Improving the catalytic effect of peroxodisulfate and peroxodiphosphate electrochemically generated at diamond electrode by activation with light irradiation

Accepted Manuscript Improving the catalytic effect of peroxodisulfate and peroxodiphosphate electrochemically generated at diamond electrode by activation with light irradiation

Danyelle Medeiros de Araújo, Cristina Sáez, Pablo Cañizares, Manuel Andrés Rodrigo, Carlos A. Martínez-Huitle PII:

S0045-6535(18)30970-6

DOI:

10.1016/j.chemosphere.2018.05.121

Reference:

CHEM 21453

To appear in:

Chemosphere

Received Date:

01 February 2018

Accepted Date:

19 May 2018

Please cite this article as: Danyelle Medeiros de Araújo, Cristina Sáez, Pablo Cañizares, Manuel Andrés Rodrigo, Carlos A. Martínez-Huitle, Improving the catalytic effect of peroxodisulfate and peroxodiphosphate electrochemically generated at diamond electrode by activation with light irradiation, Chemosphere (2018), doi: 10.1016/j.chemosphere.2018.05.121

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ACCEPTED MANUSCRIPT

RhB RB

e-

Red Direct oxidation

Photolytic activation of oxidants

Products

eOx

H2O2 + hν → 2 OH H2O

e-

H+

H2O2 +O3 + hν → 2 OH+O2

H2O

e-

S2O82 + hν → 2 (SO4)

RhB RB SO4

Products SO4

RhB RB

eS2O82-

Products

Products

H2O2 +2O32OH· +3O2

S2O82-

Mediated oxidation

P2O84- + H2O2 2 PO42-· + 2 OH· S2O82- + H2O22 SO4-· + 2 OH· Chemical activation of oxidants

Cathode

O2

RhB RB

eH2O2

Products

RhB RB

e-

2-

H2 + OH-

H2O

RhB RB

OH·  H2O2

2-

e-

P2O84 + hν → 2 (PO42-)

+ O2

O2+ 2 OH·  O3 +H2O

Anode

H2O

ACCEPTED MANUSCRIPT 1

Improving the catalytic effect of peroxodisulfate and peroxodiphosphate

2

electrochemically generated at diamond electrode by activation with light

3

irradiation

4 5

Danyelle Medeiros de Araújo1, Cristina Sáez2, Pablo Cañizares2,

6

Manuel Andrés Rodrigo2, Carlos A. Martínez-Huitle3,4

7 1 Federal

8

Povoado Base Física, Zona Rural, CEP 59508-0 - Ipanguaçu, RN, Brazil.

9 10

2 Department

3 Federal

University of Rio Grande do Norte, Instituto de Química, Lagoa Nova CEP 59078-970 - Natal, RN, Brazil.

13 14

of Chemical Engineering, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain.

11 12

Institute of Education, Science and Technology of Rio Grande do Norte,

4

Unesp, National Institute for Alternative Technologies of Detection, Toxicological

15

Evaluation and Removal of Micropollutants and Radioactives (INCT-

16

DATREM),Institute of Chemistry, P.O. Box 355, 14800-900 Araraquara, SP, Brazil.

17 18

Corresponding author: [email protected]

19 20 21 22 23 24 25

1

ACCEPTED MANUSCRIPT 26

Abstract

27

Boron doped diamond (BDD) anode has been used to oxidatively remove Rhodamine B

28

(RhB), as persistent organic pollutant, from synthetic wastewater by electrolysis,

29

photoelectrolysis and chemical oxidation containing sulfate and phosphate as supporting

30

electrolytes. RhB is effectively oxidized by electrolysis and by chemical oxidation with

31

the oxidants separately produced by electrolyzing sulfate or phosphate solutions

32

(peroxodisulfate and peroxodiphosphate, respectively). The results showed that light

33

irradiation improved the electrolysis of RhB due to the activation of oxidants under

34

irradiation at high current densities. Meanwhile, the efficiency of the chemical oxidation

35

approach by ex situ electrochemical production of oxidants was not efficient to degrade

36

RhB.

37 38

Keywords: Electrolysis, photoelectrolysis, peroxosalts, Rhodamine B, boron doped

39

diamond.

40 41 42

Introduction

43

In the last decades, the applicability of electrochemical technologies for the

44

environment protection has received great attention. Among these approaches,

45

electrochemical oxidation (EO) is the most used, and the recent studies have evidenced

46

the importance of the development of new electrode materials as well as the

47

combination with other technologies, in irder to intensify its efficiency (Bebelis et al.,

48

2013; Sirés et al., 2014). In the former, the introduction of conductive diamond coatings

49

was a significant scientific contribution in the EO (Rodrigo et al., 2001; Polcaro et al.,

50

2004; Polcaro et al., 2005; Panizza and Cerisola, 2005; Martinez-Huitle and Ferro,

2

ACCEPTED MANUSCRIPT 51

2006; Panizza and Cerisola, 2009; Sanchez-Carretero et al., 2011; Scialdone et al.,

52

2012; Sires and Brillas, 2012; Borras et al., 2013; Panizza and Martinez-Huitle, 2013;

53

Brillas and Martínez-Huitle, 2015; Martínez-Huitle et al., 2015) due to its chemical and

54

electrochemical stability as well as the efficient use of the electricity. Diamond coatings

55

are considered as “non-active” anodes (Panizza and Cerisola, 2009; Sanchez-Carretero

56

et al., 2011; Scialdone et al., 2012; Borras et al., 2013; Panizza and Martinez-Huitle,

57

2013; Brillas and Martínez-Huitle, 2015; Martínez-Huitle et al., 2015) during EO

58

process. The superiority on the oxidative character of diamond anodes in this

59

classification is related to the enthalpy of adsorption of oxidant mediators generated by

60

the water discharge, the hydroxyl radicals (Eq. 1) (Panizza and Cerisola, 2009; Isarain-

61

Chavez et al., 2013; Panizza and Martinez-Huitle, 2013; Brillas and Martínez-Huitle,

62

2015; Martínez-Huitle et al., 2015).

63

H2O  ●OH + e− + H+

(1)

64

These electrogenerated species are primordially physisorbed on diamond surface

65

(surface interacts so weakly with ●OH), playing an important role in the electrochemical

66

conversion/combustion of organic pollutants (Oturan, 2000; Panizza and Cerisola, 2009;

67

Brillas and Martínez-Huitle, 2015; Martínez-Huitle et al., 2015). Also, other reactive

68

oxygen species (H2O2 and O3) and oxidants can be electrochemically produced, such as

69

peroxo-species, when dissolved carbonates, sulfates and phosphates are present in

70

solution (Eqs. 2-4).

71

2 SO42- → S2O82–+ 2 e

(2)

72

2 PO43 → P2O84–+ 2 e

(3)

73

2 CO32→ C2O62 + 2 e

(4)

74

Although the applicability of the EO seems feasible due to its environmental

75

compatibility, amenability of automation, high energy efficiency, versatility and safety 3

ACCEPTED MANUSCRIPT 76

operating at mild conditions (Polcaro et al., 2004; Polcaro et al., 2005; Oturan et al,.

77

2008; Sanchez-Carretero et al., 2011; Scialdone et al., 2012; Panizza and Martinez-

78

Huitle, 2013); combination of this electrochemical technology with other processes

79

(oxidants production, ultraviolet light (UVC) irradiation or ultrasound (US) irradiation)

80

is becoming the new technological challenge (Polcaro et al., 2009; Scialdone et al,

81

2009a; Vacca et al., 2011; Wols and Hofman-Caris, 2012; Shih et al., 2014).

82

In this context, the combination of EO based on diamond conductive anodes with light

83

irradiation has received great interest (Souza et al., 2013a; Souza et al., 2013b) because

84

the combination of these approaches is not looking constantly for heterogeneous

85

photocatalytic processes occurring on the surface of the anode; but in many cases, it is

86

looking for the homogeneous activation of oxidants occurring in the bulk (Alves et al.,

87

2010; Malpass et al, 2010).

88

Based on the existing literature (Serrano et al, 2002; Cañizares et al., 2005a; Cañizares

89

et al., 2005b; Sanchez et al., 2013; Velazquez-Pena et al., 2013), peroxo-species (–

90

O=O–) are known to conform a very interesting group of oxidants (Sanchez et al.,

91

2013). However, these peroxo-species are not kinetically rapid when are

92

electrochemically generated and behave as non-selective soft oxidants. Therefore, their

93

activation into more reactive species (peroxo-anion radicals) is necessary (Sires et al.,

94

2014). The production of these radicals can be promoted by using light irradiation (Eqs.

95

5-7) and by the synergic interactions between the oxidants in the bulk (chemical

96

activation).

97

S2O82 + hν → 2 (SO4)

(5)

98

P2O84 + hν → 2 (PO42)

(6)

99

C2O62 + hν → 2 (CO3)

(7)

4

ACCEPTED MANUSCRIPT 100

Taking into consideration the above information, this research aims to investigate the

101

improvement on the catalytic effect of peroxodisulfate (S2O82) and peroxodiphosphate

102

(P2O84) electrochemically generated at diamond electrode (heterogeneous catalytic

103

approach) by activation with light irradiation to produce highly reactive oxidizing

104

species such as sulfate (SO4) and phosphate (PO42) radicals during the treatment of a

105

model organic compound. The oxidation approaches were also compared with chemical

106

oxidation where the oxidants were initially electrogenerated, and successively, their

107

activation with light irradiation was performed. The mechanisms involved (synergic

108

process (Scialdone, 2009b; Scialdone et al., 2010; de Oliveira, 2012) and/or oxidants

109

activation) and their catalytic effect, on the efficiency during the treatment of synthetic

110

solutions containing Rhodamine B (RhB), were established, at different oxidative

111

processes. RhB (C28H31ClN2O3) was chosen as model organic pollutant because it is

112

widely used in textiles, leathers and food stuffs with high water solubility as well as the

113

neurotoxicity, carcinogenicity, reproductive and developmental toxicity. Adittionally,

114

the chronic toxicity of RhB in human beings and animals have been proven

115

experimentally (Kornbrust and Barfknecht, 1985; IARC, 1987). For this reason, RhB is

116

considered as a persistant organic pollutant (POP).

117 118

Material and Methods

119

Chemicals

120

High purity reagents were used in this study and supplied by Fluka. Solutions of RhB

121

(2.09×10-4 M) were prepared by using Na2SO4 or H3PO4 (7.04×10-3 M) as supporting

122

electrolyte. All aqueous solutions were prepared using Milli-Q water.

123 124

Analytical procedures 5

ACCEPTED MANUSCRIPT 125

Color removal was followed by UV-vis spectrophotometric technique using a UV-1603

126

spectrophotometer Model Shimadzu, at a wavelength of 550 nm. Chemical oxygen

127

demand (COD) was determined by using pre-dosage vials with 2 mL of sample. Total

128

organic carbon (TOC) was determined with a TOC analysator Multi N/C (Analytikjena)

129

by analyzing 10 mL of each sample.

130 131

Bulk electrolysis

132

Bulk electrochemical experiments were performed by using a flow cell (undivided) with

133

a BDD disc and stainless steel, as anode and cathode, respectively (each one of them

134

with 78 cm2 of geometrical area), under galvanostatic conditions using a power supply.

135

Experiments were carried out at 25°C by applying a current density (j) of 15 or 90 mA

136

cm-2 to treat 1 L of RhB solutions with different supporting electrolytes.

137 138

Chemical oxidation tests

139

Solutions of oxidants were prepared by electrolyzing a solution of sodium sulfate or

140

phosphoric acid at 90 mA cm-2 for 240 min by irradiating (photoelectrolysis) or not

141

(electrolysis) UV light. Then, the resulting solutions were dosed (in different ratios (%

142

oxidant solution)) to synthetic wastewater containing RhB to assess the chemical

143

oxidation of this organic compound by the oxidants produced electrochemically. The

144

chemical analysis of UV-visible, TOC and HPLC were performed for all samples.

145

Oxidants concentration was determined by iodometric titration (Dionisio et al., 2018).

146 147

Light irradiation

148

A UV lamp VL-215MC (Vilber Lourmat), λ = 254 nm, of intensity of 930 µW cm-2 and

149

energy 4.43-6.20 eV was used to irradiate 15 W directly to the quartz cover of the

6

ACCEPTED MANUSCRIPT 150

electrochemical cell (in photoelectrolysis tests) or to the beakers in which the oxidants

151

previously electrogenerated were activated to perform the chemical oxidation tests.

152 153

Results and discussion

154

Figure 1 compares the color removal from the RhB solution in sulfate (Fig. 1a) and

155

phosphate (Fig. 1b) as supporting electrolytes by using photolysis, electrolysis and

156

photoelectrolysis. As it can be seen, no significant color removal was achieved by

157

photolysis (fewer than 10%). This behavior is related to the fragmentation of the

158

chromophore group of few molecules of RhB when UV light is irradiated (Fig. 1). It is

159

confirmed when no current and no light irradiation were applied (no color removal was

160

achieved), indicating that RhB is quite stable organic compound (recalcitrant pollutant),

161

and consequently, its treatment is a priority. Conversely, when a current is applied (15

162

and 90 mA cm-2) in the electrochemical cell, significant differences, in terms of color

163

removal, are achieved between electrolysis and light irradiation. These figures are

164

explained in terms of the direct and mediated EO mechanisms during electrolysis of

165

RhB solutions. In direct EO, the organic pollutants are oxidized by direct electron

166

transfer to the electrode surface. Meanwhile, in indirect EO, the heterogeneous

167

production of the oxidants occurs on the anode surface (hydroxyl radicals (●OH),

168

peroxodisulfates (S2O82) and peroxophosphates (P2O84)), and subsequently, these

169

oxidants attack the organic pollutants in solution (Labiadh et al., 2016). Another

170

interesting feature is that, when an increase on the applied current density is attained,

171

the discoloration rate is intensified. This indicates that the concentration of strong

172

(hydroxyl radicals (●OH)) and soft oxidants (peroxodisulfate (S2O82, Fig. 1a (Eq. 2))

173

and peroxophosphates (P2O84, Fig. 1b (Eq. 3)) is improved, when the applied current

174

density is increased (Serrano et al, 2002; Cañizares et al., 2008; Weiss et al, 2008). 7

ACCEPTED MANUSCRIPT 175

Similar effects were observed during the treatment of different types of wastewaters

176

containing supporting electrolyte salts (Cañizares et al., 2005b; Cañizares et al, 2009a;

177

Aquino et al., 2012).

178

It is also interesting to observe from Fig. 1 that, significant differences are attained

179

when the RhB solution is irradiated with UV light during its electrolysis with BDD

180

anode at 15 and 90 mA cm-2. Results clearly showed that the color removal efficiency

181

was intensified than the results obtained by photolysis and electrolysis approaches. This

182

improvement is due to the activation of the peroxo oxidants electrogenerated by ligh

183

irradiation, forming sulfate (SO4) and phosphate (PO42) radicals (Eqs 5 and 6),

184

which contribute to the decolorisation rate. Additionally, the efficacy of the

185

homogeneous catalytic effect showed by SO4 (Fig. 1a) was more significant than

186

PO42 (Fig. 1b) as well as this intensification is improved when an increase on the

187

applied current denty is attained (Cañizares et al., 2007; Cañizares et al., 2009b;

188

Rodrigo et al, 2010a; Rodrigo et al., 2010b). Then, an efficient production of peroxo-

189

anion radicals (SO4 and PO42) is achieved from S2O82 than P2O84 when UV light is

190

irradiated, promoting the fragmentation of chromophore group in the chemical structure

191

of RhB.

192

The participation of SO4 and PO42 in the degradation of RhB is clear when

193

the level of degradation and mineralization, in terms of COD and TOC, were

194

determined. Figure 2 shows the COD and TOC changes, as a function of time, during

195

the electrolysis and photoelectrolysis of RhB solution containing with sulfate as

196

electrolyte by applying 15 and 90 mA cm-2. In the case of electrolysis, considerable

197

organic removal (Fig. 2a) and mineralization (Fig. 2b) efficiencies were achieved, at

198

both current densities (60% of COD and 49% of TOC at 15 mA cm-2 and 80% of COD

199

and 75% of TOC at 90 mA cm-2). Conversely, a significant intensification on the 8

ACCEPTED MANUSCRIPT 200

efficiency of degradation (75% and 92% by applying 15 and 90 mA cm-2, respectively)

201

and mineralization (70% and 95% by applying 15 and 90 mA cm-2, respectively) of

202

RhB was attained when photoelectrolysis approach was used. This effect is in

203

agreement with the results achieved during the elimination of color (see, Fig. 1).

204

In the case of electrolysis, mass transport limitations affect in the effectiveness

205

of the process. At low current densities (15 mA cm-2), the concentration of pollutant in

206

solution as well as the lower production of oxidants (hydroxyl radicals (●OH) and

207

peroxodisulfates (S2O82)) limit the homogenous catalytic effect of oxidants in the

208

solution, achieving lower COD and TOC removal efficiencies. Conversely, an efficient

209

production of oxidants is attained (Fig. 2) at 90 mA cm-2, enhancing the homogeneous

210

catalytic reactions and consequently, increasing the degradation (changes in the COD,

211

see Fig. 2a) and the mineralization (changes in the TOC, see Fig. 2b) efficiencies. On

212

the contrary, the improvement on the catalytic effect of S2O82 electrochemically

213

generated at diamond electrode is a consequence of its activation to form SO4 radicals

214

by light irradiation when photoelectrolysis approach was used. Despite the mass transfer

215

limitations affect the process; this effect becomes less important because the non-

216

selective soft oxidants (S2O82) are activated to stronger oxidant species (SO4) in the

217

bulk of the solution, increasing the homogenous catalytic reactions.

218

Figure 3 shows the COD and TOC changes observed, as a function of time,

219

during the electrolysis and photoelectrolysis of RhB in phosphate solution by applying

220

15 and 90 mA cm-2. The same qualitative behavior observed at sulfate supporting

221

electrolyte can be observed in phosphate medium. Photoelectrolysis treatment is more

222

effective to degrade and mineralize the model organic pollutant than electrolysis. The

223

COD and TOC removal efficiencies was improved when an increase on the current

224

density was achieved; and the effect of light irradiation (and hence of the activation of 9

ACCEPTED MANUSCRIPT 225

oxidants, P2O84 to form PO42) was also clearly observed, especially during the RhB

226

mineralization (Fig. 3b). Nevertheless, the decontamination enhancement showed by

227

photoelectrolysis in phosphate medium was not superior to the figures achieved in

228

sulfate solution. For example, at 90 mA cm-2 in phosphate medium, 38% and 63% of

229

COD and TOC removals were reached by electrolysis, while 60% and 85% of COD and

230

TOC removals were achieved by photoelectrolysis after 240 min of treatment.

231

Meanwhile, at 90 mA cm-2 in sulfate medium, 80% and 75% of COD and TOC

232

removals were reached by electrolysis, while 92% and 95% of COD and TOC removals

233

were achieved. This result clearly indicates that the sulfate species (S2O82 and SO4)

234

are more efficient in the mineralization of RhB pollutant, being that the oxidants

235

activation plays a key role in the homogenous catalytic reactions (Elahmadi et al., 2009;

236

dos Santos et al., 2014).

237

In order to understand the activation of S2O82 and P2O84 by irradiating UV light, a new

238

set of experiments was performed. The electrolytic production of S2O82 and P2O84 was

239

performed in absence of RhB. The liquid electrolyzed was obtained from synthetic

240

solutions containing the same concentrations of sulfate or phosphate reported in Figs. 1

241

to 3. After that, the resulting solution was used to carried out the chemical oxidation

242

tests, adding a well-known volume in different RhB solutions, which allow to test

243

different oxidant ratios.

244

Figure 4 shows the concentration of oxidants electrochemically generated in absence of

245

organic matter when UV light is irradiated (photoelectrolysis) or not (electrolysis). As it

246

can be observed, the production of oxidants is rapidly achieved at both sulfate and

247

phosphate solutions, after 60 min. The plateau reached, in all cases, is the consequence

248

of the equilibrium between the electrolytic production and the decomposition of the

249

oxidants (Cañizares et al., 2005a). By using phosphate as supporting electrolyte, 10

ACCEPTED MANUSCRIPT 250

photoelectrolysis showed a slight enhancement on the production of oxidants when

251

compared to the electrolysis approach. This behavior is due to the recombination of the

252

radicals produced by light irradiation. Meanwhile, the formation of oxidants in sulfate

253

medium was significantly improved when photoelectrolysis was employed (see Fig. 4,

254

blue squares) respect to the electrolysis.

255

When the solutions of oxidants electrochemically produced by using sulfate or

256

phosphate medium (at 90 mA cm-2 for 240 min by irradiating (photoelectrolysis) or not

257

(electrolysis) UV light) were added to the solution containing with RhB, oxidants are

258

consumed. As it can be seen in Fig. 5, the higher the concentration of oxidant added, the

259

higher is the concentration of oxidant removed because it participates in the oxidation of

260

RhB and its byproducts. No great differences are observed when the oxidants were UV

261

light activated. This means that the oxidants (peroxo-species and peroxo-anion radicals)

262

promote the oxidation of organic matter but the UV light irradiation has not a clear

263

effect out of an electrolytic environment. This behavior is clearly observed when the

264

trends on the color elimination (Fig. 6a) as well as COD (Fig. 6b) and TOC (Fig. 6c)

265

decays were plotted. Although the amount of oxidants solution is increased, activated

266

by UV light irradiation or not, no improvements in the discoloration, degradation and

267

mineralization were observed. When the solutions of oxidants, previously

268

electrogenerated, were added to the RhB effluents, it seems clearly that the oxidants

269

interact chemically with RhB, but not a clear influence of light irradiation to activate

270

S2O82 and P2O84 can be drawn.

271

It is important to remark that, the formation of other oxidants is feasible due to the

272

heterogeneous reactions at BDD surface (Marselli et al., 2003; Groenen-Serrano et al.,

273

2013, Sires et al., 2014) during electrolysis and photoelectrolysis processes, such as

274

hydroxyl radicals, hydrogen peroxide and ozone, as shown in Eqs. (9-10).

11

ACCEPTED MANUSCRIPT 275

H2O  ●OH + H+ + e-

(8)

276

2 ●OH  H2O2

(9)

277

O2+ 2 ●OH  O3 +H2O

(10)

278

At the same time, ozone and hydrogen pexoxide are affected by UV light irradiation and

279

they can regenerate the hydroxyl radical in the bulk, as shown in Eqs. (11 and 12)

280

(Oturan et al., 2001; Brillas et al., 2009, Sires et al., 2014).

281

H2O2 + hν → 2 ●OH

(11)

282

H2O2 +O3 + hν → 2 ●OH +O2

(12)

283

Hydrogen peroxide can also react with ozone, S2O82 and P2O84 (Eqs. 13 to 15), and

284

their effect could be magnified by the light irradiation (Sires et al., 2014).

285

H2O2 +2O32●OH +3O2

(13)

286

P2O84- + H2O2 2 PO42-● + 2 ●OH

(14)

287

S2O82- + H2O22 SO4-● + 2 ●OH

(15)

288

Then, a significant complexity in the system is expected during the electrochemical

289

generation of oxidants by UV light irradiation or not, which can affect the action of

290

these oxidants when are introduced in the RhB solutions. Even when, the results of the

291

chemical oxidation tests are positive to eliminate RhB from solution at different oxidant

292

dosages (Fig. 6), the in situ production of oxidants as well as the UV light activation

293

play a key role, in terms of the homogeneous catalytic effect (Figs. 1, 2 and 3), during

294

photoelectrolysis approach. The most important role is associated to the synergic

295

interactions achieved during the constant exposure to the UV irradiation.

296 297

Conclusions

298

From this work the following conclusions can be drawn:

12

ACCEPTED MANUSCRIPT 299

Light irradiation promotes an enhancement on the discoloration, degradation and

300

mineralization of RhB solution during the electrolysis by using phosphate or sulfate as

301

supporting electrolytes. This effect is greater at high current densities.

302

RhB is efficiently oxidized and mineralized by peroxo-species and peroxo-anion

303

radicals produced photoelectocatalytically; however, the effect observed when sulfate is

304

used as supporting electrolyte is more aggressive than phosphate.

305

Synergic effects occur during the photoelectrolysis are not reproduced, if the process is

306

divided into two separated stages (oxidants production by electrolysis or

307

photoelectrolysis by using sulfate and phosphate solutions is performed, and after that,

308

the solutions are introduced in the RhB effluent to promote chemical oxidation);

309

indicating that the production of oxidants separately (which can be optimized) and their

310

application to the treatment of the wastewater in a later chemical oxidation, is not a very

311

interesting choice because some important mechanisms are excluded than when the

312

process is performed in one-step.

313 314

Acknowledgements

315

D.M.A. acknowledges the CAPES for PhD fellowship and CNPq for the fellowship

316

given for “doutorado sanduiche” under “Ciências sem Fronteiras” program to develop

317

the experimental research at the UCLM-Spain. Financial support of the Spanish

318

government and EU through project FEDER 2007-2013 PP201010 (Planta Piloto de

319

Estación de Regeneración de Aguas Depuradas) is gratefully acknowledged. Financial

320

supports from National Council for Scientific and Technological Development (CNPq –

321

465571/2014-0; CNPq - 446846/2014-7 and CNPq - 401519/2014-7) and FAPESP

322

(2014/50945-4) are gratefully acknowledged.

323

13

ACCEPTED MANUSCRIPT 324

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325

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326

W.F.D., Motheo, A.J., 2010. Photo-assisted electrochemical degradation of real

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textile wastewater. Water Sci. Technol., 61, 491-498.

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Aquino, J.M., Rodrigo, M.A., Rocha-Filho, R.C., Saez, C., Cañizares, P., 2012.

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Figures

1.2

a

0 mA cm-2

Abs/Abs0 / -

1.0 0.8

15 mA cm-2

0.6 0.4 90 mA cm-2

0.2 0.0 0

60

120 180 Time / min

496

240

1.2

300

b

0 mA cm-2

Abs/Abs0 / -

1.0 0.8 15 mA cm-2

0.6 0.4 0.2 0.0 0

497

90 mA cm-2

60

120 180 Time / min

240

300

498

Figure 1. Removal of color during the photolysis, electrolysis and photoelectrolysis of

499

solutions containing RhB in a) Na2SO4: 0 mA cm-2 for (■) no UV irradiation and (■)

500

with UV irradiation; 15 mA cm-2 for (■) no UV irradiation and (■) with UV irradiation;

501

90 mA cm-2 for (■) no UV irradiation and (■) with UV irradiation and b) H3PO4: 0 mA

502

cm-2 for (●) no UV irradiation and (●) with UV irradiation; 15 mA cm-2 for (●) no UV

503

irradiation and (●) with UV irradiation; 90 mA cm-2 for (●) no UV irradiation and (●)

504

with UV irradiation. 21

ACCEPTED MANUSCRIPT 1.0

a -2

COD/COD0 / -

0.8

15 mA cm

0.6 0.4

90 mA cm-2

0.2 0.0 0

60

120 180 Time / min

505

240

1.2

b

1.0

TOC/TOC0 / -

300

15 mA cm-2

0.8 0.6 0.4

90 mA cm-2

0.2 0.0 0 506

60

120 180 Time / min

240

300

507 508

Figure 2. Oxidation (in terms of normalized COD removal) and mineralization (in

509

terms of normalized TOC removal) during the electrolysis and photoelectrolysis of

510

solutions containing RhB in Na2SO4: 15 mA cm-2 for (□) no UV irradiation and (□) with

511

UV irradiation; 90 mA cm-2 for (■) no UV irradiation and (■) UV irradiation.

512 513 514 22

ACCEPTED MANUSCRIPT 1.2

a

COD/COD0 / -

1.0 15 mA cm-2

0.8 0.6

90 mA cm-2

0.4 0.2 0.0 0

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515

120 180 Time / min

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120 180 Time / min

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300

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Figure 3. Oxidation (in terms of normalized COD removal) and mineralization (in

518

terms of normalized TOC removal) during the electrolysis and photoelectrolysis of

519

solutions containing RhB in H3PO4: 15 mA cm-2 for (○) no UV irradiation and (○) with

520

UV irradiation; 90 mA cm-2 for (●) no UV irradiation and (●) with UV irradiation.

521

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ACCEPTED MANUSCRIPT 522

[Oxidants] / mol dm-3

4.010 -3 3.010 -3 2.010 -3

H3PO4

1.010 -3 0

523

Na2SO4

0

60

120 180 Time / min

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300

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Figure 4. Production of oxidants by electrolysis with conductive-diamond of solutions

525

containing Na2SO4 for (■) no UV irradiation and with (■) UV irradiation; H3PO4 for

526

(●) no UV irradiation and with (●) UV irradiation.

527

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ACCEPTED MANUSCRIPT 528 529

[Oxidant removed] / mol dm-3

530

1.010 -3 8.010 -4 6.010 -4 4.010 -4 2.010 -4 0

531

0

10

20

30

40

50

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oxidant solution / %

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Figure 5. Oxidant consumed (initial – final) in chemical oxidation tests of RhB with the

533

oxidant produced electrolytically from Na2SO4 for (■) no UV irradiation and with (■)

534

UV irradiation; H3PO4 for (●) no UV irradiation and with (●) UV Irradiation.

535 536 537 538 539 540 541 542 543 544

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ACCEPTED MANUSCRIPT 545 1.2

a

Abs/Abs0 / -

1.0 0.8 0.6 0.4 0.2 0.0 0

10

546

20 30 40 oxidant solution / %

50

1.2

60

b

COD/COD0 / -

1.0 0.8 0.6 0.4 0.2 0.0 0

10

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20 30 40 oxidant solution / %

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1.2

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c

TOC/TOC0 / -

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20 30 40 oxidant solution / %

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60

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Figure 6. Changes in the (a) color, (b) COD and (c) TOC during the chemical oxidation

550

of dye solutions with different % of oxidant solution produced by electrolyzing H3PO4

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for (■) no UV irradiation and with (■) UV irradiation; Na2SO4 for (●) no UV

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irradiation and (●) with UV irradiation. 26

ACCEPTED MANUSCRIPT Highlights -

Light irradiation promotes an enhancement on the oxidative power of oxidants.

-

RhB is oxidized by the oxidants produced electrolytically during the electrolysis.

-

Photoelectrolysis approach is more efficient than electrolysis.

-

Persulfate is activated by UV light being more efficient than peroxophosphate.

-

Synergic effects are attained when electrogenerated oxidants are UV lightactivated.