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Efficient Reductive Decomposition of Perfluorooctane Sulfonate in a High Photon Flux UV/Sulfite System Yurong Gu, Wenyi Dong, Cheng Luo, and Tongzhou LIU Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03261 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016
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Efficient Reductive Decomposition of Perfluorooctane Sulfonate in a High
2
Photon Flux UV/Sulfite System
3
Yurong GU1, Wenyi DONG1,*, Cheng LUO1, Tongzhou LIU1,**
4 5 6
1. Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, P. R. China.
7
* Corresponding author, tel & fax: +86-755-2603 2718, email:
[email protected] 8
**
9
[email protected] Corresponding
author,
tel
&
fax:
+86-755-2603
2718,
email:
10
Abstract
11
Hydrated electron (eaq-) induced reduction techniques are promising for decomposing
12
recalcitrant organic pollutants. However, its vigorous reactivity with co-present
13
scavenging species and the difficulty in minimizing the competitive reactions make
14
the proportion of eaq- participating in pollutant decomposition low, reflecting by slow
15
decomposition kinetics. In this study, a high photon flux UV/sulfite system was
16
employed to promote eaq- production. Its feasibility in enhancing a notorious
17
recalcitrant pollutant, PFOS, decomposition was investigated. The effective photon
18
flux utilized for producing eaq- was 9.93×10−8 einstein/cm2·s. At initial solution pH
19
9.2, with DO about 5 mg/L, and at around 25 oC, 98% PFOS was decomposed within
20
30 min from its initial concentration of 32 µM. The kobs of PFOS decomposition was
21
0.118 min-1 (7.08 h-1), and about 8-400 folds faster than those obtained in other
22
reductive approaches. In this system, PFOS decomposition showed can tolerate
23
co-present 7 mg N/L of NO3-. Suggested by molecular orbitals and thermodynamic
24
analyses, the mechanisms responsible for PFOS decomposition involve defluorination,
25
desulfonation, and centermost C-C bond scission. By demonstrating a more practical 1
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relevant treatment process, the outcomes of this study would be helpful for facilitating
27
future applications of eaq- induced reduction techniques for efficient recalcitrant
28
pollutants decomposition.
29 30
Abstract art
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Introduction
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Perfluorinated compounds (PFCs) have been incorporated into industrial applications
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since 1950s due to their excellent thermal and chemical stability and effective
35
surfactant
36
bioaccumulative and toxic.2 Perfluorooctane sulfonate (PFOS) and perfluorooctanoate
37
(PFOA), the two predominant PFCs, receive most environmental attention, and their
38
production have been restricted in developed countries.3 However, it’s rather difficult
39
to achieve a total restriction of PFOX (X stands for S or A) in globe, because to meet
40
the still strong demand of PFCs related products, PFOX involved production and
41
manufacturing have moved to and been keeping rising in some developing countries
42
(e.g. China) since 2000s.4,5 The manufacturers in the developing countries won’t
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voluntarily replace PFOX by more environmental friendly but costly substitutes.6 As a
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consequence, their concentrations are observed increasing in the environmental media
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adjacent to these PFOX involved industrial activities.7 Investigations have revealed
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that wastewater discharge is the principal source of PFOX releasing into the
47
environment.8,9 Hence, efficient approaches for onsite decomposing PFOX present in
48
wastewater are of great importance for minimizing their release, besides the attempts
49
of limiting their industrial use.
properties,1
but
are
notorious
by
environmentally
persistent,
50
Due to their inherent recalcitrance to microbiological and ordinary chemical
51
treatment, PFOX decomposition needs advanced treatment techniques, such as
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photochemical,10 sonochemical,11 electrochemical,12 subcritical Fe0 reductive,13 and
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mechanochemical destruction.14 Amongst them, photochemical decomposition 3
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process is the most favored one because of its operational simplicity. Photochemical
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decomposition for PFOA has been investigated in many studies,10,15-17 whereas much
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fewer study focused on PFOS decomposition. In the very limited literatures, PFOS
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decomposition displayed apparent less effectiveness than PFOA. The reported PFOS
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decomposition kinetics by photochemical approaches, including direct UV
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photolysis,18,19 UV/alkaline 2-Propanol,19 UV/Fe3+ and UV/KI systems,20,21 were slow.
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In these systems half-life time (t1/2) of PFOS decomposition ranged in 87 to 7700 min.
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It’s difficult to incorporate these techniques into onsite treatment of PFOS bearing
62
wastewater which requires short retention time.
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Decomposition techniques employing hydrated electron (eaq-) are seen as an
64
efficient
approach
for
halogenated
organic
pollutants
65
detoxification.22-24 eaq- is one of the most reactive species (Eo = -2.9 V). It can act as a
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nucleophile when reacts with organic compounds. The reactivity can be greatly
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enhanced when organic molecules contain halogen atoms, leading to C-X bond
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cleavage and halide ion release.25 eaq- can be produced by pulse radiolysis of pure
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water or photolytical methods including direct photolysis and inorganic anions
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mediated photolysis, i.e. ferrocyanide mediated laser flash photolysis,26 and KI or
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sulfite mediated UV photolysis.16,27 Inorganic anions mediated photolysis showed
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higher quantum yield of eaq- than direct photolysis.28 However, ferrocyanide and KI
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mediated photolysis suffer drawbacks of limited practical application of laser flash
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photolysis and potential detrimental effects on human induced by purposefully added
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iodides. A recent comprehensive investigation carried out by Li et al. using a model 4
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compound (monochloroacetic acid, MCAA) exhibited that eaq- production through
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sulfite mediated UV photolysis (eqs 1-3) is more practical relevant and environmental
78
friendly.28 The work of Song et al. further demonstrated the feasibility of using the
79
UV/sulfite system to decompose recalcitrant pollutant (PFOA). 17
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SO32- + hν → SO3·-+ eaq-
(1)
81
SO3·- + SO3·- → S2O62-
(2)
82
SO3·- + SO3·- + H2O → SO42- + H+ + HSO3-
(3)
83
Nevertheless, due to its high standard reduction potential, besides target
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pollutants, eaq- reacts rapidly with many co-present species (such as H+, NO2-, NO3-,
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DO, and N2O) in water matrix those have more positive reduction potentials.25 It
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causes eaq- scavenging effect and hence imposes important challenges in further
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practical applications of eaq- induced reductive decomposition treatment.28 For
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example, a relatively high solution pH (pH 10.3) accompanied by N2 purging were
89
used to attenuate the eaq- quenching reactions by H+ and DO, respectively, in a
90
previous study on PFOA decomposition by a UV/sulfite system.17 Yet, to what extent
91
the co-presence of NO3-, a common constituent in wastewater, can be tolerated in the
92
UV/sulfite system during PFOX decomposition hasn’t been studied. It seems
93
unachievable to minimize eaq- scavenging effect by excluding its competing species
94
from water matrix, since they are always abundant over the target pollutants. Thus, a
95
plausible way to increase the amount of eaq- available for pollutants decomposition is
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promoting eaq- production through an efficient manner. When using radiolytic
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generation methods, increasing reaction temperature up to 300 oC29 or lowering 5
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energy deposition density in the tracks of heavy ions were reported can enhance eaq-
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production.30 Previous photolytic studies showed that eaq- quantum yield can be
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promoted by applying higher photon flux in photo-excitation.31 Because of its higher
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emission intensity and broader emission spectrum, compared to low- and medium-
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pressure UV lamps, high-pressure UV lamp is considered promising to promote eaq-
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generation, and subsequently achieve more efficient pollutants decomposition (Text
104
S1 and Table S1). To our best knowledge, systematic investigation about the
105
enhancement of recalcitrant pollutants decomposition through photolytic promoting
106
eaq- production remains sparse.
107
In the presented study, a UV/sulfite system configured with a high-pressure UV
108
lamp as the irradiation source was utilized for promoting eaq- production and applied
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for decomposing a notorious recalcitrant pollutant, PFOS. The feasibility of achieving
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efficient PFOS decomposition through enhanced eaq- production was investigated
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without controlling DO. The promoted eaq- production and the associated improved
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tolerance to co-present NO3- were examined. Additionally, PFOS decomposition in
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the studied UV/sulfite system was compared with UV/persulfate and UV/KI treatment
114
using the same high-pressure UV lamp. The mechanisms responsible for PFOS
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decomposition were discussed with the aid of molecular orbitals and thermodynamic
116
analyses. By demonstrating a more practical relevant treatment process, the outcomes
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of this study would be helpful for facilitating future applications of eaq- induced
118
reduction techniques for efficient recalcitrant pollutants decomposition.
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Experimental Section
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Chemicals. All chemicals, including PFOA (96%), PFOS (98%), perfluorobutyric
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acid (PFBA, 98%), perfluoropentanoic acid (PFPeA, 97%), perfluorohexanoic acid
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(PFHxA, 98%), perfluoroheptanoic acid (PFHpA, 99%), potassium persulfate
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(K2S2O8, > 99%), potassium iodide (KI, > 99%), Na2SO3 (98%), NaNO3 (> 99%),
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NaNO2 (> 99%), monochloroacetic acid (MCAA, 98%), K3[Fe(C2O4)3], NaOH and
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H2SO4 were obtained from commercial sources (Sigma-Aldrich and Aladdin) and
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used as received in this study. Ultrapure water (Milipore Milli-Q) was used for
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solution preparation.
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Experimental setup. PFOS photolysis decomposition experiments were carried out
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in an opened glass reactor (50 mm in diameter and 60 mm in length) without
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controlling DO in the testing solution. A high-pressure mercury UV lamp was used as
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the irradiation source assembled with a reflector, a shutter, and a timer in a closed
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box.32 UV photon flux entering the reactor can be adjusted by transmission filters with
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different density, and were determined varied in 1.98 × 10−7 to 6.6 × 10−7
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einstein/cm2·s by ferrioxalate actinometry method.33
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25 mL of testing solutions were used in the experiments, and the respective
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initial concentrations of PFOS and Na2SO3 were 32 µM and 10 mM. The initial PFOS
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concentration falls in a typical concentration range present in the untreated
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perfluorinated organic pollutants bearing industrial wastewater.34 Besides UV/sulfite
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treatment, two blank experiments with sole UV irradiation at 100% I0 (I0 = 6.6×10−7
140
einstein/cm2·s) and sole sulfite addition in the solution without UV irradiation were 7
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also carried out. Initial solution pH was 9.2 and not adjusted unless specified. In cases
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investigating the influences of pH, initial solution pH was adjusted by dropwise
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addition of diluted NaOH or H2SO4 solution and varied from pH 10.2 to 7.0. For
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comparing the effectiveness of the studied UV/sulfite reductive PFOS decomposition
145
with other photo-oxidative or -reductive treatments, two experiments employing
146
persulfate (S2O82-) as the photochemical oxidant and iodide (I-) as the eaq-
147
photo-exciting agent, respectively, were conducted using the same high-pressure UV
148
lamp. The detailed solution conditions were described in Text S2. PFOS photolysis
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decomposition experiment was started by putting the reactor 5 cm beneath the already
150
warmed UV lamp. Experimental time was varied up to 30 min. A water cooling jacket
151
was applied to maintain the solution temperature at 25 ± 3 oC throughout the
152
experiment. At the end of the experiment, the reactor was moved away from the UV
153
lamp and the testing solution was withdrawn for immediate chemical analyses. All
154
experiments were carried out in duplicate.
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In eaq- scavenging experiments, sufficient NO3- and NO2- (10 mM) were used as
156
the scavengers to examine the dominant species responsible for PFOS decomposition.
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PFOS decomposition experiments with lower concentration of NO3- at 0.5, 1, and 2
158
mM were conducted to investigate the NO3- tolerance of the studied system.
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Meanwhile, NO3- degradation and sulfite concentration remained in the solution after
160
total NO3- degradation were monitored. Due to its good photo-stability and low
161
reactivity with many reductive species (such as ·H and SO3·-), MCAA is often chosen
162
as a model compound to assess the reductive capacity of active species in a specific 8
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treatment system.25,35 In this study, MCAA was used to indirectly examine the
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promoted eaq- production in the reductive pollutant decomposition process. Detailed
165
experimental condition of MCAA decomposition is presented in Text S2.
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Analytical methods. The concentrations of PFOS and its decomposition
167
intermediates were determined using an UPLC-MS-MS analyzer (Waters). The UPLC
168
system was equipped with a BEH C18 column (2.1×50 mm, 1.7 µm). Methanol
169
(solvent A) and 2 mM CH3COONH4 (solvent B) were used as the mobile phase. The
170
injection volume was 1 µL, and column temperature was set at 40 oC. The elution
171
flow rate was maintained at 0.3 mL/min. ESI mass spectrometry in negative mode
172
was used to identify perfluorinated compounds. The source temperature and
173
desolvation temperature were 120 oC and 400 oC, respectively, and desolvation gas
174
flow was 800 L/h. Multiple reaction monitoring (MRM) mode was used for
175
identifying intermediately generated perfluorocarboxylic acids and PFOS, and the
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elution was started with 5% methanol followed by a linear increase to 95% methanol
177
at 7 min, and reversed to original conditions at 10 min.
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Fluoride concentration was measured by a fluoride ion selective electrode
179
(INESA & Scientific instrument CO., Ltd., China) with the detection limit of 0.02
180
mg/L. Solution pH was measured using a pH meter (Sartorius). UV-vis absorbance
181
spectrum of the 10 mM Na2SO3 solution was determined by a UV spectrometer
182
(Shimadzu 2450). NO3- concentration was measured using colorimetric method. DO
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in the solution was determined using an YSI 550A DO meter. Concentrations of
184
MCAA and sulfite were analyzed by ion chromatography (Dionex, ICS-5000, USA) 9
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(see Text S3 for detailed measurement procedures).
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The relative energies of dissociation fragments with respect to eaq- attached PFOS
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radical anion (C8F17SO3·2-) (△E) was calculated at the B3LYP/6-311++G** level
188
with the ZPVE correction. The molecular orbitals of PFOS are generated by using the
189
GAUSSIAN09 program,36 and the lowest unoccupied molecular orbital (LUMO) was
190
visualized by means of the Multiwfn software Version 3.3.8 with the isovalue being
191
0.050.37
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Results and Discussion
193
PFOS decomposition kinetics. Technique PFOS has been reported18,19 and was
194
verified contains linear and branched isomers (Figure S1a). At 3 min of the
195
decomposition experiment, the peak of branched PFOS disappeared, whereas that of
196
linear PFOS decreased slightly (Figure S1b). The branched isomer decomposed much
197
faster than the linear one. This observation is consistent with previous studies.18,19 It
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shall because the tertiary C-F bonds being possessed in the branched PFOS have
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much higher electron affinity, and is hence more vulnerable to reductive
200
decomposition than their straight chain analogues.38 In the presented study, only the
201
decomposition of linear PFOS was discussed in detail, and its defluorination
202
efficiency was calculated assuming all the fluoride produced within the first 3 min
203
was attributable to branched PFOS (Text S4).
204
As shown in Figure 1, neither sole high photon flux UV irradiation nor sole
205
sulfite addition in the solution achieved observable PFOS decomposition in the 30
206
min experiment. In contrast, a fast PFOS decomposition kinetics was observed in the 10
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high photon flux UV/sulfite system. Almost all PFOS (98%) was decomposed within
208
30 min from its initial concentration of 32 µM. The decomposition kinetics can be
209
well described by a pseudo first order model and the observed reaction kinetics rate
210
constant (kobs) of PFOS decomposition was 0.118 min-1 (Table 1). It’s nearly 8-400
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folds faster than PFOS decomposition by other reported reductive approaches, and the
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time span needed for reaching 50% defluorination efficiency was decreased from
213
hours to minutes (Table 2). Meanwhile, the energy required for decomposing PFOS
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using the high photon flux UV/sulfite system showed comparable with other reductive
215
techniques (Table 2). By applying the same high photon flux UV, both persulfate
216
induced photo-oxidation and iodide induced photo-reduction hadn’t observable
217
decomposition for linear PFOS within 30 min (Figure S2). It indicated the studied
218
high photon flux UV/sulfite system is much more competitive than previously studied
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photo-oxidative or -reductive approaches in regard of PFOS decomposition.
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In a supplementary experiment where NO2- and NO3- were used as the eaq-
221
scavengers,25 the co-presence of 10 mM NO2- or NO3- totally suppressed PFOS
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decomposition (Figure S3). These results confirmed that eaq- was the predominant
223
reductive species responsible for the observed fast PFOS decomposition. Other
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species those were possibly generated in the UV/sulfite system, such as ·H and SO3·-,
225
shall have negligible effect on PFOS decomposition. The ·H scavenging capacity of
226
NO2- is about 500 folds higher than that of NO3-,25 but they caused equal inhibition on
227
PFOS decomposition at the same molar concentration. It indicated ·H played a minor
228
role in PFOS decomposition. As for SO3·-, it has been demonstrated had negligible 11
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contribution to MCAA decomposition,28 let alone more recalcitrant pollutants like
230
PFOS. In the experiments with varying initial solution pH, PFOS decomposition
231
kinetics and defluorination efficiencies showed faster and higher at initial solution pH >
232
9.2, became apparently reduced when decreased to pH 8.0, and were almost totally
233
suppressed at initial solution pH 7.0 (Table 1 and Figure S4). The strong dependence
234
on initial solution pH indirectly indicated the role of eaq- in the reductive
235
decomposition process. SO32- is the strongest UV adsorption sulfite species which
236
intrinsically governs UV photolytic eaq- generation,27 and dominates at pH above 7
237
(Figure S5) (pKa2 of sulfite is 7.2).39 eaq- generation is positively correlated to SO32-
238
concentration in UV/sulfite system.17 More SO32- in solution at higher pH (pH>8.0)
239
would lead to more eaq- generation, which was reflected by the observed faster PFOS
240
decomposition kinetics and higher defluorination efficiencies. On the other hand, at a
241
lower pH, the effect of eaq- quenching by H+ to produce ·H cannot be omitted,40 which
242
in turn suppressed the generated eaq- participating in decomposition reactions.
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One point that weakens the engineering application of eaq- induced reduction
244
process is the scavenging of eaq- by NO3-,28 which is commonly present in wastewater
245
stream with a typical concentration range of 0~20 mg N/L.41 PFOS decomposition in
246
the co-presence of 0.5, 1, and 2 mM NO3- was monitored. In the presence of 0.5 mM
247
NO3- (7 mg N/L, representing a moderate level in wastewater), efficient PFOS
248
decomposition (94.1%) was still experienced (Figure S6). This result demonstrated
249
much improved tolerance to the co-present NO3- in the high photon flux UV/sulfite
250
system. In comparison, NO3- concentration at 1.3 mg N/L was observed already 12
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inhibited MCAA dechlorination by eaq- generated in a low-pressure UV/sulfite
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system.28 Fast NO3- degradation occurred in the high photon flux UV/sulfite system,
253
especially in the first 6 min, and all NO3- was degraded from its initial concentration
254
of 0.5 mM at 15 min of the experiment (Figure S7). The obvious suppression on
255
PFOS decomposition in the first 6 min (Figure S6) echoed the strong competition of
256
eaq- by NO3-. After the total NO3- degradation, sulfite concentration remained in the
257
solution was determined as 0.57 mM. When the concentration of co-present NO3-
258
increased to 1 and 2 mM, respectively, remarkable and even total suppression on
259
PFOS decomposition was recorded in the 30 min experiment (Figure S6).
260
All the PFOS photolysis decomposition experiments were carried out in an
261
opened reactor. Initial DO in the solution was around 5 mg/L, and not controlled
262
throughout the experiment. Regardless of DO presence, the observed fast PFOS
263
decomposition and the improved tolerance to the co-present 0.5 mM NO3-
264
demonstrated the powerful treatability of the studied high photon flux UV/sulfite
265
system, and indicated its flexibility in practical application where excluding DO is
266
difficult. Detailed investigation on the effects of DO is important for further
267
optimizing this process, and is under way.
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Efficient PFOS decomposition attributable to high photon flux UV. The fast PFOS
269
decomposition and the improved tolerance to the co-present NO3- indicated the
270
abundance of eaq- generated in the high photon flux UV/sulfite system. PFOS
271
decomposition was further investigated by varying UV irradiation intensity, where
272
initial solution pH was kept at pH 9.2. PFOS decomposition followed a pseudo first 13
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order model well (Figure 2a), but kobs decreased from 0.118 min-1 to 0.020 min-1 when
274
UV intensity was adjusted from 100% I0 to 30% I0 (Table 1). PFOS defluorination
275
efficiencies were also observed decreased correspondingly (Figure 2b). In a
276
supplementary experiment (Figure S8), co-present 0.5 mM NO3- completely
277
suppressed PFOS decomposition under 30% I0, whereas it almost had no influence on
278
the final PFOS decomposition efficiency under 100% I0. It further attested that
279
applying high photon flux is essential for generating abundant eaq- in the studied
280
system.
281
eaq- is a very active reductant. In the studied system, H+, H2O, S2O62-, SO3-,
282
HSO3-, O2 and eaq- itself can be the species that react rapidly with eaq-.25, 26,42-45 They
283
exhibited 2~3 order faster reaction kinetics with eaq- than PFOA and PFOS (Table S2).
284
Because of these competing reactions, the proportion of the generated eaq- available
285
for PFOS decomposition was very low. It was reportedly about 0.1% by Park et al.21
286
and was calculated as 0.01% in the presented study (Text S5 and Figure S9). Since the
287
strong eaq- competing species are always present and abundant in the treated water, it
288
is very difficult to increase eaq- availability for PFOS decomposition by minimizing
289
the competitive reactions. Hence, increasing eaq- availability through significant
290
promotion on its production becomes a plausible way. The UV system used in this
291
study showed great promotion on eaq- production. By employing a high-pressure
292
mercury lamp and configuring a quartz reflector,32 the UV irradiation source has high
293
luminous efficiency. The photon flux in the studied UV system was determined as 6.6
294
×10−7 einstein/cm2·s. Given the effective wavelengths (200-260 nm) for producing 14
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eaq- through exciting SO32- account for 15% of the total photon flux (Text S6 and
296
Figure S10 & S11), the calculated effective photon flux being utilized for producing
297
eaq- is 9.93×10−8 einstein/cm2·s. When using MCAA as the model compound to
298
assess the reductive capacity of the active species in the studied system, the kobs of
299
MCAA decomposition obtained was found nearly 1250-2150 folds higher than those
300
values reported by other reductive approaches (Table S3). Given eaq- has been
301
identified as the predominant reductive species, this result indicated the plenty of eaq-
302
available for reductive pollutant decomposition. Besides its higher irradiation intensity,
303
comparing to low UV pressure lamp that mainly emits at 254 nm, the broader
304
emission spectrum (200-400 nm) (Figure S11) of the high-pressure UV lamp can also
305
benefit the eaq- induced reductive PFOS decomposition. Because sulfite can absorb
306
UV wavelength up to 260 nm, and absorbs strongly around 220 nm in alkaline
307
solution (Figure S10), a broader emission spectrum would enhance photo-excitation
308
of sulfite and then promote eaq- generation.
309
PFOS decomposition mechanisms. When being attacked by eaq-, complicated
310
reaction processes including defluorination, desulfonation, and centermost C-C bond
311
scission may involve in PFOS decomposition.18,19,21,46 Reaction of initial PFOS anion
312
with eaq- yields PFOS·2- (C8F17SO3·2-) (eq 4). It further dissociates and produces
313
different fragments. Based on the calculated relative energies of dissociation
314
fragments with respect to C8F17SO3·2- (△E) (Table 3), C8F17- is the most likely
315
produced species with the lowest △E. It shall be due to the lower bond energy of C-S
316
(272 kJ/mol) compared with that of C-C (346 kJ/mol). The cleavage of C-S bond of 15
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PFOS has been confirmed in a previous study with observed increasing of sulfate ion
318
concentration in a catalyst-free UV system.46
319
The dissociated C8F17- fragment was likely to transform to PFOA through
320
hydrolysis reactions (eqs 5-8). PFOA concentration showed increased and then
321
decreased quickly in the first 15 min of the experiment (Figure 3), indicating its
322
generation and the following fast decomposition. PFOA was observed decomposed
323
fast within 10 min under the same conditions of PFOS decomposition (Figure S12).
324
Its kobs was recorded 0.452 min-1, nearly 4 folds faster than that of PFOS. Besides
325
PFOA, concentration variation of other short chain PFCAs, including PFHpA,
326
PFHxA, PFPeA and PFBA, during the experiment were determined (Figure 3),
327
indicating the occurrence of stepwise defluorination (eqs 9-14).16,17 In an experiment
328
increasing the reaction time from 30 min (within which almost all PFOS (98%) was
329
decomposed, Figure 1) to 60 min, fluoride concentration in the testing solution was
330
observed kept increasing. The final defluorination efficiency was near 70%. It
331
evidenced that other F-containing intermediates still underwent defluorination
332
reactions after the total PFOS decomposition.
333
C8F17SO3- + eaq- → C8F17SO3·2-
(4)
334
C8F17SO3·2- → C8F17- + SO3·-
(5)
335
C8F17- + H3O+→ C8F17OH + ·H
(6)
336
C8F17OH → C7F15COF + HF
(7)
337
C7F15COF + H2O → C7F15COOH + HF
(8)
338
C7F15COOH + eaq- →·C7F14COOH + F-
(9) 16
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·C7F14COOH + H2O → C7F14HCOOH + ·OH
(10)
340
C7F14HCOOH + eaq- →·C7F13HCOOH + F-
(11)
341
·C7F13HCOOH + H2O → C7F13H2COOH + ·OH
(12)
342
C7F13H2COOH →·C6F13 + ·CH2 + ·COOH
(13)
343
·C6F13 + ·COOH → C6F13COOH
(14)
344
Notably, the concentration of PFBA was extraordinary higher than all other
345
PFCAs, suggesting its production might involve other mechanisms. LUMO of PFOS
346
is mainly located on the moiety of C4, C5, C6, C7 and C8 atoms in the perfluoroalkyl
347
chain with clear sigma anti-bonding nature (Figure 4). Once an external electron is
348
attached to PFOS anion, it would tend to localize in the region over C4, C5, C6, C7
349
and C8 atoms, consequently introduce reaction driving energy there, and weaken the
350
corresponding C-C sigma bonds. Moreover, as shown in Table 3, C6F13-, C5F11-, C4F9-
351
and C3F7- showed lower △E with respect to C8F17SO3·2- than other dissociation
352
fragments (i.e. C7F15-, C2F5-, CF3-), indicating their relatively thermodynamically
353
favorable formation. Based the molecular orbitals and thermodynamic analyses, the
354
formation of C3F7-, C4F9-, and C5F11- shall be the most favorable in C8F17SO3·2-
355
dissociation. It has been revealed that the external attached electrons prefer to locate
356
on the centermost C-F bond of linear perfluoroalkane,38 and C3F7- is likely to be
357
generated through reductive C-C bond scission involving the dianion produced from
358
C8F17SO3·2- dissociation from the α-positon (eq 15-17).21 The generated C3F7- would
359
further recombine with ·COOH that is produced in PFOA stepwise defluorination and
360
result in the formation of PFBA (eq 18). The carbanion produced in eq 16 retaining 17
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361
sulfonate terminal group will be protonated in presence of water and undergo further
362
H/F exchange by reacting with eaq- (eq 19).21
363
C8F17SO3·2-→C8F16SO3·- + F-
(15)
364
C8F16SO3·- + eaq-→C8F16SO32-
(16)
365
C8F16SO32- (CF3(CF2)3CF-(CF2)3SO3-) → CF3CF2 CF2-+CF2=CF(CF2)3SO3-
(17)
366
CF3CF2CF2- + ·COOH + ·OH→CF3CF2CF2COOH + OH-
(18)
367
C8F16SO32- + H2O→C8F16HSO3- + OH-
(19)
368
Technical implication. As one of the most reactive species, eaq- is promising for
369
reductively decomposing recalcitrant organic pollutants, such as PFOS. However, due
370
to its vigorous reactivity with co-present scavenging species (e.g. H+, H2O, DO, and
371
NO3-) and the difficulty in minimizing the competitive reactions, the proportion of eaq-
372
available for decomposing the target pollutant is very low, leading to slow
373
decomposition kinetics. This study demonstrated the feasibility of enhancing PFOS
374
decomposition through significantly promoting eaq- production by using a high photon
375
flux UV/sulfite system.
376
It achieved almost complete PFOS decomposition within 30 min from its initial
377
concentration of 32 µM and can tolerate co-present 0.5 mM (7 mg N/L) NO3-, a
378
common constituent and typical eaq- scavenger in wastewater. This approach is more
379
practical relevant. It uses a modified UV lamp as the irradiation source. Unlike other
380
eaq- photo-exciting agents (e.g. KI and ferrocyanide), the product of sulfite after the
381
reaction is sulfate having much less adverse environment effects. The reaction
382
condition is mild. By applying an alkaline pH of 9.2, at an initial DO about 5 mg/L, 18
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and maintaining solution temperature at around 25 oC, a fast PFOS decomposition rate
384
constant of 0.118 min-1 (7.08 h-1) was achieved. Suggested by molecular orbitals and
385
thermodynamic analyses, the mechanisms responsible for PFOS decomposition
386
involve defluorination, desulfonation, and centermost C-C bond scission. Successful
387
application of the studied system is of course dependent on many influencing factors,
388
so further detailed investigations on the effects of wastewater matrix (e.g. DO and
389
natural organic matter), hydraulics, as well as, optimizing approaches are underway.
390
Furthermore, investigations concerning the toxicity and biodegradability of the
391
decomposition products shall be carried out.
392
Associated Content
393
Supporting Information
394
Texts S1−S6, Figures S1−S12, and Table S1-S3. This information is available free of
395
charge via the Internet at http://pubs.acs.org.
396
Author Information
397
Corresponding Authors
398
*(Wenyi.Dong.) Phone & fax: +86-755-2603 2718; e-mail:
[email protected].
399
** (Tongzhou.Liu.) Phone & fax: +86-755-2603 2718;
400
e-mail:
[email protected].
401
Acknowledgement
402
The authors thank professor Chaolin LI and Qian ZHANG for providing the high 19
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photon flux UV lamp used in this study. This research was financially supported by
404
the Major Science and Technology Program for Water Pollution Control and
405
Treatment of China (Grant No. 2015ZX07206-006-04).
406
References
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photodecomposition of aqueous perfluorooctane sulfonate (PFOS) under UV
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irradiation and its mechanism. J. Hazard. Mater. 2014, 271, 9-15.
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a new class of treatment processes. Environ. Eng. Sci. 2013, 30 (5), 264-271.
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Liu, X. W. Efficient reductive dechlorination of monochloroacetic acid by
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review of physicochemical properties, levels and patterns in waters and
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Comput. Chem. 2012, 33 (5), 580-592. (38) Paul, A.; Wannere, C.S.; Schaefer, H. F. Do linear-chain perfluoroalkanes bind an electron? J. Phys. Chem. A. 2004, 108, 9428-9434. (39) Tartar, H. V.; Garretson, H. H. The Thermodynamic ionization constants of sulfurous acid at 25 oC. J. Am. Chem. Soc. 1941, 63, 808-816.
518
(40) Qu, Y.; Zhang, C. J.; Chen, P.; Zhou, Q.; Zhang, W. X. Effect of initial solution
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pH on photo-induced reductive decomposition of perfluorooctanoic acid.
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Chemosphere. 2014, 107, 218-223. 23
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521 522 523 524
(41) Tchobanoglous, G.; Burton, F. L.; Stensel, H. D., Eds. Wastewater engineering
treatment and reuse, 4th, ed.; Metcalf & Eddy. Inc: USA, 2003. (42) Dogliott, L.; Hayon, E. Flash photolysis study of sulfite, thiocyanate, and thiosulfate ions in solution. J. Phys. Chem. 1968, 72 (5), 1800-1807
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(43) Hayon, E.; Treinin, A.; Wilf, J. Electronic spectra, photochemistry, and
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autoxidationmechanism of the sulfite-bisulfite-pyrosulfite systems. The SO2-,
527
SO3-, SO4-, and SO5- radicals. J. Am. Chem. Soc. 1972, 94 (1), 47-57.
528 529 530 531
(44) Dester, U.; Warneck, P. Photooxidation of SO32- in aqueous solution. J. Phys.
Chem. 1990, 94 (5), 2191-2198. (45) Fischer, M.; Warneck, P. Photodecomposition and Photooxidation of Hydrogen Sulfite in Aqueous Solution. J. Phys. Chem. 1996, 100 (37), 15111-15117.
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(46) Lyu, X. J.; Li, W. W.; Lam, P. S.; Yu, H. Q. Insights into perfluorooctane
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sulfonate photodegradation in a catalyst-free aqueous solution. Sci. Rep. 2015, 5,
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1-6.
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Caption of Tables and Figures
536
Table 1.
537
UV/sulfite system at varied initial solution pH or varied UV emission intensity.
538
Table 2.
539
reported photo-reductive treatments
540
Table 3.
541
Figure 1.
542
irradiation, and with sole sulfite addition in the solution
543
Figure 2.
544
UV emission intensity. (I0 = 6.6×10−7 einstein/cm2·s).
545
Figure 3.
Intermediate products detected during PFOS decomposition
546
Figure 4.
Lowest unoccupied molecular orbital (LOMO) of PFOS
PFOS decomposition kinetics rate constants in the high photon flux
Comparison of reductive PFOS decomposition in this study with other
Relative energies of dissociation fragments with respect to C8F17SO3·2PFOS decomposition in the UV/sulfite system, with sole UV
(a) PFOS decomposition, and (b) defluorination efficiency at varied
547 548
25
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549
Tables and Figures
550
Table 1. PFOS decomposition kinetics rate constants in the high photon flux
551
UV/sulfite system at varied initial solution pH or varied UV emission intensity. Initial solution pH a 10.2 9.2 8.0 7.0
kobs (min )
t1/2 (min)
R
0.126±0.002 0.118±0.004 0.026±0.001 n.a.
5.50 5.87 26.66 n.a.
0.9746 0.9702 0.8032 n.a.
-1
2
UV intensity (% I0) b
kobs (min-1)
t1/2 (min)
R2
100 75 50 30
0.118±0.004 0.059±0.001 0.036±0.001 0.020±0.002
5.87 11.75 19.25 34.66
0.9702 0.9896 0.9562 0.9658
Note -7 -2 -1 a. UV intensity was kept at 100% I0 = 6.6×10 einstein cm s . b. Initial solution pH was kept at 9.2.
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Table 2. Comparison of reductive PFOS decomposition in this study with other
553
reported photo-reductive treatments Method
Direct UV (catalystfree)
Direct UV
UV-isopro panol
UV-KI
UV/sulfite
Conditions [PFOS]=37.2 µM PBS:6.0 mM pH=11.8, 100 oC V=1000 mL MPMLd: 500 W [PFOS]=20 µM pH=12.5, 25 oC V=400 mL LPMLe: 23W (185nm) [PFOS]=40 µM [NaOH]=68 mM V=750 mL (isopropanol) 38–50 oC LPMLe: 32W [PFOS]=20 µM [KI]=10 mM V=30 mL ambient temperature LPMLe: 8W [PFOS]=32 µM [Na2SO3]=10 mM V=25 mL pH 9.2, 25 oC HPMLf: 250W
ka (h-1)
0.91
0.0175
Timeb (h)
-
> 48
Energyc (kJ /µmol)
Reference
73.7
25
-
17
0.039
-
137
18
0.18
> 2.5
370
20
7.08
0.5
220
This work
Note a. Pseudo-first-order rate constants b. Reaction time needed for achieving 50% defluorination efficiency c. Energy consumptions required to decompose PFOS to half of its initial concentration d. Medium-pressure mercury lamp with a low UVC luminous efficiency, which also acted as the heat source e. Low-pressure UV lamp mainly emits at 254 nm unless otherwise specified f. High-pressure UV lamp with a high UVC luminous efficiency mainly emits at 200-400 nm
27
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Table 3. Relative energies of dissociation fragments with respect to C8F17SO3·2△E (kcal/mol)
Dissociation fragments C8F17 C7F15
-
-
SO3·
-
-30.99372
CF2SO3·
-
-11.85824 -16.92018
C6F13
-
C2F4SO3·
-
C5F11
-
C3F6SO3·
-
-16.66284
-
-
-16.50709 -15.42721
C4F9
C4F8SO3·
C3F7
-
C5F11SO3·
-
C2F5
-
C6F12SO3·
-
-11.37048
-
-
-2.866026
CF3 F
-
C7F14SO3· C8F16SO3·
-
-12.03953
28
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1.0
80
0.8
C/C0
60 0.6 sole UV - C/C0
0.4
40
sole sulfite - C/C0 UV/sulfite - C/C0
20 UV/sulfite defluorination efficiency
0.2
0.0 0
5
10
15
20
25
Defluorination efficiency (%)
100
1.2
0 30
555
Time (min)
556
Figure 1. PFOS decomposition in the UV/sulfite system, with sole UV irradiation,
557
and with sole sulfite addition in the solution.
29
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558
(a) 1.0
0.8
C/C0
0.6
0.4 100% I0 75% I0 50% I0
0.2
30% I0
0.0 0
5
10
15
20
25
30
20
25
30
Time (min)
(b)
80
Dfluorination efficiency (%)
100% I0 75% I0 50% I0
60
30% I0
40
20
0 0
5
10
15
Time (min)
559 560
Figure 2. (a) PFOS decomposition, and (b) defluorination efficiency at varied UV
561
emission intensity. (I0 = 6.6×10−7 einstein/cm2·s).
562 30
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0.14 PFOA PFHpA PFHxA PFPeA PFBA
0.12
C (µM)
0.10
0.08
0.06
0.04
0.02
0.00 0
563 564
10
20
30
40
Time (min)
Figure 3. Intermediate products detected during PFOS decomposition.
565 566
Figure 4. Lowest unoccupied molecular orbital (LOMO) of PFOS.
567 568
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