The Reactivity and Reaction Pathway of Fenton Reactions Driven by

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The reactivity and reaction pathway of Fenton reactions driven by substituted 1,2- dihydroxybenzenes Pablo Raúl Salgado, Victoria Melin, Yasna Duran, Hector D. Mansilla, and David Contreras Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05388 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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Environmental Science & Technology

1

The reactivity and reaction pathway of Fenton

2

reactions driven by substituted 1,2-

3

dihydroxybenzenes

4

Pablo Salgadoa,b, Victoria Melina,b, Yasna Durána,b, Héctor Mansillab, David

5

Contreras*a,b.

6

a

7

160-C, Concepción 4070386, Chile.

8

b

9

Casilla 160-C, Concepción 4070386, Chile.

Centro de Biotecnología, Universidad de Concepción. Barrio Universitario s/n. Casilla

Facultad de Ciencias Químicas, Universidad de Concepción. Barrio Universitario s/n.

10 11

ABSTRACT

12

Fenton systems are interesting alternative to advanced oxidation processes (AOPs) applied

13

in soil or water remediation. 1,2-Dihydroxybenzenes (1,2-DHBs) are able to amplify the

14

reactivity of Fenton systems and have been extensively studied in biological systems and

15

for AOP applications. To develop efficient AOPs based on Fenton systems driven by 1,2-

16

DHBs, the change in reactivity mediated by different 1,2-DHBs must be understood. For

17

this, a systematic study of the reactivity of Fenton-like systems driven by 1,2-DHBs with

ACS Paragon Plus Environment


18

different substituents at position 4 was performed. The substituent effect was analyzed

19

using the Hammett constant (σ), which has positive values for electron-withdrawing groups

20

(EWGs) and negative values for electron-donating groups (EDGs). The reactivity of each

21

system was determined from the degradation of a recalcitrant azo dye and hydroxyl radical

22

(HO•) production. The relationship between these reactivities and the ability of each 1,2-

23

DHB to reduce Fe(III) was determined. From these results, we propose two pathways for

24

HO• production. The pathway for Fenton-like systems driven by 1,2-DHBs with EDGs

25

depends only on the Fe(III) reduction mediated by 1,2-DHB. In Fenton-like reactions

26

driven by 1,2-DHBs with EWGs, the Fe(III) reduction is not primarily responsible for

27

increasing the HO• production by this system in the early stages.

28

INTRODUCTION

29

Advanced oxidation processes (AOPs) encompass several methods for the chemical or

30

photochemical oxidation of molecules.1,

2

31

constitute a promising technology for wastewater treatment.3, 4 Although different reaction

32

systems exist for these processes, all are based on the production of reactive oxygen species

33

(ROS), mainly hydroxyl radical (HO•).5, 6 This radical is extremely unstable and reactive

34

(E°(HO•/H2O)= 2.8 V/SHE7) and is consequently able to react quickly with different

35

organic compounds, leading to mineralization of the substrate.8, 9

AOPs performed at near ambient temperatures

36

The production of HO• from the reduction of hydrogen peroxide (H2O2) catalyzed by

37

Fe(II) is known as the Fenton reaction (1).10 The reaction between the Fe(III) produced in

38

(1) and H2O2 is known as the Fenton-like reaction (2) and involves the formation of

39

hydroperoxyl radical (HO2•).11 The Fenton-like reaction represents the limiting step in this


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40

redox system because it is three orders of magnitude slower than the Fenton reaction. Both

41

Fenton and Fenton-like reactions participate, at the same time, in a redox system.

42

Fe(II) + H2O2 → Fe(III) + HO• + HO- k= 76 molL-1s-1(ref.12)

(1)

43

Fe(III) + H2O2 → Fe(II) + HOO• + H+ k= 0.01molL-1s-1(ref. 13)

(2)

44

Fenton and Fenton-like systems are popular AOPs due to their oxidation power, low

45

toxicity, moderate cost and simple operation.9,

14

46

dependent on the reaction conditions. For example, the pH in the system can change the

47

reaction rate by changing the iron speciation.15, 16 The Fenton reaction is limited at acidic

48

pH to avoid the oxidation of Fe(II) to Fe(OH)3 or Fe2O3.17

However, these systems are highly

49

Several ligands can enhance the production of reactive species by Fenton and Fenton-like

50

systems, of which 1,2-dihydroxybenzenes (1,2-DHBs) have been studied in different

51

systems such as metabolic pathways in biological systems18-22 and AOPs for water and

52

wastewater treatment.23-26 1,2-DHBs form complexes with Fe(III) with a prooxidant or

53

antioxidant activity that is related to the coordination number, which is pH dependent.27

54

These complexes keep the iron in solution, but only monocomplexes [Fe(1,2-DHB)]+ can

55

reduce Fe(III), increasing the reactivity of Fenton systems at acidic pH up to pH 5.520 In

56

this monocomplex a tautomeric valence equilibrium is achieved,28 after that Fe(II) is

57

released (Scheme 1).Thus, 1,2-DHBs increase Fe(III) reduction, which is the limiting step

58

in the Fenton redox system (2).29

59 60

Scheme 1. Mechanism of Fe(III) reduction by 1,2-DHB.



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61

To develop an efficient AOP based on Fenton systems driven by 1,2-DHBs that can be

62

applied in water or soil remediation, the reactivity change mediated by different 1,2-DHBs

63

must be understood. Therefore, a systematic study of the reactivity of Fenton-like systems

64

driven by 1,2-DHBs with different substituents at position 4 was performed. The

65

substituent effect was analyzed using the Hammett constant (σ), which takes on positive

66

values for electron-withdrawing groups (EWGs) and negative values for electron-donating

67

groups (EDGs). The reactivity of each system was determined through the degradation of a

68

recalcitrant azo dye (methyl orange, MO) and HO• production. The relationship between

69

these reactivities and the ability of each 1,2-DHB to reduce Fe(III) was evaluated.

70

MATERIALS AND METHODS

71

Reagents. Ferric nitrate nonahydrate (Fe(NO3)3•9H2O), MO, potassium fluoride (KF), 3-

72

(2-pyridyl)-5,6-diphenyl-1,2,4-triazine-p,p′-disulfonic

acid

(ferrozine),

4-

73

morpholineethanesulfonic acid (MES), potassium nitrate (KNO3), 5,5-dimethyl-1-pyrroline

74

N-oxide (DMPO), 1,2-benzendiol (catechol), 4-tertbutylcatechol, 4-ethylcatechol, 4-

75

methylcatechol, 3,4-dihidroxibenzylamine, noradrenaline, caffeic acid, dopamine, 4-

76

chlorocatechol, adrenaline and 3,4-dihydroxybenzonitrile were purchased from Sigma-

77

Aldrich. Nitric acid (HNO3), 30 % H2O2, hydrocaffeic acid, 3,4-dihydroxybenzaldehyde,

78

3,4-dihydroxybenzoic acid and 4-nitrocatechol were purchased from Merck.

79

All reagents were used without additional purification.

80

General Procedure. All reagent solutions were prepared in the dark under an argon

81

atmosphere. The ionic strengths of all solutions were adjusted to 0.10 mol·L−1 with KNO3.

82

All experiments were performed at 20 ± 0.1 °C in triplicate (n = 3). The pH of each

83

solution was adjusted to 3.4 with HNO3 prior to the experiments using a Thermo Scientific


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84

Orion 3-Star pH meter. This pH value was selected because is the optimal pH value

85

observed for Fenton systems driven by 1,2-DHB.2, 30

86 87

All the experiments were performed at pseudo first order conditions (1,2-DHB/Fe(III) molar ratio 1:20).

88

A UV–Vis diode array spectrophotometer (Agilent 8453) coupled to a stopped-flow

89

system (applied photophysics RX2000) was used for spectrophotometric measurements.

90

The spectra (190-1100 nm) were recorded every 0.1 s for 20 s.

91

Oxidation of MO. The degradation of MO (λmax= 499 nm, Figure S1 in Supplementary

92

Information, SI) at pH 3.4 in the Fenton-like system driven by different 1,2-DHBs was

93

followed spectrophotometrically. The final concentrations in the systems were 1.0x10-

94

6

95

initiated by adding Fe(III). The kinetics data were analyzed with UV-Vis ChemStation

96

software. The pseudo-first order constant (kobs) was determined 20 s after initiating the

97

reaction (Figure S2 in SI).

mmol·L−1 1,2-DHB and 20x10-6 mol·L−1 Fe(NO3)3, MO and H2O2. The reaction was

98

Hydroxyl Radical Production. HO• was detected by a DMPO spin-trapping method

99

using EPR spectroscopy.31 The final concentrations in the systems were 50x10-6 mol·L−1

100

1,2-DHB, 20x10-3 mol·L−1 DMPO and 1.0x10-3 mol·L−1 H2O2 and Fe(NO3)3. The reactions

101

were initiated by adding Fe(III). Samples were subsequently transferred via syringe to an

102

AquaX capillary in a Bruker EMX micro instrument. The EPR spectra of the DMPO-OH

103

adduct was recorded every 15 s on the X band (~9GHz). The amount of DMPO-OH adduct

104

produced was considered proportional to the height of the second peak in the adduct spectra

105

(Figure S3 in SI). All decay plots were normalized and adjusted to pseudo-first-order

106

kinetics (3) according to Contreras et al.,32 and a pseudo-first-order constant (kp) and the

107

initial signal (I0) was determined for each system.



= − +

108

(3)

109

Reduction of Fe(III). Reduced Fe(III) was quantified in a spectrometric method by

110

measuring the levels of Fe(II) formed at different reaction times as a colored complex with

111

ferrozine (λmax=562 nm).33 The reduction of Fe(III) by each 1,2-DHB was determined at pH

112

3.4. The final concentrations in the systems were 4.0x10-4 mol·L−1 Fe(NO3)3 and 2.0x10-5

113

mol·L−1 1,2-DHB. The kinetics was assessed in a stopped-flow apparatus. The kinetics data

114

were processing with UV-Vis ChemStation software. The pseudo-first order constant (kred)

115

was determined.

116

Determination of the Redox Potential of 1,2-DHB. The redox potential of each 1,2-

117

DHB was determined by cyclic voltammetry using a method modified from Contreras et

118

al.34 The final concentration was 5.0x10-3 mol·L−1 1,2-DHB, which was prepared in an

119

aqueous solution at pH 3.0. The ionic strength was adjusted to 1.0 mol·L−1 KNO3.

120

Electrochemical measurements were performed on a computer controlled by a CHI1207A

121

potentiostat (CH Instruments, TX, USA) using a 20 mL glass chamber with a three-

122

electrode system. The working electrode was carbon graphite (3 mm in diameter), an

123

Ag/AgCl electrode was used as the reference electrode, and a platinum wire was used as the

124

auxiliary electrode. The instrumental parameters were as follows: Einitial = -0.4 V, Emax = 0.8

125

V, Emin = -0.4 V and scan rate = 0.2 V/s.

126 127 128

From the anodic (Epa) and cathodic (Epc) potential, the standard potential (E°) was estimated in equation (4).

° = −

(4)

129 130 131


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132


RESULTS AND DISCUSSION

133

Oxidation of MO. The oxidation of MO was performed for 20 s under pseudo-first-order

134

conditions. The fifteen 1,2-DHBs shown in Table 1 were utilized. Their structures are

135

shown in SI (Figure S4).

136

Table 1. 1,2-DHBs utilized in this study and their Hammett parameters. Hammett Parameters 1

1,2-DHBs

σm

σp

Σσ = (σm + σp)

4-Tert-Butylcatechol

-0.10

-0.20

-0.30

2

4-Ethylcatechol

-0.07

-0.17

-0.24

3

4-Methylcatechol

-0.07

-0.15

-0.22

4

3,4-Dihydroxybenzilamine

-0.03

-0.11

-0.14

5

Hydrocaffeic acid

-0.03

-0.07

-0.10

6

Catechol

0.00

0.00

0.00

7

Norepinephrine

0.11

0.09

0.20

8

Caffeic acid

0.14

0.09

0.23

9

Dopamine

0.23

0.17

0.40

10

4-Chlorocatechol

0.37

0.23

0.60

11

Epinephrine

0.36

0.30

0.66

12

3,4-Dihydroxybenzaldehyde

0.35

0.42

0.77

13

3,4-Dihydroxybenzoic acid

0.37

0.45

0.82

14

3,4-Dihydroxybenzonitrile

0.56

0.66

1.22

15

4-Nitrocatechol

0.71

0.78

1.49

137 138

The pseudo-first-order rate constant (kobs) was determined for each assayed system (Table

139

S1 in SI). The oxidation ability of each Fenton-like system driven by a 1,2-DHB was

140

significantly different. The Hammett equation (5) was used to determine whether the

141

substituent on 1,2-DHB had a direct influence on the observed changes in reactivity of the

142

system, where kobs is the rate constant for X substituent and k°obs is the rate when X=H

143

(catechol). The reaction constant (ρ) is a measure of the sensitivity of the reaction to

144

electronic effects and is independent of the substituent.35 The Hammett parameters (σ) for

145

meta (σm) and para (σp) substituents are defined from this equation. If the substituent is an



146

EWG on the aromatic ring, σ is greater than 0. If the substituent is an EDG, σ is lower than

147

0. The absolute values of σ indicate the relative capacity of the substituent to withdraw or

148

donate electron density to the aromatic ring.

149

=

(5)

150

Considering that 1,2-DHBs have two hydroxyl groups and one substituent, the effect of

151

this substituent was evaluated by Σσ, which includes the effect in the meta (σm) and para

152

(σp) position.

153

Figure 1 shows the dependence of log (kobs/k°obs) on the Hammett constant (∑σ), and a

154

nonlinear Hammett relationship with a concave upward deviation was observed. According

155

to the literature,36, 37 this type of deviation indicates that the mechanism of the oxidation of

156

MO in a Fenton-like system changes depending on whether the substituent on 1,2-DHB is

157

an EWG or EDG. A possible explanation for the rate processes is a dual reaction

158

mechanism, with the overall rate constant kobs being given by the sum of two rate constants

159

kl and k2.After adjusting these results to equation (6), described by Exner,37 these kinetics

160

constants are 0.1440 and 0.9998, respectively.

161

= log 10#$ % + & 10#' % (ref. 37) (6)


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0.35

log(kobs/k°obs)

0.30 0.25 0.20

14

1 2 3 10

0.15

4

11

0.10 0.05

15

7

5

8

13 12

9

Equation data

Experimental data 6 0.00 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60

Σσ 162 163

Figure 1.Hammett plot for MO oxidation (kobs) in Fenton-like systems driven by different

164

1,2-DHBs.

165

Two values for ρ were determined from the plot in Figure 1. For the Fenton-like systems

166

driven by 1,2-DHBs with EWGs, ρ=0.2145, and for the Fenton-like systems driven by 1,2-

167

DHBs with EDGs, ρ=-2.8076.

168

The magnitude of ρ indicates that the mechanism of the oxidation of MO in Fenton-like

169

systems driven by 1,2-DHBs with EDGs is most affected by the change in the substituent

170

and that the limiting step is probably highly dependent on the ability of the hydroxyl groups

171

to donate electron density. Conversely, the mechanism of the oxidation of MO in Fenton-

172

like systems driven by 1,2-DHBs with EWGs is relatively less influenced by the

173

substituent. The positive value for ρ in these systems indicates that the limiting step in the

174

mechanism includes an increase in the electron density of the transition state or

175

intermediate.38

176

Hydroxyl Radical Production. Several reports have emphasized the importance of HO•

177

in the oxidizing ability of Fenton systems. For the Fenton-like system driven by 1,2-DHBs,



178

HO• production was determined from EPR measurements. The decay kinetics of each

179

studied system was fit to a pseudo-first-order equation, obtaining values for the pseudo-

180

first-order constant (kp) and initial signal (I0) of the DMPO-OH adduct (Table S2 in SI).

181

The studied systems showed different abilities to produce HO• (Figure 2).

A

0.0000

B

-0.0005

1.20 1.10

-0.0010

1.00 0.90

-0.0020 -0.0025 -0.0030

Io

kp (s-1)

-0.0015

0.80 0.70

-0.0035

0.60

-0.0040

0.50

-0.0045

0.40

182 183

Figure2. Hydroxyl radical production by Fenton-like systems driven by different 1,2-

184

DHBs. A) Pseudo-first-order constant (kp) and B) initial signal (I0) determined from decay

185

kinetics.

186

The kinetics parameter I0 is related to the Hammett constant in a similar manner to that

187

observed for MO oxidation (Figure 3, A). According Contreras et al.,32 this parameter is

188

proportional to the initial amount of HO•. In this way, I0 linearly increases in systems

189

driven by 1,2-DHBs with EWGs and linearly decreases in systems driven by 1,2-DHBs

190

with EDGs, observing a minimum in I0 when the Fenton-like system is driven by catechol.

191

A linear relationship was observed between I0 and kobs from MO oxidation (Figure 3, B;

192

r=0.9842). Therefore, although the mechanism of the Fenton reaction differs based on the

193

1,2-DHBsubstituent, the HO• is the main oxidizing species responsible for MO oxidation.


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A

1.30

B

1.20

Io

0.50 0.40 0.30 -0.50

194

kobs (x10-3 s-1)

3

0.90

0.60

15

14

1

1.00

0.70

14

8.00

1.10

0.80

9.00

15

11

2 4

7 5

-0.25

0.00

13

8

6

12

10

0.50

0.75

1.00

1.25

1.50

1.75

1

2 10

6.00

13 11

8

5

3 12

4 9

5.00

7 6

4.00

9

0.25

7.00

3.00 0.30

0.40

0.50

0.60

Σσ

0.70

0.80

0.90

1.00

1.10

1.20

1.30

Io

195

Figure 3. Relationship between the A) initial signal of HO• production (I0) and Hammett

196

constant and the B) pseudo-first-order rate constant for MO oxidation (kobs) and I0.

197

Fe(III) Reduction. To understand the deviations in the obtained Hammett equation, the

198

ability of each 1,2-DHB to reduce Fe(III) was studied. Fe(II) production from Fe(III)

199

reduction mediated by 1,2-DHB has been considered essential to the increased oxidizing

200

ability of Fenton-like systems driven by 1,2-DHBs. The reduction rate constants (kred) were

201

obtained from the kinetics profiles of Fe(III) reduction (Table S2 in SI). A quasi-linear

202

relation was obtained between log(kred/k°red), where kred is the rate constant of Fe(III)

203

reduction mediated by different 1,2-DHBs and k°red is the rate constant of Fe(III) reduction

204

mediated by catechol (unsubstituted), and the Σσ values of the substituents on1,2-DHB

205

(Figure 4, A). This trend suggests an increase in Fe(III) reduction by 1,2-DHBs with EDGs

206

and a decrease with 1,2-DHBs with EWGs. The ρ constant for this reduction reaction is -

207

0.6701 (r=0.9839), which indicates a loss of electron density in the aromatic ring during the

208

limiting step of the reaction, according to Exner.37 The redox potential of 1,2-DHB (Table

209

S3 in SI) was significantly dependent on the type of substituent on 1,2-DHB (Figure 4, B).

210

1,2-DHB is easier to oxidize when the substituent has a more negative σ value. Overall,

211

these results suggest that an EDG on 1,2-DHB increases the electron density over the

212

hydroxyl group of the catechol portion, promoting internal electron transfer in the complex



Page 12 of 21

213

[Fe(1,2-DHB)]+, which results in the reduction of iron in the coordination sphere. This

214

result agrees with other reports,39, 40 wherein E° is strongly related to the ability of another

215

DHB molecule to reduce Fe(III).

0.20 0.00

log(kred/kored)

B

0.40 1

4 23

5

-0.20

7

8

6

0.70 0.65 15

0.60 9

E° (volt)

A

10 11

-0.40

13 12

-0.60

12

0.55

13

0.50 0.45

6 2

0.40

-0.80 14

-1.00 -0.4 -0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

3 1

0.35 15

-1.20

14

1.4

1.6

0.30 -0.4

-0.2

0.1

0.4

0.6

0.9

1.1

1.4

1.6

Σσ

Σσ

216 217

Figure 4. A) Hammett plot of Fe(III) reduction (kred) by different 1,2-DHBs. B)

218

Relationship between the Hammett constants of different 1,2-DHBs with the standard redox

219

potential (E°).

220

Proposed pathways for HO• production. According to our results, more than one

221

pathway can produce HO• in Fenton-like systems driven by 1,2-DHBs. Although several

222

publications indicate the reduction of Fe(III) as the main mechanism by which 1,2-DHBs

223

promote the Fenton reaction, it is remarkable that the relationship obtained between ∑σ and

224

MO oxidation is different than the relationship between ∑σ and the reduction of Fe(III)

225

(kred). While the first relationship follows a concave upward deviation (Figure 1, B), the

226

second has a linear relationship with a negative slope (Figure 4, A). Despite this

227

disagreement, the MO degradation is closely related to the amount of HO• produced in each

228

system. Both results indicate that only Fenton-like systems driven by 1,2-DHBs with EDGs

229

depend on the ability of 1,2-DHB to reduce Fe(III) and produce HO•. In addition, a

230

significant difference was observed between the stability of the monocomplex when the


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231

substituent was EDG or EWG (Figure S5 in SI). After 1 min, the spectrophotometric signal

232

of the monocomplex formed from 1,2-DHBs with EDGs decreased by 70.2 % (Figure 5,

233

A), where as the monocomplex formed from1,2-DHBs with EWGs did not significantly

234

change (Figure 5, B). This higher stability of the monocomplex with EWGs will allow the

235

reaction of the monocomplex with H2O2 and the subsequent production of HO•. A

B

0.35

0.30

0.30

0.25 0.20 0.15

0.25 0.20 0.15

0.10

0.10

0.05

0.05

0.00

0.00 320

236

0.40

0.35

Absorbance

Absorbance

0.40

420

520

620

720

820

920

1020

1120

320

420

520

Wavelength (nm)

620

720

820

920

1020

1120

Wavelength (nm)

237

Figure 5. Absorption spectra for solutions with Fe(III) (1x10-4 mol·L−1) and 1,2-DHB

238

(1x10-2 mol·L−1) at the initial time (t=0 s, continuous lines) and after 60 s (dotted lines): A)

239

4-tert-butylcatechol (σ = -0.30) and B) 3,4-dihydroxybenzonitrile (σ = 1.49).

240

In summary, the monocomplex stability change depending of the substituent on the 1,2-

241

DHB. This change in the reactivity affect the ability of the system to reduce Fe(III) but not

242

their ability to increase the HO• production.

243

If the tautomeric valence equilibrium is consider the substituent in the 1,2-DHB could be

244

displacing this equilibrium Fe(III)-catecholate/Fe(II)-semiquinone. In this way the EDG,

245

displaced the equilibrium from Fe(III)-catecholate to Fe(II)-semiquinone (Scheme 2, A).

246

Whereby the EDG on the 1,2-DHB, promote the internal electron transference with the

247

consequent release of Fe(II). Otherwise, the systems of 1,2-DHB with EWG shows a low

248

amount of free Fe(II) since the iron should remains inside of the monocomplex. This is

249

because the tautomeric valence equilibrium is displaced from Fe(II)-semiquinone to Fe(III)-



Page 14 of 21

250

catecholate (Scheme 2, B). Thus it can be concluded that the main iron species available to

251

react with H2O2 are different in the studied Fenton systems, depending on the kind of 1,2-

252

DHB. In Fenton systems driven by 1,2-DHB with EDGs, the H2O2 reacts with free Fe(II)

253

by a conventional Fenton reaction (Scheme 2, A). Otherwise, in Fenton systems driven by

254

1,2-DHB with EWG the H2O2 react mainly with the iron inside the monocomplex which

255

has higher stability because the EWG on the 1,2-DHB.

256

The formation of Fe(III) peroxocomplexes have been described in literature.16,

257

Scheme 2, B is postulated a possible pathway to produce HO• from iron-peroxocomplexes.

258

When the monocomplex of Fe(III) is formed with a 1,2-DHB with EWG the H2O2 reacts

259

mainly with Fe(III)-catecholate monocomplex, but also with a few portion of Fe(II)-

260

semiquinone monocomplex (Scheme 2, B). The reactivity of the peroxocomplexes of Fe(II)

261

is expected to be higher than peroxocomplexes with Fe(III) whereby the H2O2 react faster

262

with Fe(II) producing HO• and Fe(III). In this way the Fe(II)-semiquinone is consumed and

263

the tautomeric valence equilibrium is displaced. If this pathway is considered, the electron

264

is transferred from the 1,2-DHB to H2O2 through the iron.

265


41-43

In

Page 15 of 21


266

Scheme 2. Proposed pathways for Fenton-like systems driven by 1,2-DHB with A)

267

electron-donating group (EDG) and B) with electron-withdrawing group (EWG).

268

ASSOCIATED CONTENT

269

Supporting Information

270

The structure and absorption spectrum of MO, structures and E° of 1,2-DHBs employed in

271

this study, absorption spectra of complexes formed at pH 3.4 after 1 minute of mixing

272

Fe(III) with 1,2-DHB and kinetic data obtained from MO oxidation and HO• production.

273

AUTHOR INFORMATION

274

Corresponding Author

275

David Contreras, phone: +56412204601, email address: [email protected]*

276

Notes: The authors declare no competing financial interest.

277 278

ACKNOWLEDGMENT

279

The

280

(grant 1160100), FONDEQUIP (grant EQM140075), FONDAP Solar Energy Research

281

Center, SERC-Chile (grant 15110019), CONICYT (Ph.D. grant 21120966), REDOC-

282

UDEC and UDT-CCTE fellowship (grant PFT-072).

financial

support

for

this

work

was

provided

283


by

FONDECYT


284

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The Reactivity and Reaction Pathway of Fenton Reactions Driven by

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