UV/nitrilotriacetic acid process as a novel strategy for efficient … · 2018. 2. 5. · 1 1...
Transcript of UV/nitrilotriacetic acid process as a novel strategy for efficient … · 2018. 2. 5. · 1 1...
-
Subscriber access provided by Caltech Library
Environmental Science & Technology is published by the American Chemical Society.1155 Sixteenth Street N.W., Washington, DC 20036Published by American Chemical Society. Copyright © American Chemical Society.However, no copyright claim is made to original U.S. Government works, or worksproduced by employees of any Commonwealth realm Crown government in the courseof their duties.
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
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are postedonline prior to technical editing, formatting for publication and author proofing. The American ChemicalSociety provides “Just Accepted” as a service to the research community to expedite the disseminationof scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear infull in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fullypeer reviewed, but should not be considered the official version of record. They are citable by theDigital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,the “Just Accepted” Web site may not include all articles that will be published in the journal. Aftera manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Website and published as an ASAP article. Note that technical editing may introduce minor changesto the manuscript text and/or graphics which could affect content, and all legal disclaimers andethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors orconsequences arising from the use of information contained in these “Just Accepted” manuscripts.
-
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
Page 1 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
2
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
Page 2 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
3
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
Page 3 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
4
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
Page 4 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
5
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
Page 5 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
6
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
Page 6 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
7
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
Page 7 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
8
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
Page 8 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
9
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
Page 9 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
10
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
Page 10 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
11
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
Page 11 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
12
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
Page 12 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
13
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
Page 13 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
14
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
Page 14 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
15
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
Page 15 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
16
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
Page 16 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
17
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
Page 17 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
18
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
Page 18 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
19
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
Page 19 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
20
3.3. Model compound study 339
340
Page 20 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
21
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
Page 21 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
22
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
Page 22 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
23
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
Page 23 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
24
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
Page 24 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
25
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
Page 25 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
26
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
Page 26 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
27
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
Page 27 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
28
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
Page 28 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
29
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
Page 29 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
30
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
Page 30 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
31
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
Page 31 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
32
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
References 551
1. Renner, R. Growing concern over perfluorinated chemicals. Environ. Sci. 552
Technol. 2001, 35 (7), 154A-160A. 553
2. Giesy, J. P.; Kannan, K. Peer reviewed: perfluorochemical surfactants in the 554
Page 32 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
33
environment. Environ. Sci. Technol. 2002, 36 (7), 146A-152A. 555
3. Lau, C.; Butenhoff, J. L.; Rogers, J. M. The developmental toxicity of 556
perfluoroalkyl acids and their derivatives. Toxicology and Applied Pharmacology 557
2004, 198 (2), 231-241. 558
4. Lau, C.; Anitole, K.; Hodes, C.; Lai, D.; Pfahles-Hutchens, A.; Seed, J. 559
Perfluoroalkyl acids: A review of monitoring and toxicological findings. 560
Toxicological Sciences 2007, 99 (2), 366-394. 561
5. Park, H.; Vecitis, C. D.; Cheng, J.; Choi, W.; Mader, B. T.; Hoffmann, M. R. 562
Reductive Defluorination of Aqueous Perfluorinated Alkyl Surfactants: Effects 563
of Ionic Headgroup and Chain Length. J. Phys. Chem. A 2009, 113 (4), 690-696. 564
6. Gu, Y. R.; Dong, W. Y.; Luo, C.; Liu, T. Z. Efficient Reductive Decomposition of 565
Perfluorooctanesulfonate in a High Photon Flux UV/Sulfite System. Environ. Sci. 566
Technol. 2016, 50 (19), 10554-10561. 567
7. Guo, R.; Zhang, C. J.; Zhang, G.; Zhou, Q. Degradation of Perfluorooctanoic 568
Acid by UV/Chloride Process. Chem. J. Chin. Univ.-Chin. 2016, 37 (8), 569
1499-1508. 570
8. Tian, H. T.; Gao, J.; Li, H.; Boyd, S. A.; Gu, C. Complete Defluorination of 571
Perfluorinated Compounds by Hydrated Electrons Generated from 572
3-Indole-acetic-acid in Organomodified Montmorillonite. Scientific Reports 573
2016, 6, 9. 574
9. Lyu, X. J.; Li, W. W.; Lam, P. K. S.; Yu, H. Q. Insights into perfluorooctane 575
sulfonate photodegradation in a catalyst-free aqueous solution. Scientific Reports 576
Page 33 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
34
2015, 5, 1-6. 577
10. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of 578
rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl 579
radicals (⋅OH/⋅O−) in aqueous solution. Journal of physical and chemical 580
reference data 1988, 17 (2), 513-886. 581
11. Qu, Y.; Zhang, C. J.; Li, F.; Chen, J.; Zhou, Q. Photo-reductive defluorination of 582
perfluorooctanoic acid in water. Water Research 2010, 44 (9), 2939-2947. 583
12. Grossweiner, L. I.; Swenson, G. W.; Zwicker, E. F. Photochemical Generation of 584
the Hydrated Electron. Science (New York, N.Y.) 1963, 141 (3583), 805-6. 585
13. Sauer, M. C.; Crowell, R. A.; Shkrob, I. A. Electron photodetachment from 586
aqueous anions. 1. Quantum yields for generation of hydrated electron by 193 587
and 248 nm laser photoexcitation of miscellaneous inorganic anions. J. Phys. 588
Chem. A 2004, 108 (25), 5490-5502. 589
14. Elles, C. G.; Jailaubekov, A. E.; Crowell, R. A.; Bradforth, S. E. 590
Excitation-energy dependence of the mechanism for two-photon ionization of 591
liquid H2O and D2O from 8.3 to 12.4 eV. J. Chem. Phys. 2006, 125 (4), 12. 592
15. Elles, C. G.; Shkrob, I. A.; Crowell, R. A.; Bradforth, S. E. Excited state 593
dynamics of liquid water: Insight from the dissociation reaction following 594
two-photon excitation. J. Chem. Phys. 2007, 126 (16), 8. 595
16. Nikogosyan, D. N.; Angelov, D. A. Formation of free-radicals in water under 596
high-power laser UV irradiation. Chem. Phys. Lett. 1981, 77 (1), 208-210. 597
17. Gobert, F.; Pommeret, S.; Vigneron, G.; Buguet, S.; Haidar, R.; Mialocq, J. C.; 598
Page 34 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
35
Lampre, I.; Mostafavi, M. Nanosecond kinetics of hydrated electrons upon water 599
photolysis by high intensity femtosecond UV pulses. Res. Chem. Intermed. 2001, 600
27 (7-8), 901-910. 601
18. Schmidt, C. K.; Brauch, H. J. Impact of aminopolycarboxylates on aquatic 602
organisms and eutrophication: Overview of available data. Environ. Toxicol. 603
2004, 19 (6), 620-637. 604
19. Repo, E.; Warchol, J. K.; Bhatnagar, A.; Mudhoo, A.; Sillanpaa, M. 605
Aminopolycarboxylic acid functionalized adsorbents for heavy metals removal 606
from water. Water Research 2013, 47 (14), 4812-4832. 607
20. Saifullah; Meers, E.; Qadir, M.; de Caritat, P.; Tack, F. M. G.; Du Laing, G.; Zia, 608
M. H. EDTA-assisted Pb phytoextraction. Chemosphere 2009, 74 (10), 609
1279-1291. 610
21. De Luca, A.; Dantas, R. F.; Esplugas, S. Study of Fe(III)-NTA chelates stability 611
for applicability in photo-Fenton at neutral pH. Appl. Catal. B-Environ. 2015, 612
179, 372-379. 613
22. Leitner, N. K. V.; Guilbault, I.; Legube, B. Reactivity of OH center dot and 614
e(aq)(-) from electron beam irradiation of aqueous solutions of EDTA and 615
aminopolycarboxylic acids. Radiat. Phys. Chem. 2003, 67 (1), 41-49. 616
23. Heintz, W. Ueber dem Ammoniaktypus angehörige organische Säuren. Justus 617
Liebigs Annalen der Chemie 1862, 122 (3), 257-294. 618
24. Bucheli-Witschel, M.; Egli, T. Environmental fate and microbial degradation of 619
aminopolycarboxylic acids. Fems Microbiol. Rev. 2001, 25 (1), 69-106. 620
Page 35 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
36
25. Ouyang, B.; Dong, W. B.; Hou, H. Q. A laser flash photolysis study of nitrous 621
acid in the aqueous phase. Chem. Phys. Lett. 2005, 402 (4-6), 306-311. 622
26. Yamamoto, T.; Noma, Y.; Sakai, S. I.; Shibata, Y. Photodegradation of 623
perfluorooctane sulfonate by UV irradiation in water and alkaline 2-propanol. 624
Environ. Sci. Technol. 2007, 41 (16), 5660-5665. 625
27. Jin, L.; Zhang, P. Y.; Shao, T.; Zhao, S. L. Ferric ion mediated 626
photodecomposition of aqueous perfluorooctane sulfonate (PFOS) under UV 627
irradiation and its mechanism. J. Hazard. Mater. 2014, 271, 9-15. 628
28. Yang, S. W.; Cheng, J. H.; Sun, J.; Hu, Y. Y.; Liang, X. Y. Defluorination of 629
Aqueous Perfluorooctanesulfonate by Activated Persulfate Oxidation. PLoS One 630
2013, 8 (10), 10. 631
29. Chen, D.; Martell, A. E.; McManus, D. Studies on the mechanism of chelate 632
degradation in iron-based, liquid redox H2S removal processes. Can. J. 633
Chem.-Rev. Can. Chim. 1995, 73 (2), 264-274. 634
30. Sorensen, M.; Frimmel, F. H. Photodegradation of EDTA and NTA in the 635
UV/H2O2 process. Z.Naturforsch.(B) 1995, 50 (12), 1845-1853. 636
31. Anderson, R. L.; Bishop, W. E.; Campbell, R. L. A review of the environmental 637
and mammalian toxicology of the nitrilotriacetic acid. Crc Critical Reviews in 638
Toxicology 1985, 15 (1), 1-102. 639
32. Laverock, M. J.; Stephenson, M.; Macdonald, C. R. Toxicity of iodine, iodide, 640
and iodate to daphnia-magna and rainbow-trout (oncorhynchus-mykiss). 641
Archives of Environmental Contamination and Toxicology 1995, 29 (3), 344-350. 642
Page 36 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
37
33. Burgi, H.; Schaffner, T.; Seiler, J. P. The toxicology of iodate: A review of the 643
literature. Thyroid 2001, 11 (5), 449-456. 644
34. Hori, H.; Hayakawa, E.; Einaga, H.; Kutsuna, S.; Koike, K.; Ibusuki, T.; 645
Kiatagawa, H.; Arakawa, R. Decomposition of environmentally persistent 646
perfluorooctanoic acid in water by photochemical approaches. Environ. Sci. 647
Technol. 2004, 38 (22), 6118-6124. 648
35. Zhang, Z.; Chen, J. J.; Lyu, X. J.; Yin, H.; Sheng, G. P. Complete mineralization 649
of perfluorooctanoic acid (PFOA) by gamma-irradiation in aqueous solution. 650
Scientific Reports 2014, 4, 1-6. 651
36. Abramczyk, H.; Kroh, J. Absorption spectrum of the solvated electron. 2. 652
Numerical calculations of the profiles of the electron in water and methanol at 653
300 K. The Journal of Physical Chemistry 1991, 95 (16), 6155-6159. 654
37. Hart, E. J.; Boag, J. Absorption spectrum of the hydrated electron in water and in 655
aqueous solutions. Journal of the American Chemical Society 1962, 84 (21), 656
4090-4095. 657
38. Lian, R.; Oulianov, D. A.; Crowell, R. A.; Shkrob, I. A.; Chen, X.; Bradforth, S. 658
E. Electron photodetachment from aqueous anions. 3. Dynamics of geminate 659
pairs derived from photoexcitation of mono-vs polyatomic anions. The Journal 660
of Physical Chemistry A 2006, 110 (29), 9071-9078. 661
39. Huang, L.; Dong, W. B.; Hou, H. Q. Investigation of the reactivity of hydrated 662
electron toward perfluorinated carboxylates by laser flash photolysis. Chem. 663
Phys. Lett. 2007, 436 (1-3), 124-128. 664
Page 37 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
38
40. Qu, Y.; Zhang, C. J.; Chen, P.; Zhou, Q.; Zhang, W. X. Effect of initial solution 665
pH on photo-induced reductive decomposition of perfluorooctanoic acid. 666
Chemosphere 2014, 107, 218-223. 667
41. Yuan, H. X.; Pan, H. X.; Wu, Y. L.; Zhao, J. F.; Dong, W. B. Laser Flash 668
Photolysis Mechanism of Pyrenetetrasulfonate in Aqueous Solution. Acta 669
Phys.-Chim. Sin. 2012, 28 (4), 957-962. 670
42. Yao, Y. H.; Chen, D.; Zhang, S.; Li, Y. L.; Tu, P. H.; Liu, B.; Dong, M. D. 671
Building the First Hydration Shell of Deprotonated Glycine by the MCMM and 672
ab Initio Methods. J. Phys. Chem. B 2011, 115 (19), 6213-6221. 673
43. Thomsen, C. L.; Madsen, D.; Keiding, S. R.; Thogersen, J.; Christiansen, O. 674
Two-photon dissociation and ionization of liquid water studied by femtosecond 675
transient absorption spectroscopy. J. Chem. Phys. 1999, 110 (7), 3453-3462. 676
44. Sillanpaa, M. E. T.; Kurniawan, T. A.; Lo, W. H. Degradation of chelating agents 677
in aqueous solution using advanced oxidation process (AOP). Chemosphere 2011, 678
83 (11), 1443-1460. 679
45. Sahul, K.; Sharma, B. K. Gamma-radiolysis of nitrilotriacetic aicd (NTA) in 680
aqueous solutions. J. Radioanal. Nucl. Chem.-Artic. 1987, 109 (2), 321-327. 681
46. Rehorek, D.; Janzen, E. G. On the formation of arseno aminoxyls (nitroxides) by 682
spin trapping of arseno radicals. Polyhedron 1984, 3 (5), 631-634. 683
47. Buettner, G. R. Spin trapping - electron-spin resonance parameters of spin 684
adducts. Free Radic. Biol. Med. 1987, 3 (4), 259-303. 685
48. Ganiyu, S. O.; Oturan, N.; Raffy, S.; Esposito, G.; van Hullebusch, E. D.; Cretin, 686
Page 38 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
39
M.; Oturan, M. A. Use of Sub-stoichiometric Titanium Oxide as a Ceramic 687
Electrode in Anodic Oxidation and Electro-Fenton Degradation of the 688
Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway. 689
Electrochim. Acta 2017, 242, 344-354. 690
49. Antonin, V. S.; Garcia-Segura, S.; Santos, M. C.; Brillas, E. Degradation of 691
Evans Blue diazo dye by electrochemical processes based on Fenton's reaction 692
chemistry. J. Electroanal. Chem. 2015, 747, 1-11. 693
50. Chen, Y.; Hu, C.; Hu, X. X.; Qu, J. H. Indirect Photodegradation of Amine Drugs 694
in Aqueous Solution under Simulated Sunlight. Environ. Sci. Technol. 2009, 43 695
(8), 2760-2765. 696
51. Furlong, D. N.; Wells, D.; Sasse, W. H. F. Photooxidation at platinum colloidal 697
TiO2 aqueous-solution interfaces 1. Ethylenediaminetetraacetic acid and 698
related-compounds. Aust. J. Chem. 1986, 39 (5), 757-769. 699
52. Anipsitakis, G. P.; Dionysiou, D. D. Radical generation by the interaction of 700
transition metals with common oxidants. Environ. Sci. Technol. 2004, 38 (13), 701
3705-3712. 702
53. Morooka, S.; Ikemizu, K.; Kamano, H.; Kato, Y. Ozonation rate of water-soluble 703
chelates and related-compounds. J. Chem. Eng. Jpn. 1986, 19 (4), 294-299. 704
54. Martell, A. E.; Smith, R. M. Critical Stability Constants Vol. Amino Acids. 705
Plenum Press: New York, 1974. 706
55. Larson, R. A.; Stabler, P. P. Sensitized photooxidation of nitrilotriacetic and 707
iminodiacetic acids. Journal of Environmental Science & Health Part A 1978, 13 708
Page 39 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
40
(8), 545-552. 709
56. Song, Z.; Tang, H.; Wang, N.; Zhu, L. Reductive defluorination of 710
perfluorooctanoic acid by hydrated electrons in a sulfite-mediated UV 711
photochemical system. J. Hazard. Mater. 2013, 262 (22), 332-338. 712
57. Park, H.; Vecitis, C. D.; Cheng, J.; Dalleska, N. F.; Mader, B. T.; Hoffmann, M. 713
R. Reductive degradation of perfluoroalkyl compounds with aquated electrons 714
generated from iodide photolysis at 254 nm. Photochemical & Photobiological 715
Sciences 2011, 10 (12), 1945-1953. 716
Page 40 of 41
ACS Paragon Plus Environment
Environmental Science & Technology
-
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
Page 41 of 41
ACS Paragon Plus Environment
Environmental Science & Technology