Quinone Systems

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Remediation and Control Technologies

Redox Conversion of Arsenite and Nitrate in the UV/Quinone Systems Zhihao Chen, Jiyuan Jin, Xiaojie Song, Guoyang Zhang, and Shujuan Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03538 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 4, 2018

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Environmental Science & Technology

Redox Conversion of Arsenite and Nitrate in the UV/Quinone Systems

Zhihao Chen#, Jiyuan Jin#, Xiaojie Song, Guoyang Zhang, 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 8968 0389, E-mail: [email protected] Submitted to: Environmental Science & Technology

ACS Paragon Plus Environment


Table of Contents


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1

ABSTRACT

2

Whether superoxide radical anion (O2˙–) was a key reactive species in the oxidation of

3

arsenite (As(III)) in photochemical processes has long been a controversial issue. With

4

hydroquinone (BQH2) and 1,4-benzoquinone (BQ) as redox mediators, the

5

photochemical oxidation of As(III) and reduction of nitrate (NO3-) was carefully

6

– investigated. O2˙ , singlet oxygen (1O2), H2O2, and semiquinone radical (BQH.) were

7

all possible reactive species in the irradiated system. However, since the formation of

8

As(IV) is a necessary step in the oxidation of As(III), taking the standard reduction

9

potentials into account, the reactions between the above species and As(III) were

10

thermodynamically unfavorable. Based on radical scavenging experiments, hydroxyl

11

radical (.OH) was proved as the key species that led to the oxidation of As(III) in the

12

UV/BQH2 system. It should be noted that the .OH radicals were generated from the

13

– photolysis of H2O2, which came from the disproportionation of O2˙ and the reaction of

14

O2˙– with BQH2. Both the photo-ejected eaq- from 1(BQH2)* and the direct electron

15

transfer with 3(BQH2)* contributed to the reduction of NO3- in the UV/BQH2 process.

16

No synergistic effect was observed in the redox conversion of As(III) and NO3-, further

17

demonstrating that the role of BQH. was negligible in the studied systems. The results

18

here are helpful for a better understanding of the photochemical behaviors of quinones

19

in the aquatic environment.

1



20

INTRODUCTION

21

Quinones have been extensively investigated in photochemistry1-6 and redox

22

reactions,7-10 because of their roles in life processes (photosynthesis and respiration) as

23

electron shuttles and their structure characteristics as the representative moieties of

24

dissolved organic matter (DOM) in the nature.9-12 The photochemistry of quinone and

25

DOM has been extensively studied in the past decades.13-15 For example, it is reported

26

that under UV irradiation, DOM could reduce nitrate (NO3-)13 or oxidize arsenite

27

(As(III)).14,15

28

Formation of reactive oxygen species (ROS) in UV/DOM systems has been reported

29

in the literature. However, the mechanisms are unclear and are often conflicting. For

30

example, singlet oxygen (1O2) was regarded as the key ROS in the oxidation of As(III)

31

by fulvic acid under 282 nm UV irradiation,14 whereas the excited triplet states of DOM

32

and/or phenoxyl radicals, rather than ROS, were believed to play the key roles in the

33

oxidation of As(III) by UV-A irradiated Suwannee River humic acid.15 There are a few

34

reports on the oxidation of As(III) by quinones at ambient conditions (in dark without

35

UV irradiation).7 Hydrogen peroxide (H2O2), hydroxyl radical (·OH), and semiquinone

36

radical (BQH·) were believed as the key species in the quinone-involved thermal

37

oxidation of As(III).7,16 However, to the best of our knowledge, there is few work on

38

the oxidation of As(III) by UV/quinones.

39

Redox conversion of As(III) and NO3- is directly related to their potential health risks

40

on humans.17-20 Although the oxidation states of nitrogen are much more complicated 2


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than those of arsenic, the reduction of NO3- in those processes are better understood

42

than the oxidation of As(III).21,22 In the literature, there have been considerable

43

controversies over the key species that led to the oxidation of As(III).23-31 Some

44

researchers believed that superoxide radical anion (O2˙–) was the main oxidant in the

45

photocatalytic oxidation of As(III),23-26 whereas some others argued that O2˙– had little

46

or no role in the advanced oxidation conversion of As(III) in ultrasonic irradiation or

47

UV/TiO2 processes.27-30

48

Based on the quantum yields of H2O2 and As(III) as well as their relations with

49

dissolved oxygen (DO), it was clearly demonstrated that the oxidation of As(III)

50

undergoes an intermediate oxidation state, As(IV), rather than go directly to As(V).32

51

The existence of As(IV) has been evidenced by its transient absorption spectrum.33 The

52

standard reduction potential (E0) of H4AsIVO4/H3AsIIIO3 was reported to be 2.40 V,33

53

which is much higher than those of most ROS, except that of ·OH (E0 (·OH/H2O) =

54

2.73 V).34 Therefore, it is reasonable to infer that only a few species could oxidize

55

As(III). Neither O2˙– (E0 (O2˙–/H2O2) = 1.71 V)34 nor 1O2 (E0 (1O2/O2˙–) = 0.67 V)35

56

could oxidize As(III) to As(IV). This thermodynamic criterion is pivotal in untangling

57

the disagreements on the oxidation mechanism of As(III), but was unfortunately

58

ignored in the controversy.

59

NO3- and As(III) have been frequently detected together in some surface water,

60

groundwater and arsenic-related industrial wastewater.36,37 It is well known that NO3-

61

can generate ·OH through UV photolysis with a quantum yield of 0.09.38 The 3



62

concentration of NO3- ranges from 23 μM to 779 μM in arsenic-containing ground and

63

surface waters.17,36 Because of this, a UV/NO3- system has been proposed for the

64

oxidation of As(III).17 In our recent work,39 a concerted redox conversion of As(III) and

65

NO3- was observed in a photochemical process with acetylacetone (AA) as a photo-

66

activator. The two carbonyl groups in AA make it structurally similar to quinones. In

67

the photo-conversion of As(III) and NO3-, the excited AA (AA*) was believed to act as

68

a BQH·-like redox mediator.39 However, by taking the consumption of the redox

69

mediators in the photochemical processes into account, the electron shuttling abilities

70

of AA, hydroquinone (BQH2), and 1,4-benzoquinone (BQ) were quite different.39 To

71

convert the same amount of As(III), the consumption of AA was 2-4 orders of

72

magnitude lower than those of BQH2 and BQ.39

73

The mechanisms for the concerted redox conversion of As(III) and NO3- in the

74

UV/AA process were comprehensively explained based on both experimental results

75

and theoretical calculations.39 Although the photochemistry of quinones has been

76

extensively studied,1-5 it is unclear yet how As(III) is converted in the UV/quinone

77

systems and whether there was a synergistic effect in the redox conversion of As(III)

78

and NO3- in the UV/quinone processes, as observed in the UV/AA process. Therefore,

79

in this work, we used two simple chemicals, BQH2 (reduced state) and BQ (oxidized

80

state), to investigate both the oxidation of As(III) and the reduction of NO3- in the

81

UV/quinone systems to evaluate the electron shuttling ability of quinones. The

82

experimental designs are helpful to exclude the uncertainty caused by the variant 4


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compositions of DOM from different sources. The key species in the oxidation of

84

As(III) was clarified, which might be helpful to end the disputes in the oxidation of

85

As(III).

86 87

EXPERIMENTAL

88

Chemicals. Na3AsO3 (analytical grade), Na3AsO4 (analytical grade), potassium

89

hyperoxide (KO2), 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), and superoxide

90

dismutase (SOD, 5673 units/mg solid) were bought from Sigma-Aldrich, USA. BQH2

91

and 2-isopropanol (IPA) were purchased from Nanjing Chemical Regent Co. Ltd.,

92

China. KH2PO4 (HPLC grade) and 2-hydroxy-terephthalic acid (HTPA, ≥ 98.0%,

93

HPLC grade) were obtained from Aladdin Industrial Corporation (Shanghai, China).

94

BQ (analytical grade), NaNO3 (GR grade), H2O2 (30 wt%) and hydrochloric acid (HCl,

95

36-38%, GR grade) were bought from Sinopharm Chemical Reagent Co. Ltd., China.

96

Catalase (CAT, 2000-5000 units/mg protein) from bovine liver was purchased from

97

Biosharp Co. Ltd., China. N, N-diethyl-p-phenylenediamine (DPD) and peroxidase

98

(POD) from horseradish (150 U/mg) were bought from Sigma-Aldrich Germany and

99

Sigma Switzerland, respectively. Terephthalic acid (TPA, 99%) was purchased from

100

Macklin Co. Ltd., China. KBH4 was purchased from Shandong Xiya Reagent Co. Ltd.,

101

China. All the chemicals were used as received without any further treatment.

102

A water purification system (Shanghai Ulupure Industrial Co. Ltd., China) was used

103

to produce ultrapure water (18.25 MΩ·cm). All the solutions were adjusted to pH 6.8 5



104

with diluted HCl and NaOH solutions. Stock solution of KO2 (100 mM) was prepared

105

and used immediately.

106

A surface water was taken from Jiuxiang River (JXR) near Nanjing University. The

107

water was firstly filtered by a 0.22 μm filter membrane and some of the water quality

108

parameters of the surface water are listed in Table S1.

109

Irradiation Experiments. A photo-reactor equipped with a 300 W medium-mercury

110

(MP-Hg) lamp was used for irradiation experiments. The quartz sample rubes rovolved

111

around the lamp during UV irradiation. The emission spectrum of the MP-Hg lamp was

112

determined with a miniature fiber optic spectrometer (USB2000+, Ocean Optics, Inc.,

113

USA). The light intensity was determined with a radiometer (Photoelectric Instrument

114

Factory of Beijing Normal University, China) equipped with a sensor of peak sensitivity

115

at 365 nm. More details about the experimental setup are available in our previous

116

report.39

117

Sunlight irradiation experiments were carried out in a sunny day (on June 6th, 2018)

118

from 10:00 AM to 6:30 PM on the building roof of School of the Environmental at

119

Nanjing University (Nanjing, China: 32°N latitude, 118°E longitude).40 The

120

temperature ranged from 22℃ to 30 ℃.

121

In purging experiments, all the solutions were firstly bubbled with highly purified N2

122

or N2O (> 99.99%) for at least 30 min and then kept bubbling throughout the irradiation

123

process. All experiments were conducted at least in duplicate.

6


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Measurement of As(III), As(V), NO3-, NO2- and NH4+. Arsenic analysis was

125

conducted with an atomic fluorescence spectrometer (Beijing Beifen-Ruili Analytic

126

Instrument Co. Ltd., China) coupled with a liquid chromatograph (Persee Co. Ltd.,

127

China). The concentrations of NO3- and NO2- were determined with an ion

128

chromatograph (IC1100, Dionex Co. Ltd., USA) coupled with an anion suppressor

129

(ASRS 300 × 4 mm, Thermo Scientific Co. Ltd., USA) and a 4 µm × 250 mm anion-

130

exchange resin chromatographic column (AS19, Thermo Scientific Co. Ltd., USA).

131

More experimental details about the chromatography analysis are available in Text S1.

132

Measurement of BQH2 and BQ. The concentrations of BQH2 and BQ were

133

determined with a high performance liquid chromatography (HPLC) system (Ultimate

134

3000, Dionex Co. Ltd., USA) equipped with a C8 column (4.6 × 150 mm, 2.7 μm,

135

Waters Co. Ltd., USA). The main products in the UV/quinone systems were identified

136

with an ultra-performance liquid chromatograph (UPLC, Ultimate 3000, Dionex, USA)

137

combined with a tandem mass spectroscopy system (MS/MS) (Thermo Scientific Q

138

ExactiveTM Focus Orbitrap, USA). More analytical details are available in Text S2.

139

Measurement of H2O2, DO, and ROS. The concentration of H2O2 was determined

140

with the DPD-POD method.41 DO was measured with a detector equipped with a

141

lumination probe (LDO 10103, HACH Co. Ltd., USA). An in situ UV-electron spin

142

resonance (ESR) system (Bruker DRX 500, Germany) was used for the detection of

143

ROS with DMPO as the probing molecule and a 180 W MP-Hg lamp as the light source.

7



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144

The running parameters are as follows: resonance frequency, 9.772 GHz; center field,

145

3480.0 G; sweep width, 200 G; microwave power, 19.922 mW.

146

Determination of the Steady State Concentration of ·OH. The steady-state

147

concentration of ·OH ([·OH]ss) was measured with TPA (5 µM) as the probe and the

148

formation rate of HTPA as the kinetic variable:42

149

d[HTPA ] = k . OH,TPA [TPA]0 [⋅ OH]ss Y dt

(1)

150

where d[HTPA]/dt is the formation rate of HTPA. k.OH,TPA represents the second-order

151

reaction rate constant between ·OH and TPA. [TPA]0 is the initial concentration of TPA.

152

Y represents the yield of HTPA.42

153 154

RESULTS AND DICUSSION

155

Redox Conversion in the UV/Quinone Systems. The photochemical conversion of

156

quinones, As(III), and NO3- was conducted in air-equilibrated solutions of pH around

157

6.8. A two-stage kinetics was observed for the tested chemicals (Figure 1), which are

158

related with the concentration of DO (Figure 2). In the UV/quinone systems, As(III)

159

was firstly oxidized at the pseudo-first-order and then reduced back to As(III) with the

160

depletion of DO (Figure 2). In the literature, the photosensitized oxidation of As(III) by

161

Suwannee River humic acid15 and the photo-reduction of NO3- in the presence of

162

DOM13 were also reported following the pseudo-first-order kinetics. Therefore, the rate

163

constants in Table 1 were obtained by fitting the kinetic data with the pseudo-first-order

164

kinetics within the first stage to avoid the interference caused by the accumulated 8


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degradation products and the change of experimental conditions (mainly the

166

concentration of DO).

167

BQ was nearly completely transformed within several minutes (Figure 3). The

168

transformation products of BQ were mainly BQH2 (more than 60%) and

169

hydroxybenzoquinone (BQ-OH) (Figure S1). The generated BQH2 in the UV/BQ

170

system began to decompose at a pseudo-first-order rate constant (k1) (0.0075 min-1) of

171

1/55 of the k1 of BQ (0.3899 min-1), whereas the direct photo-conversion rate constant

172

of BQH2 (0.0115 min-1) was about 1/32 of that of BQ (Figure 3). As the light intensity

173

was increased from 4.0 mW/cm2 (Figure 3) to 7.1 mW/cm2 (Figure 1), the k1 values of

174

BQ and BQH2 were increased several times. However, the k1 ratio of BQH2 to BQ was

175

nearly unchanged (1/34, Table 1). Only a trace amount of BQ was detected in the

176

UV/BQH2 system (Figure 3), because of the limited formation of BQ and the rapid

177

back-conversion of BQ to BQH2.

178

Considering the fast photo-conversion of BQ, the obtained k1 values of As(III) and

179

NO3- in the UV/BQ system (Figure 1) were actually compound results of the reactions

180

involved with both BQ and its photo-products, such as BQH2 and BQ-OH. It is clearly

181

shown in Table 1 that the presence of BQH2 significantly enhanced the photo-

182

conversion of both As(III) and NO3-, whereas only a slight enhancement was observed

183

in the UV/BQ system.

184

The addition of NO3- showed obvious enhancement effects on the oxidation of

185

As(III) (Figure 1). However, the enhancement effect caused by addition of NO3- was 9



186

negligible in the UV/quinone systems. Since the absorption cross section of NO3- is

187

much smaller than that of BQH2 (Figure S2), under the given conditions, NO3- absorbed

188

less than 1% of the incident light at 254 nm. The k1 of the direct photolysis of NO3-

189

(0.0021 min-1) was 16.2% of that in the UV/BQH2 system (0.0129 min-1) (Table 1). In

190

the presence of BQH2, the direct photolysis of NO3- might be even less due to the

191

competition of BQH2 for photons. As a result, the addition of NO3- led to a negligible

192

effect on As(III) oxidation in the UV/quinone systems.

193

To make a clear comparison, a set of k1-related data was defined as below:

194

k1,Ct represents the k1 of the contaminant (Ct) in a Ct-only solution;

195

k1,Ct-RM represents the k1 of the Ct in a Ct-redox mediator (RM) binary solution;

196

k1,RM-Ct represents the k1 of the RM in a Ct-RM binary solution;

197

k1,Ct-RM(tri) represents the k1 of the Ct in a Ct-Ct’-RM ternary solution (with both a

198

RM and another Ct’).

199

Thus, the k1,Ct-RM/k1,Ct ratio reflects the efficiency of the RM in enhancing the photo-

200

conversion of Ct. The k1,RM-Ct/k1,Ct-RM ratio reflects the consumption of the RM per

201

equivalent of Ct in the UV/RM process. The k1,Ct-RM(tri)/k1,Ct-RM ratio reflects the effect

202

of the second contaminant (Ct’) on the conversion of Ct in the UV/RM process.

203

As shown in Table 1, under the given conditions, the k1 of As(III) oxidation/NO3-

204

reduction in the UV/BQ process (k1,Ct-BQ) was slightly higher than those of the UV

205

control (k1,Ct). Comparatively, BQH2 was more efficient for the reduction of NO3-

206

whereas AA was the most efficient in the oxidation of As(III). Due to the rapid photo10


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conversion of BQ to BQH2 and BQ-OH, the k1,RM-Ct/k1,Ct-RM ratio is meaningless for

208

BQ, but is valuable for BQH2. The k1,RM-Ct/k1,Ct-RM ratios of BQH2 were 1.92 and 2.62,

209

respectively, in the oxidation of As(III) and the reduction of NO3- (Table 2), indicating

210

that to convert one molecular As(III) or NO3- needs at least two molecular BQH2. The

211

k1,RM-Ct/k1,Ct-RM ratio of AA for As(III) was only 0.2, indicating that to fulfil the

212

conversion of a same amount of As(III), the consumption of AA was only 1/10 to that

213

of BQH2. In other words, in the oxidation of As(III), the electron shuttling ability of

214

BQH2 was weaker than that of AA. However, in the reduction of NO3-, BQH2 was more

215

efficient than AA. Furthermore, the k1,Ct-RM(tri-)/k1,Ct-RM ratios indicate that there was no

216

synergistic effect between the oxidation of As(III) and the reduction of NO3- in the

217

UV/BQH2 process whereas significant synergistic effects were observed in the UV/AA

218

process (Table 2). These differences suggest that the mechanisms in the UV/BQH2

219

process were different from those in the UV/AA process.

220

The Photochemistry of Quinones. The photo-conversion of BQ and BQH2 in

221

aqueous solutions have been carefully studied in the literature.1,2,6 The quantum yield

222

for the intersystem conversion (ΦISC) from 1(BQ)* to 3(BQ)* was reported as 1.0.1 The

223

3

224

0.04.2 ·OH was captured in cage by BQ to form BQ-OH·.1,2 BQ-OH· then reacted with

225

BQ at a rate constant of (6.5±0.3)×107 M-1 s-1 to form BQ-OH and BQ˙–.2 In the

226

absence of other chemicals, BQ˙– disproportionated to BQ and BQH2 at a rate constant

227

of (1.5±0.1)×108 M-1 s-1.2 BQH2 had a ΦISC of 0.39.6 Therefore, it could either emit

(BQ)* reacted with H2O to generate ·OH and BQH· (pKa = 4.0) with a Φ οf 0.47±

11



228

fluorescence or underwent direct photolysis to generate hydrated electron (eaq-).6 The

229

generated 3(BQH2)* from ISC process was strongly reductive and could react with O2

230

and NO3- through direct electron transfer.6

231

To sum up, the photochemical and photophysical processes of BQ and BQH2 are

232

schematically shown in Scheme 1. There are four pathways for the conversion of

233

3

234

reacted with H2O to generate ·OH, and (4) reduced to BQH· by accepting an electron

235

from As(III) (Scheme 1a).

(BQ)*: (1) relaxed to BQ through phosphorescence emission, (2) quenched by O2, (3)

236

Mechanism Analysis for the Oxidation of As(III) in the UV/Quinone Systems.

237

Since the experiments were conducted at pH 6.8, Eh7 values were considered to judge

238

the thermodynamic feasibility of the reactions (Figure S3 and Table S2). Based on the

239

Eh7 values, the electron transfer between 3(BQ)* and As(III) was thermodynamically

240

favorable. However, the direct electron transfer between 3(BQ)* and As(III) was

241

negligible, because the reaction between the solvent (H2O) and 3(BQ)* dominated over

242

the other three pathways. Once ·OH was generated, it quickly reacted with another

243

molecule of BQ to form BQ-OH· at a rate constant of 6.6×109 M-1 s-1.43 The formation

244

of BQ-OH· occurred in the solvent cage. As a result, few ·OH could diffuse out and

245

react with As(III). This probably explained the argument on whether there was free ·OH

246

in the UV irradiated BQ solution.1,3,44

247

In neutral solutions, BQH· deprotonated to BQ˙– and rapidly disproportionated to BQ

248

and BQH2.6 The direct electron transfer between BQ˙– and As(III) or NO3- was 12


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thermodynamically possible, but kinetically limited, as evidenced by the low k1 values

250

in the UV/BQ system (Table 1). Thus, for the oxidation of As(III) in the UV/BQH2

251

process, attention should be paid to the two ROS: 1O2 and O2˙–.

252

As illustrated in Table 3, both 1O2 and O2˙– had several possible reaction pathways.

253

Although single electron transfer between 1O2 and As(III) was proposed as a possible

254

reaction (Reaction 1.1 in Table 3),14 there was no experimental evidence to support this

255

statement. As a matter of fact, such a reaction is thermodynamically unfavorable,

256

because the Eh7 of 1O2/O2˙– was much lower than that of As(IV)/As(III) (Figure S3).

257

1

258

(Reaction 1.3 in Table 3) 46. Considering the rate constants and the concentration levels

259

of the reactants, the quenching reaction with the solvent (Reaction 1.2 in Table 3)

260

dominated in the conversion of 1O2. It is known that 1O2 has a longer life time in D2O

261

than in H2O.47 The replacement of H2O with D2O as the solvent led to no kinetic solvent

262

isotope effect on the oxidation of As(III) in the UV/BQH2 process (Figure S4),

263

excluding the contribution of 1O2 in the generation of BQH· (Reaction 1.4 in Table 3).

264

– O2˙ was proposed to play a key role in the oxidation of As(III) (Reaction 2.1 in Table

265

3).24 One evidence for this conclusion is that As(III) was oxidized in a KO2 solution.48

266

This opinion was refuted by another work,30 in which the main conclusion was that the

267

As(III) oxidation by the addition of KO2 was not due to O2˙– but due to H2O2. The

268

experimental evidence for the later conclusion is: KO2 produces not only O2˙– but also

269

H2O2. Adding 0.5 mM KO2 produced about 0.2 mM H2O2. The As(III) oxidation

O2 could be quenched by both the solvent (Reaction 1.2 in Table 3)45 and BQH2

13



270

efficiencies by addition of 0.2 mM H2O2 into 75 μM As(III) solution were very similar

271

to those by the addition of 0.5 mM KO2.30 We conducted the experiments with KO2 and

272

H2O2 at three pHs (3.2, 6.8, 10.7) (Figure S5). The oxidation of As(III) was effective at

273

pH 10.7, but ineffective at pH 3.2 and 6.8. The results are exactly the same as those in

274

the previous report.30 The generation of O2˙– from KO2 in the acidic solutions should

275

be close to that in the alkaline solution.30 The pKa of HO2· was 4.8.49 Theoretically,

276

there should not be such a large difference in As(III) oxidation between the KO2

277

solutions of pH 6.8 and 10.7, if O2˙– was able to oxidize As(III).

278

– – The role of O2˙ was further excluded by the addition of SOD. SOD reacts with O2˙

279

at a rate constant of 2.2×109 M-1 S-1 to form H2O2.50 Addition of SOD slightly enhanced

280

the oxidation of As(III) (the k1 was increased from 0.0090 min-1 to 0.0103 min-1, Table

281

S3), demonstrating that the role of O2˙– in the UV/BQH2 process for As(III) oxidation

282

was negligible.

283

Taking the rate constants and the concentration levels into account, it is reasonable

284

to infer that Reactions 2.2 and 2.3 (Table 3) dominated in the reactions of O2˙–. The

285

generated BQ-OH· and BQ˙– from the above two reactions could then react with O2˙–

286

to form BQ-OH (Reaction 2.4 in Table 3). The fraction of the disproportionation of

287

O2˙– (Reaction 2.5 in Table 3) might be small, but was crucial in the oxidation of As(III).

288

H2O2 is the disproportionation product of O2˙–, which is known as both an oxidant and

289

a reductant. Under UV irradiation, H2O2 decomposed to ·OH with a quantum yield of

290

1.0 (Reaction 3 in Table 3).51,52 The reactions of H2O2 with As(III) and NO3- were 14


Page 16 of 40

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291

thermodynamically possible but kinetically limited.27 Under the given conditions (pH

292

6.8), the dark oxidation of 0.1 mM As(III) by 0.2 mM H2O2 was less than 4% within

293

25 min, whereas the As(III) oxidation reached 100% under UV irradiation (Figure S5).

294

The pH-dependence in the dark oxidation of As(III) by H2O2 (Figure S5) could be

295

explained with the increased reduction potential difference between H2O2 and As(III)

296

with the increase of pH (Figure S6). Both H2O2 and ·OH were detected in the tested

297

solutions (Figure S7). It should be noted that the detected H2O2 concentration was the

298

residual concentration. A large amount of H2O2 generated in the UV/BQH2 process

299

have already been consumed in the oxidation process. The oxidation of As(III) was

300

drastically inhibited by the addition of either CAT (a H2O2 scavenger) or IPA (a ·OH

301

scavenger) (Table S3), indicating that ·OH played the pivotal role in the oxidation of

302

As(III).

303

The formation of ·OH in the UV/quinone systems was also verified with ESR (Figure

304

S8). DMPO-OH signals were observed in the irradiated DMPO solution, because of the

305

direct photo-ionization of DMPO.53 In the presence of BQH2, the DMPO-OH signals

306

were significantly enhanced. The addition of IPA to the UV/BQH2 system led to a

307

drastic decrease of DMPO-OH signals, demonstrating the formation of ·OH in the

308

UV/BQH2 system. Besides DMPO-OH signals, DMPO-H signals were also observed

309

in the UV/BQH2 system, which could be attributed to the photo-ejected electrons from

310

1

311

UV/BQH2 system) were observed in the UV/BQ system. However, the DMPO-OH

(BQH2)* (Scheme 1b). Strong DMPO-OH signals (almost 10 times to that in the

15



312

signals in the UV/BQ system were not affected by the addition of IPA. This is because

313

the ·OH radicals generated from 3(BQ)* was captured in cage by BQ to form BQ-

314

OH·,1,2 which is insensitive to IPA.

315

O2˙– is prone to self-disproportionation to H2O2 and H2O at a rate constant of 2.3 ×

316

106 M-1 s-1.49 The second-order rate constants for the reactions of HO2. and O2˙– with

317

DMPO are 6.6 × 103 M-1 s-1 and 10 M-1 s-1, respectively, whereas that of .OH with

318

DMPO is 1.8 ×109 M-1 s-1.54 Therefore, the low reactivity of O2˙– with DMPO might

319

account for why there is no O2˙– detected by ESR (Figure S8).

320

The steady-state concentration of ·OH in the UV/BQH2 system (4.93 × 10-14 M) was

321

one order of magnitude higher than that of the UV control (4.33 × 10-15 M, Figure S7),

322

indicating that most of the ·OH radicals in the UV/BQH2 system came from the reaction

323

of 3(BQH2)* with O2 (Scheme 1b). The photo-oxidation of As(III) was nearly

324

completely inhibited by N2-purging (Table S3), providing a further support for the

325

above conclusion (the O2  O2˙  H2O2  ·OH route) and excluding the contribution

326

of BQ˙–.

–

327

For the reaction of ·OH, there are three main competitors: BQH2, BQ, and As(III),

328

with similar rate constants (Reactions 4.1-4.3 in Table 3). Due to the rapid conversion

329

of BQ in the photochemical system (Figure 1), the concentration of BQ was much lower

330

than those of BQH2 and As(III) and therefore could be neglected. Single electron

331

transfer occurs between ·OH and As(III), leading to the formation of As(IV). As(IV) is

16


Page 18 of 40

Page 19 of 40


332

an unstable intermediate and can serve as both a strong oxidant and a strong reductant,33

333

which explains the reduction of As(V) to As(III) after the depletion of DO (Figure 2).

334

Besides the electron transfer reaction with BQH2 (Reaction 5.1 in Table 3), As(IV)

335

could also reduce O2 to O2˙– (Reaction 5.2 in Table 3) or directly disproportionate to

336

As(III) and As(V) at a rate constant of 8.4×108 M-1 s-1 (Reaction 5.3 in Table 3).33

337

– Based on the E values (Figure S3), the reduction of As(V) to As(III) by O2˙ is

338

thermodynamically possible. By adding 0.5 mM KO2 to 0.01-0.1 mM As(V) solutions

339

of pH 3.2, 6.7, and 10.7, no reduction of As(V) to As(III) was observed, demonstrating

340

that in the given system, O2˙– was not important in either the oxidation of As(III) to

341

As(V) or the reduction of As(V) to As(III). The overall results are determined by the

342

abundance of the reactive species and the reaction kinetics. As evidenced in the ESR

343

results (Figure S8), it is .OH, instead of O2˙–, that played the key role in the oxidation

344

of As(III) by UV/BQH2.

345

Mechanism Analysis for the Reduction of NO3- in the UV/Quinone Systems. In

346

all the three processes (UV, UV/BQ, UV/BQH2), NH4+ and NO2- were detected as the

347

conversion products of NO3- (Figure S9). As aforementioned, the direct photolysis of

348

NO3- in the UV/quinone systems was negligible, because of the small absorption cross

349

section of NO3-. Therefore, there were mainly two pathways for the reduction of NO3-

350

in the UV/BQH2 process: One was the direct electron transfer between 3(BQH2)* and

351

NO3- (Pathway I in Scheme 1b), and another one was the reduction by the photo-ejected

17



352

eaq- (Pathway II in Scheme 1b, Reaction 6.1 in Table 3). The generation of eaq- in the

353

UV/BQH2 process was confirmed in the ESR test (Figure S8).

354

Both 1(BQH2)* and 3(BQH2)* could be quenched by O2. N2-purging significantly

355

enhanced the photo-reduction of NO3- from 0.0123 min-1 to 0.0932 min-1 (Figure S10a),

356

demonstrating the importance of the excited BQH2 in Scheme 1b and Reaction 6.2 in

357

Table 3.55 N2O-purging also led to an enhancement effect on the photo-reduction of

358

NO3- (Figure S10a), but to a much less extent (0.0354 min-1) than that of N2-purging,

359

because of the two contrary effects of N2O-purging: the positive effect caused by the

360

elimination of DO (Reaction 6.2 in Table 3) and the negative effect caused by the

361

conversion of eaq- to ·OH (Reaction 6.3 in Table 3). The ·OH converted from eaq- could

362

also oxidize BQH2 (Figure S10b) and the reduction products of NO3- (NO2- and NH4+),

363

partially offset the enhanced Pathway I. The k1 values of NO3- reduction in N2-purged

364

and N2O-purged solutions indicate that Pathway II contributed more (about 60%) to the

365

reduction of NO3-. The contribution of Pathway I was about 40%. This is somewhat

366

different from the conclusion in a previous work,6 in which it is stated that 3(BQH2)*

367

played a major role in the photochemical transformation of BQH2. The reason for this

368

difference has already been given in this literature:6 The former conclusion was drawn

369

from a transient system. In steady-state irradiation systems, the relative importance of

370

electron ejection might be the more efficient pathway.

371

18


Page 20 of 40

Page 21 of 40


372

Environmental Implications. Using concentrations higher than the realistic level is a

373

common approach in mechanism study for the sake of convenience and accuracy in

374

determination. The concentrations of As(III) and NO3- used in this work were selected

375

based on their realistic levels in environment. It should be noted that BQ was unstable

376

at ambient conditions and would experience rapid conversion under either acidic or

377

alkaline conditions (Figure S11). Under UV irradiation, BQ was rapidly converted to

378

BQH2 and BQ-OH (Figure 3). BQH2 was stable in acidic or neutral solutions but

379

underwent significant conversion at alkaline conditions (Figure S11). Under the given

380

pH in this work (pH 6.8), there was no obvious change in the concentration of BQH2

381

within 50 hours in dark (Figure S11). Therefore, special attentions should be paid to the

382

UV/BQH2 system.

383

Considering the common concentrations of DOM in sunlit water (1-10 mg/L),11

384

the concentration of quinones in Figures 1-3 (0.5 mM) was a bit of high. To verify the

385

environmental implications of quinones, the oxidation of As(III) and reduction of NO3-

386

were checked in both an ultrapure water and a surface water (JXR water) at a quinone

387

concentration of 0.05 mM (close to the realistic concentration level in sunlit water)

388

under solar irradiation. As shown in Figure S12, both the oxidation of As(III) and the

389

reduction of NO3- were enhanced by quinones. The oxidation of As(III) was more

390

significant than the reduction of NO3-. The JXR water had a specific UV254

391

absorbance (SUVA254nm) of 3.55 ± 0.20 L/(mg·m) (Table S1), indicating a high

392

concentration of aromatic DOM. As compared to that in the ultrapure water, the photo19


nm


393

oxidation of As(III) was significantly enhanced by the JXR water and the effect of the

394

JXR matrix was similar to that of BQ, demonstrating the environmental relevance of

395

quinones in the photo-conversion of oxyanions. The results here are helpful for the

396

better understanding of the natural processes of quinones and oxyanions in aquatic

397

environment.

398 399

ASSOCIATED CONTENT

400

SUPPORTING INFORMATION

401

Chromatography analysis (Text S1 and S2), HPLC and UPLC-MS profiles (Figure S1),

402

UV spectra (Figure S2), reduction potential ladders (Figure S3), isotope effect

403

experiments (Figure S4), As(III) oxidation by KO2 and H2O2 (Figure S5), Eh-pH

404

relationship of H2O2 and arsenic species (Figure S6), generation of H2O2 and steady-

405

state concentration of ·OH (Figure S7), ESR results (Figure S8), products of nitrate

406

(Figure S9), effect of N2 and N2O-purging (Figure S10), evolution of quinones under

407

ambient conditions (Figure S11), irradiation experiments with JXR water (Figure S12),

408

water quality parameters of JXR water (Table S1), E values (Table S2), and quenching

409

experiments (Table S3). This material is available free of charge via the Internet at

410

http://pubs.acs.org.

411 412

AUTHOR INFORMATION

413

Corresponding Author 20


Page 22 of 40

Page 23 of 40


414

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

415

Notes

416

#

417

financial interests.

The two authors contributed equally to the paper. The authors declare no competing

418 419

ACKNOWLEDGMENTS

420

This work was financially supported by the National Natural Science Foundation of

421

China (21522702, 21677070). The work was also supported by the Enhancement

422

Program for Outstanding PhD candidates of Nanjing University.

423 424

References

425

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583

radicals (⋅OH/⋅O−) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17 (2),

584

513-886.

29



585

Figure, Scheme and Table Captions

586 587

Figure 1. The evolution of (a) BQ, (b) BQH2, (c) As(III), and (d) NO3- in mono-,

588

binary and ternary solutions under UV irradiation (air-equilibrated, initial

589

pH around 6.8). [BQ]0 = [BQH2]0 = 0.5 mM, [As(III)]0 = [NO3-]0 = 0.1 mM,

590

light intensity: 7.1 mW/cm2. Scatters: experimental data, curves: the

591

pseudo-first-order fitting.

592 593

Figure 2. The evolution of arsenic and DO in the UV and UV/BQH2 process under a light intensity of 5.5 mW/cm2. [BQH2]0 = 0.5 mM, [As(III)]0 = 0.1 mM.

594

Figure 3. The photo-conversion of (a) BQ and (b) BQH2 under a light intensity of 4.0

595

mW/cm2 (pH 6.8, air-equilibrated). Symbol: experiment data, Curve: the

596

pseudo first-order kinetic fitting. The data near the curves are the k1 values

597

in corresponding processes.

598

Scheme 1. Photochemical and photophysical processes of (a) BQ and (b) BQH2 in the

599

presence of As(III) and NO3-. The dotted red arrows indicate low possibility.

600

– The blue arrows indicate the disproportionation of BQ˙ .

601 602 603 604 605

Table 1. The k1 (min-1) of quinones, As(III), and NO3- in mono, binary and ternary solutions. Table 2. The k1 ratios in the redox conversion of As(III) and NO3- in the three photochemical processes. Table 3. Reaction pathways of the reactive species in the UV/quinone systems. 30


Page 32 of 40

Page 33 of 40


606 607

Figure 1. The evolution of (a) BQ, (b) BQH2, (c) As(III), and (d) NO3- in mono-, binary

608

and ternary solutions under UV irradiation (air-equilibrated, initial pH around 6.8).

609

[BQ]0 = [BQH2]0 = 0.5 mM, [As(III)]0 = [NO3-]0 = 0.1 mM, light intensity: 7.1 mW/cm2.

610

Scatters: experimental data, curves: the pseudo-first-order fitting.

611

31



612 613

Figure 2. The evolution of arsenic and DO in the UV and UV/BQH2 process under a

614

light intensity of 5.5 mW/cm2 in the presence of As(III). [BQH2]0 = 0.5 mM, [As(III)]0

615

= 0.1 mM.

32


Page 34 of 40

Page 35 of 40


616 617

Figure 3. The photo-conversion of (a) BQ and (b) BQH2 under a light intensity of 4.0

618

mW/cm2 (pH 6.8, air-equilibrated). Symbol: experiment data, Curve: the pseudo first-

619

order kinetic fitting. The data near the curves are the k1 values in corresponding

620

processes.

621

33



622

Scheme 1. Photochemical and photophysical processes of (a) BQ and (b) BQH2 in the

623

presence of As(III) and NO3-. The dotted red arrows indicate low possibility. The blue

624

arrows indicate the disproportionation of BQ˙–.

625

34


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Page 37 of 40


626

Table 1. The k1 (min-1) of quinones, As(III), and NO3- in mono, binary and ternary

627

solutions.

Solution a

k1,Q

Solution

k1,As(III)

Solution

k1,NO3-

BQH2

0.0315

As(III)

0.0059

NO3-

0.0021

BQH2/NO3-

0.0338

As(III)/NO3-

0.0098

NO3-/BQH2

0.0113

BQH2/As(III)

0.0343

As(III)/BQH2

0.0179

NO3-/As(III)

0.0129

BQH2/NO3-/As(III)

0.0364

As(III)/NO3-/BQH2

0.0190

NO3-/As(III)/BQH2

0.0135

BQ

1.0097

As(III)

0.0059

NO3-

0.0021

BQ/NO3-

1.1585

As(III)/NO3-

0.0098

NO3-/BQ

0.0079

BQ/As(III)

1.0578

As(III)/BQ

0.0089

NO3-/As(III)

0.0113

BQ/NO3-/As(III)

1.0570

As(III)/NO3-/BQ

0.0095

NO3-/As(III)/BQ

0.0077

[AA]0 = [BQ]0 = [BQH2]0 = 0.5 mM, [As(III)]0 = [NO3-]0 = 0.1 mM, light intensity:

628

a

629

7.1 mW/cm2.

35



Page 38 of 40

630

Table 2. The k1 ratios in the redox conversion of As(III) and NO3- in the three

631

photochemical processes.

k1 ratio

Ct a

BQ

BQH2

AAb

As(III)

1.51

3.03

126.4

NO3-

3.76

6.14

4.33

As(III)

118.8

1.92

0.20

NO3-

146.6

2.62

9.58

As(III)

1.07

1.06

1.29

NO3-

0.97

1.05

1.71

k1,Ct-RM/k1,Ct

k1,RM-Ct/k1,Ct-RM

k1,Ct-RM(tri-)/k1,Ct-RM

Ct represents As(III) or NO3-, [As(III)]0 = [NO3-]0 = 0.1 mM, [AA]0 = [BQ]0 = [BQH2]0

632

a

633

= 0.5 mM, light intensity: 7.1 mW/cm2.

634

b

The data are cited from reference 39.

36


Page 39 of 40


Table 3. Reaction pathways of the reactive species in the UV/quinone systems. k2 (M-1 s-1)

Reaction types

Reference

O2 + As(III) → As(IV) + O2˙– a

-- b

Electron transfer

15

H2O → O2 + *H2O c

--

Quenching

45

O2 + BQH2 → BQH2* + O2

7.0 × 106

Quenching

46

O2 + BQH2 → BQH· + O2˙– + H+

--

Electron transfer

46

O2˙– + As(III) + 2H+ → As(IV) + H2O2

3.6 × 106

Electron transfer

28

2.2

O2˙– + BQH2 + H+ → BQH· + H2O2

1.7 × 107

Electron transfer

6

2.3

O2˙– + BQ → BQ˙– + O2

9.0 × 108

Electron transfer

28

2.4

O2˙– + BQH· + H+→ BQ-OH + H2O

--

Radical recombination

6

2.5

O2˙– + O2˙– + 2H+ → O2 + H2O2

2.3 × 106

Disproportionation

49

H2O2

3

H2O2 → 2·OH (hv, Φ = 1.0)

--

Photolysis

51,52

·OH

4.1

·OH + BQH2 → BQH· + H2O

5.2 × 109

Electron transfer

55

4.2

·OH + As(III) → As(IV)

8.5 × 109

Electron transfer

33

Species

Reaction

Formulas

1O 2

1.1

1

1.2

1O + 2

1.3

1

1.4

1

2.1

O2˙–

37



As(IV)

eaq-

Page 40 of 40

4.3

·OH + BQ → BQ-OH

6.6 × 109

Electron transfer

43

5.1

As(IV) + BQH2 → BQH· + As(III)

--

Electron transfer

This work

5.2

As(IV) + O2 → As(V) + O2˙–

1.4 × 109

Electron transfer

33

5.3

2As(IV) → As(V) + As(III)

3.6 × 108

Disproportionation

33

6.1

8eaq- + NO3- + 10H+ → NH4+ + 3H2O

9.7 × 109

Electron transfer

55

6.2

eaq- + O2 → O2˙–

1.9 × 1010

Electron transfer

55

6.3

eaq- + N2O + H+ → ·OH + N2

9.1 × 109

Electron transfer

55

a

Blue font means low possibility.

b

“--” represents “not available”.

c

Bold font indicates dominant pathway.

38


Quinone Systems

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