to oxidative TCP decomposition, there would be a significant
430
distinction in the intermediate distribution between the two PMS activation systems: aqueous
431
CNT/PMS/TCP mixture and CNT and PMS physically separated via the VA-CNT membrane.
432
Note that no other (transient) chemical species (except electrons and protons) is transferable to
433
the other chamber across the VA-CNT membrane. TCP decomposition was noticeable, though it
434
was kinetically retarded when we used PDS alternatively (Figure S20), which supports our
435
hypothetic nonradical reaction pathway based on mediated electron transfer; CNT-induced
436
activation involving no radical formation is not unique to PMS and is achievable using proper
437
chemicals that can serve as electron acceptors.
438
Spectroscopic analysis suggested the possibility that PMS could form a complex on the
439
surface of graphitized nanodiamond10, and the resultant complex could effectively facilitate the
440
transfer of electrons from organic substrates to the PMS molecules involved in the surface
ACS Paragon Plus Environment
Environmental Science & Technology
441
complexation. More electronegative nitrogen atoms doped to CNTs could induce positive
442
charges on the adjacent carbons, which allowed the anionic PMS to form a reactive complex
443
with the N-doped CNTs.52 The surface complex was also suggested to initiate organic oxidation
444
by abstracting electrons from the organics through the conductive doped CNTs.52 The nonradical
445
mechanism is not far from our hypothetic mechanism, in which the CNTs likely mediate the
446
delivery of electrons from the organic compound to PMS, apart from the assumption that surface
447
complexation involving PMS would be a prerequisite for the electron exchange between the
448
organics and PMS. However, the formation of the reactive complex of PMS with CNTs is
449
unlikely to be indispensable to the electron transfer mechanism on account of the following
450
reasons. First, our previous work25 demonstrated that not only persulfate (PMS and PDS) but
451
also any oxyanions that serve as effective electron acceptors (e.g., periodate and percarboxylate)
452
could initiate oxidative organic degradation not reliant on the reactive radicals in the aqueous
453
CNT suspensions. Second, surface characterization of CNTs (after exposure to the PMS solution
454
for 1 h) using ATR-FTIR showed no occurrence of the spectral features assignable to the sulfur-
455
containing chemical moieties (Figure S22a); the infrared absorption peak at 1160 cm−1 is
456
attributed to the asymmetric stretching vibrations of C-S-C and S=O.53 Moreover, the
457
comparison of Raman spectra (Figure S22b) showed that the intensity of G-band, indicative of
458
graphitic carbon (1582 cm−1), did not change significantly after the use of CNTs in PMS
459
activation, implying no variation in defect density. Overall, the surface complexation of PMS can
460
promote the nonradical reaction pathway on carbonaceous activators, but will not contribute as
461
an essential step to PMS activation not relying on SO4•−.
462
Environmental Applications. In this study, we tested singlet oxygenation and mediated electron
463
transfer as hypothetical nonradical mechanisms underlying heterogeneous PMS activation. A
ACS Paragon Plus Environment
Page 20 of 36
Page 21 of 36
Environmental Science & Technology
464
comparison of CNT/PMS and photoirradiated RB showed that the reactivity of CNT/PMS
465
contradicted the intrinsic properties of 1O2 in terms of the effects of chemical reagents that
466
scavenge or enhance singlet oxygenation, substrate specificity, the pH dependence of
467
chlorophenol oxidation efficiency, and the effects of solvent exchange on EPR signal intensity.
468
However, simultaneous PMS reduction and TCP oxidation were demonstrated to occur when
469
these compounds were placed in two chambers physically separated by a VA-CNT membrane
470
that prevented interchamber transport of chemical species (e.g., water, hydrophilic anions, and
471
reactive oxygen species) but allowed electron conduction. Collectively, these results implied that
472
electron transfer from organics to persulfate, effectively facilitated by nanocarbon activators, was
473
primarily responsible for persulfate activation not involving reactive radicals. Here, we
474
demonstrated that PMS was superior to PDS in terms of the organic oxidation associated with
475
persulfate activation (i.e., FFA oxidation in aqueous CNT suspensions; TCP decomposition in
476
the two-chamber system). The observation was consistent with the previous finding that CNTs
477
achieved a more rapid destruction of a variety of organics in the presence of PMS rather than
478
PDS.25 Together with the general recognition that metal-induced activation results in more
479
effective SO4•− production from PMS than PDS,8, 54 the results justified the use of PMS in the
480
persulfate activation processes, even though the choice of PDS is more economically feasible.
481
The persulfate activation process utilizing SO4•− as the main oxidant (e.g., Co2+/PMS)
482
enables effective the oxidative degradation and mineralization of a wide range of organic
483
pollutants, whereas nonradical persulfate activation leads to substrate-specific oxidation. On the
484
other hand, the selective nature of the nonradical degradation pathway (typically induced by
485
carbocatalysts) has a competitive advantage over the radical-induced pathway on account of the
486
following reasons. First, a substrate-dependent oxidizing capacity allows the persulfate activation
ACS Paragon Plus Environment
Environmental Science & Technology
487
system to better target the priority pollutants present at trace levels in the complicated water
488
matrix; the majority of non-selective radicals (e.g., SO4•−, •OH) would be fruitlessly consumed
489
through the reactions with background organic and inorganic substrates. Second, since halide
490
ions are highly susceptible to one-electron oxidation by SO4•−, it is probable that the oxidative
491
treatment by SO4•− in the presence of bromide ions is inevitably accompanied by the high-yield
492
production of bromate.55 In contrast, persulfate activation based on the mediated electron transfer
493
mechanism would not result in bromate production from bromide.56 Finally, the nonradical
494
persulfate activation takes place in the co-presence of the organic pollutant (electron donor),
495
persulfate (electron acceptor), and activator (electron-transfer mediator), which likely minimizes
496
persulfate consumption; persulfate barely decomposes once the pollutant concentration is
497
significantly reduced. On the other hand, since the SO4•− in the heterogeneous persulfate
498
activation process is generated through the one-electron reduction of persulfate by the activators,
499
persulfate continues to be degraded until the residual PMS concentration reduces to virtually
500
zero.
501
The oxidative degradation of organics by CNT/PMS not reliant on SO4•− was suggested
502
to result from the CNT-induced electron exchange between the organic pollutant and PMS.
503
Carbon-based activators can kinetically enhance the transfer of electrons from organic substrates
504
to persulfates only when the electron flow is thermodynamically favored, which implies that the
505
thermodynamics of the electron transfer process is likely responsible for the selective nature of
506
the nonradical degradation pathway induced by carbocatalysts. As a result, a comparison of the
507
redox potentials of the target contaminant and persulfate will allow us to explore the treatability
508
of organic pollutants in the persulfate activation processes based on a mechanism that is not
509
radical-induced. Considering the key role of activators in the nonradical mechanism, the
ACS Paragon Plus Environment
Page 22 of 36
Page 23 of 36
Environmental Science & Technology
510
properties of the carbonaceous activators (e.g., electric conductivity, surface charge, and surface
511
affinity) that affect their capability of electron transfer mediation should be considered while
512
evaluating the performance of nanocarbon materials in heterogeneous persulfate activation. The
513
persulfate activation capacity of carbonaceous materials that can initiate oxidative degradation
514
without oxidizing radicals could possibly be improved in two ways: i) improving the electric
515
conductivity and ii) increasing the surface affinity toward organic contaminants (i.e., electron
516
donor) and persulfate (i.e., electron acceptor). Accordingly, possible strategies for developing
517
high-performance nanocarbon activators include doping with heteroatoms, combining with
518
metals/metal oxides, and surface modification with chemical moieties that interact
519
electrostatically with persulfate or organics.
520
Acknowledgements
521
This study was supported by a National Research Foundation of Korea grant funded by the
522
Korean Government (No. 2017R1A2B4002235) and a grant from the National Research
523
Foundation of Korea funded by the Ministry of Science, ICT, and Future Planning (No.
524
2016M3A7B4909318).
525
Supporting Information Available
526
Description of chemicals used in this study (Text S1), VA-CNT membrane fabrication (Scheme
527
S1), SEM images and Raman spectrum of aligned CNTs (Figures S1 and S2), XPS survey
528
spectrum of VA-CNT membrane (Figure S3), emission spectrum of fluorescent lamp and
529
absorption spectrum of RB (Figure S4), PMS decay by CNTs in H2O and D2O (Figure S5), H2O2
530
production during ascorbate oxidation by CNT/PMS and photoexcited RB (Figure S6), FFA
531
oxidation by PMS with excess L-histidine (Figure S7), reactivity of azide/PMS towards organics
532
(Figure S8), EPR spectra of azide only, PMS only, and azide/PMS with DMPO as a spin trap
ACS Paragon Plus Environment
Environmental Science & Technology
533
(Figure S9), FFA degradation by CNT/PMS and CNT/PDS (Figure S10), effects of L-histidine
534
and D2O on 4-CP oxidation by CNT/PMS and CNT/PDS (Figure S11), direct reduction of PMS
535
and PDS by L-histidine (Figure S12), direct FFA oxidation by PMS and PDS (Figure S13), effect
536
of pH on chlorophenol degradation by CNT/PMS and photoexcited RB (Figure S14), zeta
537
potential of CNTs (Figure S15), EPR spectra of CNT/PMS and photoirradiated RB with DMPO
538
as a spin trap (Figure S16), chloride permeability of VA-CNT membrane (Figure S17), time-
539
dependent changes in PMS concentration with Co2+ and PMS in chambers A and B (Figure S18),
540
concurrent TCP oxidation (chamber A) and PMS reduction (chamber B) without a phosphate
541
buffer (Figure S19), concurrent TCP oxidation (chamber A) and PDS reduction (chamber B)
542
(Figure S20), time-dependent changes in TCP concentration with aqueous TCP and pure water in
543
chambers A and B (Figure S21), and ATR-FTIR and Raman spectra of fresh and used CNTs
544
(Figure S22). This information is available free of charge via the Internet at http://pubs.acs.org/.
545 546
Literature Cited
547
1. Waclawek, S.; Lutze, H. V.; Grubel, K.; Padil, V. V. T.; Cernik, M.; Dionysiou, D. D.,
548
Chemistry of persulfates in water and wastewater treatment: A review. Chem. Eng. J. 2017, 330,
549
44-62.
550
2. Anipsitakis, G. P.; Dionysiou, D. D., Degradation of organic contaminants in water with
551
sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt. Environ. Sci.
552
Technol. 2003, 37, (20), 4790-4797.
553
3. Oh, W. D.; Dong, Z. L.; Lim, T. T., Generation of sulfate radical through heterogeneous
554
catalysis for organic contaminants removal: Current development, challenges and prospects.
555
Appl. Catal. B: Environ. 2016, 194, 169-201.
556
4. Chen, X. Y.; Wang, W. P.; Xiao, H.; Hong, C. L.; Zhu, F. X.; Yao, Y. L.; Xue, Z. Y.,
557
Accelerated TiO2 photocatalytic degradation of Acid Orange 7 under visible light mediated by
558
peroxymonosulfate. Chem. Eng. J. 2012, 193, 290-295.
ACS Paragon Plus Environment
Page 24 of 36
Page 25 of 36
Environmental Science & Technology
559
5. Kim, H.; Yoo, H. Y.; Hong, S.; Lee, S.; Lee, S.; Park, B. S.; Park, H.; Lee, C.; Lee, J., Effects
560
of inorganic oxidants on kinetics and mechanisms of WO3-mediated photocatalytic degradation.
561
Appl. Catal. B: Environ. 2015, 162, 515-523.
562
6. Chen, W. S.; Huang, C. P., Mineralization of aniline in aqueous solution by electrochemical
563
activation of persulfate. Chemosphere 2015, 125, 175-181.
564
7. Roshani, B.; Leitner, N. K. V., Effect of persulfate on the oxidation of benzotriazole and
565
humic acid by e-beam irradiation. J. Hazard. Mater. 2011, 190, (1-3), 403-408.
566
8. Ahn, Y. Y.; Yun, E. T.; Seo, J. W.; Lee, C.; Kim, S. H.; Kim, J. H.; Lee, J., Activation of
567
peroxymonosulfate by surface-loaded noble metal nanoparticles for oxidative degradation of
568
organic compounds. Environ. Sci. Technol. 2016, 50, (18), 10187-10197.
569
9. Lee, H.; Lee, H. J.; Jeong, J.; Lee, J.; Park, N. B.; Lee, C., Activation of persulfates by carbon
570
nanotubes: Oxidation of organic compounds by nonradical mechanism. Chem. Eng. J. 2015, 266,
571
28-33.
572
10. Lee, H.; Kim, H. I.; Weon, S.; Choi, W.; Hwang, Y. S.; Seo, J.; Lee, C.; Kim, J. H.,
573
Activation of persulfates by graphitized nanodiamonds for removal of organic compounds.
574
Environ. Sci. Technol. 2016, 50, (18), 10134-10142.
575
11. Zhang, T.; Chen, Y.; Wang, Y. R.; Le Roux, J.; Yang, Y.; Croue, J. P., Efficient
576
peroxydisulfate activation process not relying on sulfate radical generation for water pollutant
577
degradation. Environ. Sci. Technol. 2014, 48, (10), 5868-5875.
578
12. Wang, Y. B.; Cao, D.; Liu, M.; Zhao, X., Insights into heterogeneous catalytic activation of
579
peroxymonosulfate by Pd/g-C3N4: The role of superoxide radical and singlet oxygen. Catal.
580
Commun. 2017, 102, 85-88.
581
13. Cheng, X.; Guo, H. G.; Zhang, Y. L.; Wu, X.; Liu, Y., Non-photochemical production of
582
singlet oxygen via activation of persulfate by carbon nanotubes. Water Res. 2017, 113, 80-88.
583
14. Guan, C. T.; Jiang, J.; Pang, S. Y.; Luo, C. W.; Ma, J.; Zhou, Y.; Yang, Y., Oxidation
584
kinetics of bromophenols by nonradical activation of peroxydisulfate in the presence of carbon
585
nanotube and formation of brominated polymeric products. Environ. Sci. Technol. 2017, 51,
586
(18), 10718-10728.
587
15. Cheng, X.; Guo, H. G.; Zhang, Y. L.; Liu, Y.; Liu, H. W.; Yang, Y., Oxidation of 2,4-
588
dichlorophenol by non-radical mechanism using persulfate activated by Fe/S modified carbon
589
nanotubes. J. Colloid Interface Sci. 2016, 469, 277-286.
ACS Paragon Plus Environment
Environmental Science & Technology
590
16. Liang, P.; Zhang, C.; Duan, X. G.; Sun, H. Q.; Liu, S. M.; Tade, M. O.; Wang, S. B., An
591
insight into metal organic framework derived N-doped graphene for the oxidative degradation of
592
persistent contaminants: Formation mechanism and generation of singlet oxygen from
593
peroxymonosulfate. Environ. Sci. Nano 2017, 4, (2), 315-324.
594
17. Liang, P.; Zhang, C.; Duan, X. G.; Sun, H. Q.; Liu, S. M.; Tade, M. O.; Wang, S. B., N-
595
doped graphene from metal-organic frameworks for catalytic oxidation of p-hydroxylbenzoic
596
acid: N-functionality and mechanism. ACS Sustainable Chem. Eng. 2017, 5, (3), 2693-2701.
597
18. Tian, X. K.; Gao, P. P.; Nie, Y. L.; Yang, C.; Zhou, Z. X.; Li, Y.; Wang, Y. X., A novel
598
singlet oxygen involved peroxymonosulfate activation mechanism for degradation of ofloxacin
599
and phenol in water. Chem. Commun. 2017, 53, (49), 6589-6592.
600
19. Dai, D. J.; Yang, Z. Y.; Yao, Y. Y.; Chen, L. K.; Jia, G. S.; Luo, L. S., Highly efficient
601
removal of organic contaminants based on peroxymonosulfate activation by iron phthalocyanine:
602
Mechanism and the bicarbonate ion enhancement effect. Catal. Sci. Technol. 2017, 7, (4), 934-
603
942.
604
20. Li, D. G.; Duan, X. G.; Sun, H. Q.; Kang, J.; Zhang, H. Y.; Tade, M. O.; Wang, S. B., Facile
605
synthesis of nitrogen-doped graphene via low-temperature pyrolysis: The effects of precursors
606
and annealing ambience on metal-free catalytic oxidation. Carbon 2017, 115, 649-658.
607
21. Qi, C. D.; Liu, X. T.; Ma, J.; Lin, C. Y.; Li, X. W.; Zhang, H. J., Activation of
608
peroxymonosulfate by base: Implications for the degradation of organic pollutants. Chemosphere
609
2016, 151, 280-288.
610
22. Zhou, Y.; Jiang, J.; Gao, Y.; Ma, J.; Pang, S. Y.; Li, J.; Lu, X. T.; Yuan, L. P., Activation of
611
peroxymonosulfate by benzoquinone: A novel nonradical oxidation process. Environ. Sci.
612
Technol. 2015, 49, (21), 12941-12950.
613
23. Zhou, Y.; Jiang, J.; Gao, Y.; Pang, S. Y.; Yang, Y.; Ma, J.; Gu, J.; Li, J.; Wang, Z.; Wang, L.
614
H.; Yuan, L. P.; Yang, Y., Activation of peroxymonosulfate by phenols: Important role of
615
quinone intermediates and involvement of singlet oxygen. Water Res. 2017, 125, 209-218.
616
24. Lange, A.; Brauer, H. D., On the formation of dioxiranes and of singlet oxygen by the
617
ketone-catalysed decomposition of Caro's acid. J. Chem. Soc. Perkin Trans. 2 1996, (5), 805-
618
811.
ACS Paragon Plus Environment
Page 26 of 36
Page 27 of 36
Environmental Science & Technology
619
25. Yun, E. T.; Yoo, H. Y.; Bae, H.; Kim, H. I.; Lee, J., Exploring the role of persulfate in the
620
activation process: Radical precursor versus electron acceptor. Environ. Sci. Technol. 2017, 51,
621
(17), 10090-10099.
622
26. Duan, X. G.; Sun, H. Q.; Wang, Y. X.; Kang, J.; Wang, S. B., N-doping-induced nonradical
623
reaction on single-walled carbon nanotubes for catalytic phenol oxidation. ACS Catal. 2015, 5,
624
(2), 553-559.
625
27. Haag, W. R.; Hoigne, J., Singlet oxygen in surface waters .3. Photochemical formation and
626
steady-state concentrations in various types of waters. Environ. Sci. Technol. 1986, 20, (4), 341-
627
348.
628
28. Lee, K. J.; Park, H. D., The most densified vertically-aligned carbon nanotube membranes
629
and their normalized water permeability and high pressure durability. J. Membrane Sci. 2016,
630
501, 144-151.
631
29. Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R., Perspectives on
632
carbon nanotubes and graphene Raman spectroscopy. Nano Lett. 2010, 10, (3), 751-758.
633
30. Du, F.; Qu, L. T.; Xia, Z. H.; Feng, L. F.; Dai, L. M., Membranes of vertically aligned
634
superlong carbon nanotubes. Langmuir 2011, 27, (13), 8437-8443.
635
31. Liang, C. J.; Huang, C. F.; Mohanty, N.; Kurakalva, R. M., A rapid spectrophotometric
636
determination of persulfate anion in ISCO. Chemosphere 2008, 73, (9), 1540-1543.
637
32. Gorman, A. A.; Rodgers, M. A. J., Singlet molecular oxygen. Chem. Soc. Rev. 1981, 10, (2),
638
205-231.
639
33. Kramarenko, G. G.; Hummel, S. G.; Martin, S. M.; Buettner, G. R., Ascorbate reacts with
640
singlet oxygen to produce hydrogen peroxide. Photochem. Photobiol. 2006, 82, (6), 1634-1637.
641
34. Lee, J.; Hong, S.; Mackeyev, Y.; Lee, C.; Chung, E.; Wilson, L. J.; Kim, J. H.; Alvarez, P. J.
642
J., Photosensitized oxidation of emerging organic pollutants by tetrakis C60 aminofullerene-
643
derivatized silica under visible light irradiation. Environ. Sci. Technol. 2011, 45, (24), 10598-
644
10604.
645
35. Thompson, R. C.; Wieland, P.; Appelman, E. H., Oxidation of azide and
646
azidopentaaminechromium(III) by peroxymonosulfate in aqueous solution. Inorg. Chem. 1979,
647
18, (7), 1974-1977.
648
36. Alfassi, Z. B.; Schuler, R. H., Reaction of azide radicals with aromatic compounds. Azide as
649
a selective oxidant. J. Phys. Chem. 1985, 89, (15), 3359-3363.
ACS Paragon Plus Environment
Environmental Science & Technology
650
37. Kalyanaraman, B.; Janzen, E. G.; Mason, R. P., Spin trapping of the azidyl radical in
651
azide/catalase/H2O2 and various azide/peroxidase/H2O2 peroxidizing systems. J. Biol. Chem.
652
1985, 260, (7), 4003-4006.
653
38. Ram, M. S.; Stanbury, D. M., Electron-transfer reactions involving the azidyl radical. J.
654
Phys. Chem. 1986, 90, (16), 3691-3696.
655
39. Yang, Y.; Banerjee, G.; Brudvig, G. W.; Kim, J.-H.; Pignatello, J. J., Oxidation of organic
656
compounds in water by unactivated peroxymonosulfate. Environ. Sci. Technol. 2018, 52, 5911-
657
5919.
658
40. Kim, W.; Park, J.; Jo, H. J.; Kim, H. J.; Choi, W., Visible light photocatalysts based on
659
homogeneous and heterogenized tin porphyrins. J. Phys. Chem. C 2008, 112, (2), 491-499.
660
41. Kim, H.; Kim, W.; Mackeyev, Y.; Lee, G. S.; Kim, H. J.; Tachikawa, T.; Hong, S.; Lee, S.;
661
Kim, J.; Wilson, L. J.; Majima, T.; Alvarez, P. J. J.; Choi, W.; Lee, J., Selective oxidative
662
degradation of organic pollutants by singlet oxygen-mediated photosensitization: Tin porphyrin
663
versus C60 aminofullerene systems. Environ. Sci. Technol. 2012, 46, (17), 9606-9613.
664
42. Scully, F. E.; Hoigne, J., Rate constants for reactions of singlet oxygen with phenols and
665
other compounds in water. Chemosphere 1987, 16, (4), 681-694.
666
43. Wilkinson, F.; Helman, W. P.; Ross, A. B., Rate constants for the decay and reactions of the
667
lowest electronically excited singlet-state of molecular oxygen in solution - An expanded and
668
revised compilation. J. Phys. Chem. Ref. Data 1995, 24, (2), 663-1021.
669
44. Serjeant, E. P.; Dempsey, B., Ionisation Constants of Organic Acids in Aqueous Solution.
670
Pergamon: Oxford, U.K., 1979.
671
45. Cessna, A. J.; Grover, R., Spectrophotometric determination of dissociation constants of
672
selected acidic herbicides. J. Agric. Food Chem. 1978, 26, 289-292.
673
46. Wang, Y. X.; Sun, H. Q.; Ang, H. M.; Tade, M. O.; Wang, S. B., 3D-hierarchically
674
structured MnO2 for catalytic oxidation of phenol solutions by activation of peroxymonosulfate:
675
Structure dependence and mechanism. Appl. Catal. B: Environ. 2015, 164, 159-167.
676
47. Nardi, G.; Manet, I.; Monti, S.; Miranda, M. A.; Lhiaubet-Vallet, V., Scope and limitations
677
of the TEMPO/EPR method for singlet oxygen detection: The misleading role of electron
678
transfer. Free Radical Biol. Med. 2014, 77, 64-70.
ACS Paragon Plus Environment
Page 28 of 36
Page 29 of 36
Environmental Science & Technology
679
48. Pilgrim, G. A.; Amori, A. R.; Hou, Z. T.; Qiu, F.; Lampa-Pastirk, S.; Krauss, T. D., Carbon
680
nanotube-based membrane for light-driven, simultaneous proton and electron transport. ACS
681
Energy Lett. 2017, 2, (1), 129-133.
682
49. Walther, J. H.; Ritos, K.; Cruz-Chu, E. R.; Megaridis, C. M.; Koumoutsakos, P., Barriers to
683
superfast water transport in carbon nanotube membranes. Nano Lett. 2013, 13, (5), 1910-1914.
684
50. Pilgrim, G. A.; Leadbetter, J. W.; Qiu, F.; Siitonen, A. J.; Pilgrim, S. M.; Krauss, T. D.,
685
Electron conductive and proton permeable vertically aligned carbon nanotube membranes. Nano
686
Lett. 2014, 14, (4), 1728-1733.
687
51. Yang, S. Y.; Wang, P.; Yang, X.; Shan, L.; Zhang, W. Y.; Shao, X. T.; Niu, R., Degradation
688
efficiencies of azo dye Acid Orange 7 by the interaction of heat, UV and anions with common
689
oxidants: Persulfate, peroxymonosulfate and hydrogen peroxide. J. Hazard. Mater. 2010, 179,
690
(1-3), 552-558.
691
52. Duan, X. G.; Sun, H. Q.; Shao, Z. P.; Wang, S. B., Nonradical reactions in environmental
692
remediation processes: Uncertainty and challenges. Appl. Catal. B: Environ. 2018, 224, 973-982.
693
53. Chen, L. S.; Cui, X. Z.; Wang, Y. X.; Wang, M.; Qiu, R. H.; Shu, Z.; Zhang, L. X.; Hua, Z.
694
L.; Cui, F. M.; Weia, C. Y.; Shi, J. L., One-step synthesis of sulfur doped graphene foam for
695
oxygen reduction reactions. Dalton Transact. 2014, 43, (9), 3420-3423.
696
54. Anipsitakis, G. P.; Dionysiou, D. D., Radical generation by the interaction of transition
697
metals with common oxidants. Environ. Sci. Technol. 2004, 38, (13), 3705-3712.
698
55. Fang, J. Y.; Shang, C., Bromate formation from bromide oxidation by the UV/persulfate
699
process. Environ. Sci. Technol. 2012, 46, (16), 8976-8983.
700
56. Yun, E.-T.; Moon, G.-H.; Lee, H.; Jeon, T. H.; Lee, C.; Choi, W.; Lee, J., Oxidation of
701
organic pollutants by peroxymonosulfate activated with low-temperature-modified
702
nanodiamonds: Understanding the reaction kinetics and mechanism. Appl. Catal. B: Environ. In
703
press.
704
ACS Paragon Plus Environment
Environmental Science & Technology
(a)
CNT/PMS CNT/PMS w / L-histidine CNT/PMS w / azide
1.0
CNT/PMS w / D2O
0.8 FFA Conc. (C/C0 )
Page 30 of 36
CNT only PMS only 0.6
0.4
0.2
0.0 0
10
20
30
40
50
60
Reaction Time (min) (b)
RB RB w / L-histidine RB w / azide RB w / D2 O
1.0
FFA Conc. (C/C0)
0.8
0.6
0.4
0.2
0.0 0
705
10
20
30
40
50
60
Fluorescent Light Irradiation Time (min)
706
FIGURE 1. Degradation of furfuryl alcohol (FFA) by (a) CNT/PMS (in the dark) and (b) RB
707
(under photoirradiation) in the absence and presence of L-histidine, azide, and D2O ([CNT]0 =
708
0.1 g L−1; [RB]0 = 0.05 mM; [PMS]0 = 1 mM; [FFA]0 = 0.05 mM; [L-histidine]0 = [azide]0 = 100
709
mM; [phosphate buffer]0 = 1 mM; pHi = 7.0).
ACS Paragon Plus Environment
Page 31 of 36
Environmental Science & Technology
(a)
1.0
5 mM L-histidine 10 mM L-histidine 25 mM L-histidine 100 mM L-histidine
PMS Conc. (C/C0 )
0.8
0.6
0.4
0.2
0.0 0
10
20
30
40
50
60
(b) 1.0 FFA Conc. (C/C0)
1.0
PMS Conc. (C/C0 )
0.8
0.6
Azide only PMS/5 mM azide PMS/25 mM azide PMS/100 mM azide CNT /PMS/100 mM azide
0.8
0.6
5 mM azide 10 mM azide 25 mM azide 100 mM azide
0.4
0.2
0.0
0.4
0
10
20
30
40
50
60
Reaction Time (min)
0.2
0.0 0
10
20
30
40
50
60
Reaction Time (min)
710 711
FIGURE 2. Effects of initial concentrations of (a)
712
decomposition ([PMS]0 = 1 mM; [furfuryl alcohol (FFA)]0 = 0.05 mM; [phosphate buffer]0 = 1
713
mM; pHi = 7.0). Inset: FFA degradation during PMS activation with increasing concentrations of
714
azide.
L-histidine
715
ACS Paragon Plus Environment
and (b) azide on PMS
Environmental Science & Technology
(a)
Benzoic acid
1.0 Organic Compound Conc. (C/C0 )
Page 32 of 36
Bisphenol A Carbamazepine Cimetidine
0.8
Propranolol 0.6
0.4
0.2
0.0 0
10
20
30
40
50
60
Reaction Time (min) (b)
Benzoic acid Bisphenol A
Organic Compound Conc. (C/C0)
1.0
Carbamazepine Cimetidine
0.8
Propranolol
0.6
0.4
0.2
0.0 0
716
10
20
30
40
50
60
Fluorescent Light Irradiation Time (min)
717
FIGURE 3. Degradation of various organic compounds by (a) CNT/PMS (in the dark) and (b)
718
RB (under photoirradiation) ([CNT]0 = 0.1 g L−1; [RB]0 = 0.05 mM; [PMS]0 = 1 mM; [benzoic
719
acid]0 = [bisphenol A]0 = [cimetidine]0 = [propranolol]0 = [carbamazepine]0 = 0.05 mM;
720
[phosphate buffer]0 = 1 mM; pHi = 7.0).
ACS Paragon Plus Environment
Page 33 of 36
Environmental Science & Technology
Pseudo-First-Order Rate Constant (min-1 )
2.0
(a)
pH 4.5 pH 7 pH 11
1.5
1.0
0.5
0.0 4-CP
TCP
PCP
0.20 Pseudo-First-Order Rate Constant (min-1 )
(b)
pH 4.5 pH 7 pH 11
0.15
0.10
0.05
0.00 4-CP
TCP
PCP
721 722
FIGURE 4. Pseudo-first-order rate constants for chlorophenol degradation by (a) CNT/PMS (in
723
the dark) and (b) RB (under photoirradiation) under various pH conditions ([CNT]0 = 0.1 g L−1;
724
[RB]0 = 0.05 mM; [PMS]0 = 1 mM; [4-chlorophenol (4-CP)]0 = [trichlorophenol (TCP)]0 = 0.05
725
mM; [pentachlorophenol (PCP)]0 = 0.04 mM; [phosphate buffer]0 = [carbonate buffer]0 = 1 mM).
ACS Paragon Plus Environment
Environmental Science & Technology
(a)
Page 34 of 36
CNT only PMS only CNT/PMS (in H2O)
Intensity (Arb. Unit)
CNT/PMS (in D2O)
330.4
330.6
330.8
331.0
331.2
(b)
RB (in H2O)
Intensity (Arb. Unit)
RB (in D2O)
330.6
330.7
330.8
330.9
331.0
331.1
Magnetic Field (mT)
726 727
FIGURE 5. EPR spectra recorded in H2O- and D2O-based (a) CNT/PMS suspensions (in the
728
dark) and (b) RB solutions (under photoirradiation) ([CNT]0 = 0.1 g L−1; [RB]0 = 0.05 mM;
729
[PMS]0 = 1 mM; [TEMP]0 = 1 mM; [phosphate buffer]0 = 1 mM; pHi = 7.0). TEMP was used as
730
a spin trap.
731 732
ACS Paragon Plus Environment
Page 35 of 36
Environmental Science & Technology
733 (a)
734 735 736 737 738 739 740 741 1.5
(b)
1.0
TCP (chamber A) TCP (chamber B) PMS (chamber A) PMS (chamber B)
1.2
0.9
0.6
0.6
0.4
0.3
0.2
0.0 0
742
PMS Conc. (mM)
TCP Conc. (C/C0 )
0.8
100
200
300
400
0.0 500
Reaction Time (min)
743
FIGURE 6. (a) Experimental set-up for PMS activation in the reaction system partitioned into
744
two chambers by a vertically aligned CNT membrane and (b) simultaneous trichlorophenol (TCP)
745
oxidation (chamber A) and PMS reduction (chamber B) ([PMS]0 = 1.5 mM; [TCP]0 = 0.01 mM;
746
[phosphate buffer]0 = 1 mM; pHi = 7.0).
ACS Paragon Plus Environment
Environmental Science & Technology
747
Table of Contents Figure:
748 749 750 751 752 753
ACS Paragon Plus Environment
Page 36 of 36