Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel

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Environmental Processes

Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel Photosensitizer and Electron Shuttle Jiyuan Jin, Shujuan Zhang, Bingdang Wu, Zhihao Chen, Guoyang Zhang, and Paul G. Tratnyek Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02751 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Environmental Science & Technology

Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel Photosensitizer and Electron Shuttle

Jiyuan Jin1, Shujuan Zhang1*, Bingdang Wu1, Zhihao Chen1, Guoyang Zhang1, and Paul G. Tratnyek2

1

State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing, 210023, China

2 OHSU-PSU

School of Public Health, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland, OR 97239, USA

*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

Quinones are important electron shuttles as well as micropollutants in the nature.

3

Acetylacetone (AA) is a newly recognized electron shuttle in aqueous media exposed to

4

UV irradiated. Herein, we studied the interactions between AA and hydroquinone (QH2)

5

under steady-state and transient photochemical conditions to clarify the possible reactions

6

and consequences if QH2 and AA co-exist in a solution. Steady-state experimental results

7

demonstrate that the interactions between AA and QH2 were strongly affected by dissolved

8

oxygen. In O2-rich solutions, the photo-transformation of QH2 was AA-independent. Both

9

QH2 and AA utilize O2 as the electron acceptor; but in O2-insufficient solutions, AA

10

became an important electron acceptor for the oxidation of QH2. In all cases, the co-

11

existence of AA increased the photo-transformation of QH2, whereas the decomposition of

12

AA in O2 saturated and over-saturated solutions was inhibited by the presence of QH2. The

13

underlying mechanisms were investigated by a combination of laser flash photolysis (LFP)

14

and reduction potential analysis. The LFP results show that the excited AA serves as a

15

better electron shuttle than QH2. As a consequence, AA might regulate the redox cycling

16

of quinones, leading to significant effects on many processes, ranging from photosynthesis

17

and respiration to photodegradation.

1



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19

INTRODUCTION

20

Quinones are among the most characteristic reactive moieties in biological and

21

environmental

22

(dihydroxybenzene-containing) structures is an essential link in the electron transport chain

23

for both photosynthesis and cellular respiration.3,4 The interconversion between the

24

quinone and quinol structures is also the reaction mechanism responsible for the

25

anthraquinone autoxidation process that is used in industrial production of hydrogen

26

peroxide (H2O2).5 With their extensive use in a wide range of industries, quinones become

27

important micropollutants of the aquatic environment, including in effluents from textile

28

dying, photo-processing, coal-tar production, and paper manufacturing.6-9

chemistry.1,2

The

interconversion

of

quinone

and

quinol

29

The reactivity of quinones in aquatic media has been extensively studied from various

30

perspectives, including their photo-transformation10-14 and role as electron shuttles (redox

31

mediators) in natural and engineered systems.1,15-17 As electron shuttles, quinones can

32

produce reactive oxygen species (ROS) in photochemical redox cycling via either

33

oxidation of water or reduction of dissolved oxygen (DO).15-17 The ROS generated from

34

quinones mediates a wide range of environmental redox processes in the subsurface, soils,

35

sediments, surface and atmospheric waters.18-20

36

So far, more attention has been paid to the photo-transformation of quinones than

37

quinols.10-14 In a recent work,21 it was found that hydroquinone (QH2) is more

38

photochemically stable than 1,4-benzoquinone (BQ). Under identical conditions (0.5 mM

39

quinones, pH 6.8, air-saturated, 4.0-7.1 mW/cm2 at 365 nm, medium pressure mercury 2


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40

lamp), the pseudo-first-order rate constant (k1) of QH2 was in the range of 0.002-0.032 min-

41

1, whereas the k of BQ was in the range of 0.39-1.01 min-1. In BQ-arsenite binary solutions, 1

42

BQ was rapidly reduced to QH2 under UV irradiation within a few minutes, while the

43

oxidation of arsenite occurred after the depletion of BQ,21 indicating that the ROS

44

generated from the photolysis of QH2 rather than BQ was responsible for the oxidation of

45

arsenite. Therefore, taking these secondary reactions into account, it appears that the

46

photochemistry of quinols is environmentally important and deserves further research.

47

The two carbonyl groups and the keto-enol tautomerization make acetylacetone (AA)

48

structurally similar to quinone/quinol. As the simplest β-diketone, AA is widely used as a

49

precursor in organic synthesis and as an additive in gasoline, lubricants, inks, and dyes.22-

50

24

51

UV irradiation, AA can rapidly decolorize several types of dyes, including quinone-based

52

dyes such as 1-aminoanthraquinone-2-sulfonic acid, 1-amino-2-hydroxyl anthraquinone-

53

2-sulfonic acid, 4,4’-diamino-1,1’-dianthraquinone-3,3’-disulfonate and alizarin red.26,27

54

Based on this observation, we proposed that AA might be an environmentally significant

55

photoactivator.28 In follow-up work, we found that the UV-excited AA reacted similarly to

56

semiquinone radicals (QH•−) in redox reactions involving arsenite and nitrate.21,29 In the

57

photo-oxidation of arsenite, the k1 of arsenite oxidation in the UV/AA process was 10 and

58

60 times to those in the UV/QH2 and UV/BQ processes, whereas the consumption of AA

59

was less than 1/25 and 1/1000 of those of QH2 and BQ, respectively.29 The efficient

60

electron shuttling between arsenite and DO via AA and excited AA was attributed to the

AA is also reported as an oxidation product in the treatment of sludge liquors.25 Under

3



61

higher efficiency but lower consumption of AA as compared with quinones.29 Since then,

62

we have further shown that AA under dark conditions can serve as an electron donor for

63

BQ, resulting in the formation of QH· and the subsequent reduction to QH2.30

64

QH2 with a standard reduction potential (E0QH·/QH2) of 1.04 V vs SHE is a mild

65

reductant relative to many environmental species.17 The reduction potential (E(AA+H)·/AA)

66

and oxidation potential (EAA/(AA-H)·) of AA have been determined to be -1.172 V vs SHE at

67

pH 2.0 and 0.969 V vs SHE at pH 7.0, respectively.29 These E values suggest that electron

68

transfer between QH2 and AA may be thermodynamically favorable under environmental

69

conditions, but there appears to be no prior work documenting direct reaction between QH2

70

and AA.

71

Although the photochemistry of QH2 and AA have been investigated

72

individually,12,21,26-29 it is still unclear what will happen if QH2 and AA co-exist in a

73

solution. Specifically, both QH2 and AA are known as electron shuttles, but it is not clear

74

which will act as the electron donor and which will act as the acceptor under UV irradiation.

75

The interactions between QH2 and AA are expected to affect not only the formation of

76

ROS, with or without photoactivation, but also many biological and environmental redox

77

processes. Therefore, in the present work, we studied the interactions of QH2 and AA in

78

both steady-state and transient photochemical systems. The objectives were: (1) to clarify

79

the electron shuttling performances of QH2 and AA in the absence and presence of O2, and

80

(2) to elucidate the mechanisms of the interactions between QH2 and AA. To the best of

4


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81

our knowledge, this is the first work on the interactions between QH2 and diketones in

82

aquatic photochemical systems.

83 84

EXPERIMENTAL

85

Materials. All materials were used as received without any purification. QH2 and AA

86

of analytical grade were purchased from Nanjing Chemical Reagent Co. Ltd., China. H2O2

87

(30 wt%) was purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd., China.

88

Phosphoric acid and 2,4-dinitrophenylhydrazine (2,4-DNPH) of analytical grade were

89

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

90

phenylenediamine (DPD) and peroxidase (POD) from horseradish (150 U/mg) were

91

bought from Sigma-Aldrich, Germany and Sigma, Switzerland, respectively. 5,5-dimethyl-

92

1-pyrroline-1-oxide (DMPO) was purchased from Sigma-Aldrich Co. Ltd., USA. KH2PO4

93

(≥ 99.5%, HPLC grade) was purchased from Aladdin Industrial Corporation (Shanghai,

94

China). Formic acid (HPLC grade) was purchased from Roe Scientific Inc., USA.

95

Methanol and acetonitrile of chromatography grade were purchased from Merck Co. Ltd.,

96

Germany. Ultrapure water (18.25 MΩ·cm) made from a water purification system

97

(Shanghai Ulupure Industrial Co. Ltd., China) was used throughout the experiments.

98

The pH of all solutions was about 6.0, without any adjustment. High purity N2, O2 and

99

N2O (99.99%) were used to purge the solutions whenever needed. Solutions were bubbled

100

for at least 20 min prior to UV irradiation and was continuingly bubbled throughout the

101

whole irradiation process. 5



102

Steady-state Irradiation Experiments. The irradiation experiments were conducted

103

with a rotating disk photoreactor (Nanjing Stone Tech Electric Equipment, China). A

104

medium-pressure mercury lamp was used as the light source. The emission spectrum was

105

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

106

USA). More details about the reactor were available in a previous report.31

107

Laser Flash Photolysis Experiments. Transient absorption experiments were

108

conducted with an LP980 Edinburgh instrument (UK) with laser excitation at 266 nm from

109

a Surelite I-10 Q-Switched Nd: YAG laser (35 mJ). A xenon lamp was used as detecting

110

light source, and a 100 MHz Tektronix TDS3012C digital oscilloscope was employed to

111

record the transient signals of aqueous solutions in a 1 cm path length quartz cell. The

112

software L900 provided by Edinburgh was used to analyze the digitized signal.

113

Prior to laser flash photolysis experiments, the N2-purged solutions were added to fill

114

quartz cells (1 cm in diameter) and were further deoxygenated in a glovebox (Super

115

(1220/750/900), Mikrouna Co. Ltd., Shanghai, China). O2-saturated solutions were

116

transferred to quartz cells immediately after purging with high purity oxygen and were

117

sealed with screw caps to conduct the photolysis experiments. The reaction progress was

118

characterized by subtracting the absorbance of the raw solution from the absorbance of the

119

laser pulsed solution. The absorbance difference is denoted as △OD, which is indicative of

120

the intensities of generated species from laser pulse. The △OD reaches the highest point

121

within a short time after laser pulse and then decays exponentially with time.

122

Analytical Approach. Absorption spectra of the solutions were recorded with a 6


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123

double beam spectrophotometer (UV-2700, Shimadzu Co., Japan). The DO content was

124

determined with a dissolved oxygen meter (SG9-ELK, Mettler Toledo). Solution pH was

125

measured with a pH meter (intelliCALTM MTC101, HACH). The concentration of H2O2

126

was determined with the DPD-POD method.32

127

The concentration of AA was analyzed with a high-performance liquid

128

chromatography (HPLC, Ultimate 3000, Dionex Co. Ltd., USA) system. Prior to analysis,

129

AA-containing solutions were mixed with 2,4-DNPH (0.6 g/L in a 0.2% phosphoric acid-

130

methanol solution) for at least 3 h for the derivatization of AA.33 The derivation products

131

were recorded with a UV detector at 254 nm. A C18 column (SunFire, 5 μm, 4.6 × 150

132

mm, Waters, Ireland) was equipped to separate AA derivatives from QH2 at 25 oC. The

133

mobile phase was composed of methanol and water at a ratio of 6:4 and was pumped at a

134

flow rate of 0.8 mL·min-1. The concentration of QH2 was determined with the HPLC at

135

225 nm with an Eclipse Plus C8 column at 25 oC (5 μm, 4.6 × 150 mm, Agilent, USA).

136

The mobile phase was composed of methanol and 5 mM KH2PO4 at a ratio of 3:7 and was

137

pumped at a flow rate of 0.6 mL·min-1.

138

The reaction products were identified with a Thermo Fisher Scientific Q ExactiveTM

139

Focus Orbitrap LC-MS/MS system. A ZORBAX Eclipse Plus column C18 (4.6×150 mm,

140

3.5 μm, Aglient, USA) was used for separation. Acetonitrile and 0.1% phosphoric acid

141

(30%/70%, V/V) at a flow rate of 0.2 mL·min-1 were used as the mobile phase.

142

Electron spin resonance (ESR) experiments were conducted with a DRX500

143

spectrometer (Bruker Co. Ltd., Germany) and a 180 W MP-Hg lamp as the light source. 7



Page 10 of 33

144

The running parameters were as follows: center field of 3480.0 G, sweep width of 200 G,

145

microwave frequency of 9.773 GHz and power of 19.922 mW.

146

Calculation of Inner Filter Effect. The inner filter effect was quantified with the

147

fraction of available photons to the tested chemicals by the presence of other light-

148

absorbing substances. Basically, the absorption spectra of the chemicals and the emission

149

spectrum of the light source was used for the calculation of the inner filter effect. The inner

150

filter effect (ICF) was calculated with the following equation:

   Q     Q

,w/

I CF 151

(1)

152

where  is the photonic efficiency, i.e., the ratio of the number of converted molecules to

153

the total number of photons absorbed by the system; λ (nm) is the wavelength; Q is the

154

absorbed number of photons of a given λ per unit time; “w/” indicates the coexistence of

155

light-absorbing substances. More details for the calculation are available in previous

156

work.31

157 158

RESULTS AND DISCUSSION

159

Photo-transformation of QH2 and AA. Quinones generally are stable in acidic to

160

neutral solutions, but are much more labile to reduction under alkaline conditions.34 In the

161

absence of UV irradiation, the solutions of AA and QH2 under ambient conditions were

162

stable over 6 hr (Figure S1). However, as illustrated in Figure 1a-c, even in acidic to neutral

163

solutions, reaction between QH2 and AA is rapid under UV irradiation. The photo8


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164

transformation of QH2 was dependent on AA concentration, but also the concentration of

165

DO (Figure 1a-c and Table 1). In N2-purged solutions ([O2] < 0.01 mM), the photo-

166

transformation of QH2 was negligible in the absence of AA, but was significantly enhanced

167

by the addition of AA (Figure 1a). There was a positive correlation between the k1,QH2 and

168

the concentration of AA (inset of Figure 1a). In O2-saturated solutions ([O2] ≈ 1.4 mM),

169

QH2 underwent a rapid photo-transformation and the enhancement effect of AA on the

170

photo-transformation of QH2 became less significant (Figure 1b).

171

Interestingly, a two-stage photo-transformation process was observed in the air-

172

saturated solutions ([O2] = 0.28 mM) when the concentrations of AA and QH2 were in the

173

range of 0.1-0.2 mM (Figure 1c). The thermal reaction of QH2 is sensitive to solution pH,

174

with the deprotonated form of QH2 usually being more reactive than the protonated

175

species.12 After irradiation, the solution pH had decreased from 5.8 to around 4.0

176

(depending on the concentration of AA) (Figure 1d). In this pH range, the speciation of

177

QH2 (pKa = 9.85) was unaltered. Consequently, the reaction was not affected by such a pH

178

change. This is further confirmed by the experiment run in buffered solutions. As shown

179

in Figure S2, the two-stage kinetics exist in both buffered and unbuffered solutions.

180

Therefore, it is reasonable to infer that the two-stage kinetics was not caused by the change

181

in solution pH. Instead, the depletion of DO was attributed to the decreased rate of photo-

182

transformation of QH2 in the later stage (Figure 1e). This conclusion is supported by the

183

positive correlation between the photo-transformation of QH2 and the concentration level

184

of O2 (Table 1). This conclusion was also supported by the segmentation of the k1, DO vs 9



185

[AA]/[QH2] profile, with a break point at the AA/QH2 molar ratio of 1 (Figure 1f). The

186

absence of segmentation in the kinetic curve of the 0.5 mM AA-0.2 mM QH2 solution

187

could be explained by the combined roles of DO and AA: the inhibition caused by the

188

depletion of DO was compensated by enhancement from the residual AA. In short, the

189

photo-transformation of QH2 in the presence of AA is a two-step process: the first step is

190

the AA-independent conversion of QH2 under conditions of sufficient O2; the second step

191

is the AA-dependent transformation of QH2 under O2-deficiency conditions.

192

Accompanying with the photo-transformation of QH2, AA was simultaneously

193

decomposed (Table 1). AA decomposition was fastest in air-saturated solutions. Both

194

deoxygenation (N2-purging) and oxygenation (O2-purging) resulted in slower

195

decomposition of AA. In all cases, the co-existence of AA was favorable for the photo-

196

transformation of QH2, whereas the presence of QH2 inhibited the decomposition of AA in

197

aerated and oxygenated solutions (Table 1). The ratio of k1 values for the disappearance of

198

QH2 and DO was close to 1 (0.095 min-1 vs 0.081 min-1), as was the ratio of k1 values for

199

the disappearance of AA and DO (0.137 min-1 vs 0.126 min-1) (Table S1). Therefore, it is

200

reasonable to infer that the inhibition that occurred in the aerated solutions arose from the

201

competition between QH2 and AA for O2. However, this competition should be

202

insignificant in the oxygenated solution, because the concentration of DO was far larger

203

than that of QH2 and AA.

204

Besides the competition for O2, QH2 and AA also competed for photons (Figure S3). In

205

the mixture, the number of photons available to AA was reduced by QH2 and the photolysis 10


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206

products (Figure S3). The fraction of available photons to AA was in the range of 0.68-

207

0.36 (Figure S3), which is consistent with the k1,AA(w/ QH2)/k1,AA(w/o QH2) ratios (0.59-0.31)

208

(Table 1). Therefore, the observed inhibitory effect of QH2 on the photo-transformation of

209

AA derived from the inner filter effect. This inner filter effect was mutual because AA, in

210

turn, exerted an inner filter effect on the photo-transformation of QH2. Comparatively, the

211

inner filter effect for QH2 was weaker than that for AA, because the products of QH2 had

212

much stronger absorption than those of AA. The enhancement effect of AA on the photo-

213

transformation of QH2 compensates for the inner filter effect caused by AA. As a result,

214

faster rates of photo-transformation of QH2 were caused by the co-existence of AA.

215

As noted above, H2O2 could be generated by the reaction of QH2 with O2. The complex

216

effect of DO on the photo-transformation was reflected in the formation rate of H2O2 (Table

217

S1). More H2O2 was generated in the oxygenated QH2-containing solutions (from 1.542 to

218

3.601 M/min in QH2 solution and from 1.288 to 1.810 M/min in QH2-AA solution),

219

whereas oxygenation decreased the formation of H2O2 in the AA solution (from 1.256 to

220

0.642 M/min). With the addition of AA, the yields of H2O2 in both air-saturated and O2-

221

saturated solutions were decreased about 16.5% (from 1.542 to 1.288 M/min) and 49.7%

222

(from 3.601 to 1.810 M/min), respectively, although AA could also produce H2O2 (Table

223

S1).

224

Other than H2O2, other products were detected in the solutions using LC-MS/MS

225

(Figure S4). In O2-saturated solutions, the species generated in the irradiated QH2-AA

226

solution were the same as those in the irradiated QH2 solution, and included hydroxyl11



227

benzoquinone (QOH), 2,5,2’,5’-tetrahydroxybiphenyl (THBP), and hydroxylated THBP

228

derivatives (M1-M4) (Table S2 and S3). In N2-purged or air-saturated solutions, the

229

addition of AA led to the formation of additional products from the adducts of AA-derived

230

radicals with the above mentioned quinoid molecules (Table S2 and S3). The above results

231

demonstrate that in the presence of sufficient O2, the photo-transformation of QH2 and AA

232

proceeded independently. In low O2 or anoxic systems, the interaction between QH2 and

233

AA became predominant.

234

Primary Photochemical and Photophysical Processes in QH2 Solution. To better

235

understand the interactions between QH2 and AA, time-resolved transient absorption

236

spectra and kinetic traces under various conditions were obtained. The primary

237

photochemistry and photophysics of QH2 and AA individually have been reported in the

238

literature.12,23,28 After a photon is absorbed, QH2 is activated to the first excited state

239

1(QH

240

photoionization, and intersystem crossing (ISC) to 3(QH2)* with a quantum yield (ΦISC) of

241

0.39.12 The generated 3(QH2)* is a strong reductant (Eh7(QH·/3(QH2)*) = -2.84 V vs SHE)

242

and could react with O2 and other applicable chemicals through direct electron transfer.12,21

243

AA is non-fluorescent with a ΦISC of 1.0.23 Upon photoexcitation, the triplet excited state

244

of AA, i.e., 3(AA)*, was the main transient species, which could be quenched by O2 or

245

heterolytically degraded into carbon-centered or oxygen-centered radicals.28

*,

2)

which decays by three possible pathways: fluorescence quenching, direct

246

After the laser pulse, three absorption bands were observed at 312, 410 and 650 nm

247

(Figure 2a). A shoulder peak at 428 nm appeared at pH 4.0 or above and was absent at pH 12


Page 14 of 33

Page 15 of 33


248

3.0 (Figure 2a). The band at 650 nm was enhanced by deoxygenation and was a

249

characteristic absorption of eaq–. The species at 312, 410, and 428 nm have been assigned

250

to QH•/Q•−.11,12 As illustrated in the kinetic traces (Figure 2b), the 410 nm and 650 nm

251

species appeared first, followed by the formation of the 312 nm and 428 nm species. In

252

contrast to the 312 nm and 428 nm traces, the 410 nm trace showed a sharp decline at the

253

very beginning (< 200 ns). The maximum △OD of the 312 nm and 428 nm species were

254

insensitive to DO, whereas the maximum △OD of the 410 nm species was reduced by DO

255

(Table 2). After the rapid decline, the secondary maximum △OD of the 410 nm species

256

was also DO-independent.

257

On the basis of the above observation, we propose that the 410 nm species was the

258

cation radical of QH2 (QH2•+), coming from the photoionization of 1(QH2)*. QH2•+ should

259

deprotonate relatively rapidly in neutral solutions (pKa = -0.8) and the generated QH· will

260

deprotonate further to Q•− (pKa = 4.0).12 Both QH2•+ and QH· had a peak absorption at 410

261

nm, whereas the maximum absorption of Q•− was at 428 nm. There are three possible

262

reaction pathways between 3(QH2)* and O2: thermal quenching, energy transfer, and

263

electron transfer. Energy transfer leads to the formation of singlet oxygen (1O2), whereas

264

direct electron transfer results in the formation of QH2•+. Compared with the N2-purged

265

counterpart, less QH2•+ radical was generated in the aerated and oxygenated solution,

266

excluding the direct electron transfer between 3(QH2)* and O2. The higher maximum △OD

267

values at 410 nm and 428 nm in the oxygenated solution compared with the aerated results

268

suggest that electron transfer between 3(QH2)* and O2 did occur, but in a proton coupled 13



269

pathway. As a result, the product was QH·, rather than QH2•+. It should be noted that all of

270

the quinone radicals could react with O2. Therefore, the lifetimes of these species were DO-

271

dependent because a higher DO concentration led to a short lifetime (Table 2). In aerated

272

and oxygenated solutions, the addition of AA had a negligible effect on the lifetimes of the

273

quinone radicals. However, the lifetimes of the quinone radicals in the N2-purged solution

274

was significantly reduced by the addition of AA, providing a direct evidence for the

275

interaction between quinone radicals and AA.

276

The effect of AA was also observed in the decay of eaq– (Figure 3). The addition of

277

AA reduced both the maximum △OD at 650 nm and the lifetime of the eaq– (Table 2). The

278

reduced △OD at 650 nm was due to the inhibited photoionization of QH2 caused by the

279

inner filter effect from AA. The shortened lifetime was a result of the reaction between eaq–

280

and AA, whose contribution was more significant in N2-purged solution than those in O2-

281

containing solutions, because O2 is a powerful scavenger of eaq–.35

282

Difference in reduction potential is the main driving force for the electron transfer

283

between a reductant-oxidant pair. The reduction potentials of O2, AA, and QH2 related

284

species are illustrated in Figure 4. On the basis of the E values, there were six main possible

285

electron transfer pathways for the decomposition of QH2: from 3(QH2)* to O2 (Path a), from

286

3(QH

287

d), from QH2 to 3(AA)* (Path e), and from QH2 to (AA-H)· (Path f). As for the

288

decomposition of AA, there were only two main possible electron transfer pathways: from

*

2)

to AA (Path b), from 3(QH2)* to 3(AA)* (Path c), from 3(QH2)* to (AA-H)· (Path

14


Page 16 of 33

Page 17 of 33


to O2 (Path g) and from 3(AA)* to Q•− (Path h). The contributions of these pathways

289

3(AA)*

290

were dependent on the relative abundance of the reactants and the △E. Both the abundance

291

of the reactants and the △E were much more favorable for QH2 than for AA. This also

292

explains why in all cases, the co-existence of AA was favorable for the photo-

293

transformation of QH2, whereas inhibition effect was observed in the decomposition of AA.

294

The above mechanisms are summarized in Scheme 1. In short, the photoexcitation of

295

QH2 leads to the formation of 1(QH2)* and 3(QH2)*, which undergoes a series of further

296

reactions: physical quenching (collisional deactivation), energy transfer to generate 1O2,

297

and electron transfer to generate O2•− and H2O2. The reactive species in Scheme 1,

298

including eaq−, ·OH and Q • −, were confirmed with ESR determination (Figure S5).

299

Although no direct detection of O2•− was conducted, there was a solid evidence for the

300

formation of O2•−: The kinetic traces for eaq− varied with the change of DO (Figures 2 and

301

3), indicating the reaction between eaq− and O2, with O2•− as the product. O2•− will then

302

disproportionate to H2O2 and O2. Similar to O2, AA serves as an electron acceptor for QH2.

303

Therefore, in O2-deficient solutions, AA plays a significant role in the decomposition of

304

QH2 as an alternative of O2.

305

Environmental Implications. In the present work, we verified that AA can serve as

306

an electron acceptor for QH2 in UV irradiated systems. The role of AA in the photo-

307

transformation of QH2 was much more significant in anoxic solutions than in aerated

308

solution, indicating that the electron accepting ability of AA was weaker than that of O2.

309

However, because excited AA serves as both an electron donor and acceptor (O2 can serve 15



310

only as an electron acceptor), it plays an important role in the interconversion between BQ

311

(AA acted as an electron donor in the reduction of BQ30) and QH2. Since AA was much

312

more stable and abundant than semiquinone radicals, AA may play a more important role

313

in photochemical systems. For example, intracellular AA might participate in the cycling

314

of quinones and consequently influence oxidative stress in living organisms. For

315

environmental engineering applications, QH2 has been found to greatly increase the

316

degradation rate of substituted benzenes in Fenton-like reactions (through enhanced

317

formation of ROS),15 AA might also be able to affect the Fenton and Fenton-like reactions.

318

The possibility for AA to be used in regulation of artificial photosynthesis and

319

photodynamic action are also topics deserving of attention.

320 321

ASSOCIATED CONTENT

322

Supporting Information

323

Further details of the experimental data are presented free of charge on the internet at

324

http://pubs.acs.org. These materials include: UV spectra (Figure S1), QH2 degradation in

325

buffered solutions (Figure S2), UV spectra and correction factor of inner filter effect

326

(Figure S3), total ion current spectra (Figure S4), ESR spectra (Figure S5), rate constants

327

of O2 and H2O2 (Table S1), products in the UV irradiated solutions (Table S2) and ion

328

current intensity of products (Table S2).

329 330

AUTHOR INFORMATION 16


Page 18 of 33

Page 19 of 33


331

Corresponding Author

332

*Phone: +86 25 8968 0389; E-mail: [email protected]

333

Notes

334

The authors declare no competing financial interests.

335 336

ACKNOWLEDGMENTS

337

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

338

China (No. 21677070) and the National Key Research and Development Program of

339

China (No. 2018YFC1802003).

340 341

Reference

342

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of phenolic micropollutants within polyamide composite membranes. Environ. Sci.

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Biomed. Anal. 2014, 87, 261-270.

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(10) Ononye, A. I.; McIntosh, A. R.; Bolton, J. R. Mechanism of the photochemistry of

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p-benzoquinone in aqueous-solutions. 1. spin trapping and flash-photolysis electron-

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paramagnetic resonance studies. J. Phys. Chem. 1986, 90 (23), 6266-6270.

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(11) Ononye, A. I.; Bolton, J. R. Mechanism of the photochemistry of p-benzoquinone in

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aqueous-solutions. 2. optical flash-photolysis studies. J. Phys. Chem. 1986, 90 (23),

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6270-6274.

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(12) Boule, P.; Rossi, A.; Pilichowski, J. F.; Grabner, G. Photoreactivity of hydroquinone 18


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1,4-benzoquinone in aqueous solution revisited. Chem.-Eur. J. 2004, 10 (2), 440-451.

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(15) Chen, R.; J. J. Pignatello. Role of quinone intermediates as electron shuttles in Fenton

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and photoassisted Fenton oxidations of aromatic compounds. Environ. Sci. Technol.

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photolysis of dilute aqueous quinone solutions. Photochem. Photobiol. 2007, 83 (4),

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(17) Song, N.; Gagliardi, C. J.; Binstead, R. A.; Zhang, M.-T.; Thorp, H.; Meyer, T. J.

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Role of proton-coupled electron transfer in the redox interconversion between

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benzoquinone and hydroquinone. J. Am. Chem. Soc. 2012, 134 (45), 18538-18541.

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(18) Van der Zee, F. P.; Cervantes, F. J. Impact and application of electron shuttles on the

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redox (bio)transformation of contaminants: A review. Biotechnol. Adv. 2009, 27(3),

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256-277.

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(19) Brose, D. A.; James, B. R. Oxidation-reduction transformations of chromium in

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aerobic soils and the role of electron-shuttling quinones. Environ. Sci. Technol. 2010,

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44 (24), 9438-9444.

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(20) Scharko, N. K.; Martin, E. T.; Losovyj, Y.; Peters, D. G.; Raff, J. D. Evidence for 19



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conversion on soil surfaces. Environ. Sci. Technol. 2017, 51 (17), 9633-9643.

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(21) Chen, Z. H.; Jin, J. Y.; Song, X. J.; Zhang, G. Y.; Zhang, S. J. Redox conversion of

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arsenite and nitrate in the UV/quinone systems. Environ. Sci. Technol. 2018, 52 (17),

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10011-10018.

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(22) Siegel, H.; Eggersdorfer, M. Ketones. In Ullmann's Encyclopedia of Industrial Chemistry; Elvers, B., Eds.; Wiley: German 2012; pp 187-208.

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(23) Verma, P. K.; Koch, F.; Steinbacher, A.; Nuernberger, P.; Brixner, T. Ultrafast UV-

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induced photoisomerization of intramolecularly H-bonded symmetric beta-diketones.

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J. Am. Chem. Soc. 2014, 136 (42), 14981-14989.

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(24) Rastogi, S. C. Levels of organic-solvents in printer’s inks. Arch. Environ. Contam. Toxicol. 1991, 20 (4), 543-547.

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(25) Boyle, L. L.; McCullough, N. H. Ozonation of sludge-press liquors: Determination

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of carbonyl compounds by the PFBOA method and the effect on the chemical oxygen

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demand. Natl. Meet. Am. Chem. Soc. Div. Environ. Chem. 1996, 36 (2), 70-74.

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(26) Zhang, S. J.; Liu, X. T.; Wang, M. S.; Wu, B. D.; Pan, B. C.; Yang, H; Yu, H. Q.

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Diketone-mediated photochemical processes for target-selective degradation of dye

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pollutants. Environ. Sci. Technol. Lett. 2014, 1, 167-171.

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(27) Liu, X. T.; Song, X. J.; Zhang, S. J.; Wang, M. S.; Pan, B. C. Non-hydroxyl radical

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mediated photochemical processes for dye degradation. Phys. Chem. Chem. Phys.

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2014, 16 (16), 7571-7577. 20


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(28) Wu, B. D.; Zhang, G. Y.; Zhang, S. J. Fate and implication of acetylacetone in

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photochemical processes for water treatment. Water Res. 2016, 101, 233-240.

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(29) Chen, Z. H.; Song, X. J.; Zhang, S. J.; Wu, B. D.; Zhang, G. Y; Pan, B. C.

417

Acetylacetone as an efficient electron shuttle for concerted redox conversion of

418

arsenite and nitrate in the opposite direction. Water Res. 2017, 124, 331-340.

419

(30) Jin, J. Y.; Chen, Z. H.; Song, X. J.; Wu, B. D.; Zhang, G. Y.; Zhang, S. J. Effects of

420

acetylacetone on the thermal and photochemical conversion of benzoquinone in

421

aqueous solution. Chemosphere 2019, 223, 628-635.

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(31) Wu, B. D.; Yin, M. H.; Yin, R.; Zhang, S. J. Applicability of light sources and the

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inner filter effect in UV/acetylacetone and UV/H2O2. J. Hazard. Mater. 2017, 335,

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100-107.

425

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

426

low concentrations of hydrogen peroxide by the peroxidase catalyzed oxidation of

427

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

428

(33) Cardoso, D. R.; Bettin, S. M.; Reche, R. V.; Lima-Neto, B. S.; Franco, D. HPLC-

429

DAD analysis of ketones as their 2, 4-dinitrophenylhydrazones in Brazilian sugar-

430

cane spirits and rum. J. Food Compos. Anal. 2003, 16 (5), 563-573.

431

(34) Sadykh-Zade, S. I.; Ragimov, A. V.; Suleimanova, S. S.; Liogon'kii, V. I. The

432

polymerization of quinones in an alkaline medium and the structure of the resulting

433

polymers. Polym. Sci. U.S.S.R. 1972, 14 (6), 1395-1403.

434

(35) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B.; Tsang, W. Review of 21



435

rate constants for reactions of hydrated electrons chemical kinetic data base for

436

combustion chemistry, part 3: propane. J. Phys. Chem. Ref. Data 1988, 17, 513-886.

437

22


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438

Figure, Scheme and Table Captions

439

Figure 1. The evolution of QH2 (a-c), pH (d), and DO (e) in UV irradiated QH2 and QH2-

440

AA solutions. (f) The k1 of DO consumption vs the concentration ratio of AA

441

to QH2 in air-saturated solutions. [QH2]0: 0.2 mM, UV: 6.0 mW·cm-2 at 365

442

nm.

443

Figure 2. (a) The transient absorption spectra of QH2 (0.2 mM) in N2-purged and air-

444

saturated solutions of different pH. (b) The kinetic traces of the species at 312

445

nm, 410 nm, 428 nm, and 650 nm in a N2-purged QH2 solution (0.2 mM). Laser

446

(266 nm): 71.2 mJ/pulse.

447

Figure 3. (a) The kinetic traces of the species at 650 nm in (a) a long time interval and (b)

448

a short time interval after the laser pulse. [QH2]0 = [AA]0 = 0.2 mM, Laser (266

449

nm): 71.2 mJ/pulse.

450

Figure 4. The redox potential ladder and the possible electron transfer pathways of

451

quinones, O2, and AA related species at pH 6.0. The gray shadow represents

452

the stability area of H2O. Solid arrows: main pathways, dashed arrows: minor

453

pathways. The Eh6 values were calculated from the reported E0 values.17,21,29

454

Scheme 1. The photochemistry and photophysics of QH2 and AA in UV irradiated

455 456

solutions. Table 1.

457 458

The k1 values of the photo-transformation of QH2 (0.2 mM) and AA (0.2 mM) in mono- (w/o) and binary (w/) solutions. UV: 6.0 mW·cm-2 at 365 nm.

Table 2.

The maximum △OD and lifetime () of laser pulse generated species in QH2 23



459

(0.2 mM) and QH2-AA (0.2 mM) solutions. Laser (266 nm): 71.2 mJ/pulse.

24


Page 26 of 33

Page 27 of 33


461 462

Figure 1. The evolution of QH2 (a-c), pH (d), and DO (e) in UV irradiated QH2 and QH2-

463

AA solutions. (f) The k1 of DO consumption vs the concentration ratio of AA to QH2 in

464

air-saturated solutions. [QH2]0: 0.2 mM, UV: 6.0 mW·cm-2 at 365 nm.

25



466 467

Figure 2. (a) The transient absorption spectra of 0.2 mM QH2 in N2-purged and air-

468

saturated solutions of different pH. (b) The kinetic traces of the species at 312 nm, 410 nm,

469

428 nm, and 650 nm in a N2-purged QH2 solution (0.2 mM). Laser (266 nm): 71.2 mJ/pulse.

26


Page 28 of 33

Page 29 of 33


471 472

Figure 3. (a) The kinetic traces of the species at 650 nm in (a) a long time interval and (b)

473

a short time interval after the laser pulse. [QH2]0 = [AA]0 = 0.2 mM, Laser (266 nm): 71.2

474

mJ/pulse.

475

27



476 477

Figure 4. The redox potential ladder and the possible electron transfer pathways of

478

quinones, O2, and AA related species at pH 6.0. The gray shadow represents the stability

479

area of H2O. Solid arrows: main pathways, dashed arrows: minor pathways. The Eh6 values

480

were calculated from the reported E0 values.17,21,29

28


Page 30 of 33

Page 31 of 33


482

Scheme 1. The photochemistry and photophysics of QH2 and AA in UV irradiated

483

solutions.

484

29



Page 32 of 33

486

Table 1. The k1 values of the photo-transformation of QH2 (0.2 mM) and AA (0.2 mM) in

487

mono- (w/o) and binary (w/) solutions. UV: 6.0 mW·cm-2 at 365 nm. [AA]0

N2

Air

O2

0

0.1 mM

0.2 mM

0.3 mM

0.003

0.022

0.048

0.186

k1,AA (w/ QH2)

0.083

0.073

0.050

k1,AA (w/o QH2)

0.023

0.020

0.018

k1,AA(w/)/k1,AA(w/o)

3.55

3.74

2.83

0.107

0.109

0.111

k1,AA (w/ QH2)

0.054

0.056

0.046

k1,AA (w/o QH2)

0.177

0.137

0.078


0.31

0.41

0.59

0.138

0.150

0.151

k1,AA (w/ QH2)

0.038

0.041

0.038

k1,AA (w/o QH2)

0.107

0.099

0.084


0.36

0.41

0.45

k1,QH2

k1,QH2

k1,QH2

0.095

0.126

30


Page 33 of 33


489

Table 2. The maximum △OD and lifetime () of laser pulse generated species in QH2 (0.2

490

mM) and QH2-AA (0.2 mM) solutions. Laser (266 nm): 71.2 mJ/pulse. QH2

QH2+AA

Species

△OD

 (s)

N2

Air

O2

N2

Air

O2

312 nm

0.161

0.168

0.162

0.161

0.149

0.156

410 nm (p)a

0.111

0.082

0.098

0.088

0.086

0.093

410 nm (s)b

0.065

0.061

0.064

0.060

0.058

0.065

428 nm

0.066

0.067

0.072

0.059

0.068

0.074

650 nm

0.145

0.128

0.116

0.105

0.066

0.068

312 nm

69.1

22.5

19.9

48.9

21.9

20.9

410 nm (p)

0.56

0.21

0.10

0.45

0.22

0.11

410 nm (s)

47.4

22.8

19.6

40.5

22.4

19.5

428 nm

60.4

22.6

20.1

37.2

22.8

15.8

650 nm

606

134

43

246

101

44

491

a The

primary maximum △OD immediately after the laser pulse.

492

b The

secondary maximum △OD after the rapid decline at the very beginning.

31


Enhanced Photooxidation of Hydroquinone by Acetylacetone, a Novel

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