Nitrilotriacetic Acid Process as a Novel Strategy for Efficient

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UV/nitrilotriacetic acid process as a novel strategy for efficient photoreductive degradation of perfluorooctane sulfonate Zhuyu Sun, Chaojie Zhang, Lu Xing, Qi Zhou, Wenbo Dong, and Michael R Hoffmann Environ. 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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Environmental Science & Technology

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UV/nitrilotriacetic acid process as a novel strategy for

2

efficient photoreductive degradation of perfluorooctane

3

sulfonate

4 5

Zhuyu Sun,†‡ Chaojie Zhang,*†‡ Lu Xing,†‡ Qi Zhou,†‡ Wenbo Dong,&

6

Michael R. Hoffmann§

7

†

State Key Laboratory of Pollution Control and Resources Reuse, College

8

of Environmental Science and Engineering, Tongji University, Shanghai

9

200092, China

10

‡

Shanghai 200092, China

11 12

Shanghai Institute of Pollution Control and Ecological Security,

&

Shanghai Key Laboratory of Atmospheric Particle Pollution and

13

Prevention, Department of Environmental Science & Engineering, Fudan

14

University, Shanghai 200433, China

15

§

Linde-Robinson Laboratories, California Institute of Technology,

16

Pasadena, California 91125, United States

17

*Corresponding author. Tel: +86 21 65981831; fax: +86 21 65983869;

18

E-mail address: [email protected]

19

1

ACS Paragon Plus Environment


20

Abstract: Perfluorooctane sulfonate (PFOS) is a toxic, bioaccumulative and highly

21

persistent anthropogenic chemical. Hydrated electrons (eaq–) are potent nucleophiles

22

that can effectively decompose PFOS. In previous studies, eaq– are mainly produced

23

by photoionization of aqueous anions or aromatic compounds. In this study, we

24

proposed a new photolytic strategy to generate eaq– and in turn decompose PFOS,

25

which utilizes nitrilotriacetic acid (NTA) as a photosensitizer to induce water

26

photodissociation and photoionization, and subsequently as a scavenger of hydroxyl

27

radical (·OH) to minimize the geminate recombination between ·OH and eaq–. The net

28

effect is to increase the amount of eaq– available for PFOS degradation. The UV/NTA

29

process achieved a high PFOS degradation ratio of 85.4% and a defluorination ratio of

30

46.8% within 10 h. A pseudo first-order rate constant (k) of 0.27 h-1 was obtained. The

31

laser flash photolysis study indicates that eaq– is the dominant reactive species

32

responsible for PFOS decomposition. The generation of eaq– is greatly enhanced and

33

its half-life is significantly prolonged in the presence of NTA. The electron spin

34

resonance (ESR) measurement verified the photodissociation of water by detecting

35

·OH. The model compound study indicates that the acetate and amine groups are the

36

primary reactive sites.

2


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37

1.

Introduction

38

Perfluorooctane sulfonate (PFOS, C8F17SO3−) is a useful anthropogenic chemical

39

that has been extensively utilized in industrial and commercial applications for

40

decades.1,2 However, due to its high toxicity, environmental persistence,

41

bioaccumulation and global distribution, PFOS is considered hazardous to

42

environmental and human health.3,4 Moreover, because of the high thermal and

43

chemical stability of C–F bond (~116 kcal/mol, almost the strongest in nature2), PFOS

44

is extremely resistant to traditional chemical and biological degradations.

45

Recently, hydrated electrons (eaq–) mediated photoreductive approaches have

46

garnered special attention for PFOS decomposition owing to their extraordinarily high

47

efficiency and relatively mild conditions. However, in order to produce sufficient

48

amount of eaq– to defluorinate PFOS, chemicals such as iodide5, sulfite6, chloride7 and

49

indole derivatives8 are essential. Lyu et al. developed a catalyst-free PFOS

50

photodecomposition method, but it relied on using strong alkaline conditions (pH =

51

11.8) and high temperatures (100 °C).9 Furthermore, due to the rapid reaction between

52

eaq– and oxygen (Eq. 1, rate constant = 1.9×1010 M-1·s-1)10, the reduction efficiency of

53

eaq– is significantly affected by dissolved oxygen (O2). Thereby, previous eaq–

54

-mediated photoreductive approaches normally require strict anoxic conditions.9, 11 In

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view of the drawbacks of current photoreductive technologies, we were motivated to

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propose a new strategy for PFOS removal, which can be conducted under more

57

environmentally relevant conditions and with minimization of undesirable

58

byproducts. 3



In addition to electron photodetachment from anions and aromatic compounds12,

59 60

13

, UV photolysis of liquid water generates eaq– as well (Eq. 2).14, 15 However, direct

61

photoionization of liquid water under 254 nm irradiation is difficult due to a

62

negligible absorption coefficient at λ > 200 nm.16 In addition, the quantum yield of

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eaq– from water photoionization is low because the radical species undergo significant

64

geminate recombination. In pure water photolysis, energy deposition takes place in

65

well-separated local volumes called spurs, where the primary products including eaq–,

66

hydroxyl radicals (·OH), hydrogen atoms (·H) and H3O+ are formed in close vicinity

67

with high initial local concentrations.17 However, as the spurs expand through

68

diffusion, a large proportion of the primary products undergo very efficient back

69

reactions (Eq. 3-5). Thus, only a small fraction of eaq– actually escape into the bulk

70

solution to initiate reductive chemical processes.10 Therefore, there is scarcely any net

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generation of eaq– in pure water photolysis upon low energy excitation. Based on these

72

facts, we now propose a new strategy to enhance the generation of eaq–, i.e., by

73

minimizing the geminate recombination of eaq– with oxidative species after inducing

74

water photoionization. – + O → O∙

(1)

75

– 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 4


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carboxylate groups bound to one or more nitrogen atoms. Due to their excellent

77

chelating properties18, interests in previous studies were mainly focused in their

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chelation with metal ions.19-21 Furthermore, APCAs contain several donor atoms

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whose free electron pairs are easily attacked by ·OH. Leitner et al.22 reported that the

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rate constants for the reactions of APCAs with ·OH can exceed 1010 M-1·s-1. These

81

near diffusion controlled values are 2 to 3 orders of magnitude higher than reactions

82

of APCAs with eaq–. Therefore, APCAs are considered to be excellent candidates to

83

scavenge ·OH, and thus increase the apparent quantum yields of eaq– and in turn

84

promote the photoreductive degradation of PFOS.

85

In this study, we chose to use nitrilotriacetic acid (NTA) as a representative

86

APCA to explore its impact on the photoreductive degradation of PFOS. NTA is the

87

first chemical synthetic APCA23 and is more biodegradable than other APCAs24. In

88

order to unravel the underlying reaction mechanisms, laser flash photolysis, ESR

89

measurement and a model compound study were carried out. Furthermore, the effects

90

of NTA concentration, pH and air were investigated. To our best knowledge, this is

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the first study that utilizes APCAs as ·OH scavengers to promote the photoreductive

92

degradation of refractory organic pollutants. The findings in this study can provide a

93

new strategy for PFOS remediation and a novel application field for APCAs.

5



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2.

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2.1. Chemicals

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Materials and methods

Perfluorooctanesulfonic acid (PFOS, ~40% in water), HPLC grade methanol

97

(≥99.9%),

5,5-Dimethyl-1-pyrroline N-oxide

(DMPO,

≥98.0%)

and

model

98

compounds including trisodium citrate (≥99.0%), ethylenediamine-N,N’-disuccinic

99

acid trisodium salt solution (EDDS, ~35% in water) and methylglycine diacetic acid

100

(MGDA, ≥99.0%) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, Mo,

101

USA). Nitrilotriacetic acid (NTA, ≥98.5%), ammonium hydroxide solution (25%),

102

ammonium chloride (≥99.8%), ethylenediaminetetraacetic acid (EDTA, ≥99.5%),

103

iminodiacetic acid (IDA, ≥98.0%), glycine (≥99.5%), oxamic acid (≥98.0%) and

104

sodium oxalate (≥99.8%) were obtained from Sinopharm Chemical Reagent Co.

105

(Shanghai, China). HPLC grade ammonium acetate (97.0%) was purchased from

106

TEDIA (Fairfield, OH, USA). Sodium perfluoro-1-[1,2,3,4-13C4]octanesulfonate

107

(MPFOS, ≥99%,

108

Canada) was used as the internal standard for the quantification of PFOS. Milli-Q

109

water and deionized water were used throughout the whole experiment.

110

2.2. Reductive defluorination

13

C4) acquired from Wellington Laboratories Inc. (Guelph, ON,

111

The photoreductive degradation of PFOS was conducted under anoxic conditions

112

in a stainless steel cylindrical reactor (Fig. S1, SI). The outer and inner diameters of

113

the reactor were 100 mm and 60 mm, respectively. A low-pressure mercury lamp (14

114

W, Heraeus, Germany) with quartz tube protection was placed in the center of the

115

reactor, emitting 254 nm UV light. 720 mL solution was added to the reactor. Before 6


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the reaction, the mixture was bubbled with highly purified nitrogen for 20 mins to

117

remove oxygen. The solution pH was adjusted with ammonium hydroxide-ammonium

118

chloride (NH3·H2O-NH4Cl) buffer. The reaction temperature was held constant at 30

119

o

120

liquid were analyzed after filtration through 0.22 µm nylon filter (ANPEL Laboratory

121

Technologies, Shanghai, China).

122

2.3. Analytical methods

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C by the circulating cooling system. After various time intervals, samples of the

The concentrations of PFOS and possible aqueous-phase intermediate analytes

124

were determined

125

spectrometry (HPLC−MS/MS, TSQTM Quantum AccessTM, Thermo Finnigan, San

126

Jose, CA, USA). Ion Chromatography (Dionex, ICS-3000, Thermo Fisher Scientific,

127

USA) equipped with a conductivity detector was used for the analysis of fluoride,

128

nitrate and short chain organic acids. NTA and its degradation products including IDA,

129

oxamate and oxalate were detected by UPLC−MS/MS (ACQUITY UPLC&SCIEX

130

SelexION Triple Quad 5500 System, Waters, MA, USA). More detailed information

131

is available in the SI.

132

2.4. Laser flash photolysis experiment and electron spin resonance (ESR)

133

by high-performance liquid chromatography/tandem mass

measurement

134

Nanosecond laser flash photolysis for the detection of eaq– was performed using a

135

Quanta Ray LAB-150-10 Nd:YAG laser at an excitation wavelength of 266 nm. The

136

components of the laser flash photolysis apparatus were as described by Ouyang et

137

al..25 Detailed information is available in the SI. 7



138

The ESR spectra were obtained on a Bruker EMXplus-10/12 ESR spectrometer

139

at room temperature. The instrumental parameters for ESR analysis were as follows:

140

microwave frequency, 9.852 GHz; microwave power, 20 mW; modulation amplitude

141

1 G; modulation frequency, 100 kHz; center field, 3500 G; sweep width, 100 G. The

142

simulations of ESR spectra were obtained with the use of Spinfit in Xenon software.

143

Details are provided in the SI.

144

8


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3.

Results and Discussion

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3.1. NTA-assisted photoreductive degradation and defluorination of PFOS

147 148

Figure 1. The time profiles of PFOS degradation (a) and defluorination (b) under different 9



149

conditions. The treatment experiment: PFOS (0.01 mM), NTA (2 mM), UV irradiation, N2

150

saturated, pH (10.0); the 1st control experiment (direct photolysis): PFOS (0.01 mM), UV

151

irradiation, N2 saturated; the 2nd control experiment (UV/buffer): PFOS (0.01 mM), UV irradiation,

152

N2 saturated, pH (10.0); the 3rd control experiment (without UV irradiation): PFOS (0.01 mM),

153

NTA (2mM), N2 saturated, pH (10.0); the 4th control experiment (UV/sulfite): PFOS (0.01 mM),

154

SO32- (2mM), UV irradiation, N2 saturated, pH (10.0). Error bars represent standard deviations of

155

triplicate assays.

156

The photochemical decomposition of PFOS was conducted under 254 nm light

157

irradiation in the presence of NTA under anoxic condition. In order to determine the

158

effect of each parameter, four control experiments were conducted. The results of

159

experiments under different conditions are shown in Fig. 1. Since PFOS has a weak

160

absorption at 254 nm and the quantum yield of eaq– by pure water photolysis is

161

negligible26, the direct photolysis of PFOS was poor. Only 12.9% of the initial PFOS

162

was decomposed after 10 h of irradiation, with a low defluorination ratio of 3.7%.

163

Adjusting solution pH to 10.0 with NH3·H2O-NH4Cl buffer somewhat enhanced the

164

decomposition of PFOS, with the 10-h PFOS degradation and defluorination ratios

165

increased to 20.9% and 9.9%, respectively. Compared with the UV/buffer process,

166

addition of 2 mM NTA significantly accelerated the degradation and defluorination of

167

PFOS. After 10 h of irradiation, the degradation and defluorination ratios of PFOS in

168

the presence of NTA were 85.4% and 46.8%, respectively. In particular, PFOS

169

degraded rapidly during the initial 0.5 h. The 0.5-h PFOS photodegradation and

170

defluorination ratios by UV/NTA process achieved 45.1% and 18.6%, respectively, 10


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which are 5.2-fold and 8.5-fold, respectively higher than by UV/buffer process. The

172

results of the 3rd control experiment indicate that PFOS does not degrade without UV

173

irradiation, even in the presence of NTA. Therefore, both UV and NTA are necessary

174

for the efficient degradation and defluorination of PFOS.

175

Gu et al. (2016) demonstrated that UV/sulfite system exhibited a high efficiency

176

in decomposing PFOS.6 In order to better evaluate the efficiency of UV/NTA process

177

for PFOS degradation, the newly developed approach was compared with UV/sulfite

178

process (the 4th control). The 10-h degradation and defluorination ratios of PFOS by

179

UV/sulfite process were 60.1% and 29.6%, respectively, which were 0.7-fold and

180

0.63-fold, respectively lower than by UV/NTA process. PFOS degradation by the

181

UV/NTA process follows pseudo first-order kinetics (Fig. S4, SI), with an apparent

182

reaction rate constant (k) of 0.27 h-1, and a half-life (t1/2) of 2.6 h. The k value for the

183

UV/NTA process was higher than other photochemical approaches including UV/KI

184

process5, UV/sulfite process, UV/Fe3+ process27, UV/alkaline 2-propanol process26

185

and UV/K2S2O8 process28 (Table S1, SI). Therefore, UV/NTA process exhibited a

186

high efficiency in the degradation and defluorination of PFOS.

187

As PFOS decomposed, short-chain-length perfluorinated intermediates including

188

PFBS, PFBA, PFHxS, PFHxA, PFHpA and PFOA were detected (Fig. S5, SI). Based

189

on the concentrations of PFOS, intermediates and F−, the mass balance of F was

190

calculated (Fig. S6, SI). The loss of F recovery during the reaction implies the

191

formation of partially fluorinated intermediates. Therefore, based on the product

192

distribution and F balance, three possible degradation pathways of PFOS are likely to 11



193

occur in UV/NTA process: a) PFOS undergoes direct defluorination to form

194

polyfluorinated intermediates; b) PFOS firstly undergoes desulfonation to form PFOA,

195

and then degrades to shorter-chain-length perﬂuoroalkyl carboxylic acids (PFCAs); c)

196

C-C bond in PFOS molecules cleaves, in which case short-chain-length

197

perﬂuoroalkane sulfonates (PFSAs), such as PFHxS and PFBS, were produced. The

198

proposed PFOS degradation pathways in UV/NTA process are similar with other

199

photochemical processes5, 9, 26, and accord well with the mechanisms suggested by

200

molecular orbitals and thermodynamic analyses6.

201

As mentioned above, most photochemical technologies for PFOS degradation are

202

faced with challenges of secondary pollution due to by-product formation. In order to

203

evaluate the potential post-treatment impact of the UV/NTA process, the degradation

204

products of NTA were quantified. As is shown in Fig. S7, SI, NTA degraded rapidly

205

during the first 1 h of PFOS degradation. 97.7% of the initial NTA decomposed within

206

1 h, along with the concomitant formation of three intermediates including IDA,

207

oxamate and oxalate. The maximum concentrations of IDA, oxamate and oxalate

208

were 0.75 mM, 0.26 mM and 0.68 mM, respectively, observed at 0.5 h, 2 h and 6 h,

209

respectively. IDA and oxamate underwent nearly complete decomposition after 10 h,

210

with their concentrations falling below 0.03 mM. Oxalate was completely

211

decomposed after 14 h (data not shown). Ammonium (NH4+) and nitrate (NO3−) were

212

the major N-containing end products. Based on the product distribution and the NTA

213

photooxidation mechanism in literatures29, 30, a possible NTA decomposition pathway

214

is illustrated in Fig. S8, SI. The end products of NTA degradation are NH4+, NO3− and 12


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carbon dioxide (CO2), which are innocuous and can be easily removed by biological

216

transformation. Furthermore, NTA is reported to have relatively low environmental

217

risks to sewage treatment or aquatic life.18, 31 It is readily biodegradable by natural

218

microbial processes, and the biodegradation is complete without accumulation of

219

unwanted intermediates.24 In contrast, the secondary pollutions of other approaches

220

appear to be severer. For example, the iodide by-products include residual iodide,

221

iodine, polyiodide and iodate, which may cause detrimental effects to aquatic

222

organisms32 and human health33. The various iodide species are also potential sources

223

for iodinated disinfection byproducts (iodo-DBPs). Therefore, UV/NTA process is

224

expected to provide a relatively green alternative for efficient photoreductive

225

degradation of PFOS.

226

As is shown in Fig. 1, the PFOS decomposition rate attenuated after 1 h. This is

227

primarily due to the nearly complete degradation of NTA within 1 h. The subsequent

228

PFOS degradation and defluorination after 1 h is attributed to the effect of NTA

229

degradation products such as IDA. The effects of NTA degradation products are

230

discussed in section 3.3.

13



231

3.2. Mechanism of NTA-assisted UV photoreductive degradation of PFOS

232

Photodegradation of organic compounds can take place through direct and/or

233

indirect photolysis. Since direct PFOS photolysis under UV irradiation is very slow,

234

PFOS degradation in UV/NTA process is believed to occur through indirect pathway.

235

In order to ascertain the dominant reactive species that is responsible for the

236

degradation of PFOS, experiments with nitrous oxide (N2O) were conducted. As is

237

shown in Fig. S9 in the SI, PFOS degradation and defluorination were substantially

238

suppressed in the presence of N2O. N2O is a well-known scavenger of eaq–, which

239

quenches eaq– rapidly to form ·OH (Eq. 6, rate constant = 9.1×109 M-1·s-1) 10, whereas

240

·OH has a poor reactivity towards perfluorinated acids that it cannot decompose PFOS

241

effectively.34, 35 Therefore, these results imply the crucial role of eaq– in the PFOS

242

degradation in UV/NTA process. Besides eaq–, N2O can also react with ·H. 10 However,

243

under alkaline conditions, the yield of ·H is much lower than that of eaq–, and the rate

244

constant for the reaction of N2O and ·H at alkaline pH is 2.1×106 M-1·s-1, which is

245

over 3 orders of magnitude lower than that of N2O and eaq–. 10 Therefore, the effect of

246

·H can be excluded. – + N O → OH + ∙ OH + N

(6)

247

14


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248 249

Figure 2. Transient absorption spectra following the laser flash photolysis of 10 mM NTA

250

solution at pH 10.0. Insertion in Figure 2 is the decay of eaq– detected at 630 nm in the presence of

251

PFOS with different concentrations. The transient absorption curves are fitted.

252

The N2O scavenging experiment provides an indirect proof for the potential role

253

of eaq–. However, a more direct proof is needed to reveal the formation and decay of

254

eaq– in UV/NTA process. Therefore, a laser flash photolysis study was conducted. The

255

absorption spectrum of intermediates was obtained at 10 nm intervals between 260

256

nm and 700 nm upon the photolysis of 10 mM NTA in N2 saturated water (Fig. 2).

257

The broad optical absorption band with a peak at around 630 nm is attributed to eaq–

258

since it is identical to the eaq– absorption spectrum acquired both theoretically36 and

259

experimentally37. The absorption peak of eaq– decayed continuously with monitoring

260

time, indicating eaq– gradually reacts back with other primary species. The decays of 15



261

eaq– in the presence of PFOS with different concentrations are shown in the inserted

262

figure in Fig. 2. In the absence of NTA (i.e., the laser photolysis of pure water), eaq–

263

was not produced. By adding 10 mM NTA, the absorbance at 630 nm increased

264

significantly, confirming the enhanced production of eaq– in the presence of NTA. The

265

addition of PFOS accelerated the decay of eaq–, and its decay rate increased at higher

266

PFOS concentration level. These results further verify the role of eaq– in PFOS

267

degradation.

268

Thus, the generation of eaq– in UV/NTA process has been clearly confirmed by

269

the laser flash photolysis study. However, compared with common eaq– source

270

chemicals like sulfite38, ferrocyanide39, iodide40, and aromatic compounds like

271

pyrenetetrasulfonate41, the transient yield of eaq– by UV/NTA process is much lower.

272

The relatively low transient yield of eaq– cannot explain the high efficiency of

273

UV/NTA process in PFOS degradation and defluorination. For instance, the 10-h

274

degradation and defluorination ratios of PFOS by UV/NTA process were 1.42-fold

275

and 1.58-fold, respectively higher than by UV/sulfite process. Therefore, we speculate

276

that besides direct photoejection of eaq– from NTA, there may be another dominant

277

mechanism for the efficient generation of eaq–.

278

Grossweiner et al. once proposed an alternative mechanism for the generation

279

of eaq– based on their observations, that is the photoionizaton of water itself sensitized

280

by the light-absorbing substances.12 In this case, a hydroxyl radical is produced by

281

dissociation of the photoionized water.12 According to the UV-Vis spectra (Fig. S10,

282

SI), direct photoionization of water under 254 nm light irradiation can be excluded 16


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283

since water is transparent in the near UV spectral domain (λ > 200 nm)16. However,

284

NTA has an absorbance at 254 nm, which means NTA is able to absorb photons and

285

possibly sensitize water ionization. Furthermore, APCAs can form hydration

286

complexes with several water molecules through hydrogen bonding. For instance,

287

eight water molecules are required to fully hydrate the first hydration shell of

288

deprotonated glycine.42 The solvation process may make it easier for the interactions

289

between NTA and water. Therefore based on foregoing facts, we speculate that water

290

photoionization sensitized by NTA is another major source of eaq– in UV/NTA

291

process.

292

In addition, an important reason for the low quantum yield of eaq– from pure

293

water photolysis is the rapid recombination of eaq– with ·OH, ·H and H3O+ (Eq. 3-5),

294

especially the reaction with ·OH, which accounts for more than 82% ± 3% of the

295

recombination of eaq–.43 The geminate recombination between eaq– and ·OH greatly

296

decreases the survival probability of eaq–. However in UV/NTA process, NTA is

297

highly reactive towards ·OH,30, 44 with a high reaction rate constant of 4.2×109 M-1·s-1

298

at pH of 10.0.22,

299

effectively protect eaq– from being quenched by ·OH, which increases the steady-state

300

concentration of eaq– and in turn facilitates the decomposition of PFOS. The eaq–

301

protection mechanism can be demonstrated by the long lifetime of eaq– in UV/NTA

302

process (Fig. 2). For instance, the half-life of eaq– in UV/NTA process is about 11.6 µs,

303

whereas it is only 1 ns in UV/sulfite process38 and lower than 2 ns in UV/KI process40.

304

Therefore, the presence of NTA can dramatically prolong the survival time of eaq– by

45

The strong scavenging capacity of NTA towards ·OH can

17



305

scavenging ·OH, thus promoting the degradation of PFOS. This explains why

306

UV/NTA process has a low transient quantum yield of eaq–, whereas has a high

307

efficiency in PFOS degradation.

308

Overall, the generation of eaq– during the UV/NTA process appears to involve

309

three steps. In aqueous solution, NTA is fully hydrated with both primary and

310

secondary hydration spheres. The fully hydrated NTA induces water photodissociation

311

and photoionization as a photosensitizer with the generation of eaq– and ·OH. Finally,

312

the NTA core scavenges ·OH to decrease the geminate recombination between ·OH

313

and eaq–, thus increasing the amount of eaq– available for PFOS degradation.

314 315

Figure 3. (A) DMPO spin-trapping ESR spectra recorded in the UV/NTA and UV/buffer

316

(NH3·H2O-NH4Cl) processes. (B) Simulated data of various radicals trapped by DMPO in

317

UV/NTA process after 60 s irradiation: (c) total ESR signal; (e) simulation of DMPO-·OH; (f)

318

simulation of DMPO-·CH2COOH; (g) simulation of NO·.

319

In order to verify our speculation, DMPO was used as an ESR spin-trap to

320

identify possible short-lived radicals produced in the UV/NTA process. As shown in

321

Fig. 3, after 30 s irradiation, several peaks were clearly observed in the ESR spectra, 18


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322

indicating formation of radicals and their involvement in the reaction. As the

323

irradiation time increased to 60 s, the ESR signal intensity also increased. No

324

well-defined ESR signal was observed in the UV/buffer process, indicating that the

325

radicals were not produced in the absence of NTA. In order to further confirm the

326

radical species, the ESR data of UV/NTA-60s reaction were simulated. The ESR

327

signal of 1:2:2:1 quartets is thus assigned to DMPO-·OH (curve e). The spectrum of

328

curve f is speculated to be the signal of DMPO-·CH2COOH. The three-line spectrum

329

(curve g) indicates the detection of free nitroxide (NO·).46, 47 The occurrence of ·OH

330

suggests that water indeed undergoes photodissociation in the UV/NTA process. NO·

331

and ·CH2COOH are presumably the reaction products of NTA with ·OH, which is

332

consistent with the reactive sites in NTA molecule (discussed in detail in section 3.3)

333

and also consistent with the known steady-state products of NTA degradation (Fig. S7,

334

SI). The ESR data of UV/NTA-30s has a similar fitting result with UV/NTA-60s

335

(shown in Fig. S11, SI). Therefore, the ESR measurement detected the occurrence of

336

·OH, ·CH2COOH and NO·, further confirming the mechanism that NTA induces water

337

photodissociation and photoionization, and scavenges ·OH to increase the quantum

338

yield of eaq–.

19



339

3.3. Model compound study

340

20


Page 20 of 41

Page 21 of 41


341

Figure 4. The time profiles of PFOS degradation and defluorination in the presence of model

342

compounds (NTA, IDA, EDTA, EDDA, citrate, glycine, oxamate, EDDS and MGDA): PFOS

343

(0.01 mM), model compounds (2 mM), UV irradiation, N2 saturated, pH (10.0). Error bars

344

represent standard deviations of triplicate assays.

345

In order to further validate the proposed mechanism and to test the effects of

346

other chelating agents, IDA, EDTA, EDDA, citric acid, glycine, oxamic acid, oxalic

347

acid, EDDS and MGDA were chosen as model compounds to investigate the

348

structure-activity relationship of APCAs in terms of impacting the photodegradation

349

of PFOS. The structural formulas of the model compounds are shown in Table S2, SI.

350

The results of the model compound study are shown in Fig. 4. Except oxamate and

351

oxalate, all of the tested model compounds accelerated the PFOS degradation and

352

defluorination compared with the control experiment, although their enhancement

353

effects varied a lot. For example, NTA was clearly better than IDA in accelerating

354

PFOS degradation. The 10-h PFOS degradation and defluorination ratios in presence

355

of NTA were 85.4% and 46.8%, respectively, while in the case of IDA they were only

356

54.8% and 25.6%, respectively. NTA and IDA are APCAs with only one nitrogen

357

atom. However, the nitrogen atom in NTA molecule is attached one more acetate

358

group (-CH2COOH) than IDA. Similarly, in the comparison of EDDA to EDTA, the

359

PFOS degradation and defluorination ratios were found to be higher in the presence of

360

EDTA (10-h values of 78.5% and 43.7%, respectively for EDTA and 51.1% and

361

24.2%, respectively for EDDA). This is because each nitrogen atom in EDTA

362

molecule is connected to one more acetate group than EDDA. Therefore, the acetate 21



363

group density appears to be a factor that determines the efficiency of the specific

364

APCA in promoting PFOS photodegradation.

365

Citric acid is an important organic tricarboxylic acid. According to the results

366

shown in Fig. 4, citrate also promotes PFOS degradation and defluorination as

367

compared with the control experiment in its absence. However, the enhancement

368

effect of citrate is lower than NTA. The 10-h PFOS degradation ratios in the presence

369

of NTA and citrate were 85.4% and 50.7%, respectively, with the corresponding 10-h

370

defluorination ratios of 46.8% and 24.1%, respectively. Citric acid has a similar

371

chemical structure with NTA, both containing acetate groups, whereas citric acid has

372

no amine group. Therefore, the greater acceleration in the presence of NTA than

373

citrate can be probably attributed to the electron-rich center of amine group, whose

374

lone pair on the nitrogen atom favors the electrophilic attack of ·OH.

375

Glycine and oxamic acid both contain an amine group and a carboxyl group, but

376

their relative impacts on PFOS degradation are totally different. Glycine clearly

377

promoted PFOS degradation compared with the control experiment, whereas oxamate

378

has a significant inhibitory effect. The 10-h PFOS degradation and defluorination

379

ratios in the presence of glycine were 61.5% and 29.4%, respectively, whereas they

380

were only 1.8% and 0.4%, respectively in the presence of oxamate. These results are

381

mainly due to the different moieties present in glycine and oxamic acid that connect

382

amine group and carboxyl group, i.e., methylene group (CH2) for glycine and

383

carbonyl group (C=O) for oxamic acid. Therefore, it is presumably the methylene

384

group in acetate group that offers a reactive site for ·OH attack. The significant 22


Page 22 of 41

Page 23 of 41


385

inhibition of oxamate is resulted from the electron-withdrawing property of

386

C=O-COOH group, which inductively withdraws electron density from the

387

neighboring N atom, thus reducing its reactivity with ·OH. Oxamic acid is reported to

388

be highly persistent during ·OH oxidation process.48, 49 In contrast, oxamic acid has a

389

high reactivity towards eaq–. The rate constant for the reaction of oxamate with eaq– is

390

reported to be 5.7×109 M-1·s-1 (pH = 9.2). The second-order rate constant for the

391

reactions between oxamate and eaq– is more than 3 orders of magnitude higher than

392

the corresponding rate constant for eaq– with glycine (1.7×106 M-1·s-1, pH = 11.8).

393

Therefore, oxamic acid competes with PFOS for eaq–, and thus inhibits PFOS

394

degradation. Similar to oxamic acid, the presence of oxalate significantly inhibited

395

PFOS degradation and defluorination (Fig. S12, SI). This result coincides with the

396

low reaction rate constant for oxalate with ·OH (7.7×106 M-1·s-1, pH = 6.0)10 and

397

further indicates that carboxyl group alone was unable to accelerate PFOS

398

degradation. In other words, the methylene group in acetate moieties is essential for

399

the effective attack by ·OH via H-atom abstraction.

400

In comparing EDTA to EDDS, we find that EDTA is more efficient with respect

401

to acceleration of PFOS degradation than EDDS. Although EDTA and EDDS both

402

have four acetate groups, two acetate groups in EDDS molecule are attached to the

403

α-C instead of the N atom. Therefore, EDTA is a tertiary amine while EDDS is a

404

secondary amine. The hydrogen bonding to the N atom in EDDS decreases the

405

availability of N-electrons and hinders the N-electrons transfer. Furthermore, the

406

presence of hydrogen on carbon next to the amine (α-hydrogen) was reported to be a 23



407

key factor for electron-transfer interactions.50 Therefore, an extra α-hydrogen in

408

EDTA may result in a higher electron-transfer capability. Compared with NTA, the

409

enhancement is more favored for MGDA. This is because MGDA has an additional

410

CH3 group on the carbon α of amine, which increases the electron density of N due to

411

the electron-donating inductive effect, and adds a reactive site for ·OH attack.

412

Therefore, the electronic distribution of N atom, especially the availability of the lone

413

pair on N atom also makes a difference for the efficiencies of APCAs.

414

In summary, the acetate group and amine group in APCAs play important roles

415

in accelerating the photodegradation of PFOS. This conclusion is consistent with the

416

reactive sites for the reactions of APCAs with ·OH as pointed out in literatures.22, 29, 45,

417

50-53

418

effect on ·OH by APCAs is the primary mechanism for the enhanced

419

photodegradation of PFOS.

Therefore, the results of our model compound study confirm that the scavenging

420

The effects of NTA degradation products including IDA, glycine, oxamate and

421

oxalate were summarized in Fig. S12, SI. Their efficiencies for the degradation and

422

defluorination of PFOS followed the order of NTA > glycine ≈ IDA > control >

423

oxamate ≈ oxalate. Compared with the control experiment, NTA, glycine and IDA

424

obviously accelerated the photodegradation of PFOS, whereas oxamate and oxalate

425

significantly inhibited. Since NTA degradation products have either lower efficiencies

426

than NTA or inhibiting effect, the PFOS degradation and defluorination rates

427

decreased gradually as NTA degraded, especially after 1 h of reaction.

24


Page 24 of 41

Page 25 of 41


428

3.4. Effects of pH and air

429 430

Figure 5. The effect of pH on the degradation (a) and defluorination (b) of PFOS: PFOS (0.01

431

mM), NTA (2 mM), UV irradiation, N2 saturated. Error bars represent standard deviations of 25



432

triplicate assays.

433

As is shown in Fig. 5, the degradation and defluorination of PFOS by UV/NTA

434

process is strongly dependent on pH. The PFOS degradation ratios under the pH

435

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%,

436

56.6%, 85.4% and 99.5%, respectively. The 10-h defluorination ratios of PFOS under

437

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%,

438

46.8% and 72.3%, respectively. After 2 h irradiation, the degradation and

439

defluorination ratios of PFOS at the pH value of 11.0 had reached 82.2% and 48.8%,

440

respectively, whereas they were only 2.1% and 0.1%, respectively at the pH value of

441

6.0. The strong dependence of PFOS degradation on pH is mainly attributed to the

442

following two reasons. First, at low pH values, eaq– is quickly quenched by H+ with a

443

high reaction rate constant of 2.3×1010 M−1·s−1 (Eq. 5). 10 The excessive consumption

444

of eaq– by H+ inhibits the degradation of PFOS and results in a low defluorination

445

efficiency under acidic condition. Second, pH affects the reactivity of NTA towards

446

·OH. The pKa values for successive deprotonation of NTA are 0.8, 1.9, 2.48 and

447

9.65.45, 54 The principal forms of NTA at pH 2.0 are HN+(CH2COOH)2(CH2COO−)

448

and HN+(CH2COOH)(CH2COO−)2; its major form at pH 6.0 is HN+(CH2COO−)3;

449

while at pH 10.0, NTA is near fully deprotonated in the form of :N+(CH2COO−)3. As

450

concluded in the model compound study, the lone pair on the nitrogen atom is one of

451

the primary reactive sites for the electrophilic attack of ·OH on APCAs molecules. At

452

lower pH values, the addition of a single proton to :N(CH2COO−)3 or to

453

HN+(CH2COO−)3 can decrease the rate constant for ·OH reaction by about one order 26


Page 26 of 41

Page 27 of 41


454

of magnitude.45 Therefore, the deprotonated form of NTA is the more reactive species

455

towards ·OH than its protonated or partially protonated counterparts. Leitner et al.

456

drew a similar conclusion that the reactivity of ·OH with NTA is related to the amount

457

of deprotonated nitrogen.22 The rate constant for the reaction of ·OH with NTA was

458

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

459

the pH values of 2.0, 6.0 and 10.0.45 Furthermore, since the pKa4 value of NTA is

460

9.65,45 the PFOS degradation rate increased significantly as pH increased from 9.0 to

461

10.0. Therefore, alkaline conditions are most favorable for the reaction of ·OH with

462

NTA, thus making more eaq– available for the degradation of PFOS.

27



463 464

Figure 6. The effect of air on the degradation (a) and defluorination (b) of PFOS: PFOS (0.01

465

mM), NTA (2 mM), UV irradiation, pH (10.0). Error bars represent standard deviations of

466

triplicate assays. 28


Page 28 of 41

Page 29 of 41


467

Besides pH, air is another important parameter that affects the photoreductive

468

degradation efficiency of PFOS. In general, the eaq–-mediated degradation of PFOS

469

requires strict anoxic conditions to prevent eaq– from being quenched by molecular

470

oxygen (Eq. 1).9, 11 For instance, Park et al. reported that PFOS was degraded rapidly

471

by UV/Iodide process in the presence of Ar with a rate constant of 6.5×10-3 min-1,

472

whereas it was not degraded in the presence of air.5 Therefore in this study, PFOS

473

degradation was conducted under both N2 and air saturated conditions to evaluate the

474

effect of air. Under N2 saturation, the solution was pre-bubbled with N2 for 20 mins

475

and kept bubbling during the whole reaction period. Under air saturation, the solution

476

was not pre-bubbled and the reactor was kept open with the solution exposed to air

477

during the whole reaction period. The results are shown in Fig. 6. The degradation

478

ratios of PFOS under N2 and air saturated conditions after 10 h irradiation were 85.4%

479

and 75.2%, respectively. The 10-h defluorination ratios of PFOS under N2 and air

480

saturation were 46.8% and 35.8%, respectively. The PFOS degradation and

481

defluorination efficiencies under N2 saturation were just slightly higher than under air

482

condition. No significantly negative impact of air was observed. This is because the

483

excited NTA under UV irradiation (NTA*) can react with O2.30 Meanwhile, ·OH can

484

abstract the alpha hydrogen from the methylene group in NTA molecule.45 The

485

hydrogen abstracted radical intermediate can also react with O2.45 As a reductant,

486

NTA can scavenge a variety of oxidizing species produced in the presence of O2 such

487

as superoxide radicals (O2·−) and HO2 radicals (HO2·), which are also active

488

quenchers of eaq–.55 The sequestration of these oxidants by NTA protects eaq–, and thus 29



489

enhances the degradation of PFOS. Therefore, the photodegradation of PFOS by

490

UV/NTA process is less sensitive to air than previously reported photoreductive

491

approaches like UV/KI11 and UV/sulfite56 processes. This result bodes well for future

492

applications of in situ or ex situ PFOS photo-remediation since strict anoxic

493

conditions are often difficult to achieve in practical engineering applications.

494

The effect of NTA concentration was also evaluated in this study. A higher NTA

495

concentration over the range of 0 to 4.0 mM resulted in higher PFOS degradation and

496

defluorination ratios (Fig. S13, SI). A detailed discussion is available in the SI.

497

4.

Environmental Implications

498

This study presents a new strategy for efficient photoreductive degradation and

499

defluorination of PFOS. Compared with previous photochemical approaches, the

500

newly developed UV/NTA process has three remarkable advantages. First, PFOS

501

undergoes rapid photodegradation in the presence of NTA, with a high defluorination

502

rate. Second, no significant detrimental impact of air was observed, which means

503

UV/NTA process has a certain tolerance to oxygen. Third, the end products of NTA

504

degradation are CO2, NH4+ and NO3−, which are easily biodegradable with minimized

505

secondary pollution risks. Therefore, UV/NTA may provide a novel, efficient and

506

relatively green technology for future in situ or ex situ PFOS remediation.

507

In previous studies, the most common way for the generation of eaq– to

508

decompose PFOS was by adding eaq– source chemicals such as iodide, sulfite and

509

indole derivatives. Meanwhile, attentions were more often paid on the improvement

510

of the transient generation of eaq–, such as increasing the reaction temperature or 30


Page 30 of 41

Page 31 of 41


511

applying high photon flux. Actually, the apparent quantum yield of eaq– or its ultimate

512

efficiency is not just determined by its transient quantum yield. It is also greatly

513

influenced by the twinborn species such as ·OH and oxidative byproducts. For

514

instance, the unsatisfactory activity of UV/iodide process towards PFOS degradation

515

is likely a result of triiodide scavenging of eaq–.57 In this study, we proposed a new

516

way to generate eaq–, which utilizes NTA to sensitize water photoionization, and

517

subsequently to scavenge ·OH, in order to minimize the geminate recombination

518

between ·OH and eaq–. The net effect is to increase the steady-state level of eaq– that is

519

available for PFOS degradation. The laser flash photolysis results indicate that

520

although UV/NTA process has a low transient yield of eaq–, it has a long half-life of

521

eaq–, which is considered to be the main reason for the high efficiency of UV/NTA

522

process.

523

APCAs is a class of compounds acting as chelating agents. Interests in previous

524

studies were mainly focused in their chelating effect with metal ions. Other APCAs’

525

properties receive little attention. For example, they are excellent electron donors and

526

they have a high reactivity with ·OH. In this study, we revealed that the

527

photoreductive degradation of PFOS can be enhanced by various APCAs such as

528

NTA, IDA, EDTA, EDDA, EDDS and MGDA. The acetate and amine groups are the

529

primary

530

photoreductive process may be a novel application field for APCAs, which is also

531

expected to be a promising replacement for the current photoredutive technologies for

532

PFOS decomposition.

reactive

sites

in

APCAs

molecules.

Therefore,

31


APCAs-mediated


533

Associated content

534

Supporting Information

535

Schematic diagram of the photochemical reactor; Technical data of the low-pressure

536

mercury lamp; additional details on analytical methods; kinetics for PFOS

537

degradation; abbreviation and chemical structure of model compounds. Figures

538

showing time profiles of PFOS degradation products; mass balance of F; NTA and

539

NTA degradation products; NTA degradation pathway; effect of N2O; UV-Vis

540

absorption spectra; ESR spectra; effect of NTA degradation products and effect of

541

NTA concentration. Discussion on the effect of NTA concentration. This information

542

is available free of charge via the Internet at http://pubs.acs.org.

543

Acknowledgements

544

The authors greatly thank Dr. Jiahui Yang from Bruker (Beijing) Scientific

545

Technology Co., Ltd for her kind assistance on the ESR result analysis. The authors

546

also gratefully acknowledge Prof. Side Yao and Dr. Huijie Shi for their valuable

547

comments on the discussion. This study has been supported by the National Natural

548

Science Foundation of China (Project No. 21677109), and the State Key Laboratory

549

of Pollution Control and Resource Reuse Foundation (No. PCRRT16001).

550 551

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552

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CO2, NH4+, NO3−

hv

O

H H

C

C

O

H H C

O

N H C

C O

C

NTA scavenges ∙OH

∙OH

H

O

O

Geminate recombination NTA sensitized

H 2O H

C

water photoionization

N

O

F

eaq−

S

PFOS Degradation & Defluorination


Nitrilotriacetic Acid Process as a Novel Strategy for Efficient

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