Sulfate Radical Photogeneration Using Fe-EDDS: Influence of Critical

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Sulfate Radical Photogeneration using Fe-EDDS : Influence of Critical Parameters and Naturally Occurring Scavengers Yanlin Wu, Angelica Bianco, Marcello Brigante, Wenbo Dong, Pascal De Sainte-Claire, Khalil Hanna, and Gilles Mailhot Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03316 • Publication Date (Web): 11 Nov 2015 Downloaded from http://pubs.acs.org on November 20, 2015

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Environmental Science & Technology

1 2

Sulfate Radical Photogeneration using Fe-EDDS:

3

Influence of Critical Parameters and Naturally

4

Occurring Scavengers

5 6

Yanlin Wua,b,c, Angelica Biancoa,b, Marcello Brigantea,b, Wenbo Dongc, Pascal de

7

Sainte-Clairea,b, Khalil Hannad, Gilles Mailhota,b

8 9 a

10

Ferrand, BP 10448, F-63000 CLE Clermont-Ferrand, France

11

b

12 13

c

16

CNRS, UMR 6296, ICCF, F-63171 Aubière, France

Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention, Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China

14 15

Université Clermont Auvergne, Université Blaise Pascal, Institut de Chimie de Clermont-

d

Ecole Nationale Supérieure de Chimie de Rennes UMR CNRS 6226, 11 Allée de Beaulieu, CS 50837, F-35708 RENNES Cedex 7, France

17 18 19 20

A revised manuscript to Environmental Science and Technology

21

ACS Paragon Plus Environment

1


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22

Abstract

23

In this study the activation of persulfate induced by Fe(III)-ethylenediamine-N,N’-disuccinic

24

acid (EDDS) under dark and irradiation conditions and the reactivity of the generated sulfate

25

radical (SO4●–) under a wide range of experimental conditions were investigated by means of

26

experimental kinetic analyses and modelling. The Fe(III)-EDDS induced activation of

27

persulfate was found to be efficient across a wide range of pH value (3–7), whereas the

28

second order rate constant of SO4●– with 4-tert-butylphenol (4tBP) k SO• − , 4tBP = (4.21 ± 0.22) 4

29

×109 M-1 s-1 was found to be unchanged between pH 2.5 and 8.5. Experimental and theoretical

30

investigations showed clearly that the 4tBP degradation was enhanced in the presence of

31

chloride (10 mM), whereas an almost complete inhibition was observed in the presence of

32

carbonates (10 mM). For the first time, second order rate constants evaluated by laser flash

33

photolysis experiments revealed that SO4●– has a similar reactivity with EDDS (6.21 × 109 M-

34

1

35

but also greater amounts of hydroxyl radicals formed in the presence of chloride can likely

36

explain the enhancement of the 4tBP degradation rate. These results may have strong

37

implications for the removal of organic pollutants via sulfate radical generation from

38

contaminated waters, especially if the wastewater possesses carbonate and chloride

39

concentrations

s-1) and 4tBP (4.21 × 109 M-1 s-1). However, the secondary generated radicals (mainly Cl2●–)

consistent

with

those

present

in


aquatic

environments.

2

Page 3 of 23

40


Introduction

41

Advanced Oxidation Processes (AOPs) based on the production of hydroxyl radical

42

(HO●) were proposed for the degradation of organic contaminants with the final goal of

43

wastewater decontamination.1,

44

activation of added persulfate (S2O82-) to generate the strongly oxidizing sulfate radicals,

45

SO4●- (E0(SO4●-/SO42-) = 2.43 V),3 was recently proposed for treating pharmaceutical drugs,4, 5

46

azo dyes 6 and other organic compounds.7-9 The activation of persulfate (E0= 2.01 V) leads to

47

the formation of sulfate radicals, which can, in general, react with organic compounds with a

48

second order rate constant in the range 106−108 M-1 s-1.10 Such sulfate radical generation can

49

be performed via thermal,9, 11,

50

activation. Among the activation mechanisms, the one involving Fe2+ is similar to Fenton’s

51

reaction and is efficient for sulfate radical formation. However, the reaction between Fe2+ and

52

persulfate has several defects, especially limitations in an appropriate pH range and iron

53

precipitation as in the traditional Fenton-like system.

2

12

A new radical-based oxidative process involving the

ultraviolet light

7, 8, 13

and transition metal

6, 14

mediated

54

Fe(III) with UV could be considered as a source of Fe2+. However, Fe(III) is unstable at

55

pH > 4.0, and the formation of insoluble iron oxy-hydroxides is expected. To overcome this

56

drawback, organic complexing agents can be used at circumneutral pH values closer to

57

natural conditions. Among the used Fe-complexing agents, ethylenediamine-N,N’-disuccinic

58

acid (EDDS), a structural isomer of EDTA, is biodegradable and has been reported to be a

59

safe and environmentally benign replacement for EDTA.15, 16 We have recently investigated

60

the stoichiometry and physicochemical properties of the Fe(III)-EDDS complex in water and

61

showed that Fe(III)-EDDS can be easily photolyzed in a wide pH range (3 to 9) to generate

62

Fe2+.17-19 In the present work, the ability of the Fe(III)-EDDS complex to activate persulfate

63

and generate sulfate radicals was evaluated for the first time. The effect of irradiation time

64

(300 nm < λ < 500 nm), pH, Fe(III)-EDDS and S2O82- concentrations on the degradation


3


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65

performance of 4-tert-butylphenol (4tBP) used as a target pollutant was investigated. 4tBP is

66

an alkylphenol (AP) and is an endocrine disrupting chemical (EDC) with highly oestrogenic

67

effects.20, 21 First, to understand and explain the different results obtained, the second-order

68

reaction rate constant of the reaction between 4tBP and SO4●- was evaluated by laser flash

69

photolysis. A kinetic model was developed to estimate the sulfate radical formation rates

70

under the adopted irradiation conditions. The effect of the naturally occurring anions such as

71

carbonates (HCO3–/CO32–) and chloride ions (Cl–) on the transformation of 4tBP was also

72

experimentally investigated. The scavenging effects on SO4●– by carbonates and chloride ions

73

were then estimated using a kinetic modelling approach incorporated in Matlab to assess the

74

radical species involving during the oxidative process.22, 23 The experimental evaluation of

75

radical rate constants, by laser flash photolysis, and theoretical calculations were compared

76

and discussed in the context of wastewater treatments.

77 78 79

Materials and methods

80

Chemicals.

81

ethylenediamine-N,N’-disuccinic acid trisodium salt (EDDS-Na) solution (35% in water), and

82

sodium chorine (NaCl) were obtained from Sigma, France. Ferric perchlorate (Fe(ClO4)3) was

83

obtained from Fluka, France. Perchloric acid (HClO4) and sodium hydroxide (NaOH) were

84

used to adjust the pH of the solutions. All chemicals were used without further purification.

85

Fe(III)-EDDS solutions were prepared by mixing appropriate volumes (1/1, v/v) of freshly

86

prepared aqueous solutions of Fe(ClO4)3 and EDDS. These solutions were used maximum

87

two hours after their preparation,

Potassium

persulfate

(K2S2O8),

4-tert-butylphenol

(4tBP)

and

S,S’-

88 89

Irradiation setup and experimental procedure. An home-made photoreactor placed in a

90

cylindrical stainless steel container equipped with four fluorescent light tubes (Philips TL D


4

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91

15W/05) was used for the irradiation of the aqueous solutions (total volume of 50 mL). The

92

four tubes were separately placed in the four different axes, whereas the photoreactor, a water-

93

jacketed Pyrex tube of 2.8 cm diameter, was placed in the centre of the setup.

94

The emission spectrum (see Figure S1) reaching the solution was determined using an optical

95

fiber coupled with a CCD spectrophotometer (Ocean Optics USD 2000+UV-VIS) which was

96

calibrated using a DH-2000-CAL Deuterium Tungsten Halogen reference lamp. The energy

97

has been normalized to the actinometry results using paranitroanisole (PNA)/pyridine

98

actinometer. 24 Over the wavelength range 300-370 nm, a total flux of 578 W m-2 reaching the

99

solution was determined.

100

The solutions were magnetically stirred with a magnetic bar during the reaction. All the

101

experiments were performed at room temperature (293 ± 2 K). Figure S1 displays the

102

measured spectral irradiance of the four tubes used during these experiments, as well as the

103

UV-visible spectra of S2O82- at pH 5.1 in water and of the Fe(III)-EDDS complex at pH 4.0

104

(taken with a Cary 300 scan UV-visible spectrophotometer).

105

Generally, the initial concentration of 4tBP, Fe(III)-EDDS and S2O82- were 50, 100 and

106

500 µM, respectively in all steady-state irradiation experiments. Samples were taken from the

107

reaction photoreactor at fixed intervals and stored in the dark at 283 K before

108

chromatographic analysis.

109 110

Quantification of the chemical species. The concentration of the 4tBP remaining in the

111

aqueous solution was determined by high performance liquid chromatography (HPLC)

112

equipped with a photodiode array detector (Waters 996, USA), an HPLC pump (Waters 515,

113

USA) and an autosampler (Waters 717, USA). The experiments were performed by UV

114

detection at 221 nm. The flow rate was 1 mL min-1 and the mobile phase was a mixture of

115

water and methanol (20/80, v/v). The column was a Zorbax RX-C8 of 250 mm × 4.6 mm × 5


5


116

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µm. Under these conditions, the retention time of 4tBP was equal to 6.5 min.

117 118

Laser Flash Photolysis. The laser flash photolysis apparatus has been previously described.

119

25

120

= 266 nm) for the 266 nm excitation and the excitation energy was set to approximately 45

121

mJ/pulse. An appropriate volume of chemical stock solutions (4tBP, 2-propanol, tert-butanol,

122

S2O82-, HCO3-/CO32-, Cl-) was mixed just before each experiment to obtain the d

123

esired mixtures and concentrations. Moreover, a peristaltic pump was used to continuously

124

replace the solution inside the quartz cell to avoid sample degradation after the LFP pulse. All

125

experiments were performed at ambient temperature (293 ± 2 K) and in aerated solutions.

126

Sulfate radical decay was followed at 450 nm corresponding to the maximum absorption of

127

this species.26 Dichloride radical anion species (Cl2●–) was generated using 10 mM of chloride

128

ion in the presence of 10 mM of persulfate. The reaction has been previously described by

129

George and Chovelon

130

of 340 nm (ε340nm ≈ 8800 M-1 cm-1). To determine the second-order rate constant for the

131

quenching of SO4●– and Cl2●–, plots were made of the first-order decay constants of the

132

radicals, determined from the regression lines of the logarithmic decays of SO4●– and Cl2●–

133

monitored at 450 nm and 340 nm, respectively, as a function of the quencher concentration.

134

Each value was the average of 4 consecutive laser pulses and the reported error is ± 3σ, which

135

was obtained from the scattering of the experimental data from the fitting line.

Presently, the only difference is that the current experiments used the fourth harmonic (λexc

27

and generates the transient species Cl2●- that absorbs at a maximum

136 137

Kinetic modelling. The reactions considered for kinetic modelling are reported in Table S1

138

(K1 to K34). The pseudo-first order decay or second-order rate constants have been obtained

139

either from the literature or they have been experimentally evaluated in this work (see LFP

140

experiments).


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141

The numerical differential equation of the reaction rates were integrated using the ode15s

142

solver function of Matlab. The initial species concentration was implemented in the m-file as

143

the input data. The primary kinetic pathways from the iron-complex photolysis up to the

144

reactivity of the hydroxyl and sulfate radicals on 4tBP were implemented. In the model we

145

considered that the 4tBP oxidation product has a similar reactivity constant to the

146

photogenerated radicals. Possible pH effects were not included in the model because no

147

significant effect was observed under acidic and circumneutral pH, in agreement with the

148

obtained experimental results. Moreover, it is important to mention that after the complete

149

disappearance of Fe(III)-EDDS complex (20 min of irradiation), Fe(III)/Fe(II) species are still

150

present in solution and so photocatalytic cycle continue to generate radical species (mainly

151

HO●). This second step of radical species generation is not taken into account in our model

152

but participate to 4tBP disappearance for longer irradiation times.

153

The effects of the inorganic ions (i.e., carbonates and chloride ions) on the sulfate radical

154

reactivity were evaluated by using a kinetic model with Matlab code.

155 156

Results and Discussion

157

Sulfate radical detection and kinetic constants determination. Figure 1 shows the transient

158

absorption spectra obtained after 266 nm laser excitation of S2O82- in the absence or presence

159

of 4tBP. Maximum absorption of SO4●– occurred at 450 nm, and a decrease of the transient

160

5 -1 ' species was observed with a pseudo-first order constant k SO s . After the • − of 1.2 × 10 4

161

addition of 4tBP, the transient decay increased to 2.4 ×106 s-1, indicating that SO4●– and 4tBP

162

react with each other in high yield. The new transition species, which absorbs between 350

163

and 430 nm with a maximum at 410 nm, has been attributed to the phenoxy radical anion

164

(4tBP●–).28 This signal was also observed, albeit with a lower intensity, upon direct excitation

165

at 266 nm of 4tBP in pure water. Using a linear regression of the pseudo-first order decay


7


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166

constant of SO4●– (k’, s-1) versus the 4tBP concentration we can estimate the second order rate

167

constant k SO• − , 4tBP = (4.21 ± 0.22) ×109 M-1 s-1, which is close to the values reported for the 4

168

reactivity of SO4●– with aromatic compounds.10, 29 The k SO• − , 4tBP was measured from pH 2.5–8 4

169

and no significant variation was observed (insert Figure 1). Using the same method, the

170

second order rate constants of SO4●– were estimated in the presence of different chemical

171

compounds used in this work: 2-propanol ( k SO•− ,2 − pr = (6.9 ± 0.2) ×107 M-1 s-1), tert-butanol 4

172

( k SO•− ,t −but = (8.4 ± 0.4) ×105 M-1 s-1), HCO3– ( k SO•− , HCO− (4.3 ± 0.2) ×106 M-1 s-1) and Cl– 4

173

4

3

( kSO•− ,Cl − = (2.0 ± 0.2) ×108 M-1 s-1). These constants were also estimated at different pH 4

174

values (see Figure S2) and are in agreement with those reported in the literature.10

175 176

Fe(III)-EDDS complex photolysis and degradation of 4tBP. Under the present irradiation

177

conditions, the Fe(III)-EDDS complex is quickly photolyzed to generate Fe2+ and an oxidized

178

ligand (R1) by ligand-to-metal charge transfer (LMCT) transitions. Photogenerated ferrous

179

iron can react with persulfate with a second order rate constant ( k Fe2+ ,S O2− ) of 27 M-1 s-1

30

2 8

180

(R2):

181

hν (290−500nm ) Fe( III ) − EDDS   → Fe2+ + EDDSox

(R1)

182

Fe2+ + S2O82− → Fe3+ + SO42− + SO4•−

(R2)

183

The pseudo-first order constant for the Fe2+ formation ( k Fef 2+ ) can be considered to be equal to

184

d that of the disappearance of Fe(III)-EDDS ( kFe ( III ) − EDDS ). The latter was then determined at

185

different pH values (see Fig. S3), (3.97±0.15)×10-3 s-1 at pH 4.0, (3.42±0.09)×10-3 s-1 at pH

186

5.9 and (2.65±0.24)×10-3 s-1 at pH 8.0. Such difference can be attributed to the chemical

187

speciation of the Fe(III)-EDDS complex vs pH. At pH < 6.0 Fe(III)-EDDS–, that is the most

188

photoactive form of the complex, predominates, while at pH 8.0 a less photoactive form,


8

Page 9 of 23


189

monohydroxylated Fe(III)(OH)-L2–, prevails 19.

190

In the presence of 500 µM of persulfate the disappearance rate of Fe(III)-EDDS was

191

unchanged, if you compare without persulfate, at pH 4.0 and 5.9 ((3.94 instead of

192

3.97±0.15)×10-3 s-1 and (3.45 instead of 3.42±0.08)×10-3 s-1, respectively), while at pH 8.0 it

193

slightly increased ((3.29 instead of 2.65±0.12)×10-3 s-1). In contrast to pH 8.0, the contribution

194

of sulfate radicals is minor at acidic pH due to the higher photolysis yield of Fe(III)-EDDS

195

complex.

196

Experiments showed that UV irradiation of Fe(III)-EDDS with 500 µM of persulfate could

197

effectively degrade 4tBP (Figure S4). To verify that 4tBP degradation is primarily caused by

198

the reactivity of photogenerated sulfate radicals and not because of the possible formation of

199

hydroxyl radicals (see Table S1), 10 mM of tert-butanol were added to the system (i.e.,

200

Fe(III)-EDDS 100 µM + S2O82– 500 µM + 4tBP 50 µM) at pH 4.0 under irradiation

201

conditions. The addition of tert-butanol slightly modified the initial degradation rate of 4tBP

202

(Figure S4) rate constants from (2.22±0.50)×10-3 s-1 to (1.88±0.47)×10-3 s-1 suggesting that

203

hydroxyl radicals, which have a strong second order rate constant with 4tBP ( k HO• ,t −but = 4

204

×109 M-1 s-1 >> kSO•− ,t −but = 8.4 ×105 M-1 s-1) 31, are not the main radical involved in the initial 4

205

degradation of 4tBP under our experimental conditions.

206 207

Effects of initial pH and Fe(III)-EDDS concentrations. Experiments of 4tBP

208

photodegradation in two systems (i.e., S2O82–/Fe(III)-EDDS and S2O82–/Fe3+) were performed

209

at pH values ranging from 2.2 to 8.8. The initial degradation rates of 4tBP ( R4dtBP ) at different

210

pH values are shown in Figure 2. The efficiency of 4tBP degradation decreased with

211

increasing pH for both reaction systems because of the precipitation of iron at high pH values.

212

The precipitation of Fe3+ occurring after pH 4.0 led to the obstruction of activation

213

processes.32 However, the efficiency of 4tBP degradation decreased much more rapidly for the


9


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214

UV/S2O82–/Fe3+ system than the UV/ S2O82–/Fe(III)-EDDS system. Indeed, EDDS

215

complexation can retain Fe3+ in its dissolved form even at neutral or alkaline pH. In addition,

216

f, Fe(II) the quantum yield of Fe(II) formation ( Φ290 −400nm ) following the irradiation of Fe(III)-EDDS

217

Fe(II) was stable across a large range of pH values. The Φf,290 −400nm values have been estimated to be

218

0.09, 0.11 and 0.10 at pH values of 4.0, 6.0 and 8.6, respectively.19 However, the decrease of

219

R4dtBP , which is also observed with Fe(III)-EDDS, can be rationalized by the higher amount of

220

Fe2+ that can be oxidized by dissolved oxygen when the pH value increases.33

221

The photolysis of Fe(III)-EDDS generates Fe2+ that activates S2O82– to generate SO4●–.34 By

222

varying the Fe(III)-EDDS concentration from 100 µM to 1 mM, R4dtBP increased from 1.25 ×

223

10-8 to 3.86 × 10-8 M s-1, and then decreased to 2.07 × 10-8 M s-1 when the concentration of

224

Fe(III)-EDDS was 1.5 mM (Figure S5). Increasing the Fe(III)-EDDS concentration may

225

enhance the formation of Fe2+ and therefore the generation of SO4●–, whereas scavenging

226

SO4●– by Fe(III)-EDDS or the EDDS oxidation by-products may occur at much higher

227

concentrations.

228 229

Effect of carbonates and chloride ions

230

The effects of naturally occurring anions such as carbonates and chloride ions on the

231

reactivity of SO4●– generated from R3 in the Fe(III)-EDDS/S2O82-/UV system were

232

investigated.

233

The degradation of 4tBP was almost completely inhibited by addition of 10 mM of carbonates

234

at pH 8.2 (corresponding to 98% in the form of HCO3-) in the Fe(III)-EDDS/S2O82- system

235

under polychromatic irradiation (Figure S6). In fact, carbonates are able to quench the sulfate

236

radical via reactions R5 and R6 to generate the carbonate radical (CO3●–).

237

Notably, the presence of 10 mM of chloride ions significantly enhanced the degradation rate


10

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238

of 4tBP (+ 58% see Figure 3), though the chloride ions react strongly with SO4●– (R7 and R8)

239

and generate other radical species as well. These results indicate that chloride ions act as a

240

promoter and not inhibitor of the degradation of 4tBP.

241 242

243

hν Fe ( III ) − EDDS + S2O82−  → → SO4•−

SO4•− + 4tBP → SO42− + 4tBPox

-1 f RSO •− (M s )

(R3)

4

kSO•− ,4tBP = 3.9 × 109 M-1 s-1

(R4)

244

work]

245

SO4•− + HCO3− → CO3•− + H + + SO42−

k SO•− , HCO − = 9.1 × 106 M-1 s-1

(R5) 10

SO4•− + CO32− → CO3•− + SO42−

k SO•− ,CO 2 − =4.1 × 106 M-1 s-1

(R6) 35

SO4•− + Cl − → Cl • + SO42−

kSO•− ,Cl − =2.0 × 108 M-1 s-1

(R7) 10

Cl • + Cl − → Cl2•−

kCl• ,Cl − =0.8-2.1 × 1010 M-1 s-1

(R8) 36, 37

246

247

248

[this

4

4

4

3

3

4

249 250

To simulate the effects of carbonates and chloride ions on the sulfate radical chemistry,

251

theoretical calculations using the second order rate constants of the reactions noted in Table

252

S1 and those of reactions R5 to R7 were performed. The inhibition (I in E1) is determined by

253

considering a simple kinetic approach where Fe(III)-EDDS photolysis leads to the formation

254

of Fe2+, followed by the activation of S2O82- by Fe2+ to generate SO4●–:

255

I HCO− / CO2− or Cl − , SO•− (%) = (1 − 3

3

4

kSO•− ,4tBP [4tBP] 4

(kSO•− ,4tBP [4tBP]) + ∑ i kSO•− ,S [ Si ] 4

4

) ×100

(E1)

i

256 257

where kSO•− ,4tBP and [4tBP] are the second order rate constants between SO4●– and 4tBP and 4

∑k

258

the initial 4tBP concentration, respectively. The

259

the contributions of each scavenger (considering that the pKa of HCO3–/CO32– = 10.32) with

i

SO4•− , Si

[ Si ] was calculated as the sum of


11


Page 12 of 23

260

their reaction rate constants. The fraction of reacted sulfate radicals can be then determined

261

versus pH and the carbonate concentration (Figure 4A), and as a function of hydrogen

262

carbonate and chloride ions concentration at pH 8.2 (Figure 4B). It is noteworthy that in

263

aqueous solutions containing elevated carbonate concentrations (e.g., 10 mM, corresponding

264

to ~ 600 mg L-1), approximately 30% of the generated SO4●–: may react with carbonates but

265

this amount reduces to 5% when the carbonate concentration is 2 mM.

266

The reactivity of SO4●– with chloride ions (R7) is much stronger than with carbonates (R5 and

267

R6). For chloride ion concentrations higher than 3 mM, more than 80% of the SO4●– species

268

are trapped, and this percentage becomes more than 90% with 10 mM of chloride ions.

269

However, these theoretical calculations are not consistent with the experimental observations

270

(i.e., enhanced 4tBP degradation in the presence of chloride ions).

271

To explain this unusual behavior, a new kinetic model (using kinetic constants of Table S1)

272

developed in Matlab code was used to quantify the generation of hydroxyl and sulfate radicals

273

from Fe(III)-EDDS photolysis without 4tBP. The calculation results revealed a significant

274

discrepancy in the formation of SO4●– and HO● with and without 10 mM Cl– (Figure 5). It is

275

interesting to note that without chloride, SO4●- represents the main radical generated during

276

irradiation while HO● concentration remains about one order of magnitude lower (reaching a

277

maximum of ~ 2.5 × 10-13 M). The latter can be formed through reactions of sulfate radical

278

with water and hydroxide ions (see reactions K4 and K5 in table S1).

279

In the presence of 10 mM Cl–, greater amounts of HO● were estimated reaching a

280

concentration of ~3.7 × 10-12 M after about 5 min of irradiation, while only ~ 8.0 × 10-14 M

281

was found for SO4●–. In the presence of chloride, the decay in sulfate radical formation is

282

probably due to the reactivity of SO4●– with Cl– generating Cl● (K23 in Table S1). Other

283

secondary radical species such as ClOH●– or Cl2●– can also be formed and consequently the

284

hydroxyl radical through the reactions K20 to K30 (Table S1). The formation of the latter,


12

Page 13 of 23


285

corroborated by the efficient quenching observed in the presence of tert-butanol (see Figure

286

3), may explain the enhancement of 4tBP degradation observed in the presence of chloride

287

(kHO•, 4tBP = 1.6 1010 M-1 s-1, K12).

288

Moreover, in the presence of chloride ions, new radical species are formed and among them

289

Cl2●– are the most concentrated radicals in the first times of irradiation (Figure 5). To explain

290

the possible impact of chloride ions, the reactivity of SO4●- and a secondary generated radical

291

(i.e., Cl2●-) was investigated by determining their second order rate constants with 4tBP and

292

EDDS using laser flash photolysis (LFP). The LFP results revealed that SO4●- has a similar

293

reactivity with EDDS (6.21 × 109 M-1 s-1) and 4tBP (4.21 × 109 M-1 s-1), whereas the

294

dichloride radical (Cl2●-) exhibits a reactivity with 4tBP that is higher than with EDDS (2.78 ×

295

108 M-1 s-1 versus 5.2 × 107 M-1 s-1, respectively). The higher selectivity of Cl2●- could also

296

explain the greater 4tBP degradation observed with this radical, even though it is less reactive

297

than the sulfate or hydroxyl radical.

298

Environmental implications for wastewater treatments

299

This paper shows, for the first time, that Fe-EDDS can activate persulfate to generate

300

SO4•− and induce the effective degradation of 4tBP across a wide range of pH conditions. The

301

chemical or photochemical activation of persulfate using Fe species often requires

302

acidification to prevent iron precipitation. Herein, we have demonstrated that EDDS was able

303

to stabilize Fe(III) across a large range of pH values (i.e., 4 to 8), leading to persulfate

304

activation. The application of Fe(III)-complexes in AOPs may have several advantages such

305

as relatively low process costs15, wide irradiation wavelengths (λ < 580 nm) and relatively

306

wider pH range applicability.

307

The sulfate radical can react with naturally occurring anions such as carbonates and chloride

308

to generate other types of radical species. Although the rate constant of SO4•− with 4tBP (4.21

309

× 109 M-1 s-1) is more than ten times larger than with Cl2●– (2.78 × 108 M-1 s-1), an increase of


13


Page 14 of 23

310

4tBP degradation was obtained in the presence of chloride ions. Experimental and theoretical

311

investigations showed clearly that more hydroxyl radicals were formed, thereby explaining

312

the enhancement of 4tBP degradation observed in the presence of chloride. In contrast, the

313

presence of carbonates attenuated the 4tBP degradation rate because of the low reactivity of

314

CO3●–- with the target contaminant. Considering that the reactivity of CO3●– is similar to that

315

of phenol, CO3●– is expected to react with 4tBP with a second order rate constant of ~ 107 M-1

316

s-1.38 Consequently, the presence of carbonates has a detrimental effect to the degradation of

317

4tBP whereas the presence of chloride promotes the removal of 4tBP in the Fe(III)-EDDS

318

system. This observation should increase the awareness of the effects of secondary generated

319

radical species on the performance of treatment processes, especially when complexing agents

320

are used to maintain Fe in its dissolved form. These results are very promising for the

321

application of EDDS-driven persulfate activation for the treatment of contaminated waters in

322

more natural conditions (i.e., circumneutral pH with the presence of chloride and carbonates).

323

More generally, the role of such ubiquitous ions could be counterintuitive and their influence

324

of advanced oxidation processes must be considered carefully in different wastewater

325

treatment strategies.

326 327

Acknowledgements

328

The authors gratefully acknowledge financial support from China Scholarship Council for.

329

Yanlin Wu to study at the Blaise Pascal University in Clermont-Ferrand, France. This work

330

was supported by the National Natural Science Foundation of China (NSFC 21077027),

331

Shanghai Natural Science

332

Environmental Science & Technology Development Foundation (STGEF) and the Graduate

333

Innovative Fund of Fudan University (13). This work was also supported by the “Federation

334

des Recherches en Environnement” through the CPER “Environnement” founded by the

Fund

(12ZR1402000), Shanghai Tongji Gao Tingyao


14

Page 15 of 23

335


“Région Auvergne,” the French government and FEDER from European community.

336 337

Supporting Information Available

338

Details of the analytical and LFP methods, and additional results for radical formation and

339

kinetic degradation of 4tBP. This information is available free of charge via the Internet at

340

http://pubs.acs.org/.

341 342

References

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18. Li, J.; Mailhot, G.; Wu, F.; Deng, N., Photochemical efficiency of Fe(III)-EDDS complex: OH radical production and 17β-estradiol degradation. J. Photochem. Photobiol., A 2010, 212, (1), 1-7. 19. Wu, Y.; Brigante, M.; Dong, W.; de Sainte-Claire, P.; Mailhot, G., Toward a better understanding of Fe(III)-EDDS photochemistry: Theoretical stability calculation and experimental investigation of 4-tertButylphenol degradation. J. Phys. Chem. A 2014, 118, (2), 396-403. 20. Barse, A. V.; Chakrabarti, T.; Ghosh, T. K.; Pal, A. K.; Jadhao, S. B., One-tenth dose of LC50 of 4-tertbutylphenol causes endocrine disruption and metabolic changes in Cyprinus carpio. Pestic. Biochem. Physiol. 2006, 86, (3), 172-179. 21. Myllymaki, S.; Haavisto, T.; Vainio, M.; Toppari, J.; Paranko, J., In vitro effects of diethylstilbestrol, genistein, 4-tert-butylphenol, and 4-tert-octylphenol on steroidogenic activity of isolated immature rat ovarian follicles. Toxicol. Appl. Pharmacol. 2005, 204, (1), 69-80. 22. Yang, Y.; Pignatello, J. J.; Ma, J.; Mitch, W. A., Comparison of halide impacts on the efficiency of contaminant degradation by sulfate and hydroxyl radical-based advanced oxidation processes (AOPs). Environ. Sci. Technol. 2014, 48, (4), 2344-2351. 23. Lutze, H. V.; Kerlin, N.; Schmidt, T. C., Sulfate radical-based water treatment in presence of chloride: Formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate. Water Res. 2015, 72, 349-360. 24. Dulin, D.; Mill, T., Development and evaluation of sunlight actinometers. Environ. Sci. Technol. 1982, 16, (11), 815-820. 25. Brigante, M.; Charbouillot, T.; Vione, D.; Mailhot, G., Photochemistry of 1-Nitronaphthalene: A potential source of singlet oxygen and radical species in atmospheric waters. J. Phys. Chem. A 2010, 114, (8), 2830-2836. 26. Hayon, E.; Treinin, A.; Wilf, J., Electronic spectra, photochemistry, and autoxidation mechanism of the sulfite-bisulfite-pyrosulfite systems. SO2-, SO3-, SO4-, and SO5- radicals. J. Am. Chem. Soc. 1972, 94, (1), 47-57. 27. George, C.; Chovelon, J.-M., A laser flash photolysis study of the decay of SO4- and Cl2- radical anions in the presence of Cl- in aqueous solutions. Chemosphere 2002, 47, (4), 385-393. 28. De Laurentiis, E.; Maurino, V.; Minero, C.; Vione, D.; Mailhot, G.; Brigante, M., Could triplet-sensitised transformation of phenolic compounds represent a source of fulvic-like substances in natural waters? Chemosphere 2013, 90, (2), 881-884. 29. Warneck, P.; Ziajka, J., Reaction mechanism of the iron(III)-catalyzed autoxidation of bisulfite in aqueous solution: Steady state description for benzene as radical scavenger. Ber. Bunsen-Ges. Phys. Chem. 1995, 99, 59-65. 30. Woods, R.; Kolthoff, I. M.; Meehan, E. J., Arsenic (IV) as an intermediate in the induced oxidation of arsenic (III) by the iron (II)-hydrogen peroxide reaction. J. Am. Chem. Soc. 1964, 86, (9), 1698-1700. 31. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B., Critical review of rate constants for reactions • • − of hydrated electrons, hydrogen atoms and hydroxyl radicals ( OH/ O ) in aqueous solution. J. Phys. Chem. Ref. Data 1988, 17, 513-886. 32. Xu, X.-R.; Zhao, Z.-Y.; Li, X.-Y.; Gu, J.-D., Chemical oxidative degradation of methyl tert-butyl ether in aqueous solution by Fenton's reagent. Chemosphere 2004, 55, (1), 73-79. 33. Morgan, B.; Lahav, O., The effect of pH on the kinetics of spontaneous Fe(II) oxidation by O2 in aqueous solution – basic principles and a simple heuristic description. Chemosphere 2007, 68, 2080-2084. 34. Anipsitakis, G. P.; Dionysiou, D. D., Radical generation by the interaction of transition metals with common oxidants. Environ. Sci. Technol. 2004, 38, (13), 3705-3712. 35. Padmaja, S.; Neta, P.; Huie, R. E., Rate constants for some reactions of inorganic radicals with inorganic ions. Temperature and solvent dependence. Int. J. Chem. Kinet. 1993, 25, (6), 445-455. 36. Jayson, G. G.; Parsons, B. J.; Swallow, A. J., Some simple, highly reactive, inorganic chlorine derivatives in aqueous solution. Their formation using pulses of radiation and their role in the mechanism of the Fricke dosimeter. J. Chem. Soc. Faraday Trans. 1 1973, 69, (0), 1597-1607. 37. Nagarajan, V.; Fessenden, R. W., Flash photolysis of transient radicals. 1. X2 with X = Cl, Br, I, and SCN. J. Phys. Chem. 1985, 89, (11), 2330-2335. 38. Busset, C.; Mazellier, P.; Sarakha, M.; De Laat, J., Photochemical generation of carbonate radicals and their reactivity with phenol. J. Photochem. Photobiol., A 2007, 185, (2-3), 127-132.


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Figures

435 436

Figure 1: Decay of SO4●– transient followed at 450 nm as a function of 4tBP concentration.

437

Transient signals were obtained upon LFP (266 nm, 45 mJ pulse) for S2O82- at 10

438

mM in aerated water solutions. Insert: dependence as a function of 4tBP

439

concentration of the pseudo-first order decay of SO4●- followed at 450 nm. The

440

solid line represents the linear fit of the experimental data and the dashed lines

441

denote the 95% confidence interval of this fit.

442 443

Figure 2: Effect of pH on the initial degradation rate of 4tBP ( R4dtBP ( M s −1 ) using persulfate

444

activation with Fe(III) (filled circle) or Fe(III)-EDDS (empty circle). The errors

445

bars represent the 3σ based on the linear fit of the experimental data.

446 447

Figure 3: 4tBP (50 µM) degradation profile using persulfate (500 µM) activated by the

448

Fe(III)-EDDS complex (100 µM) under UV irradiation at pH 8.2 (empty triangles)

449

with Cl– 10 mM; (filled triangles) with Cl– 10 mM + tert-butanol; (empty circles)

450

without Cl–; (filled circle) without Cl– + tert-butanol.

451 452

Figure 4: (A) Fraction of sulfate radical (%) reacting with carbonates as a function of pH and

453

carbonate concentration. (B) Fraction of sulfate radical (%) reacting with hydrogen

454

carbonate ( HCO3- ) and chloride ions as a function of their concentrations at pH

455

8.2. The initial 4tBP concentration is 50 µM.

456 457

Figure 5: Estimated amounts of SO4●–(A), HO● (B) with and without 10 mM of chloride ions.

458

(C) and (D) represent the chlorine radical species (Cl2●– ,Cl● ,ClOH●–) evolution in

459

the presence of 10mM of chloride ions. Reactions reported in table S1 and the

460

following conditions: [Fe(III)-EDDS] = 100 µM, [S2O82-] = 500 µM and pH 8.2,

461

were used in the modeling approach investigated here.

462


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Figure 1

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Figure 2

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472 473

Figure 3

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475 476

Figure 4A

477 478

Figure 4B

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Figure 5

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Sulfate Radical Photogeneration Using Fe-EDDS: Influence of Critical

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