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Article
UV/nitrilotriacetic acid process as a novel strategy for
efficientphotoreductive degradation of perfluorooctane
sulfonate
Zhuyu Sun, Chaojie Zhang, Lu Xing, Qi Zhou, Wenbo Dong, and
Michael R HoffmannEnviron. Sci. Technol., Just Accepted Manuscript
• DOI: 10.1021/acs.est.7b05912 • Publication Date (Web): 03 Feb
2018
Downloaded from http://pubs.acs.org on February 5, 2018
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1
UV/nitrilotriacetic acid process as a novel strategy for 1
efficient photoreductive degradation of perfluorooctane 2
sulfonate 3
4
Zhuyu Sun,†‡
Chaojie Zhang,*†‡
Lu Xing,†‡
Qi Zhou,†‡
Wenbo Dong,&
5
Michael R. Hoffmann§ 6
†State Key Laboratory of Pollution Control and Resources Reuse,
College 7
of Environmental Science and Engineering, Tongji University,
Shanghai 8
200092, China 9
‡ Shanghai Institute of Pollution Control and Ecological
Security, 10
Shanghai 200092, China 11
&Shanghai Key Laboratory of Atmospheric Particle Pollution
and 12
Prevention, Department of Environmental Science &
Engineering, Fudan 13
University, Shanghai 200433, China 14
§Linde-Robinson Laboratories, California Institute of
Technology, 15
Pasadena, California 91125, United States 16
*Corresponding author. Tel: +86 21 65981831; fax: +86 21
65983869; 17
E-mail address: [email protected] 18
19
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Abstract: Perfluorooctane sulfonate (PFOS) is a toxic,
bioaccumulative and highly 20
persistent anthropogenic chemical. Hydrated electrons (eaq–) are
potent nucleophiles 21
that can effectively decompose PFOS. In previous studies, eaq–
are mainly produced 22
by photoionization of aqueous anions or aromatic compounds. In
this study, we 23
proposed a new photolytic strategy to generate eaq– and in turn
decompose PFOS, 24
which utilizes nitrilotriacetic acid (NTA) as a photosensitizer
to induce water 25
photodissociation and photoionization, and subsequently as a
scavenger of hydroxyl 26
radical (·OH) to minimize the geminate recombination between ·OH
and eaq–. The net 27
effect is to increase the amount of eaq– available for PFOS
degradation. The UV/NTA 28
process achieved a high PFOS degradation ratio of 85.4% and a
defluorination ratio of 29
46.8% within 10 h. A pseudo first-order rate constant (k) of
0.27 h-1
was obtained. The 30
laser flash photolysis study indicates that eaq– is the dominant
reactive species 31
responsible for PFOS decomposition. The generation of eaq– is
greatly enhanced and 32
its half-life is significantly prolonged in the presence of NTA.
The electron spin 33
resonance (ESR) measurement verified the photodissociation of
water by detecting 34
·OH. The model compound study indicates that the acetate and
amine groups are the 35
primary reactive sites. 36
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1. Introduction 37
Perfluorooctane sulfonate (PFOS, C8F17SO3−) is a useful
anthropogenic chemical 38
that has been extensively utilized in industrial and commercial
applications for 39
decades.1,2
However, due to its high toxicity, environmental persistence,
40
bioaccumulation and global distribution, PFOS is considered
hazardous to 41
environmental and human health.3,4
Moreover, because of the high thermal and 42
chemical stability of C–F bond (~116 kcal/mol, almost the
strongest in nature2), PFOS 43
is extremely resistant to traditional chemical and biological
degradations. 44
Recently, hydrated electrons (eaq–) mediated photoreductive
approaches have 45
garnered special attention for PFOS decomposition owing to their
extraordinarily high 46
efficiency and relatively mild conditions. However, in order to
produce sufficient 47
amount of eaq– to defluorinate PFOS, chemicals such as
iodide
5, sulfite
6, chloride
7 and 48
indole derivatives8 are essential. Lyu et al. developed a
catalyst-free PFOS 49
photodecomposition method, but it relied on using strong
alkaline conditions (pH = 50
11.8) and high temperatures (100 °C).9 Furthermore, due to the
rapid reaction between 51
eaq– and oxygen (Eq. 1, rate constant = 1.9×10
10 M
-1·s
-1)10
, the reduction efficiency of 52
eaq– is significantly affected by dissolved oxygen (O2).
Thereby, previous eaq
–53
-mediated photoreductive approaches normally require strict
anoxic conditions.9, 11
In 54
view of the drawbacks of current photoreductive technologies, we
were motivated to 55
propose a new strategy for PFOS removal, which can be conducted
under more 56
environmentally relevant conditions and with minimization of
undesirable 57
byproducts. 58
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In addition to electron photodetachment from anions and aromatic
compounds12,
59
13, UV photolysis of liquid water generates eaq
– as well (Eq. 2).
14, 15 However, direct 60
photoionization of liquid water under 254 nm irradiation is
difficult due to a 61
negligible absorption coefficient at λ > 200 nm.16
In addition, the quantum yield of 62
eaq– from water photoionization is low because the radical
species undergo significant 63
geminate recombination. In pure water photolysis, energy
deposition takes place in 64
well-separated local volumes called spurs, where the primary
products including eaq–, 65
hydroxyl radicals (·OH), hydrogen atoms (·H) and H3O+ are formed
in close vicinity 66
with high initial local concentrations.17
However, as the spurs expand through 67
diffusion, a large proportion of the primary products undergo
very efficient back 68
reactions (Eq. 3-5). Thus, only a small fraction of eaq–
actually escape into the bulk 69
solution to initiate reductive chemical processes.10
Therefore, there is scarcely any net 70
generation of eaq– in pure water photolysis upon low energy
excitation. Based on these 71
facts, we now propose a new strategy to enhance the generation
of eaq–, i.e., by 72
minimizing the geminate recombination of eaq– with oxidative
species after inducing 73
water photoionization. 74
���– + O� → O�
∙ (1)
H�O��� ���
– +∙ OH +∙ H + H�O� (2)
���– +∙ OH → OH (3)
���– +∙ H → H� + OH
(4)
���– + H�O
� →∙ H + H�O (5)
Aminopolycarboxylic acids (APCAs) are compounds that contain
several 75
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carboxylate groups bound to one or more nitrogen atoms. Due to
their excellent 76
chelating properties18
, interests in previous studies were mainly focused in their
77
chelation with metal ions.19-21
Furthermore, APCAs contain several donor atoms 78
whose free electron pairs are easily attacked by ·OH. Leitner et
al.22
reported that the 79
rate constants for the reactions of APCAs with ·OH can exceed
1010
M-1
·s-1
. These 80
near diffusion controlled values are 2 to 3 orders of magnitude
higher than reactions 81
of APCAs with eaq–. Therefore, APCAs are considered to be
excellent candidates to 82
scavenge ·OH, and thus increase the apparent quantum yields of
eaq– and in turn 83
promote the photoreductive degradation of PFOS. 84
In this study, we chose to use nitrilotriacetic acid (NTA) as a
representative 85
APCA to explore its impact on the photoreductive degradation of
PFOS. NTA is the 86
first chemical synthetic APCA23
and is more biodegradable than other APCAs24
. In 87
order to unravel the underlying reaction mechanisms, laser flash
photolysis, ESR 88
measurement and a model compound study were carried out.
Furthermore, the effects 89
of NTA concentration, pH and air were investigated. To our best
knowledge, this is 90
the first study that utilizes APCAs as ·OH scavengers to promote
the photoreductive 91
degradation of refractory organic pollutants. The findings in
this study can provide a 92
new strategy for PFOS remediation and a novel application field
for APCAs. 93
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2. Materials and methods 94
2.1. Chemicals 95
Perfluorooctanesulfonic acid (PFOS, ~40% in water), HPLC grade
methanol 96
(≥99.9%), 5,5-Dimethyl-1-pyrroline N-oxide (DMPO, ≥98.0%) and
model 97
compounds including trisodium citrate (≥99.0%),
ethylenediamine-N,N’-disuccinic 98
acid trisodium salt solution (EDDS, ~35% in water) and
methylglycine diacetic acid 99
(MGDA, ≥99.0%) were purchased from Sigma-Aldrich Chemical Co.
(St. Louis, Mo, 100
USA). Nitrilotriacetic acid (NTA, ≥98.5%), ammonium hydroxide
solution (25%), 101
ammonium chloride (≥99.8%), ethylenediaminetetraacetic acid
(EDTA, ≥99.5%), 102
iminodiacetic acid (IDA, ≥98.0%), glycine (≥99.5%), oxamic acid
(≥98.0%) and 103
sodium oxalate (≥99.8%) were obtained from Sinopharm Chemical
Reagent Co. 104
(Shanghai, China). HPLC grade ammonium acetate (97.0%) was
purchased from 105
TEDIA (Fairfield, OH, USA). Sodium perfluoro-1-[1,2,3,4-13
C4]octanesulfonate 106
(MPFOS, ≥99%, 13
C4) acquired from Wellington Laboratories Inc. (Guelph, ON,
107
Canada) was used as the internal standard for the quantification
of PFOS. Milli-Q 108
water and deionized water were used throughout the whole
experiment. 109
2.2. Reductive defluorination 110
The photoreductive degradation of PFOS was conducted under
anoxic conditions 111
in a stainless steel cylindrical reactor (Fig. S1, SI). The
outer and inner diameters of 112
the reactor were 100 mm and 60 mm, respectively. A low-pressure
mercury lamp (14 113
W, Heraeus, Germany) with quartz tube protection was placed in
the center of the 114
reactor, emitting 254 nm UV light. 720 mL solution was added to
the reactor. Before 115
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the reaction, the mixture was bubbled with highly purified
nitrogen for 20 mins to 116
remove oxygen. The solution pH was adjusted with ammonium
hydroxide-ammonium 117
chloride (NH3·H2O-NH4Cl) buffer. The reaction temperature was
held constant at 30
118
oC by the circulating cooling system. After various time
intervals, samples of the 119
liquid were analyzed after filtration through 0.22 µm nylon
filter (ANPEL Laboratory 120
Technologies, Shanghai, China). 121
2.3. Analytical methods 122
The concentrations of PFOS and possible aqueous-phase
intermediate analytes 123
were determined by high-performance liquid chromatography/tandem
mass 124
spectrometry (HPLC−MS/MS, TSQTM
Quantum AccessTM
, Thermo Finnigan, San 125
Jose, CA, USA). Ion Chromatography (Dionex, ICS-3000, Thermo
Fisher Scientific, 126
USA) equipped with a conductivity detector was used for the
analysis of fluoride, 127
nitrate and short chain organic acids. NTA and its degradation
products including IDA, 128
oxamate and oxalate were detected by UPLC−MS/MS (ACQUITY
UPLC&SCIEX 129
SelexION Triple Quad 5500 System, Waters, MA, USA). More
detailed information 130
is available in the SI. 131
2.4. Laser flash photolysis experiment and electron spin
resonance (ESR) 132
measurement 133
Nanosecond laser flash photolysis for the detection of eaq– was
performed using a 134
Quanta Ray LAB-150-10 Nd:YAG laser at an excitation wavelength
of 266 nm. The 135
components of the laser flash photolysis apparatus were as
described by Ouyang et 136
al..25
Detailed information is available in the SI. 137
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The ESR spectra were obtained on a Bruker EMXplus-10/12 ESR
spectrometer 138
at room temperature. The instrumental parameters for ESR
analysis were as follows: 139
microwave frequency, 9.852 GHz; microwave power, 20 mW;
modulation amplitude 140
1 G; modulation frequency, 100 kHz; center field, 3500 G; sweep
width, 100 G. The 141
simulations of ESR spectra were obtained with the use of Spinfit
in Xenon software. 142
Details are provided in the SI. 143
144
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3. Results and Discussion 145
3.1. NTA-assisted photoreductive degradation and defluorination
of PFOS 146
147
Figure 1. The time profiles of PFOS degradation (a) and
defluorination (b) under different 148
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conditions. The treatment experiment: PFOS (0.01 mM), NTA (2
mM), UV irradiation, N2 149
saturated, pH (10.0); the 1st control experiment (direct
photolysis): PFOS (0.01 mM), UV 150
irradiation, N2 saturated; the 2nd
control experiment (UV/buffer): PFOS (0.01 mM), UV irradiation,
151
N2 saturated, pH (10.0); the 3rd
control experiment (without UV irradiation): PFOS (0.01 mM),
152
NTA (2mM), N2 saturated, pH (10.0); the 4th
control experiment (UV/sulfite): PFOS (0.01 mM), 153
SO32-
(2mM), UV irradiation, N2 saturated, pH (10.0). Error bars
represent standard deviations of 154
triplicate assays. 155
The photochemical decomposition of PFOS was conducted under 254
nm light 156
irradiation in the presence of NTA under anoxic condition. In
order to determine the 157
effect of each parameter, four control experiments were
conducted. The results of 158
experiments under different conditions are shown in Fig. 1.
Since PFOS has a weak 159
absorption at 254 nm and the quantum yield of eaq– by pure water
photolysis is 160
negligible26
, the direct photolysis of PFOS was poor. Only 12.9% of the
initial PFOS 161
was decomposed after 10 h of irradiation, with a low
defluorination ratio of 3.7%. 162
Adjusting solution pH to 10.0 with NH3·H2O-NH4Cl buffer somewhat
enhanced the 163
decomposition of PFOS, with the 10-h PFOS degradation and
defluorination ratios 164
increased to 20.9% and 9.9%, respectively. Compared with the
UV/buffer process, 165
addition of 2 mM NTA significantly accelerated the degradation
and defluorination of 166
PFOS. After 10 h of irradiation, the degradation and
defluorination ratios of PFOS in 167
the presence of NTA were 85.4% and 46.8%, respectively. In
particular, PFOS 168
degraded rapidly during the initial 0.5 h. The 0.5-h PFOS
photodegradation and 169
defluorination ratios by UV/NTA process achieved 45.1% and
18.6%, respectively, 170
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which are 5.2-fold and 8.5-fold, respectively higher than by
UV/buffer process. The 171
results of the 3rd
control experiment indicate that PFOS does not degrade without
UV 172
irradiation, even in the presence of NTA. Therefore, both UV and
NTA are necessary 173
for the efficient degradation and defluorination of PFOS.
174
Gu et al. (2016) demonstrated that UV/sulfite system exhibited a
high efficiency 175
in decomposing PFOS.6 In order to better evaluate the efficiency
of UV/NTA process 176
for PFOS degradation, the newly developed approach was compared
with UV/sulfite 177
process (the 4th
control). The 10-h degradation and defluorination ratios of PFOS
by 178
UV/sulfite process were 60.1% and 29.6%, respectively, which
were 0.7-fold and 179
0.63-fold, respectively lower than by UV/NTA process. PFOS
degradation by the 180
UV/NTA process follows pseudo first-order kinetics (Fig. S4,
SI), with an apparent 181
reaction rate constant (k) of 0.27 h-1
, and a half-life (t1/2) of 2.6 h. The k value for the 182
UV/NTA process was higher than other photochemical approaches
including UV/KI 183
process5, UV/sulfite process, UV/Fe
3+ process
27, UV/alkaline 2-propanol process
26 184
and UV/K2S2O8 process28
(Table S1, SI). Therefore, UV/NTA process exhibited a 185
high efficiency in the degradation and defluorination of PFOS.
186
As PFOS decomposed, short-chain-length perfluorinated
intermediates including 187
PFBS, PFBA, PFHxS, PFHxA, PFHpA and PFOA were detected (Fig. S5,
SI). Based 188
on the concentrations of PFOS, intermediates and F−, the mass
balance of F was 189
calculated (Fig. S6, SI). The loss of F recovery during the
reaction implies the 190
formation of partially fluorinated intermediates. Therefore,
based on the product 191
distribution and F balance, three possible degradation pathways
of PFOS are likely to 192
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occur in UV/NTA process: a) PFOS undergoes direct defluorination
to form 193
polyfluorinated intermediates; b) PFOS firstly undergoes
desulfonation to form PFOA, 194
and then degrades to shorter-chain-length perfluoroalkyl
carboxylic acids (PFCAs); c) 195
C-C bond in PFOS molecules cleaves, in which case
short-chain-length 196
perfluoroalkane sulfonates (PFSAs), such as PFHxS and PFBS, were
produced. The 197
proposed PFOS degradation pathways in UV/NTA process are similar
with other 198
photochemical processes5, 9, 26
, and accord well with the mechanisms suggested by 199
molecular orbitals and thermodynamic analyses6. 200
As mentioned above, most photochemical technologies for PFOS
degradation are 201
faced with challenges of secondary pollution due to by-product
formation. In order to 202
evaluate the potential post-treatment impact of the UV/NTA
process, the degradation 203
products of NTA were quantified. As is shown in Fig. S7, SI, NTA
degraded rapidly 204
during the first 1 h of PFOS degradation. 97.7% of the initial
NTA decomposed within 205
1 h, along with the concomitant formation of three intermediates
including IDA, 206
oxamate and oxalate. The maximum concentrations of IDA, oxamate
and oxalate 207
were 0.75 mM, 0.26 mM and 0.68 mM, respectively, observed at 0.5
h, 2 h and 6 h, 208
respectively. IDA and oxamate underwent nearly complete
decomposition after 10 h, 209
with their concentrations falling below 0.03 mM. Oxalate was
completely 210
decomposed after 14 h (data not shown). Ammonium (NH4+) and
nitrate (NO3
−) were 211
the major N-containing end products. Based on the product
distribution and the NTA 212
photooxidation mechanism in literatures29, 30
, a possible NTA decomposition pathway 213
is illustrated in Fig. S8, SI. The end products of NTA
degradation are NH4+, NO3
− and 214
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carbon dioxide (CO2), which are innocuous and can be easily
removed by biological 215
transformation. Furthermore, NTA is reported to have relatively
low environmental 216
risks to sewage treatment or aquatic life.18, 31
It is readily biodegradable by natural 217
microbial processes, and the biodegradation is complete without
accumulation of 218
unwanted intermediates.24
In contrast, the secondary pollutions of other approaches
219
appear to be severer. For example, the iodide by-products
include residual iodide, 220
iodine, polyiodide and iodate, which may cause detrimental
effects to aquatic 221
organisms32
and human health33
. The various iodide species are also potential sources 222
for iodinated disinfection byproducts (iodo-DBPs). Therefore,
UV/NTA process is 223
expected to provide a relatively green alternative for efficient
photoreductive 224
degradation of PFOS. 225
As is shown in Fig. 1, the PFOS decomposition rate attenuated
after 1 h. This is 226
primarily due to the nearly complete degradation of NTA within 1
h. The subsequent 227
PFOS degradation and defluorination after 1 h is attributed to
the effect of NTA 228
degradation products such as IDA. The effects of NTA degradation
products are 229
discussed in section 3.3. 230
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3.2. Mechanism of NTA-assisted UV photoreductive degradation of
PFOS 231
Photodegradation of organic compounds can take place through
direct and/or 232
indirect photolysis. Since direct PFOS photolysis under UV
irradiation is very slow, 233
PFOS degradation in UV/NTA process is believed to occur through
indirect pathway. 234
In order to ascertain the dominant reactive species that is
responsible for the 235
degradation of PFOS, experiments with nitrous oxide (N2O) were
conducted. As is 236
shown in Fig. S9 in the SI, PFOS degradation and defluorination
were substantially 237
suppressed in the presence of N2O. N2O is a well-known scavenger
of eaq–, which 238
quenches eaq– rapidly to form ·OH (Eq. 6, rate constant =
9.1×10
9 M
-1·s
-1)
10, whereas 239
·OH has a poor reactivity towards perfluorinated acids that it
cannot decompose PFOS 240
effectively.34, 35
Therefore, these results imply the crucial role of eaq– in the
PFOS 241
degradation in UV/NTA process. Besides eaq–, N2O can also react
with ·H.
10 However, 242
under alkaline conditions, the yield of ·H is much lower than
that of eaq–, and the rate 243
constant for the reaction of N2O and ·H at alkaline pH is
2.1×106 M
-1·s
-1, which is 244
over 3 orders of magnitude lower than that of N2O and eaq–.
10
Therefore, the effect of 245
·H can be excluded. 246
���– +N�O → OH
+∙ OH + N� (6)
247
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248
Figure 2. Transient absorption spectra following the laser flash
photolysis of 10 mM NTA 249
solution at pH 10.0. Insertion in Figure 2 is the decay of eaq–
detected at 630 nm in the presence of 250
PFOS with different concentrations. The transient absorption
curves are fitted. 251
The N2O scavenging experiment provides an indirect proof for the
potential role 252
of eaq–. However, a more direct proof is needed to reveal the
formation and decay of 253
eaq– in UV/NTA process. Therefore, a laser flash photolysis
study was conducted. The 254
absorption spectrum of intermediates was obtained at 10 nm
intervals between 260 255
nm and 700 nm upon the photolysis of 10 mM NTA in N2 saturated
water (Fig. 2). 256
The broad optical absorption band with a peak at around 630 nm
is attributed to eaq– 257
since it is identical to the eaq– absorption spectrum acquired
both theoretically
36 and 258
experimentally37
. The absorption peak of eaq– decayed continuously with
monitoring 259
time, indicating eaq– gradually reacts back with other primary
species. The decays of 260
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eaq– in the presence of PFOS with different concentrations are
shown in the inserted 261
figure in Fig. 2. In the absence of NTA (i.e., the laser
photolysis of pure water), eaq– 262
was not produced. By adding 10 mM NTA, the absorbance at 630 nm
increased 263
significantly, confirming the enhanced production of eaq– in the
presence of NTA. The 264
addition of PFOS accelerated the decay of eaq–, and its decay
rate increased at higher 265
PFOS concentration level. These results further verify the role
of eaq– in PFOS 266
degradation. 267
Thus, the generation of eaq– in UV/NTA process has been clearly
confirmed by 268
the laser flash photolysis study. However, compared with common
eaq– source 269
chemicals like sulfite38
, ferrocyanide39
, iodide40
, and aromatic compounds like 270
pyrenetetrasulfonate41
, the transient yield of eaq– by UV/NTA process is much lower.
271
The relatively low transient yield of eaq– cannot explain the
high efficiency of 272
UV/NTA process in PFOS degradation and defluorination. For
instance, the 10-h 273
degradation and defluorination ratios of PFOS by UV/NTA process
were 1.42-fold 274
and 1.58-fold, respectively higher than by UV/sulfite process.
Therefore, we speculate 275
that besides direct photoejection of eaq– from NTA, there may be
another dominant 276
mechanism for the efficient generation of eaq–. 277
Grossweiner et al. once proposed an alternative mechanism for
the generation 278
of eaq– based on their observations, that is the photoionizaton
of water itself sensitized 279
by the light-absorbing substances.12
In this case, a hydroxyl radical is produced by 280
dissociation of the photoionized water.12
According to the UV-Vis spectra (Fig. S10, 281
SI), direct photoionization of water under 254 nm light
irradiation can be excluded 282
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since water is transparent in the near UV spectral domain (λ
> 200 nm)16
. However, 283
NTA has an absorbance at 254 nm, which means NTA is able to
absorb photons and 284
possibly sensitize water ionization. Furthermore, APCAs can form
hydration 285
complexes with several water molecules through hydrogen bonding.
For instance, 286
eight water molecules are required to fully hydrate the first
hydration shell of 287
deprotonated glycine.42
The solvation process may make it easier for the interactions
288
between NTA and water. Therefore based on foregoing facts, we
speculate that water 289
photoionization sensitized by NTA is another major source of
eaq– in UV/NTA 290
process. 291
In addition, an important reason for the low quantum yield of
eaq– from pure 292
water photolysis is the rapid recombination of eaq– with ·OH, ·H
and H3O
+ (Eq. 3-5), 293
especially the reaction with ·OH, which accounts for more than
82% ± 3% of the 294
recombination of eaq–.43
The geminate recombination between eaq– and ·OH greatly 295
decreases the survival probability of eaq–. However in UV/NTA
process, NTA is 296
highly reactive towards ·OH,30, 44
with a high reaction rate constant of 4.2×109 M
-1·s
-1 297
at pH of 10.0.22, 45
The strong scavenging capacity of NTA towards ·OH can 298
effectively protect eaq– from being quenched by ·OH, which
increases the steady-state 299
concentration of eaq– and in turn facilitates the decomposition
of PFOS. The eaq
–300
protection mechanism can be demonstrated by the long lifetime of
eaq– in UV/NTA 301
process (Fig. 2). For instance, the half-life of eaq– in UV/NTA
process is about 11.6 µs, 302
whereas it is only 1 ns in UV/sulfite process38
and lower than 2 ns in UV/KI process40
. 303
Therefore, the presence of NTA can dramatically prolong the
survival time of eaq– by 304
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scavenging ·OH, thus promoting the degradation of PFOS. This
explains why 305
UV/NTA process has a low transient quantum yield of eaq–,
whereas has a high 306
efficiency in PFOS degradation. 307
Overall, the generation of eaq– during the UV/NTA process
appears to involve 308
three steps. In aqueous solution, NTA is fully hydrated with
both primary and 309
secondary hydration spheres. The fully hydrated NTA induces
water photodissociation 310
and photoionization as a photosensitizer with the generation of
eaq– and ·OH. Finally, 311
the NTA core scavenges ·OH to decrease the geminate
recombination between ·OH 312
and eaq–, thus increasing the amount of eaq
– available for PFOS degradation. 313
314
Figure 3. (A) DMPO spin-trapping ESR spectra recorded in the
UV/NTA and UV/buffer 315
(NH3·H2O-NH4Cl) processes. (B) Simulated data of various
radicals trapped by DMPO in 316
UV/NTA process after 60 s irradiation: (c) total ESR signal; (e)
simulation of DMPO-·OH; (f) 317
simulation of DMPO-·CH2COOH; (g) simulation of NO·. 318
In order to verify our speculation, DMPO was used as an ESR
spin-trap to 319
identify possible short-lived radicals produced in the UV/NTA
process. As shown in 320
Fig. 3, after 30 s irradiation, several peaks were clearly
observed in the ESR spectra, 321
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indicating formation of radicals and their involvement in the
reaction. As the 322
irradiation time increased to 60 s, the ESR signal intensity
also increased. No 323
well-defined ESR signal was observed in the UV/buffer process,
indicating that the 324
radicals were not produced in the absence of NTA. In order to
further confirm the 325
radical species, the ESR data of UV/NTA-60s reaction were
simulated. The ESR 326
signal of 1:2:2:1 quartets is thus assigned to DMPO-·OH (curve
e). The spectrum of 327
curve f is speculated to be the signal of DMPO-·CH2COOH. The
three-line spectrum 328
(curve g) indicates the detection of free nitroxide (NO·).46,
47
The occurrence of ·OH 329
suggests that water indeed undergoes photodissociation in the
UV/NTA process. NO· 330
and ·CH2COOH are presumably the reaction products of NTA with
·OH, which is 331
consistent with the reactive sites in NTA molecule (discussed in
detail in section 3.3) 332
and also consistent with the known steady-state products of NTA
degradation (Fig. S7, 333
SI). The ESR data of UV/NTA-30s has a similar fitting result
with UV/NTA-60s 334
(shown in Fig. S11, SI). Therefore, the ESR measurement detected
the occurrence of 335
·OH, ·CH2COOH and NO·, further confirming the mechanism that NTA
induces water 336
photodissociation and photoionization, and scavenges ·OH to
increase the quantum 337
yield of eaq–.338
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3.3. Model compound study 339
340
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Figure 4. The time profiles of PFOS degradation and
defluorination in the presence of model 341
compounds (NTA, IDA, EDTA, EDDA, citrate, glycine, oxamate, EDDS
and MGDA): PFOS 342
(0.01 mM), model compounds (2 mM), UV irradiation, N2 saturated,
pH (10.0). Error bars 343
represent standard deviations of triplicate assays. 344
In order to further validate the proposed mechanism and to test
the effects of 345
other chelating agents, IDA, EDTA, EDDA, citric acid, glycine,
oxamic acid, oxalic 346
acid, EDDS and MGDA were chosen as model compounds to
investigate the 347
structure-activity relationship of APCAs in terms of impacting
the photodegradation 348
of PFOS. The structural formulas of the model compounds are
shown in Table S2, SI. 349
The results of the model compound study are shown in Fig. 4.
Except oxamate and 350
oxalate, all of the tested model compounds accelerated the PFOS
degradation and 351
defluorination compared with the control experiment, although
their enhancement 352
effects varied a lot. For example, NTA was clearly better than
IDA in accelerating 353
PFOS degradation. The 10-h PFOS degradation and defluorination
ratios in presence 354
of NTA were 85.4% and 46.8%, respectively, while in the case of
IDA they were only 355
54.8% and 25.6%, respectively. NTA and IDA are APCAs with only
one nitrogen 356
atom. However, the nitrogen atom in NTA molecule is attached one
more acetate 357
group (-CH2COOH) than IDA. Similarly, in the comparison of EDDA
to EDTA, the 358
PFOS degradation and defluorination ratios were found to be
higher in the presence of 359
EDTA (10-h values of 78.5% and 43.7%, respectively for EDTA and
51.1% and 360
24.2%, respectively for EDDA). This is because each nitrogen
atom in EDTA 361
molecule is connected to one more acetate group than EDDA.
Therefore, the acetate 362
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group density appears to be a factor that determines the
efficiency of the specific 363
APCA in promoting PFOS photodegradation. 364
Citric acid is an important organic tricarboxylic acid.
According to the results 365
shown in Fig. 4, citrate also promotes PFOS degradation and
defluorination as 366
compared with the control experiment in its absence. However,
the enhancement 367
effect of citrate is lower than NTA. The 10-h PFOS degradation
ratios in the presence 368
of NTA and citrate were 85.4% and 50.7%, respectively, with the
corresponding 10-h 369
defluorination ratios of 46.8% and 24.1%, respectively. Citric
acid has a similar 370
chemical structure with NTA, both containing acetate groups,
whereas citric acid has 371
no amine group. Therefore, the greater acceleration in the
presence of NTA than 372
citrate can be probably attributed to the electron-rich center
of amine group, whose 373
lone pair on the nitrogen atom favors the electrophilic attack
of ·OH. 374
Glycine and oxamic acid both contain an amine group and a
carboxyl group, but 375
their relative impacts on PFOS degradation are totally
different. Glycine clearly 376
promoted PFOS degradation compared with the control experiment,
whereas oxamate 377
has a significant inhibitory effect. The 10-h PFOS degradation
and defluorination 378
ratios in the presence of glycine were 61.5% and 29.4%,
respectively, whereas they 379
were only 1.8% and 0.4%, respectively in the presence of
oxamate. These results are 380
mainly due to the different moieties present in glycine and
oxamic acid that connect 381
amine group and carboxyl group, i.e., methylene group (CH2) for
glycine and 382
carbonyl group (C=O) for oxamic acid. Therefore, it is
presumably the methylene 383
group in acetate group that offers a reactive site for ·OH
attack. The significant 384
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inhibition of oxamate is resulted from the electron-withdrawing
property of 385
C=O-COOH group, which inductively withdraws electron density
from the 386
neighboring N atom, thus reducing its reactivity with ·OH.
Oxamic acid is reported to 387
be highly persistent during ·OH oxidation process.48, 49
In contrast, oxamic acid has a 388
high reactivity towards eaq–. The rate constant for the reaction
of oxamate with eaq
– is 389
reported to be 5.7×109 M
-1·s
-1 (pH = 9.2). The second-order rate constant for the 390
reactions between oxamate and eaq– is more than 3 orders of
magnitude higher than 391
the corresponding rate constant for eaq– with glycine
(1.7×10
6 M
-1·s
-1, pH = 11.8). 392
Therefore, oxamic acid competes with PFOS for eaq–, and thus
inhibits PFOS 393
degradation. Similar to oxamic acid, the presence of oxalate
significantly inhibited 394
PFOS degradation and defluorination (Fig. S12, SI). This result
coincides with the 395
low reaction rate constant for oxalate with ·OH (7.7×106 M
-1·s
-1, pH = 6.0)
10 and 396
further indicates that carboxyl group alone was unable to
accelerate PFOS 397
degradation. In other words, the methylene group in acetate
moieties is essential for 398
the effective attack by ·OH via H-atom abstraction. 399
In comparing EDTA to EDDS, we find that EDTA is more efficient
with respect 400
to acceleration of PFOS degradation than EDDS. Although EDTA and
EDDS both 401
have four acetate groups, two acetate groups in EDDS molecule
are attached to the 402
α-C instead of the N atom. Therefore, EDTA is a tertiary amine
while EDDS is a 403
secondary amine. The hydrogen bonding to the N atom in EDDS
decreases the 404
availability of N-electrons and hinders the N-electrons
transfer. Furthermore, the 405
presence of hydrogen on carbon next to the amine (α-hydrogen)
was reported to be a 406
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key factor for electron-transfer interactions.50
Therefore, an extra α-hydrogen in 407
EDTA may result in a higher electron-transfer capability.
Compared with NTA, the 408
enhancement is more favored for MGDA. This is because MGDA has
an additional 409
CH3 group on the carbon α of amine, which increases the electron
density of N due to 410
the electron-donating inductive effect, and adds a reactive site
for ·OH attack. 411
Therefore, the electronic distribution of N atom, especially the
availability of the lone 412
pair on N atom also makes a difference for the efficiencies of
APCAs. 413
In summary, the acetate group and amine group in APCAs play
important roles 414
in accelerating the photodegradation of PFOS. This conclusion is
consistent with the 415
reactive sites for the reactions of APCAs with ·OH as pointed
out in literatures.22, 29, 45,
416
50-53 Therefore, the results of our model compound study confirm
that the scavenging 417
effect on ·OH by APCAs is the primary mechanism for the enhanced
418
photodegradation of PFOS. 419
The effects of NTA degradation products including IDA, glycine,
oxamate and 420
oxalate were summarized in Fig. S12, SI. Their efficiencies for
the degradation and 421
defluorination of PFOS followed the order of NTA > glycine ≈
IDA > control > 422
oxamate ≈ oxalate. Compared with the control experiment, NTA,
glycine and IDA 423
obviously accelerated the photodegradation of PFOS, whereas
oxamate and oxalate 424
significantly inhibited. Since NTA degradation products have
either lower efficiencies 425
than NTA or inhibiting effect, the PFOS degradation and
defluorination rates 426
decreased gradually as NTA degraded, especially after 1 h of
reaction. 427
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3.4. Effects of pH and air 428
429
Figure 5. The effect of pH on the degradation (a) and
defluorination (b) of PFOS: PFOS (0.01 430
mM), NTA (2 mM), UV irradiation, N2 saturated. Error bars
represent standard deviations of 431
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triplicate assays. 432
As is shown in Fig. 5, the degradation and defluorination of
PFOS by UV/NTA 433
process is strongly dependent on pH. The PFOS degradation ratios
under the pH 434
conditions of 6.0, 7.0, 8.0, 9.0, 10.0 and 11.0 after 10 h were
17.4%, 29.9%, 47.7%, 435
56.6%, 85.4% and 99.5%, respectively. The 10-h defluorination
ratios of PFOS under 436
the pH conditions of 6.0, 7.0, 8.0, 9.0, 10.0 and 11.0 were
8.5%, 8.0%, 15.1%, 34.9%, 437
46.8% and 72.3%, respectively. After 2 h irradiation, the
degradation and 438
defluorination ratios of PFOS at the pH value of 11.0 had
reached 82.2% and 48.8%, 439
respectively, whereas they were only 2.1% and 0.1%, respectively
at the pH value of 440
6.0. The strong dependence of PFOS degradation on pH is mainly
attributed to the 441
following two reasons. First, at low pH values, eaq– is quickly
quenched by H
+ with a 442
high reaction rate constant of 2.3×1010
M−1
·s−1
(Eq. 5). 10
The excessive consumption 443
of eaq– by H
+ inhibits the degradation of PFOS and results in a low
defluorination 444
efficiency under acidic condition. Second, pH affects the
reactivity of NTA towards 445
·OH. The pKa values for successive deprotonation of NTA are 0.8,
1.9, 2.48 and 446
9.65.45, 54
The principal forms of NTA at pH 2.0 are
HN+(CH2COOH)2(CH2COO
−) 447
and HN+(CH2COOH)(CH2COO
−)2; its major form at pH 6.0 is HN
+(CH2COO
−)3; 448
while at pH 10.0, NTA is near fully deprotonated in the form of
:N+(CH2COO
−)3. As 449
concluded in the model compound study, the lone pair on the
nitrogen atom is one of 450
the primary reactive sites for the electrophilic attack of ·OH
on APCAs molecules. At 451
lower pH values, the addition of a single proton to :N(CH2COO−)3
or to 452
HN+(CH2COO
−)3 can decrease the rate constant for ·OH reaction by about one
order 453
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of magnitude.45
Therefore, the deprotonated form of NTA is the more reactive
species 454
towards ·OH than its protonated or partially protonated
counterparts. Leitner et al. 455
drew a similar conclusion that the reactivity of ·OH with NTA is
related to the amount 456
of deprotonated nitrogen.22
The rate constant for the reaction of ·OH with NTA was 457
reported to be 6.1×107
M−1
·s−1
, 5.5×108
M−1
·s−1
and 4.2×109
M−1
·s−1
, respectively, at 458
the pH values of 2.0, 6.0 and 10.0.45
Furthermore, since the pKa4 value of NTA is 459
9.65,45
the PFOS degradation rate increased significantly as pH
increased from 9.0 to 460
10.0. Therefore, alkaline conditions are most favorable for the
reaction of ·OH with 461
NTA, thus making more eaq– available for the degradation of
PFOS. 462
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463
Figure 6. The effect of air on the degradation (a) and
defluorination (b) of PFOS: PFOS (0.01 464
mM), NTA (2 mM), UV irradiation, pH (10.0). Error bars represent
standard deviations of 465
triplicate assays. 466
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Besides pH, air is another important parameter that affects the
photoreductive 467
degradation efficiency of PFOS. In general, the eaq–-mediated
degradation of PFOS 468
requires strict anoxic conditions to prevent eaq– from being
quenched by molecular 469
oxygen (Eq. 1).9, 11
For instance, Park et al. reported that PFOS was degraded
rapidly 470
by UV/Iodide process in the presence of Ar with a rate constant
of 6.5×10-3
min-1
, 471
whereas it was not degraded in the presence of air.5 Therefore
in this study, PFOS 472
degradation was conducted under both N2 and air saturated
conditions to evaluate the 473
effect of air. Under N2 saturation, the solution was pre-bubbled
with N2 for 20 mins 474
and kept bubbling during the whole reaction period. Under air
saturation, the solution 475
was not pre-bubbled and the reactor was kept open with the
solution exposed to air 476
during the whole reaction period. The results are shown in Fig.
6. The degradation 477
ratios of PFOS under N2 and air saturated conditions after 10 h
irradiation were 85.4% 478
and 75.2%, respectively. The 10-h defluorination ratios of PFOS
under N2 and air 479
saturation were 46.8% and 35.8%, respectively. The PFOS
degradation and 480
defluorination efficiencies under N2 saturation were just
slightly higher than under air 481
condition. No significantly negative impact of air was observed.
This is because the 482
excited NTA under UV irradiation (NTA*) can react with O2.30
Meanwhile, ·OH can 483
abstract the alpha hydrogen from the methylene group in NTA
molecule.45
The 484
hydrogen abstracted radical intermediate can also react with
O2.45
As a reductant, 485
NTA can scavenge a variety of oxidizing species produced in the
presence of O2 such 486
as superoxide radicals (O2·−) and HO2 radicals (HO2·), which are
also active 487
quenchers of eaq–.55
The sequestration of these oxidants by NTA protects eaq–, and
thus 488
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enhances the degradation of PFOS. Therefore, the
photodegradation of PFOS by 489
UV/NTA process is less sensitive to air than previously reported
photoreductive 490
approaches like UV/KI11
and UV/sulfite56
processes. This result bodes well for future 491
applications of in situ or ex situ PFOS photo-remediation since
strict anoxic 492
conditions are often difficult to achieve in practical
engineering applications. 493
The effect of NTA concentration was also evaluated in this
study. A higher NTA 494
concentration over the range of 0 to 4.0 mM resulted in higher
PFOS degradation and 495
defluorination ratios (Fig. S13, SI). A detailed discussion is
available in the SI. 496
4. Environmental Implications 497
This study presents a new strategy for efficient photoreductive
degradation and 498
defluorination of PFOS. Compared with previous photochemical
approaches, the 499
newly developed UV/NTA process has three remarkable advantages.
First, PFOS 500
undergoes rapid photodegradation in the presence of NTA, with a
high defluorination 501
rate. Second, no significant detrimental impact of air was
observed, which means 502
UV/NTA process has a certain tolerance to oxygen. Third, the end
products of NTA 503
degradation are CO2, NH4+ and NO3
−, which are easily biodegradable with minimized 504
secondary pollution risks. Therefore, UV/NTA may provide a
novel, efficient and 505
relatively green technology for future in situ or ex situ PFOS
remediation. 506
In previous studies, the most common way for the generation of
eaq– to 507
decompose PFOS was by adding eaq– source chemicals such as
iodide, sulfite and 508
indole derivatives. Meanwhile, attentions were more often paid
on the improvement 509
of the transient generation of eaq–, such as increasing the
reaction temperature or 510
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applying high photon flux. Actually, the apparent quantum yield
of eaq– or its ultimate 511
efficiency is not just determined by its transient quantum
yield. It is also greatly 512
influenced by the twinborn species such as ·OH and oxidative
byproducts. For 513
instance, the unsatisfactory activity of UV/iodide process
towards PFOS degradation 514
is likely a result of triiodide scavenging of eaq–.57
In this study, we proposed a new 515
way to generate eaq–, which utilizes NTA to sensitize water
photoionization, and 516
subsequently to scavenge ·OH, in order to minimize the geminate
recombination 517
between ·OH and eaq–. The net effect is to increase the
steady-state level of eaq
– that is 518
available for PFOS degradation. The laser flash photolysis
results indicate that 519
although UV/NTA process has a low transient yield of eaq–, it
has a long half-life of 520
eaq–, which is considered to be the main reason for the high
efficiency of UV/NTA 521
process. 522
APCAs is a class of compounds acting as chelating agents.
Interests in previous 523
studies were mainly focused in their chelating effect with metal
ions. Other APCAs’ 524
properties receive little attention. For example, they are
excellent electron donors and 525
they have a high reactivity with ·OH. In this study, we revealed
that the 526
photoreductive degradation of PFOS can be enhanced by various
APCAs such as 527
NTA, IDA, EDTA, EDDA, EDDS and MGDA. The acetate and amine
groups are the 528
primary reactive sites in APCAs molecules. Therefore,
APCAs-mediated 529
photoreductive process may be a novel application field for
APCAs, which is also 530
expected to be a promising replacement for the current
photoredutive technologies for 531
PFOS decomposition. 532
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Associated content 533
Supporting Information 534
Schematic diagram of the photochemical reactor; Technical data
of the low-pressure 535
mercury lamp; additional details on analytical methods; kinetics
for PFOS 536
degradation; abbreviation and chemical structure of model
compounds. Figures 537
showing time profiles of PFOS degradation products; mass balance
of F; NTA and 538
NTA degradation products; NTA degradation pathway; effect of
N2O; UV-Vis 539
absorption spectra; ESR spectra; effect of NTA degradation
products and effect of 540
NTA concentration. Discussion on the effect of NTA
concentration. This information 541
is available free of charge via the Internet at
http://pubs.acs.org. 542
Acknowledgements 543
The authors greatly thank Dr. Jiahui Yang from Bruker (Beijing)
Scientific 544
Technology Co., Ltd for her kind assistance on the ESR result
analysis. The authors 545
also gratefully acknowledge Prof. Side Yao and Dr. Huijie Shi
for their valuable 546
comments on the discussion. This study has been supported by the
National Natural 547
Science Foundation of China (Project No. 21677109), and the
State Key Laboratory 548
of Pollution Control and Resource Reuse Foundation (No.
PCRRT16001). 549
550
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water photoionization
O
Geminate
recombination
hv
H2O
eaq−
NTA
scavenges
∙OH
CO2, NH4+, NO3
−
∙OH
NTA sensitized
H C N O F S
CC
CC
C
C
N
O
O
O
OO
HH
H
H
HH
O
Degradation & DefluorinationPFOS
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