Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis

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Environmental Processes

Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis of Aqueous Nitrous Oxide Guoyang Zhang, Shijie Wei, Bingdang Wu, Zhihao Chen, and Shujuan Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02145 • Publication Date (Web): 03 Aug 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Environmental Science & Technology

Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis of Aqueous Nitrous Oxide Guoyang Zhang, Shijie Wei, Bingdang Wu, Zhihao Chen, Shujuan Zhang*

State Key Laboratory of Pollution Control and Resource Reuse, School of the

Environment, Nanjing University, 163 Xianlin Avenue, Nanjing, 210023, China

*Correspondence author. Phone: +86 25 89690389, E-mail: [email protected]

Submitted to: Environmental Science & Technology

ACS Paragon Plus Environment


Table of Contents (TOC) Art

1


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1

ABSTRACT

2

Nitrous oxide (N2O) is widely used in radiation-chemistry and photochemistry as a scavenger to

3

convert hydrated electron (eaq-) to hydroxyl radical (·OH). However, few investigation pays

4

attention to the photochemistry of dissolved N2O itself. The effects of purged N2O on

5

photochemical processes are unclear and neglected. In the present work, the effects of N2O on the

6

hydroxylation of terephthalic acid (TPA) were investigated with both medium-pressure and low-

7

pressure mercury lamps as the light sources. Under short-wavelength UV (200-300 nm) irradiation,

8

N2O accelerated the decay of TPA and the formation of 2-hydroxylterephthalic acid (hTPA). The

9

effective quantum yield of ·OH from the photolysis of dissolved N2O at 254 nm was determined

10

as 1.15-1.63, which was far larger than those of NO3- (0.09) and NO2- (0.046). Based on kinetic

11

analysis in N2 and N2O purged solutions, isotope fractionation with heavy oxygen water, and ·OH

12

scavenging experiments with tert-butyl alcohol, the contribution of the ·OH radicals generated

13

from photolysis of N2O to the formation of hTPA (61.7%) was determined to be one order of

14

magnitude higher than that from the converted eaq- (6.5%). These results demonstrate that using

15

N2O and ·OH probes to quantify photogenerated eaq- in UVC irradiation might lead to false results.

16

The work here is helpful for the proper design of scavenging and probing experiments by the

17

combination of N2O and ·OH probes.

18

2



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19

INTRODUCTION

20

Nitrous oxide (N2O) is widely used as a scavenger of hydrated electron (eaq-) in radiation-chemistry

21

and photochemistry studies.1-4 For example, a highly sensitive approach was developed to quantify

22

the photoproduction of eaq- from humic substances by purging N2O into the solution.5 Based on

23

the rationale that eaq- can be efficiently transformed to hydroxyl radical (·OH) by N2O (Reaction

24

1), the combination of N2O with some typical ·OH probes, such as coumarin (COU) and benzene,

25

was used to assess whether eaq- was formed as a primary photolysis product of NO2-.6,7 The above

26

photolysis experiments were conducted with light sources containing UVC (200-280 nm) photons.

27

The photochemistry of gaseous N2O in the wavelength (l) range of 185-230 nm (denoted as s-UV)

28

has been studied extensively.8-11 However, few investigation has paid attention to the

29

photochemistry of the dissolved N2O.

30

eaq - + N2O + H2O ® × OH + OH- + N2

31

Under s-UV irradiation, N2O can decompose into either dinitrogen and atomic oxygen or nitric

32

oxide and atomic nitrogen (Reactions 2-5).12 The generated atomic oxygen can then react with

33

N2O to produce nitric oxide or molecular oxygen/nitrogen (Reactions 6 and 7).9 hv

34

N2O ® N2 (1Σ) + O( 3 P)

35

N2O ® N2 (1Σ) + O( 1 D)

36

N2O ® N2 (1Σ) + O( 1S)

37 38

k2 = 9.1 × 109 M-1 s-1

(1)

l ≤ 742 nm

(2)

l ≤ 341 nm

(3)

l ≤ 212 nm

(4)

N2O ® NO(2P) + N( 4 S)

l ≤ 252 nm

(5)

N 2 O + O( 1 D) ® 2NO

k2 = 6.7 × 10-11 cm3 molecule-1 s-1

(6)

hv

hv

hv

3


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39

N 2O + O( 1 D) ® N 2 + O 2

40

Reaction 3 has a quantum yield near 1 and has been considered as the primary pathway in the

41

photolysis of gaseous N2O.11-13 Once dissolved into water or complexed with a H2O molecule, the

42

electronic absorption spectrum of N2O was changed.14-16 However, the generation of O(1D) was

43

still regarded as a key step in the photolysis of aqueous N2O under s-UV irradiation.17 The O(1D)

44

inserted into a H2O molecule to form H2O2 (Reaction 8) or directly to ·OH (Reaction 9).

k2 = 4.9 × 10-11 cm3 molecule-1 s-1

(7)

45

O(1D) + H 2O ® H 2O 2

(8)

46

O(1D) + H 2 O ® 2 × OH

(9)

47

Another possible pathway in the photolysis of water-bounded N2O was proposed as the

48

decomposition of excited H2O·N2O complex to either H2O2 (Reaction 10) or ·OH (Reaction 11).18 hv

49

[ N 2O × H 2O]

® N 2 + H 2O 2

50

[ N 2O × H 2O]

® N 2 + 2 × OH

51

H2O2 ® 2 × OH

52

The photolysis of H2O2 (Reaction 12) is known as a typical way for the generation of ·OH

53

with a quantum yield around 1.19 Taking Reactions 8-12 into account, no matter decomposed

54

through which way, the final products of aqueous N2O under s-UV irradiation would be ·OH.

(10)

hv

(11)

hv

(12)

55

In most photochemical studies for water treatment, low-pressure (LP) and medium-pressure

56

(MP) mercury lamps are widely used as light sources. The main effective emission is in the range

57

of 250-405 nm. To the best of our knowledge, the photochemistry of aqueous N2O in the UV range

58

of 250-405 nm is unclear yet. Considering the known photodissociation reactions of the H2O·N2O

4



59

complex or the dissolved N2O, N2O-purging is applicable to quench eaq-, but might lead to artifacts

60

if the quantification of eaq- was conducted with ·OH probes.

61

To clarify the suitability of using N2O as a scavenger for reductive species in photochemical

62

systems, the photo-degradation of terephthalic acid (TPA), a widely used ·OH probe,20-23 in

63

aqueous solutions with and without N2O purging were investigated. The main objectives of this

64

work are to investigate the effects of N2O on the hydroxylation of TPA and consequently to

65

evaluate the applicability of using N2O and ·OH probes as an approach to quantify photogenerated

66

eaq- in UVC irradiation.

67 68

EXPERIMENTAL

69

Material. H2O2 of analytical grade was purchased from Shanghai Reagent Station, China.

70

NaOH, HCl, KI, KIO3, Na2B4O7, NaH2PO4·2H2O and Na2HPO4·12H2O of analytical grade were

71

obtained from Nanjing Reagent Station, China. NaNO3, NaNO2 and H3PO4 of analytical grade

72

were purchased from Sinopharm Chemical Reagent Co., Ltd., China. N, N-diethyl-p-

73

phenylenediamine (DPD), methanol and formic acid of chromatographic grade were purchased

74

from Sigma-Aldrich, China. Tert-butyl alcohol (TBA) (99.5%), peroxidase (POD) (≥ 3000 u mg-

75

1

76

TPA (99%, CAS number: 100-21-0) and heavy oxygen water (H218O) (99.99%) were purchased

77

from Macklin, China. 2-hydroxylterephthalic acid (hTPA) (98%, CAS number: 636-94-2) were

78

purchased from Aladdin, China. COU (99%, CAS number: 91-64-5) and 7-hydroxycoumarin

79

(hCOU) (99%, CAS number: 93-35-6) were obtained from J&K, China. Benzoic acid (BA) (99%,

) from horseradish were purchased from Shanghai Yuanye Biology Technology Co., Ltd., China.

5


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80

CAS number: 65-85-0) and 2-hydroxybenzoic acid (2hBA) (99%, CAS number: 69-72-7) were

81

purchased from Sigma-Aldrich, China. More detailed information about the tested probes is listed

82

in Table S1.

83

High purity N2 (99.999%) and N2O (99.95%) were purchased from Nanjing Tianze Gas Co.,

84

Ltd., China. Ultrapure water (18.25 MW cm) was made with a water purification system (Shanghai

85

Ulupure Industrial Co., Ltd., China) and used for the preparation of sample solutions.

86

Prior to use, 10 g L-1 DPD (in 0.05 M H2SO4) and 1 g L-1 POD stock solutions were prepared

87

according to the literature24 and were stored in the dark at 4oC. TPA stock solution of 1 mM was

88

prepared in a diluted NaOH solution (5 mM) to ensure its full dissolution. Prior to irradiation, all

89

the solutions were adjusted with NaOH or HClO4 solutions to pH 9.0.

90

Irradiation Experiments. UV irradiation experiments were carried out in a rotating disk

91

photoreactor (Nanjing StoneTech Electric Equipment, China) with a 300 W medium-pressure

92

mercury lamp (MP-Hg, Shanghai Hongguang Tungsten & Molybdenum Technology Co., Ltd.) or

93

with a 10 W low-pressure mercury lamp (LP-Hg, GPH212T5L, Heraeus Noblelight) as the light

94

sources (Figure S1). The MP-Hg lamp was polychromatic with a maximum light emission at 365

95

nm and the LP-Hg lamp was monochromatic at 254 nm. The light intensity was measured with a

96

radiometer (Photoelectric Instrument Factory of Beijing Normal University, China) equipped with

97

two sensors of peak sensitivity at 365 nm for MP-Hg lamp and 254 nm for LP-Hg lamp. Unless

98

otherwise stated, the irradiation experiments in this work were conducted with the MP-Hg lamp.

99

In purging experiments, the sample solutions were firstly purged with the corresponding gas

100

for 30 min prior to irradiation and then continuously purged during photo-irradiation (Figure S1). 6



101

In order to control the concentration of N2O, a series of N2O and N2 mixtures of different volume

102

percentage (v/v%) was inflated with variable-area flow meters (Kede Thermos-Technical

103

Instrument Co., Ltd., China) and the flow rate of the mixed gas was about 25 L h-1. The dissolution

104

of N2O conformed to the Henry’s law (Figure S2a). The concentrations of N2O were 3.3, 7.9, 12.3,

105

16.8, 19.3, and 24.1 mM when the volume fractions of N2O were 16.7%, 31.4%, 40.0%, 50.0%,

106

60.0%, and 70.1%, respectively. A glove box system (Mikrouna Co., Ltd., China) was used to

107

prepare solutions free of dissolved O2 (DO).

108

Analytical Methods. UV-Vis spectra were recorded with a double beam spectrophotometer

109

(UV-2700, Shimadzu, Japan). Total organic carbon (TOC) was determined with a Multi N/C TOC

110

apparatus (TOC-L, Shimadzu, Japan). DO was determined with an HQ30d apparatus (HACH,

111

USA). The concentrations of TPA, COU, hTPA, hCOU, BZ and 2hBZ were determined with a

112

high performance liquid chromatography (HPLC) system (Waters 1525, USA) equipped with an

113

Agilent C18 reversed phase column (150 mm × 4.6 mm, 5 µm) at 25oC. The injection volume was

114

5 µL. The detailed HPLC conditions are listed in Table S1.

115

16

O-hTPA and

18

O-hTPA were identified with a Thermo Scientific Q ExactiveTM Focus

116

Orbitrap LC-MS/MS System (Thermo, USA) without separation and were detected in a negative

117

ion mode with an electrospray ionization (ESI) source. The settings for ESI were: capillary

118

temperature: 350oC, spray voltage: 3.0 kV, sheath gas flow: 35, auxiliary gas flow: 20 (arbitrary

119

units), capillary voltage: 25 V, and tube lens offset: 100 V.

120

H2O2 could oxidize POD to a higher valent state, which in turn oxidized DPD to a pink colored

121

radical cation DPD·+ with a maximum absorption at 551 nm.24 Other organic hydroperoxides could 7


Page 8 of 35

Page 9 of 35


122

also oxidize POD. According to the stoichiometric ratio, the overall peroxides could be

123

approximately estimated with the DPD/POD method based on the standard curve of H2O2 (Figure

124

S2b).

125

The concentration of dissolved N2O was determined using the headspace method with a gas

126

chromatograph (GC, Thermo Scientific Fisher Trace 1310, USA).25, 26 The GC was equipped with

127

an electron capture detector (ECD) and a 5 Å molecular sieve column (30 m × 0.32 mm × 30 µm).

128

The carrier gas was N2 at a flow rate of 5 mL min-1. The injector, oven, and detector temperatures

129

were 110, 200, and 250oC, respectively. Sample solutions of 1 mL were equilibrated in 20 mL

130

serum vials at 25oC for 24 h prior to injection for GC analysis. The injection volume of headspace

131

gas was 100 µL. Total N2O concentration, including gaseous and dissolved N2O, was calculated

132

based on the solubility constant.

133

Quantum Yield Determination. In order to obtain the effective quantum yield of ·OH (Φ·OH)

134

from N2O photolysis in aqueous solution, a monochromatic LP-Hg lamp was used as the light

135

source with the formation of hTPA as an index. Since N2O-purging can reduce DO, N2-purging

136

experiments were conducted and employed as the background reference, which will be revisited

137

later.

138

hTPA is a primary product in the photolysis of TPA and in the reaction of TPA with ·OH. The

139

interaction of ·OH with TPA will not exclusively generate hTPA, but also some other products.27

140

Therefore, the hydroxylation yield of TPA into hTPA is defined as YhTPA, which is the ratio between

141

the initial formation rate of hTPA (k’0hTPA) and the initial degradation rate of TPA ([TPA]0 k’1TPA).

142

YhTPA = k’0hTPA / ([TPA]0 k’1TPA)

(13) 8



Page 10 of 35

143

k’0hTPA = k0, N2OhTPA – k0, N2hTPA

(14)

144

k’1TPA = k1, N2OTPA – k1, N2TPA

(15)

145

where k0, N2OhTPA and k0, N2hTPA are the initial pseudo-zero-order formation rate constants of hTPA

146

in N2O-purged and N2-purged solutions, respectively. k1, N2OTPA and k1, N2hTPA are the pseudo-first-

147

order degradation rate constants of TPA in N2O-purged and N2-purged solutions, respectively.

148

[TPA]0 is the initial concentration of TPA.

149

The deduction in Equations 14 and 15 ensures that the k’0hTPA and [TPA]0 k’1TPA come

150

exclusively from the reaction of TPA with ·OH that is produced from the photolysis of N2O. In

151

this case, the formation and decay of ·OH follows the steady-state assumption, i.e., the

152

concentration of ·OH remains constant over time and is the ratio of ·OH formation rate to ·OH

153

quenching rate. Therefore, k’0hTPA can be expressed as:

154

k '0 hTPA = k2 [TPA]0 [ × OH]ss YhTPA

(16)

155

where [·OH]ss is the steady-state concentration of ·OH, k2 is the second-order reaction rate

156

constants of TPA and ·OH, which is reported to be 4.4 × 109 M-1 s-1.21

157

In the literature, YhTPA is reported as 0.35 in the UV/NO2- systems.21 In the N2O-purged

158

solution, YhTPA was calculated to be 0.36 ± 0.07 (Table S2). With k’0hTPA and YhTPA, the Φ·OH in the

159

photolysis of N2O could be obtained as28:

160

F×OH =

k '0 hTPA ´ V I 0 ´ (1 - 10-e CL ) ´ YhTPA

(17)

9


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161

where I0 is the photon flux entering the solution (einstein s-1), L is the effective path length (cm),

162

V is solution volume (L), ε is the molar absorption coefficient of N2O, and C is the concentration

163

of N2O (M).

164

The I0 at 254 nm was determined to be 5.08 ± 0.01 × 10-8 einstein s-1 by the KI/KIO3 method

165

(Text S1 and Figure S3).29,30 The effective path length (L) was obtained to be 2.22 ± 0.02 cm from

166

the photolysis kinetics of a diluted H2O2 solution (Text S2 and Figure S4).30,31

167 168

RESULTS AND DISCUSSION

169

Effects of Dissolved N2O on the Photolysis of TPA. The photodegradation of TPA was

170

conducted side-by-side in ambient, N2-purged, and N2O-purged aqueous solutions. After N2 or

171

N2O-purging, the residual DO in the TPA solution was reduced from 8.56 mg L-1 (ambient) to

172

0.26-0.30 mg L-1 (N2-purged) or 0.45-0.46 mg L-1 (N2O-purged). As shown in Figure 1a, the

173

photodegradation of TPA was enhanced by N2 or N2O-purging. This was consistent with the

174

observed DO effect on the photodegradation of some pharmaceuticals.32 The pseudo-first-order

175

degradation rate constant (k1) of TPA in the N2O-purged solution was 2.7 times to that in N2-purged

176

solution, indicating that the effects of N2O-purging were not merely the elimination of DO. The

177

contribution of other reactive species in the photochemical process of N2O could not be ignored.

178

There was a linear relationship between the N2O concentration and the k1 of TPA (k1 = 0.0048 ×

179

[N2O] + 0.7843, R2 = 0.96) (Figure 1b). As the N2O dosage was increased from 3.2 to 24.1 mM,

180

the k1 of TPA was increased from 0.088 to 0.185 min-1.

10



Page 12 of 35

181

The formation of hTPA in the irradiated TPA solutions followed the pseudo-zero-order kinetics

182

(Figure S5). The k0 of hTPA in the N2O-purged solution (0.255 µM min-1) was 11.1 times to the

183

ambient counterpart (0.023 µM min-1) and was 4.5 times to that in the N2-purged solution (0.057

184

µM min-1), indicating that more ·OH radicals were generated in the N2O-purged solutions (Figure

185

S5). This was also evidenced by the larger TOC reduction in the N2O-purged solutions (Figure S6).

186

Enhanced generation of hydroxylation products in N2O-purged solutions was also observed for the

187

irradiated systems with COU and BA as the target molecules, which are also widely used ·OH

188

probes in photochemical studies (Figure S7).6,33-35

189

The [·OH]ss in N2O-purged solution was calculated to be 5.50 × 10-15 M (Table 1). Based on

190

the k values, the Φ·OH of hydrated N2O at 254 nm was determined as 1.15-1.63, indicating the

191

occurrence of Reaction 11. To verify the reliability of the determination method, NO3- was

192

employed as a reference.

193

It is well known that the photolysis of nitrate (NO3-) and nitrite (NO2-) can generate ·OH with

194

quantum yields of 0.090 and 0.046, respectively.36, 37 The Φ·OH of NO3- at 254 nm was calculated

195

as 0.06, which is close to the reported value. The photolysis of TPA in the N2O-purged solution

196

was compared with those in the UV/NO2- and UV/NO3- systems. The addition of NO2- or NO3- in

197

the concentration range of 5-100 µM slightly increased the k1 of TPA (Figure 2a). Correspondingly,

198

the k0 of hTPA formation was increased by the addition of NO2- or NO3-, but was still much lower

199

than that in the N2O-purged solution (Figure 2b).

200 201

It should be noted that a further increase of NO2- or NO3- would not necessarily increase the k0 of hTPA, because the unfavorable inner filter effect of NO2- or NO3-. 11


Page 13 of 35


hv

202

NO3- + H+ ® × OH + × NO2

203

NO2- + H+ ® × OH + × NO

204

Effective Wavelength Range. According to the Grotthuss-Draper law, only the absorbed light

205

can cause a photochemical change. The absorption of dissolved N2O was very weak above 200

206

nm. This might be the reason why the previous reports on the photochemistry of N2O were limited

207

in the short wavelength range. From 200 nm to 300 nm, the molar extinction coefficient (e) of

208

aqueous N2O gradually reduces from 11.6 to 0.001 M-1 cm-1. The e of aqueous N2O at 254 nm is

209

in the range of 0.0012-0.0017 M-1 cm-1 (Table S3).

(18)

hv

(19)

210

All the above kinetic experiments were conducted with a MP-Hg lamp as the light source. Its

211

emission spectrum is shown in Figure S8. To distinguish which fraction of the emission spectrum

212

played the key role in the photolysis of N2O aqueous solution, two filters were inserted individually

213

into the light path between the MP-Hg lamp and the sample tubes. The 365 nm filter (BP365) and

214

420 nm filter (CF420) allow the penetration of photons in the wavelength range of 300-365 nm

215

and > 420 nm, respectively. The use of filters also suppressed the intensity of the light (Figure S8).

216

In the presence of the CF420 filter, hTPA was not detected in the N2O-purged solution, even

217

irradiated for a much longer time (80 min). With the use of the BP365 filter, the formation rate

218

constant of hTPA in the N2O-purged solution was decreased from 0.255 µM min-1 to 0.0007 µM

219

min-1 (Table S4). Compared with the N2-purged solution, no more hTPA was formed in the N2O-

220

purged solution (Table S4). All the above results demonstrate that the photons with wavelength

221

longer than 300 nm took no effect on N2O. Similar results have been reported in the literature.6, 38

222

For example, with a Xenon lamp as the light source equipped with a 305 nm high-pass filter, the 12



Page 14 of 35

223

purging of N2O had no effect on the photoionization of anthracene, benz[a]anthracene and 9-

224

methylanthracene.38

225

Role of N2O in hTPA Formation. There are at least two routes for the formation of hTPA in

226

the irradiated TPA solution: ·OH-independent (the direct photolysis of TPA to TPA˙+, which then

227

reacts with H2O to form hTPA) and ·OH-dependent (formation of ·OH first, followed with the

228

electrophilic addition of ·OH to TPA). In the photoionization of TPA, a hydrated electron (eaq-)

229

was ejected, accompanying with the formation of TPA˙+. The photo-ejected eaq- could react with

230

N2O to form ·OH (in N2O-purged solution) or react with O2 to form O2˙- (in ambient and N2-purged

231

solution). The O2˙- could then disproportionate to H2O2 and consequently photo-dissociate to ·OH.

232

As shown in Figure 3a, after UV irradiation, a considerable amount of H2O2 was detected in

233

the N2O-purged ultrapure water (in the absence of any organic matter, H2O2 was the only possible

234

peroxide), whereas there was no H2O2 formation in the ambient water. These results indicate that

235

N2O is the only source for the formation of H2O2 in the irradiated ultrapure water. The detected

236

peroxide concentration in the N2O-purged solution was decreased by the presence of 20 µM TPA

237

(Figure 3b). There were negligible peroxides in the N2-purged TPA solution. However, a

238

considerable amount of peroxides was formed in the ambient TPA solution. This is reasonable,

239

because the formation of organic peroxides from TPA needs the involvement of DO. The organic

240

peroxides might also serve as a source of ·OH23 and account for the formation of hTPA.

241

For clarity, all the possible pathways for the formation of hTPA are illustrated in Scheme 1.

242

Route 1 (·OH-independent):

13


Page 15 of 35


H2 O

hv

243

(I) TPA ® TPA* ® TPA×+ ® hTPA

244

Route 2 (·OH-dependent):

245

® × OH ® hTPA (II) TPA ® TPA* ® eaq - ¾¾

246

(III) N2O + H2O ® H2O2 or × OH ® hTPA

247

(IV) TPA ® TPA* ® Organic peroxides ® × OH ® hTPA

248

Path I occurred in all the systems, whereas the other pathways were highly dependent on the

249

gas conditions. Theoretically, in N2-purged solution, Path I is the only one accounting for the

250

formation of hTPA. Since a H2O molecule was involved in this pathway, if H218O was used as the

251

solvent, all the formed hTPA should be 18O-hTPA. As shown in Table 2, the generation of 18O-

252

hTPA in the N2-purged solution was negligible as the solvent was H216O (0.7%), whereas the

253

fraction of 18O-hTPA was significantly increased as H218O was used as the solvent (16.8%) (Table

254

2 and Figure S9). The abundance of 18O in H216O was 0.2%.39 The detected fraction of 18O-hTPA

255

in the H216O solution was reasonable. However, considering the fraction of

256

solution (99%), the fraction of 18O-hTPA in the H218O solution (16.8%) was significantly lower

257

than the expected value (99%), indicating that about 83.2% of the hTPA was generated through

258

other pathways.

N2 O or O2

hv

hv

hv

(20)

TPA

(21)

TPA

O2

hv

(22) TPA

(23)

18

O in the H218O

259

We speculate that the residual DO in the N2-purged solution (0.26 mg L-1) was attributable to

260

the unexpected formation of hTPA. To check this speculation, an O2-free TPA solution was

261

prepared in a glove box system. The DO in this O2-free TPA solution was below the detection limit

262

(DO < 0.01 mg L-1). As shown in Figure 4, the k0 values of hTPA in the O2-free solution were 19%

263

and 17% of those in the N2-purged solution with the MP-Hg lamp and the LP-Hg lamp as the light 14



Page 16 of 35

264

sources, respectively. These results fully demonstrate the role of the residual DO in the formation

265

of hTPA. It should be noted that the DO had a dual-effect on the formation of hTPA. On the one

266

hand, the DO could enhance the formation of hTPA through the generation of peroxides (Path IV).

267

On the other hand, the DO could quench the excited TPA, leading to reduced formation of hTPA

268

through Path I. When the concentration of DO was high, the quenching effect dominated, which

269

is evidenced by the lower k0 values of hTPA in the ambient solution as compared with those in the

270

N2-purged solution (Figure 4, Tables 2 and 3). The positive effect of DO on hTPA formation

271

dominated over the quenching effect in the N2-purged and N2O-purged solutions, and its

272

contributions in the two purged systems should be similar.

273

Besides Path I, Path III was also related with H2O molecules. The fraction of 18O-hTPA in the

274

N2O-purged solution (38.6%) was significantly higher than that in the N2-purged solution (16.8%)

275

(Table 2), because the photo-ejected eaq- was converted to ·OH by N2O. The N2O-purged and N2-

276

purged solutions had similar residual DO. Therefore, it is reasonable to infer that the contributions

277

of Paths I, II, and IV in the N2O-purged solution were identical to those in the N2-purged solution.

278

Thus, the difference in k0

279

contribution of Path III. The stoichiometric ratios of the 18O-hTPA to the total hTPA formed in

280

Path I and Path III were 1 and 2, respectively. Based on the k0

281

contribution of Path III in the N2O-purged solution (fN2O, III) could be calculated as:

18O-hTPA

18O-hTPA

between the N2O-purged and N2-purged solutions reflected the

- k0, N2

18O-hTPA

) / ( k0, N2O

18OH

18O-hTPA

+ k0, N2O

16OH

values in Table 1, the

282

fN2O, III = 2 × (k0, N2O

283

The contribution of ·OH to the formation of hTPA was further evaluated by using TBA as

284

a ·OH scavenger. The formation of hTPA was significantly inhibited by the addition of TBA (Table 15


) = 61.7%

(24)

Page 17 of 35


285

3). Based on the rate constants with ·OH (6.0 × 108 M-1 s-1 for TBA4 and 4.4 × 109 M-1 s-1 for

286

TPA21) and the concentrations of TBA and TPA used in the scavenging experiments (100 mM TBA

287

and 20 µM TPA), theoretically, more than 99% of ·OH radicals could be scavenged by TBA.

288

Calculated with a previously reported approach,25 the inner filter effect caused by TBA to the tested

289

solutions were negligible. The scavenging effects might be exclusively attributed to the scavenging

290

reaction between TBA and ·OH.

291

In the N2-purged solution, the inhibition of TBA on hTPA formation (Table 3) was derived

292

from Path II and Path IV, which (fN2, II + fN2, IV) account for 26.1% of the total yield. Except the

293

contribution of Path III, the contributions of the other pathways in the N2O-purged solution, as

294

aforementioned, should be the same as those in the N2-purged solution. In the N2O-purged solution,

295

the ·OH radicals came from Paths II, III and IV. The fN2O, III has been quantified as 61.7% in the

296

H218O experiments. Therefore, the contributions of Path II and Path IV in the N2O-purged solution

297

(fN2O, II + fN2O, IV) could be calculated as:

298

fN2O, II + fN2O, IV = (1 - fN2O, III) × (fN2, II + fN2, IV) = 10.0%

299

Thus, the sum of the contributions of Paths II, III and IV in the N2O-purged solution (fN2O, II +

300

fN2O, III + fN2O, IV) was 71.7%, which was consistent with the inhibition rate caused by TBA (71.3%

301

in Table 3).

302 303 304

(25)

The contribution of Path I in the N2-purged solution (fN2, I) was obtained as 16.8% (Table 2). Thus, the contribution of Path I in the N2O-purged solution (fN2O, I) could be calculated as: fN2O, I = (1 - fN2O, III) × fN2, I = 6.5%

(26)

16



Page 18 of 35

305

The stoichiometric ratio of eaq- to TPA˙+ was 1. Therefore, the contribution of Path II in the

306

N2O-purged solution (fN2O, II) should be identical to fN2O, I (6.5%). Thus, the contribution of Path

307

IV in the N2O-purged solution (fN2O, IV) accounted for 3.5%:

308

fN2O, IV = (fN2O, II + fN2O, IV) - fN2O, II = 3.5%

(27)

309

Now, the sum of the known contributions (Paths I, II, III, and IV) in the N2O-purged solution

310

was totally 78.2%. The pathways for the remaining part (21.8%) were unclear yet. As shown in

311

Reactions 1, 10, and 11, N2 was the main N-containing product in the photolysis of N2O. Neither

312

NO3- nor NO2- was detectable in the irradiated N2O solution.

313

Environmental Implication. The generated ·OH radicals from the photolysis of dissolved

314

N2O in the natural aquatic environment might be not important for the following reasons: (a) In

315

estuaries, rivers or sea water, the concentrations of dissolved N2O typically ranges from 5 to 50

316

nM,40 (b) The UVC light accounts for only a small fraction of the solar energy (less than 5%), and

317

(c) N2O aqueous solution has a low absorption cross section in the UVC band. However, the work

318

here is of great significance in photochemical studies for the following reasons:

319

(1) In photochemical systems, N2O-purging has been widely used as an experimental approach

320

to detect the formation of eaq-. The work here found that the dissolved N2O could produce

321

nonnegligible ·OH under UVC irradiation despite of its extremely low absorption cross section.

322

The effective quantum yield of ·OH from the photolysis of hydrated N2O at 254 nm was

323

determined to be 1.15-1.63, which was far larger than those of NO3- and NO2-. These results

324

demonstrate that using N2O and ·OH probes as an approach to quantify photogenerated eaq- in

17


Page 19 of 35


325

UVC irradiation might lead to false data, because N2O could not only convert eaq- to ·OH, but also

326

directly generate ·OH from UVC photolysis.

327

(2) TPA is one of the most widely used ·OH probes in photochemical studies. By careful

328

calculation of the k values, the ·OH radicals generated from the photolysis of N2O accounted for

329

more than half of the generated hTPA and the contribution of the N2O-derived ·OH was more than

330

9 times to that of eaq-. Some unnoticed pathways in the hydroxylation of TPA were actually

331

noneligible. The misuse of TPA and N2O might lead to false mechanism analysis. Close attention

332

should be paid to the combination of N2O and ·OH probes in the determination of reactive species

333

in UVC systems.

334 335

Considering the above facts, this work is helpful for the proper design of scavenging and probing experiments in photochemical systems.

336 337

ASSOCIATED CONTENT

338

Supporting Information

339

Determination of photon flux and effective path length of light (Texts S1 and S2), analytical

340

conditions for the studied probe (Table S1), the yield of hTPA in N2O-purged solution (Table S2),

341

molar extinction coefficient of N2O (Table S3), the k0 of hTPA formation under different irradiation

342

conditions (Table S4), the diagrammatic sketch of the photo-reactor (Figure S1), the standard

343

curves of dissolved N2O and H2O2 (Figure S2), formation of I3- in the KI/KIO3 chemical

344

actinometer under irradiation at 254 nm (Figure S3), photolysis of a diluted H2O2 solution (Figure

345

S4), the formation of hTPA over time under various gas conditions (Figure S5), the reduction of 18



Page 20 of 35

346

TOC (Figure S6), the time profiles of hCOU and 2hBA (Figure S7), emission spectra of the light

347

sources (Figure S8), ESI-MS spectrum of a UV treated TPA solution (Figure S9).

348

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

349 350

AUTHOR INFORMATION

351

Corresponding Author

352

*Correspondence author. Phone: +86 25 8968 0389, E-mail: [email protected]

353

Notes

354

The authors declare no competing financial interests.

355 356 357 358

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21522702, 21677070).

359 360

REFERENCES

361

(1) Jortner, J.; Ottolenghi, M.; Stein, G. Solvent effects on the photochemistry of the iodide ion.

362

J. Phys. Chem. 1963, 67 (6), 1271-1274.

363

(2) Jortner, J.; Ottolenghi, M.; Stein, G. The effect of nitrous oxide and the nature of intermediates

364

in the photochemistry of the iodide ion in aqueous solution. J. Phys. Chem. 1962, 66 (10),

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2037-2042.

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(3) Dainton, F. S.; Peterson, D. B. Use of nitrous oxide to discriminate between the forms of

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hydrogen atoms (H· and H2+ ?) produced by γ-irradiation of aqueous solutions. Nature 1960,

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(4) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants

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aqueous solution. J. Phys. Chem. Ref. Data 1988, 17 (2), 513-886.

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(5) Thomas-Smith, T. E.; Blough, N. V. Photoproduction of hydrated electron from constituents of natural waters. Environ. Sci. Technol. 2001, 35 (13), 2721-2726. (6) Kim, D. H.; Lee, J.; Ryu, J.; Kim, K.; Choi, W. Arsenite oxidation initiated by the UV photolysis of nitrite and nitrate. Environ. Sci. Technol. 2014, 48 (7), 4030-4037. (7) Fischer, M.; Warneck, P. Photodecomposition of nitrite and undissociated nitrous acid in aqueous solution. J. Phys. Chem. 1996, 100 (48), 18749-18756. (8) Noyes Jr, W. A. Photochemical studies. XXV. The direct photochemical decomposition of nitrous oxide. J. Chem. Phys. 1937, 5 (10), 807-812.

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(9) Sander, S. P.; Golden, D. M.; Kurylo, M. J.; Moortgat, G. K.; Ravishankara, A. R.; Kolb, C.

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E.; Molina, M. J.; Finlayson-Pitts, B. J. Chemical Kinetics and Photochemical Data for Use

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in Atmospheric Studies; NASA Jet Propulsion Laboratory: Pasadena, Calif., 2003;

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https://jpldataeval.jpl.nasa.gov/pdf/JPL_Publication_15-10.pdf.

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(10) Rajappan, M.; Büttner, M.; Cox, C.; Yates Jr, J. T. Photochemical decomposition of N2O by

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Lyman-α radiation: scientific basis for a chemical actinometer. J. Phys. Chem. A 2010, 114

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(11) Hanisco, T. F.; Kummel, A. C. State-resolved photodissociation of nitrous oxide. J. Phys. Chem. 1993, 97 (28), 7242-7246.

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(12) Okabe, H. Photochemistry of Small Molecules; Wiley: New York, 1978; pp 219.

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(13) Nakayama, T.; Takahashi, K.; Matsumi, Y.; Taniguchi, N.; Hayashida, S. Quantum yield for

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N(4S) production in the ultraviolet photolysis of N2O. J. Geophys. Res. 2003, 108 (D21), 4668.

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(14) Kjaergaard, H. G.; Robinson, T. W.; Howard, D. L.; Daniel, J. S.; Headrick, J. E.; Vaida, V.

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Complexes of importance to the absorption of solar radiation. J. Phys. Chem. A 2003, 107

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(15) Kolb, C. E.; Jayne, J. T.; Worsnop, D. R.; Molina, M. J.; Meads, R. F.; Viggiano, A. A. Gas

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phase reaction of sulfur trioxide with water vapor. J. Am. Chem. Soc. 1994, 116 (22), 10314-

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10315.

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(16) Morokuma, K.; Muguruma, C. Ab initio molecular orbital study of the mechanism of the gas

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phase reaction SO3 + H2O: importance of the second water molecule. J. Am. Chem. Soc. 1994,

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116 (22), 10316-10317.

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(17) Dainton, F. S.; Fowles, P. The photolysis of aqueous systems at 1849 Å. I. Solutions containing nitrous oxide. Proc. R. Soc. Lond. A 1965, 287, 295-311.

403

(18) Tanaka, N.; Nagashima, U.; Takayanagi, M.; Kim, H. L.; Hanazaki, I. Photochemical reaction

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dynamics of the N2O·H218O van der Waals complex. J. Phys. Chem. A 1997, 101 (4), 507-

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512.

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(19) Zellner, R.; Exner, M.; Herrmann, H. Absolute ·OH quantum yields in the laser photolysis of

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nitrate, nitrite and dissolved H2O2 at 308 and 351 nm in the temperature range 278-353 K. J.

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Atmos. Chem. 1990, 10 (4), 411-425.

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(20) Charbouillot, T.; Brigante, M.; Mailhot, G.; Maddigapu, P. R.; Minero, C.; Vione, D.

410

Performance and selectivity of the terephthalic acid probe for ·OH as a function of

411

temperature, pH and composition of atmospherically relevant aqueous media. J. Photochem.

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Photobiol. A 2011, 222 (1), 70-76.

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(21) Page, S. E.; Arnold, W. A.; McNeill, K. Terephthalate as a probe for photochemically generated hydroxyl radical. J. Environ. Monit. 2010, 12 (9), 1658-1665.

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(22) Kwon, B. G.; Kim, J. O.; Kwon, J. K. An advanced kinetic method for HO2·/O2·-

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determination by using terephthalate in the aqueous solution. Environ. Eng. Res. 2012, 17 (4),

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205-210.

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(23) Tafer, R.; Sleiman, M.; Boulkamh, A.; Richard, C. Photomineralization of aqueous salicylic

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acids. Photoproducts characterization and formation of light induced secondary ·OH

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precursors (LIS-OH). Water Res. 2016, 106, 496-506.

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(24) Bader, H.; Sturzenegger, V.; Hoigne, J. Photometric method for the determination of low

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concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of N, N-diethyl-

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p-phenylenediamine (DPD). Water Res. 1988, 22 (9), 1109-1115.

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(25) Xiong, Z. Q.; Xing, G. X.; Zhu, Z. L. Water dissolved nitrous oxide from paddy agroecosystem in China. Geoderma 2006, 136 (3-4), 524-532.

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(26) Ma, T.; Chen, Q.; Gui, M. Y.; Li, C.; Ni, J. R. Simultaneous denitrification and phosphorus

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removal by Agrobacterium sp. LAD9 under varying oxygen concentration. Appl. Microbiol.

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Biotechnol. 2016, 100 (7), 3337-3346.

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(27) Fang, X.; Mark, G.; von Sonntag, C. ·OH radical formation by ultrasound in aqueous solutions

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Part I: the chemistry underlying the terephthalate dosimeter. Ultrason. Sonochem. 1996, 3 (1),

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57-63.

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(28) Herrmann, H. On the photolysis of simple anions and neutral molecules as sources of ·O-/·OH, SOx- and Cl in aqueous solution. Phys. Chem. Chem. Phys. 2007, 9 (30), 3935-3964. (29) Rahn, R. O. Potassium iodide as a chemical actinometer for 254 nm radiation: use of iodate as an electron scavenger. Photochem. Photobiol. 1997, 66 (4), 450-455.

436

(30) Li, X. C.; Ma, J.; Liu, G. F.; Fang, J. Y.; Yue, S. Y.; Guan, Y. H.; Chen, L. W.; Liu, X. W.

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Efficient reductive dechlorination of monochloroacetic acid by sulfite/UV process. Environ.

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Sci. Technol. 2012, 46 (13), 7342-7349.

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(31) Beltran, F. J.; Ovejero, G.; Garcia-Araya, J. F.; Rivas, J. Oxidation of polynuclear aromatic

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hydrocarbons in water. 2. UV radiation and ozonation in the presence of UV radiation. Ind.

441

Eng. Chem. Res. 1995, 34 (5), 1607-1615.

442 443 444 445

(32) Zhang, G. Y.; Wu, B. D.; Zhang, S. J. Effects of acetylacetone on the photoconversion of pharmaceuticals in natural and pure waters. Environ. Pollut. 2017, 225, 691-699. (33) Xiang, Q.; Yu, J.; Wong, P. K. Quantitative characterization of hydroxyl radicals produced by various photocatalysts. J. Colloid Interface Sci. 2011, 357 (1), 163-167.

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(34) Rosario-Ortiz, F. L.; Canonica, S. Probe compounds to assess the photochemical activity of

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dissolved organic matter. Environ. Sci. Technol. 2016, 50 (23), 12532-12547.

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(35) Page, S. E.; Arnold, W. A.; McNeill, K. Assessing the contribution of free hydroxyl radical

449

in organic matter-sensitized photohydroxylation reactions. Environ. Sci. Technol. 2011, 45 (7),

450

2818-2825.

451 452 453 454 455 456

(36) Thorn, K. A.; Cox, L. G. Ultraviolet irradiation effects incorporation of nitrate and nitrite nitrogen into aquatic natural organic matter. J. Environ. Qual. 2012, 41 (3), 865-881. (37) Mack, J.; Bolton, J. R. Photochemistry of nitrite and nitrate in aqueous solution: a review. J. Photochem. Photobiol. A 1999, 128 (1), 1-13. (38) Fasnacht, M. P.; Blough, N. V. Mechanisms of the aqueous photodegradation of polycyclic aromatic hydrocarbons. Environ. Sci. Technol. 2003, 37 (24), 5767-5772.

457

(39) Water with heavy oxygen; Nature 1935, 135, 575. www.nature.com/articles/135575d0.pdf.

458

(40) Karl, D. M.; Michaels, A.F. Nitrogen cycle. In Encyclopedia of Ocean Sciences (Second

459

Edition); Steele, J. H.; Turekian, K. K.; Thorpe, S. A., Eds.; Academic Press: Oxford 2001;

460

pp 32-39.

461

24



Figure, Scheme and Table Captions

462 463

Table 1.

466

The effective quantum yields and the steady-state concentrations of ·OH in UVC irradiated N2O and NO3- solutions.

464 465

Page 26 of 35

Table 2.

The pseudo-zero-order formation rate constants (k0) of

18

O-hTPA and 16O-hTPA in

photochemical processes with H216O or H218O as the solvent.

467

Table 3. The inhibition effect of TBA on the pseudo-zero-order formation rate constant of hTPA.

468

Scheme 1. The proposed pathways for the formation of hTPA in UV irradiated TPA solutions.

469

Figure 1. (a) The evolution of TPA (20 µM) under UV irradiation in ambient, N2-purged and

470

N2O-purged solutions. k1 (min-1): the pseudo-first-order degradation rate constant. (b)

471

The k1 of TPA as a function of N2O concentration. [TPA]0 = 20 µM, light intensity: 7.89

472

mW cm-2. Error bars represent the standard deviation from at least duplicate

473

experiments.

474

Figure 2. (a) The k1 of TPA degradation and (b) the k0 of hTPA formation in photochemical

475

processes with the addition of nitrogen oxide. [TPA]0 = 20 µM, [NO2-]0 = [NO3-]0

476

= 5-100 µM, [N2O] = 24.1 mM. The dashed line represents the k1 and k0 in the

477

absence of nitrogen oxide.

478

Figure 3. The generation of H2O2 in UV irradiated ultrapure water (pH 6.0) and (b) the

479

generation of peroxides in UV irradiated TPA solutions (20 µM, pH: 9.0) under

480

various gas conditions.

25


Page 27 of 35


481

Figure 4. The k0 of hTPA formation under different irradiation and gas conditions. [TPA]0

482

= 20 µM, light intensity: 4.72 mW cm-2 (MP-Hg at 365 nm) and 0.40 mW cm-2

483

(LP-Hg at 254 nm).

484

26



Page 28 of 35

485

Table 1. The effective quantum yields and the steady-state concentrations of ·OH in UVC

486

irradiated N2O and NO3- solutions. k’0hTPA

C0

ε254 × 10-2

Φ·OH

(nM min-1)

(mM)

(M-1 cm-1)

moles/einstein (fM)

N2O

10.45 ± 0.85

24.10

0.12-0.17

1.15-1.63

5.50

NO3-

1.97 ± 0.15

0.05

300

0.06

1.07

Solution

487

27


[·OH]ss

Page 29 of 35


488

Table 2. The pseudo-zero-order formation rate constants (k0) of

489

photochemical processes with H216O or H218O as the solvent.

18

O-hTPA and

16

O-hTPA in

k0hTPA (nM min-1) Solution a H216O (0.2%b) k0

16O-hTPA

k0

18O-hTPA

H218O (99%b) f

18O-hTPA c

k0

16O-hTPA

k0

18O-hTPA

f

18O-hTPA c

Ambient

30.26

0.20

0.6%

NAd

NA

NA

N2-purged

39.85

0.28

0.7%

27.56

5.50

16.8%

N2O-purged

214.23

2.18

1.0%

46.38

28.62

38.6%

490

a

[TPA]0 = 10 µM, light intensity: 4.35 mW cm-2 (365 nm).

491

b

The fraction of 18O in the solvent.

492

c

The fraction of 18O-hTPA: f

493

d

“NA” represents “not available”

18O-hTPA

= k0

18O-hTPA

/ (k0

16O-hTPA

18O-hTPA

+ k0

494

28


).


495

Page 30 of 35

Table 3. The inhibition effect of TBA on the pseudo-zero-order formation rate constant of hTPA. Solution a

k0hTPA (µM min-1) × 10-2

Inhibition Rate b

w/ TBA

w/o TBA

Ambient

0.13 ± 0.00

1.44 ± 0.09

91.0%

N2-purged

2.26 ± 0.37

3.06 ± 0.09

26.1%

N2O-purged

3.96 ± 0.10

13.82 ± 0.19

71.3%

496

a

[TPA]0 = 20 µM, [TBA]0 = 100 mM, light intensity: 4.72 mW cm-2 (365 nm).

497

b

Inhibition rate = 1- k0, w/ TBA/k0, w/o TBA.

498

29


Page 31 of 35

499


Scheme 1. The proposed pathways for the formation of hTPA in UV irradiated TPA solutions

500 501

30



Page 32 of 35

502

503 504

Figure 1. (a) The evolution of TPA (20 µM) under UV irradiation in ambient, N2-purged and N2O-

505

purged solutions. k1 (min-1): the pseudo-first-order degradation rate constant. (b) The k1 of TPA as

506

a function of N2O concentration. [TPA]0 = 20 µM, light intensity: 7.89 mW cm-2. Error bars

507

represent the standard deviation from at least duplicate experiments.

508

31


Page 33 of 35


509 510

Figure 2. (a) The k1 of TPA degradation and (b) the k0 of hTPA formation in photochemical

511

processes with the addition of nitrogen oxide. [TPA]0 = 20 µM, [NO2-]0 = [NO3-]0 = 5-100

512

µM, [N2O] = 24.1 mM. The dashed line represents the k1 and k0 in the absence of nitrogen

513

oxide.

514

32



Page 34 of 35

515 516

Figure 3. (a) The generation of H2O2 in UV irradiated ultrapure water (pH 6.0) and (b) the

517

generation of peroxides in UV irradiated TPA solutions (20 µM, pH: 9.0) under various gas

518

conditions.

33


Page 35 of 35


519 520

Figure 4. The k0 of hTPA formation under different irradiation and gas conditions. [TPA]0

521

= 20 µM, light intensity: 4.72 mW cm-2 (MP-Hg at 365 nm) and 0.40 mW cm-2 (LP-Hg at

522

254 nm).

34


Nonnegligible Generation of Hydroxyl Radicals from UVC Photolysis

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