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Formation and stabilization of environmentally persistent free radicals induced by the interaction of anthracene with Fe(III)-modified clays Hanzhong Jia, Gulimire Nulaji, Hongwei Gao, Fu Wang, Yunqing Zhu, and Chuanyi Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00527 • Publication Date (Web): 25 May 2016 Downloaded from http://pubs.acs.org on May 29, 2016
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Environmental Science & Technology
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Formation and stabilization of environmentally persistent free
2
radicals induced by the interaction of anthracene with
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Fe(III)-modified clays
4
a, b
Hanzhong Jiaa, Gulimire Nulaji
a
a
a
, Hongwei Gao , Fu Wang , Yunqing Zhu , and Chuanyi Wanga *
5 a
6
Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of
7
Physics & Chemistry; Key Laboratory of Functional Materials and Devices for Special
8
Environments, Chinese Academy of Sciences, Urumqi 830011, China.
9
b
School of Geology and Mining Engineering, Xinjiang University, Urumqi 830046, China.
10 11 12 13 14 15 16 17 18 19
*To whom correspondence should be addressed.
20 21 22 23 24 25
Xinjiang Technical Institute of Physics & Chemistry Chinese Academy of Sciences 40-1 South Beijing road, Urumqi, Xinjiang, 830011, China Phone: +86-911-3835879 Fax: +86-911-3838957 E-mails:
[email protected] (CYW);
[email protected] (HZJ)
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ABSTRACT:
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detected in Superfund sites but the formation of EPFRs induced by polycyclic
29
aromatic hydrocarbons (PAHs) is not well understood. In the present work, the
30
formation of EPFRs on anthracene-contaminated clay minerals was quantitatively
31
monitored
32
surface/interface-related environmental influential factors were systematically
33
explored. The obtained results suggest that EPFRs are more readily formed on
34
anthracene-contaminated Fe(III)-montmorillonite than in other tested systems.
35
Depending on the reaction condition, more than one type of organic radicals including
36
anthracene-based radical cations with g-factors of 2.0028-2.0030 and oxygenic
37
carbon-centered radicals featured by g-factors of 2.0032-2.0038 were identified. The
38
formed EPFRs are stabilized by their interaction with interlayer surfaces, and such
39
surface-bound EPFRs exhibit slow decay with 1/e-lifetime of 38.46 days.
40
Transformation pathway and possible mechanism are proposed on the basis of
41
experimental results and quantum mechanical simulations. Overall, the formation of
42
EPFRs involves single-electron-transfer from anthracene to Fe(III) initially, followed
43
by H2O addition on formed aromatic radical cation. Due to their potential exposure in
44
soil and atmosphere, such clay surface-associated EPFRs might induce more serious
45
toxicity than PAHs and exerts significant impacts on human health.
46
KEYWORD:
47
Environmentally persistent free radicals (EPFRs); Clay minerals; Polycyclic aromatic
48
hydrocarbons (PAHs); Electron transfer; Surface interaction.
via
Environmentally persistent free radicals (EPFRs) are occasionally
electron
paramagnetic
resonance
(EPR)
spectroscopy,
and
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TOC Art
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INTRODUCTION
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Environmentally persistent free radicals (EPFRs) are considered as a new class of
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emerging pollutants due to their potential of inducing the formation of biologically
54
damaging reactive oxygen species (ROS), which may be responsible for the oxidative
55
stress causing cardiopulmonary disease and probably cancer.1 EPFRs have been
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previously observed in combustion-generated particles and airborne particulate matter
57
with d < 2.5 µm (PM2.5).2 These EPFRs are produced by substituted aromatic
58
molecules (e.g., chlorophenols and chlorobenzenes) on the surfaces of transition
59
metal-containing particles at temperatures between 150 and 500 oC.2 The thermal
60
reaction processes are accomplished within a few seconds, but the formed EPFRs
61
persist in ambient air for days.3,
62
pentachlorophenol-contaminated soil from a former wood treatment facility sites.5
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The reactions are quite facile, occurring at room temperature, and have also been
64
found in Superfund sites contaminated with polycyclic aromatic hydrocarbons
65
(PAHs), polychlorinated biphenyls (PCBs), and polybrominated biphenyl ethers
66
(PBDEs).5-7 This inspired us to consider that EPFRs might be more ubiquitous than
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previously suspected or envisioned, especially at sites contaminated with organic
68
pollutants.
4
Recently, EPFRs were detected in
3
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In soil phase, the formation of EPFRs correlates with the interaction between
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selective aromatic compounds and soil components, such as inorganic minerals, soil
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organic matter, and the biological components.8-10 Sequestering and binding of
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organic contaminations to clay minerals play a significant role in their transformation
73
and persistence.11 Smectite, including montmorillonite, is a representative clay
74
mineral, which generally consists of a center octahedral Al-O sheet sandwiched
75
between two tetrahedral Si-O sheets. The planar aluminosilicate layers typically exist
76
in stacked assemblages, which are often referred to as tactoids. The unique properties,
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such as negatively charged layers, high cation exchangeable capacity (CEC), and
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expansible interlayer spaces, enable smectite to provide desired active sites for
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organic pollutants bonding on its surface, thereby leading to various physicochemical
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processes.12-14 Saturation of various cations is expected to modify the structural and
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physicochemical properties of the clay minerals, and thus influences the interaction
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between organic pollutants and clay surfaces.15 When exchanged with certain
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transition metal ions (e.g., Cu(II) and Fe(III)), clay minerals can make a variety of
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aromatic molecules, such as chlorinated phenols and anisoles, transform via interface
85
electron transfer, often followed by further reactions including dechlorination and
86
polymerization.6,
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through the formation of organic radicals as intermediates with simultaneous
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reduction of surface cations.7 Such organic radical intermediates are typically unstable
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and short-lived species. However, surface-bound organic radicals associated with
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mineral particles are occasionally persistent and are relatively long-lived in the
7, 16, 17
Transformation of these pollutants is possibly achieved
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environment, i.e., EPFRs.4
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Besides substituted phenols or benzenes, PAHs could also be transformed on clay
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surfaces.18 PAH molecules, which possess highly delocalized π-electrons, may act as
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strong electron-donors when interacting with electron-deficient species such as
95
exchangeable cations via electron-donor–acceptor interactions.19-22 Such “cation-π”
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interaction has been demonstrated as an important factor regulating PAHs availability
97
and transformation on mineral surfaces.12 Among commonly found cations (Na(I),
98
K(I), Ca(II), Mg(II), Al(III), and Fe(III)), the presence of transition metal ions such as
99
Fe(III) on clay surfaces facilitates the transformation of PAHs due to their strong
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cation-π interactions.22 The transformation is accompanied by the electron transfer
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from the aromatic species to surface cations.23-25 Therefore, free radicals are very
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likely to be generated during the interaction between modified clay minerals and
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molecular PAHs. The formed free organic radicals associated with the clay surfaces
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retain their additional stabilization, might allowing them to persist in the
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environment.26, 27 However, limited work has been conducted to assess the formation
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of PAHs-induced free radicals and their persistence on clay surfaces, and thus critical
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information is missing for the evaluation of potential risks from PAHs-contaminated
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soil in association with EPFRs.
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In this work, we demonstrate for the first time the potential of EPFRs formation
110
induced by PAHs, and in particular, by anthracene on clay mineral surfaces under
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environmentally relevant conditions. Conversion of molecular PAHs to EPFRs as
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well as persistency of the EPFRs was monitored via electron paramagnetic resonance 5
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(EPR) analysis. The principal objectives of the work are to 1) probe the role of
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interactions between PAHs and clay minerals in PAHs transformation and EPFRs
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formation; 2) reveal the influence of ionization potential of the organic molecule and
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clay surface properties on clay-mediated EPFRs formation; and 3) further gain insight
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into the mechanisms of PAHs-induced EPFRs formation on Fe(III)-clay surface. This
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work will provide useful information for the evaluation of potential risks from
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PAHs-contaminated clay minerals in association with free radicals.
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EXPERIMENTAL SECTION
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Chemicals and materials. Detailed information on the chemicals used in this
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study is supplied in the Supporting Information (SI). Reference montmorillonite, illite,
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and kaolinite were obtained from Zhejiang Feng-Hong Clay Chemicals Co., Ltd
124
(ZheJiang, China). These clays were dissimilar in CEC, specific surface area, and
125
interlayer swelling properties (Table S1).
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EPFRs formation. PAHs-contaminated clay minerals were prepared by adopting a
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protocol as previously reported.28 Briefly, clay minerals (< 2 µm) were obtained by
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centrifugation of clay suspension for 6 min at 600 rpm, and then treated with 0.1 M
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FeCl3 solution for four times. The clay samples before and after Fe(III) saturation
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were digested by the mixture of hydrofluoric acid, nitric acid and perchloric acid at
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250 oC for 90 min, and Fe contents were determined using a Perkin-Elmer PinAAcle
132
900T Atomic Absorption Spectrophotometer (Nor-walk, CT). The reaction mixtures
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of 0.1 mg/g PAHs-contaminated clay minerals were prepared by mixing 1 g of
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Fe(III)-modified clays with 5 mL of various PAHs in methanol solution, in which 6
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anthracene, phenanthrene, pyrene, and benzo[a]pyrene were, respectively, employed
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in each sample. Methanol was used as solvent of PAHs to allow its evaporation under
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ambient conditions.
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One gram of obtained PAHs-contaminated clay was laid onto a Petri dish, and then
139
placed inside a desiccator without light irradiation to prevent any light-induced
140
chemical reactions. The relative humidity (RH) was controlled by the saturated salt
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solutions in reaction cells. To conduct the anoxic reaction, the reaction system was
142
transferred into an anoxic chamber without free O2 and H2O molecules. To investigate
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the effect of reaction temperature, the desiccators with reaction systems were
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transferred into an oven controlled to various temperatures. At pre-selected intervals
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(such as 1 d, 2 d, 3 d, 5 d, 8 d, and max. 35 d), the samples were sacrificed and
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transferred into 50 mL Teflon centrifuge tubes. The residual PAHs and produced
147
EPFRs were extracted with 10 mL of extraction solution (mixture of 5 mL acetone
148
and 5 mL dichloromethane) and analyzed immediately.
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EPR Characterization. All EPR measurements were performed using a Bruker
150
E500 EPR spectrometer at room temperature. Instrument and operating parameters
151
are as follows: center field, 3470 G; microwave frequency, 9.7 GHz; microwave
152
power, 2.0 mW; modulation frequency, 4.0 G; modulation amplitude, 4.0 G; sweep
153
width, 200 G; receiver gain, 3.54 * 104; time constant, 41.0 ms; sweep time, 167.7 s;
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and three scans. Radical concentrations were calculated by comparing the signal peak
155
area, as derived from (∆Hp-p)2 multiplied by the relative intensity, to a
156
2,2-diphenyl-1-picrylhydrazyl standard. 7
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Products Analysis. PAHs were quantified using a Thermo Fisher Ultra 3000
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HPLC equipped with a 25 cm × 4.6 mm Cosmosil C18 column. A 85:15 (v/v) mixture
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of methanol:water was employed as mobile effluent. The flow rate was 1.0 mL min−1,
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and the ultraviolet detector was set at 254 nm. The PAHs intermediate products were
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identified using a Agilent 7890A-5975C gas chromatograph incorporated with a mass
162
spectrometer operated on a full scan mode (30-500 amu), where a HP-5MS capillary
163
column (length = 30 m; internal diameter = 250 µm; film thickness = 0.25 µm) was
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employed. Helium was used as carrier gas at a flow rate of 1.2 mL/min with splitless
165
injection at 230 oC. The oven temperature was programmed from 80 oC to 200 oC (20
166
o
167
in the reacted system was measured with following procedures. The reacted
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Fe(III)-clay sample was mixed with d.i. water using Vortex for 30 s. Then 0.5 mL of
169
suspension was collected and added to 1 mL of ferrozine solution (100 mM), and the
170
volume of the mixture was adjusted to 15 mL. The suspension was agitated for 2 h
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and filtered through a 0.45 µm filter. Concentration of ferrozine-complexed Fe(II) was
172
measured by UV–Vis spectrophotometer at 562 nm.
C min−1, 2 min hold), and then to 260 oC (20 oC min−1, 2 min hold). Content of Fe(II)
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Decay study. Kinetic studies were performed to determine the persistency of free
174
radicals in air. The samples were exposed to ambient air, and EPR signal was
175
measured periodically to determine the radical concentration as a function of time.
176
For reaction rate calculations, a first order kinetic expression was used:
177 178
−dR / dt = k[R] where R is the concentration of detected EPFRs. The 1/e lifetime ( t1/e ) of EPFRs for 8
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the first-order decay was evaluated as following:
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Ln(R / R0 ) = −kt , and t1/e = 1/ k
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Rate constant k was derived from the slop of the correlation between logarithm of
182
radical concentration change (R/R0) vs time, and 1/e lifetime was thereby derived.
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Theoretical modeling. Density functional theory (DFT) calculations were carried
184
out to model the reaction energies and activation energies associated with the
185
proposed reaction pathway using Dmol3 program29, 30 from the Material Studio (MS)
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of Dassault Systèmes Biovia Corp. The geometry optimizations of all intermediate
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and transition state structures were performed using the Becke exchange31 plus
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Perdew-Wang approximation functional32 within Generalized gradient approximation
189
(GGA). The energies were unscaled and zero-point corrected. Transition states were
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located by performing relaxed potential energy surface scans followed by
191
implementation of a complete linear synchronous transit (LST) and quadratic
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synchronous transit (QST) method.
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RESULTS AND DISCUSSION
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EPFRs formation on Fe(III)-montmorillonite. The potential generation of EPFRs
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on Fe(III)-montmorillonite and Na(I)-montmorillonite contaminated by various PAHs
196
was studied by EPR under relatively dehydrated condition (RH ~8%). The
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non-polluted clay minerals and Na(I)-montmorillonite contaminated by various PAHs
198
do not show any detectable EPR signals (Fig. S1). For Fe(III)-montmorillonite
199
systems, significant EPR signals were observed in anthracene-contaminated
200
Fe(III)-montmorillonite (Figs. 1a and S1). The total EPFRs yields increase with 9
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reaction time in 10 d, and then gradually decrease with reaction time (Fig. 1b).
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Interestingly, more than 30% of EPR signals remain even after one month, indicating
203
the persistent nature of these detected free radicals. However, the EPFRs were not
204
observed in the system of Fe(III)-montmorillonite contaminated by other PAHs such
205
as phenanthrene, pyrene, and benzo[a]pyrene (Fig. S1). Formation of EPFRs might
206
relate to the PAHs transformation on clay surface. As shown in Fig. S2a, almost 65 %,
207
90 %, and 100 % of anthracene, pyrene, and benzo[a]pyrene are transformed in 3 d by
208
Fe(III)-montmorillonite, while insignificant changes in phenanthrene are observed
209
during the experimental period. For control experiments performed with
210
Na-montmorillonite, changes in PAHs were undetectable, implying that adsorption
211
and microbiological reactions therein were negligible in the present work time frame.
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The varied transformation rates for individual PAHs can be ascribed to the fact that
213
PAHs with ionization potential lower than 7.6, such as anthracene (7.44 eV), pyrene
214
(7.43 eV), and benzo[a]pyrene (7.12 eV), readily undergo a single-electron transfer
215
reaction between PAHs and Fe(III) on clay surfaces18, 33 This is further supported by
216
the
217
PAHs-Fe(III)-montmorillonite system (Fig. S2b). In this electron-transfer process,
218
organic radical intermediates might be produced. However, the EPFRs cannot be
219
detected in the reaction systems associated with pyrene and benzo[a]byrene, which
220
are perhaps due to their less persistency making them hardly observable except at
221
very short time. On the other hand, Fe(III)-montmorillonite is unable to degrade
222
PAHs, such as phenanthrene, with ionization potential > 7.6 (see Fig S2a).18 Thus,
increase
in
Fe(II)
concentration
with
reaction
time
in
10
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electron transfer appears to be unfeasible in phenanthrene-Fe(III)-montmorillonite
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system. Therefore, among those four PAHs, persistent free radicals can only be
225
observed on Fe(III)-montmorillonite contaminated by anthracene.
226
The g-factor of EPR signal is a useful parameter for identifying the type of free
227
radicals.34, 35 As shown in Fig. 1b, the g-factor in a range from 2.0033 to 2.0036
228
increases with reaction time initially and decreases afterwards. In addition, the
229
asymmetrical EPR spectral profiles indicate more than one type of EPFRs generated.
230
The de-convolution of the EPR spectra implies 3 different spectral components
231
therein, denoted as g1, g2, and g3 with the g-factors of 2.0028-2.0030, 2.0036-2.0038,
232
and ~ 2.0032, respectively (Fig. S3). Previous studies suggest that the PAHs-based
233
radical cations with g-factor of ~ 2.0028 are readily formed through the direct
234
electron-transfer from aromatic compounds to transition-metal ions on clay surface.36,
235
37
236
cations. The oxygen-centered radicals, such as semiquinone radical anions, possess a
237
g-factor > 2.0040. 35 Radical signals with g-factors of 2.0030-2.0040 are attributed to
238
oxygenic carbon-centered EPFRs or/and carbon-centered radicals with a nearby
239
heteroatom, such as oxygen or halogen, which increase the spin-orbit coupling
240
constant.38, 39 In the present study, therefore, the produced g2 and g3 signals were
241
originated from carbon-centered EPFRs with an adjacent oxygen atom or/and
242
oxygenic carbon-centered EPFRs. As shown in Figs. 1c and S3, the EPR spectra
243
shape and peak areas of the g1, g2, and g3 signals vary with reaction time. Initially,
244
the free radical of g1 dominates the EPR spectra within 2 d. After that, the peak area
Thus, the g1 EPR signal can be ascribed to the formation of anthracene-type radical
11
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of g1 signal gradually decreases to undetectable level in 6 d, while the peak area of g2
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signal increases rapidly in the same time frame. The result suggests that the formation
247
of g2 radical can be ascribed to the decomposition of aromatic radical cations, which
248
is in agreement with previous studies.40 After 10 d, the peak area of g2 signal
249
generally decreases with further increase in reaction time, accompanied by the
250
increase in the yield of g3 signal. This suggests that the in situ-formed EPFRs can be
251
a precursor to form a new type of carbon-centered radicals with g-factor of ~ 2.0032
252
(Fig. 1c). After 17 d, most of g2 radicals are consumed. Meanwhile, the g3 signal is
253
also gradually decreased with increasing reaction time, implying the decomposition of
254
g3 radicals to the final product. The development of free radicals correlates with the
255
transformation processes of anthracene. The analyses of the GC-MS extracts of
256
anthracene-contaminated Fe(III)-montmorillonite are presented in Fig. S4. The main
257
transformation products of anthracene could be identified as anthraquinone formed by
258
ketonizing the intermediate benzene ring of anthracene (Fig. S5), while anthrone is
259
considered as a major detectable intermediate for the transformation from anthracene
260
to anthraquinone, suggesting the possible formation of carbon-centered EPFRs with
261
an adjacent oxygen atom (such as g2 radical) by partially ketonizing the benzene ring
262
of anthracene. Thus, the transformation from g1 radical to g2 radical is probably
263
accompanied with the formation of anthrone from anthracene. On the other hand, g3
264
radical might be involved in the ketonizing of the other carbon (i.e., C10) in the
265
intermediate benzene ring of anthracene, which may result in the formation of final
266
product, i.e., anthraquinone. 12
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In addition, the obtained three types of EPFRs exhibit varied persistency on the clay
268
surfaces. The decays of g1, g2, and g3 signals are depicted from the reaction time at
269
their highest yield, i.e., 3 d for g1, 10 d for g2, and 15 d for g3. As shown in Fig. 1d,
270
g1 signal exhibits a “fast” decay with 1/e lifetime of 1.41 d, indicating that aromatic
271
radical cations are relatively unstable on Fe(III)-saturated clay surfaces. The half-lives
272
of carbon-centered EPFRs with an adjacent oxygen atom, such as g2 radical, are
273
much longer than the corresponding radical cation species on Fe(III)-montmorillonite,
274
with a 1/e lifetime of 3.45 days. On the other hand, the g3 radical exhibits the
275
“slowest” decay, with a 1/e lifetime of 38.46 d (Fig. 1d). As reported previously, the
276
carbon-centered radical may remain stable when the para positions of the benzene
277
ring are occupied by stable substituents.17 Thus, the g3 signal with “slower” decay
278
and lower g-factor than g2 signal might be attributed to the formation of
279
carbon-centered radicals with para oxygen atom, such as anthroxyl radical.
280
Role of clay minerals on EPFRs formation. Although common free radicals are
281
typically unstable and of short-lived species, some free organic radicals generated at
282
or near the solid particle may have strong interactions with the particle surfaces,
283
thereby making them stable and persistent.41,
284
minerals on EPFRs formation, various Fe(III)-saturating clay minerals (i.e.,
285
Fe(III)-kaolinite,
286
anthracene was studied by EPR. As shown in Table S1, the CEC of montmorillonite is
287
greater than that of other clays, and the exchangeable cations are located on both
288
external and interlayer surfaces of montmorillonite. Kaolinite clay has essentially no
Fe(III)-illite,
and
42
To investigate the role of clay
Fe(III)-montmorillonite)
contaminated
by
13
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isomorphic substitution and the small amount of negative charges result mostly from
290
the dissociation of hydroxides at edge sites, hence a small quantity of surface Fe(III)
291
resides primarily on the edge sites. Though negatively-charged illite has high surface
292
density, most of structural charges in clay interlayers are compensated by fixed K+
293
which cannot be replaced by the added Fe(III), thus surface Fe(III) ions are mainly
294
located on external surfaces. Therefore, EPFRs formation on Fe(III)-saturated
295
kaolinite was used to examine the reactive sites on edge surfaces, while illite was used
296
to reflect the reactivity on external surfaces. Compared with kaolinite and illite, Fe(III)
297
located on montmorillonite could refer to the reactive sites in clay interlayers.
298
As displayed in Table S1, the surface Fe(III) contents were ca. 3.12 %, 1.54 %, and
299
0.44 % for Fe(III)-saturated montmorillonite, illite, and kaolinite, respectively, which
300
is derived by the difference in Fe content between original clays and Fe(III)-saturated
301
clays. However, the amount of EPFRs formed on anthracene-contaminated
302
Fe(III)-montmorillonite is > 4 orders of magnitude greater than that on Fe(III)-illite
303
and Fe(III)-kaolinite clays (Fig. S6). In other words, the difference in surface Fe(III)
304
content for the three Fe(III)-saturated clay systems is less than 1 order of magnitude,
305
while the difference in EPFRs yields is more than 4 orders of magnitude. Therefore,
306
microenvironment of Fe(III) located in the clay interlayers plays vital role in EPFRs
307
stabilization among the reactive Fe(III) sites on clay surfaces, which agrees well with
308
what reported previously.16, 43 Generally, the EPFRs are located at specific sites on
309
mineral surfaces, in which stable free radicals are readily formed or in which the
310
produced radicals are easily stabilized.37 During the PAHs transformation process, the 14
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electron-transfer reaction could induce the formation of organic radical cations and
312
their oxygenic radicals.44, 45 The formed positively charged species would be strongly
313
bound to the negatively charged silicate surface.37, 46, 47 The large electric fields in the
314
interlayer region of clays favor an optimum dispersal of surface charge, which lowers
315
the electrostatic energy of various interaction or complexes in layered silicates.37 Thus,
316
free radicals formed from selected organic molecules at localized sites on clay
317
surfaces are more stable in clay interlayer than in outer spaces. On the other hand, the
318
aromatic molecules are tend to orientate in the interlayer region, which are favorable
319
in electron-transfer reactions and free radicals formation.37 Therefore free radicals
320
located at interlayer sites are relatively long-lived in the environment, i.e.,
321
environmentally persistent.
322
Effect of environmental condition
323
EPFRs formed in anoxic condition. The formation of EPFRs may be affected by
324
atmospheric O2 or/and H2O molecules, which may participate in the oxidation of
325
organic contaminants or/and the decomposition of free radical intermediates.4 To
326
explore the role of O2/H2O on EPFRs formation, anthracene transformation on
327
Fe(III)-montmorillonite clay was initially conducted in an anoxic chamber without
328
free O2 and H2O molecules. Under anoxic condition, the yield of EPFRs gradually
329
increases up to 15 d (Fig. 2). The de-convolution of their EPR spectra indicates that
330
the g1 signal, which is defined as aromatic radical cations, is initially formed and
331
relatively stable under anoxic condition compared to that under ambient condition at
332
RH of ~ 8% (Fig. S7). With increasing reaction time, the produced radical cations are 15
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partially transformed to more oxygenic radicals such as carbon-centered radicals with
334
an adjacent oxygen atom, which is consistent with observed increasing in g-factor
335
from 2.0028 to 2.0032 (Figs. 2 and S7). Although O2 and H2O molecules are limited
336
in an anoxic chamber, Fe(III) on clay surfaces may combine with OH- and/or H2O
337
molecules and readily form small oligomers such as [Fe(OH)1-4]n-1~2+ during its
338
preparation. As reported previously, the hydroxyl group from available H2O
339
molecules is incorporated into the radical products, and the reaction of radical cations
340
with OH- is dependent on the structure of precursor molecules, such as PAHs.33, 37
341
Radical cations localized at 9, 10 positions in the anthracene and similar compounds
342
react preferentially with OH- to result in quinines or/and diphenols as primary
343
products.33 Thus, the partial formation of oxygenic free radicals might be due to the
344
reaction between aromatic radical cations and OH-/water complexing with Fe(III) on
345
clay surfaces. Exposure of the radical cations, formed in anoxic chamber, to ambient
346
air (RH is ~ 8%) affects both their EPR signal positions and intensities. The g-factor
347
rapidly increases from 2.0032 to 2.0038 after prolonged air exposure (Fig. 2). Such
348
g-factor change indicates the trend of aromatic radical cations rapidly converting to
349
carbon-centered EPFRs with an adjacent oxygen atom.48 In addition, the peak area
350
shows little change for the first few days of air exposure, and then gradually decreases
351
after prolonged air exposure, accompanied with the decrease in g-factor. The result
352
indicates the gradual decomposition of carbon-centered radical with nearby oxygen to
353
the other carbon-centered radical and further generation of final product, i.e.,
354
anthraquinone. 16
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The effect of temperature. The transformation of organic pollutants on clay
356
surfaces and formation of EPFRs is likely to be temperature-dependent.49 Fig. 3a
357
depicts the derivative EPR spectra of EPFRs observed for 15 d transformation of
358
anthracene as a function of reaction temperature ranging from 25 to 90 °C. The total
359
yields of EPFRs exhibit insignificant changes with increasing reaction temperature
360
from 25 °C to 40 °C. However, further increase in reaction temperature induces
361
decreasing in radical yields (Fig. 3b). When raising temperature above 75 °C, the
362
EPFRs yields become approximately constant and relatively low, suggesting very
363
limited conversion of anthracene to its EPFRs. The increase in the reaction
364
temperature reduces the yields of EPFRs, which might be due to either lower initial
365
yield of free radicals or high reactivity for the decomposition of free radicals on clay
366
minerals. It is noted that the transformation rate of anthracene is significantly
367
enhanced when the reaction temperature increases from 40 °C to 90 °C (Fig. 3c). The
368
obtained results indicate that the low yield of EPFRs can be mainly attributed to their
369
easier decomposition under higher temperature, inducing rapid conversion of
370
anthracene to its final product.
371
Although a slight change of EPR g-factors was observed under various reaction
372
temperatures, interestingly, the g-factor gradually increases from 2.00335 to 2.00351
373
as the reaction temperature increases from ~ 40 to 90 °C (Fig. 3b). The
374
temperature-dependent evolution of g-factor is related to the type and relative yields
375
of EPFRs. As discussed above, newly formed oxygenic carbon-centered EPFRs (i.e.,
376
g3 radical) dominates the EPR spectra at 25 oC, accompanied by the formation of a 17
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377
small amount of g2 radical (i.e., carbon-centered EPFRs with an adjacent oxygen
378
atom). The result suggests a higher contribution of g3 radical at ≤ 40 °C, while
379
oxidation or losing of more g3 radicals readily occurs at higher temperature. Thus,
380
carbon-centered radical with adjacent oxygen atom might be the predominant species
381
at relatively high reaction temperature, which induce the increase in g-factor of
382
EPFRs. This is also consistent with the change of EPFRs yields at various reaction
383
temperatures, in which less EPFRs yields were observed at > 40 °C (Fig. 3b).
384
Meanwhile, increase in the EPFRs yields at ambient temperatures such as 25-40 °C
385
simply indicates that the transformation rate of anthracene exceeds the rate of
386
carbon-centered radical decomposition on clay surface.
387
The effect of relative humidity. Besides the reaction temperature, the nature and
388
amount of produced free organic radicals also depends on the RH in the reaction
389
medium.37 As shown in Figs. 3d and S8, the increase in the RH that ranges from 8%
390
to 11% leads to a small amount of improvement in EPFRs yields accompanied with
391
increased transformation rate of anthracene. This might be due to that the ligand water
392
molecules around surface cations participate in the EPFRs formation reaction.33, 37
393
Further increase in RH above 11% results in a steep decrease in both of the EPFRs
394
yields and PAHs transformation rate. When RH is > 43%, the transformation of
395
anthracene to EPFRs on Fe(III)-montmorillonite surfaces is almost completely
396
retarded (Fig. S8). This result is in agreement with previous report, in which the
397
addition of water to a system associated with arene transformation on silica-alumina
398
resulted in a rapid decay of reaction rate.50 The suppress effect by water is attributed 18
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399
to a competition between arene and water molecule for the Lewis acid site (such as
400
Fe(III) on clay surfaces).50 Generally, cations on clay surfaces tend to be hydrated,
401
resulting in the formation of water layer in interfacial region.14 The coverage of water
402
molecules leads to an increased detachment of anthracene from the inner-sphere
403
coordination sites of cations, which influences the interaction between organic
404
contaminants and clay surfaces.22 The decreased anthracene-Fe(III) interaction on
405
clay surface induces the decrease in electron transfer reaction rate and EPFRs
406
formation
407
Fe(III)-montmorillonite enhances PAHs chemisorptions, which, in turn, allows the
408
reaction of electron transfer.51 Therefore, the presence of free water blocks the active
409
sites and hinders the catalytic effect of the clay surface; the interlayer water must be
410
removed to certain extent for the oxidation reaction of organic contaminants and the
411
formation of EPFRs to proceed.
on
clay
surface.44
On
the
other
hand,
dehydration
of
412
Theoretical prediction of possible EPFRs formation mechanism. Reaction of
413
aromatic compounds with montmorillonite saturated by transition metal cations (e.g.,
414
Fe3+ and Cu2+) has been previously studied.6, 52, 53 The results lead to the proposal that
415
during the reaction, electrons were donated by the unsaturated organic compounds to
416
the surface cations, resulting in the formation of aromatic radical cations and reduced
417
metal ions such as Fe(II) and Cu(I).6, 7 Similar to the mechanism proposed in previous
418
studies, PAHs transformation and EPFRs formation are also due to the electron
419
transfer process between PAHs and surface cations.18, 45 In order to further understand
420
the mechanism of EPFRs formation, the initial Fe(III) impact on the anthracene 19
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421
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process is modeled by the transformation state theory.
422
Potential energy surface (PES) and optimized geometries of the anthracene reaction
423
with Fe(III) are shown in Fig. 4. Fe(III) and anthracene form a pre-reaction complex,
424
IM1, by means of two intermolecular bonding (4.282 Å) interaction. The Fe(III)
425
addition to the C9 or C10 atom on the anthracene molecule processes through the
426
transition state TS with an activation barrier of 0.355 kcal/mol. This reaction can be
427
considered as a barrierless pathway. As a result of stabilizing the resonance ring
428
structure, the adducts of Fe(III) and anthracene leads to the formation of the
429
intermediate IM2 with 22.591 kcal/mol lower energy than the reactants, thereby
430
resulting in the formation of cation–π complexes on clay surfaces. In this process,
431
attack by Fe(III) at the C9 or C10 atom leads to –Fe···C– distance of 2.12 Å. Then,
432
the initial single-electron-transfer reaction is exoenergetic by 0.142 kcal/mol. The
433
associated electron-transfer within the complex results in the formation of
434
anthracene-type radical cation (radical A) and reduction of transition metal ions (as
435
shown in Scheme 1).36 The redox reaction is mainly facilitated and enhanced by the
436
planar negatively charged silicate layers of the montmorillonite clay.36 Although the
437
formed radical cations are stabilized on clay interlayer surface, the unpaired electrons
438
in the free radicals can be oxidized by atmospheric oxygen or hydrolyzed by H2O
439
molecules, resulting in the formation of other intermediate products.7, 16 According to
440
theoretical simulation, O2 and OH- can be added to C9 (or C10) atom of
441
anthracene-type radical cation via the transition state TS with an activation barrier of
442
1.803 and 0.118 kcal/mol, respectively. Finally, the –O···C– distance decreases from 20
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443
1.869 to 1.544 in the case of O2 addition and from 2.76 to 1.46 Å for OH- addition,
444
inducing the formation of final oxygenic radicals with energy of -15.618 and -30.777
445
kcal/mol, respectively. Thus, the reaction between H2O and radical A is more readily
446
than that between O2 and radical A, resulting in the formation of radical B (Scheme 1).
447
As
448
Fe(III)-montmorillonite, the radical B (hydroxyanthracene-type radical) deprotonates
449
to form the 9-hydroxy-anthracene, which is further rapidly tautomerized to the
450
thermodynamically favored anthrone (Scheme 1).54 Then, electron transfer between
451
anthrone and Fe(III)-montmorillonite induces the formation of anthrone-type radical
452
(radical C), followed by hydrolysis to generate oxanthrone, which can be further
453
transformed to anthraquinone.55 Radical B might also be directly transformed to
454
radical C through the intramolecular electron-transfer and deprotonation processes
455
accompanied by the formation of the intermediate products A and B. The present
456
study indicates that radical A is relatively unstable in natural environment and readily
457
transform to radical B. Compared with radical B, radical C (a carbon-centered
458
anthroxyl radical) are more stable due to the para positions of the benzene ring
459
occupied by ketone group.17
460
Environmental importance
the
pathway
proposed
for
anthracene
oxidative
degradation
on
461
The precursor molecules (PAHs) employed in this study are produced worldwide
462
via fossil fuel deposit and incomplete combustion, have been recognized as one class
463
of primary contaminants in naturally hydrophobic phases such as soil and sediment.56
464
Previous work reports the detection of EPFRs in PAHs-contaminated soil under
21
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465
ambient environmental conditions.57 But limited evidence is available to prove its
466
formation process. This study provides, for the first time, experimental evidence for
467
EPFRs formation on the clay minerals contaminated by the PAHs such as anthracene
468
under environmentally relevant conditions, i.e., presence of water and ambient
469
temperature. This study implies that many PAHs-contaminated soils may be at risk
470
for EPFRs production. Moreover, large production and propensity for volatilization
471
could lead to ubiquitous atmospheric contamination by PAHs. This suggests that the
472
highly stable EPFRs on clay mineral particles might be transported in the atmosphere
473
for a long distance from the source, eventually participate in atmospheric reactions, or
474
directly exert health and environmental impacts.58
475
Previous studies suggest that clay-based system such as Fe(III)- and Cu(II)-smectite
476
may be useful in the alteration and degradation of aromatic molecules present in
477
waste or contaminated sites.59 Such clay-catalyzed electron-transfer reactions were
478
considered as detoxification of organic toxicants under mild reaction conditions.
479
However, the toxicity of EPFRs has not been measured during the transformation of
480
those organic contaminants. Biochemical and biomedical studies on Fe-EPFRs
481
complexes indicate that such surface-associated EPFRs can induce the formation of
482
biologically damaging ROS such as peroxide, superoxide, and hydroxyl radical by
483
redox-cycles process, which may be responsible for the oxidative stress causing
484
cardiopulmonary disease and probably cancer that has been attributed to the humane
485
exposure to clay particles.60 Thus, transformation of PAHs on clay minerals could
486
potentially give rise to more toxic PAHs-type EPFRs. Therefore, the potential 22
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487
environmental risks from PAHs-contaminated clay minerals should be re-evaluated
488
due to their association with EPFRs.
489
ASSOCIATED CONTENT
490
Supporting Information Available
491 492
Additional details as noted in text. This information is available free of charge via the Internet at http://pubs.acs.org.
493
AUTHOR INFORMATION
494
Corresponding Author
495
*Phone: +86-911-3835879; e-mails:
[email protected].
496
Notes
497
The anthors declare no competing financial interest.
498
ACKNOWLEDGMENTS
499
Financial support by the National Natural Science Foundation of China (Grants No.
500
41571446 and 41301543), the West Light Foundation of Chinese Academy of
501
Sciences (2015-XBQN-A-03), the Xinjiang Program of Introducing High-Level
502
Talents (Y539031601), and the CAS Youth Innovation Promotion Association
503
(2016380) are gratefully acknowledged.
504
REFERENCES
505 506 507 508 509 510 511 512 513 514 515 516
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Figure Captions 1.5
80
2.0038
g-Factor
0.5 0.0
8d 10d 12d 17d 25d
-0.5 -1.0
100
b
1d 2d 3d 4d 7d
1.0
Intensity (a.u.)
2.0040
a
g-factor Peak area
40
2.0034
-1.5
20
2.0032 1.99
2.00
2.01
0
0
2.02
5
10
g-Factor
g1 (2.0028-2.0030) g2 (2.0036-2.0038) g3 (2.0031-2.0032)
25
30
35
d k3 = 0.026 t1/e= 38.46 d
-0.5 ln(C/C0)
80
20
0.0
c 100
15
Time (d)
120
Peak area
60
2.0036
Peak area
675
Page 28 of 32
60 40
-1.0 g1 g2 g3
-1.5
20 -2.0 0 -2.5 0
5
10
15
20
25
30
35
k2 = 0.29 t1/e= 3.45 d
k1 = 0.71 t1/e= 1.41 d
0
2
4
6
676
8
10
12
14
16
Time (d)
Time(d)
677
Fig. 1. The evolution of (a) EPR spectra and (b) their g-factor and peak area as a
678
function of reaction time in the reaction system of anthracene-contaminated
679
Fe(III)-montmorillonite at relative humidity (RH) of 8% and room temperature (~ 25
680
o
681
time. (d) Normalized pseudo-first-order decay kinetics of EPFRs derived from the
682
reaction time at their highest yield for various radicals, i.e., 3d for g1, 10d for g2, and
683
15d for g3.
C). (c) The evolution of peak area of g1, g2, and g3 signals as function of reaction
684 685 686 687 688 689 690 691 692
28
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Page 29 of 32
Environmental Science & Technology
2.0044
70 In glovebox
2.0042
In air 60
2.0040 50
2.0036 40 2.0034 2.0032
Peak area
g-Factor
2.0038
30
2.0030 20
g-factor Peak area
2.0028 2.0026
10 0
10
20
30
40
50
Time (d) 693 694
Fig. 2. The evolution of g-factor and peak area of EPR spectra as a function of
695
reaction time in the reaction system of anthracene-contaminated Fe(III)-
696
montmorillonite under anoxic and oxic conditions at room temperature (~ 25 oC).
697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718
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Environmental Science & Technology
Intensity (a.u.)
0.6 0.4 0.2 0.0
60 2.00360
a
g-factor Peak area
2.00355
-0.2
b
50 40
2.00350 2.00345
30
2.00340
20
2.00335
10
Peak area
25 oC o 40 C 50 oC 60 oC 70 oC 80 oC 90 oC
g-Factor
0.8
Page 30 of 32
-0.4 -0.6 -0.8
2.00330
0 20
1.990 1.995 2.000 2.005 2.010 2.015
1.0
25 oC 40 oC
80
100
0.8
50 oC 60 oC
0.6
70 oC 90 oC
1.0
c
d 0.8
C/C0
C/C0
60
Temprature (oC)
g-Factor
0.4
0.6 RH = 8% RH = 11% RH = 30% RH = 60% RH = 90%
0.4
0.2
0.2
0.0
0.0 0
719
40
20
40
60
80
0
20
Time (h)
40
60
80
Time (h)
720
Fig. 3. (a) EPR spectra under different reaction temperatures, and (b) Temperature
721
dependence of g-factors and peak area in the 15 d reaction system of
722
anthracene-contaminated Fe(III)-montmorillonite. The transformation of anthracene
723
as a function of reaction time at Fe(III)-montmorillonite surface under various
724
reaction temperatures (c) and relative humidity (d).
725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 30
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Environmental Science & Technology
Relative energy (kcal/mol)
1.945 TS + O2 0.355 IM1
IM2
0.142
+ H2O 0.260
TS
.
+
TS
Radical A
-13.673
-22.591 Radical B -30.517
740 741
Fig. 4. Profile of the reaction of anthracene with Fe(III). The energies of anthracene
742
complexes with Fe(III) were set to zero.
743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763
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ACS Paragon Plus Environment
Environmental Science & Technology
H Fe(III)O+
-H + H2 O
Radical A
H
OH
OH Fe(II)OH+ Fe(III)(H2O)
Fe(II)O Fe(II)OH+
Fe(II)O
Page 32 of 32
-H
Radical B
OH
OH
Fast
Intermediate B
Intermediate A
O
O
O
O Fe(II)O Fe(III)O+
Fe(III)(H2O) Fe(II)O
-H + H2 O
764
O
H
OH
Radical C
765
Scheme 1. Proposed mechanism for the transformation of anthracene and formation of
766
EPFRs on Fe(III)-modified montmorillonite.
767 768
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