Electro-oxidation of methyl paraben on DSA®-Cl2: UV irradiation, mechanistic aspects and energy consumption

Journal Pre-proof Electro-oxidation of methyl paraben on DSA®-Cl2: UV irradiation, mechanistic aspects and energy consumption Dawany Dionisio, Lucas H.E. Santos, Manuel A. Rodrigo, Artur J. Motheo PII:

S0013-4686(20)30293-0

DOI:

https://doi.org/10.1016/j.electacta.2020.135901

Reference:

EA 135901

To appear in:

Electrochimica Acta

Received Date: 2 December 2019 Accepted Date: 12 February 2020

Please cite this article as: D. Dionisio, L.H.E. Santos, M.A. Rodrigo, A.J. Motheo, Electro-oxidation of methyl paraben on DSA®-Cl2: UV irradiation, mechanistic aspects and energy consumption, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2020.135901. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Electro-oxidation of methyl paraben on DSA®-Cl2: UV

2

irradiation, mechanistic aspects and energy consumption

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Dawany Dionisioa,b, Lucas H. E. Santosa, Manuel A. Rodrigob,

5

Artur J. Motheoa,*

6 7

(a)

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São Carlos, SP, Brazil

9

(b)

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São Carlos Institute of Chemistry, University of São Paulo, P.O. Box 780, CEP 13560-970,

Department of Chemical Engineering, Faculty of Chemical Sciences & Technologies,

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

11 12 13 14

*Corresponding author

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Phone: +55 16 33739932

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E-mail: [email protected]

17 18

1

19

Abstract

20 21

The electro-oxidation of methylparaben (MeP) was studied using a DSA®-Cl2 in simple

22

electrolysis and hybrid process (electrolysis followed by UV light irradiation), aiming at

23

evaluating the oxidation mechanism, the removal of organic matter and the energy

24

consumption. Analysis of the results revealed that MeP removal is rapid in both processes.

25

However, the formation of a solid byproduct of a polymeric nature, which is probably related

26

to MeP oxidation products, has occurred. Irradiation of UV light in the solution improved the

27

mineralization process by facilitating the degradation of byproducts, including the solid one.

28

Using the hybrid process, mineralization increased by 40% with low additional energy costs.

29

In addition, new aspects about the MeP electrooxidation mechanism were found. The use of

30

the DSA®-Cl2 appears to favor oxidative coupling reactions, resulting in higher molecular

31

weight products prior to aromatic ring breakdown and further mineralization.

32 33 34

Keywords

35

Hybrid process; chloride medium; buffer solution; oxidation pathway; oligomerization.

36

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37

1

Introduction

38 39

Parabens are part of an important class of preservatives widely used in food, pharmaceutical

40

and cosmetic industries. Darbre et al. [1] reported in 2004 that these compounds are related to

41

human breast cancer, which was restated by Dagher et al. [2] in 2012. Since that, parabens

42

are classified as endocrine disrupting compounds and various studies have been published

43

considering the adverse effects of these substances on the environment and human health [3–

44

9]. According to that, some organizations such as US Food and Drug Administration (FDA),

45

World Health Organization (WHO), European Union (EU), restricted the permitted

46

concentrations of parabens in several products [9,10]. However, due to the high benefit-cost

47

ratio, these preservatives are still largely used, resulting in the contamination of several

48

environmental matrices [11–14].

49

Different types of methods are reported for the removal of organic pollutants, such as

50

parabens, from wastewater. Electrochemical advanced oxidation processes (EAOPs) have

51

showed higher efficacy to achieve complete mineralization and good versatility for

52

technologies coupling [15–20]. Combining other technologies to electrolysis, for instance

53

irradiation of UV light or ultrasound, it is becoming a common strategy to enhance the

54

production of oxidants in the media and, thus, improving the removal of contaminants [21–

55

23]. Nevertheless, the efficiencies of these processes are strongly dependent on the system

56

coupling, the electrodes material and the matrix composition.

57

A few types of electrodes have been used to study the electrochemical oxidation of parabens

58

[15,17,24–30]. However, boron-doped diamond (BDD) anodes are still the most employed,

59

which are very powerful but also expensive. On the other hand, dimensionally stable anodes

60

(DSA®-Cl2) are a commercial type of mixed metal oxides electrode (MMO), which are

61

cheap, robust and very stable. These electrodes are widely used in the chlor-alkali industry

3

62

and several studies reported their good performance for the degradation of pollutants in

63

chloride media [31–36].

64

Both, DSA®-Cl2 and BDD, represent the boundaries in the behavior of electrodes for the

65

electrolysis of organics. In fact, a very important model proposed in the nineties about the

66

mechanisms in the oxidation of organics suggests diamond and mixed metal oxides as

67

examples of active and non-active electrodes [37]. For active electrodes, the formation of

68

hydroxyl radicals by anodic oxidation of water was not as effective as in the case non-active

69

electrodes. This is a consequence of these radicals’ characteristics, which are key to

70

understand the performance of the electrolysis of wastewater. They interact with the metal

71

oxides, leading to the formation of unstable metal oxide species at higher oxidation state, and

72

this unstable oxides becomes the real responsible for the oxidation of organics [38]. This

73

means that the oxidation is really chemical and non radicalary or electrochemical and this

74

explains its softer character. Conversely, in the case of the non-active electrodes, hydroxyl

75

radicals are not combined with the species contained on the surface of the electrode and they

76

attack directly to the organic pollutants in a hasher way [39]. In addition, from the beginning

77

of the research studies focused on the treatment of wastewater [40–43], the significance of

78

mediated oxidation has been pointed out, in particular in the presence of chlorides. These

79

anions allows the formation of chlorine and other oxidants [44–46], which does not always

80

have a positive impact on the treatment because of the reported formation of hazardous

81

byproducts. At this point, key differences between the mechanisms involved in the oxidation

82

of organics using active and non-active electrodes are not related to the oxidative reactions

83

but to the significance of the progress in each of them. Thus, in non-active electrodes,

84

combination of intermediates for form polymers is not promoted and carboxylic acids are

85

easily degraded [47]. Conversely, in active electrodes, with the same oxidation routes,

86

accumulation of carboxylic acid is typically found, because of the difficulties found in the

4

87

chemical oxidation of these highly oxidized organic molecules. In addition, stability of

88

polymers is much higher than that of the parent molecules and because of that they remain in

89

the treated solution, sometimes fouling the electrode surface and leading to important

90

inefficiencies [48].

91

It is possible to find several mechanistic proposals of parabens oxidation using either

92

electrochemical or non-electrochemical processes [15,18,20,28,49–53]. However, to the best

93

knowledge of the authors, for the electrooxidation process, all-mechanistic considerations are

94

proposed using BDD anodes, i.e., there is a lack of information on the use of MMO as

95

anodes, specifically for this system. According to this, the aim of the present study was to

96

analyze the electrooxidation mechanism of methyl paraben (MeP) using a DSA®-Cl2, in order

97

to verify the feasibility of this technology for the removal of parabens from wastewaters. A

98

hybrid process with irradiation of UV light during electrolysis was also considered, focusing

99

on the removal of organic matter and energy consumption.

100 101

2

Experimental

102 103

Chemicals. Methyl paraben was obtained from Sigma-Aldrich. NaCl (Synth) and H2SO4

104

(PanReac) were used as supporting electrolyte and for pH adjustment. Monochloroacetic acid

105

(AnalytiCals) was used to prepare a buffer solution with pH = 3 (named monochloroacetate

106

buffer, BMCAc). Acetonitrile and methanol, HPLC grade were obtained from PanReac. All

107

reactants were used as received. High-purity water obtained from a Millipore Milli-Q system

108

(resistivity >18M cm at 25 ºC) was used for the preparation of all solutions.

109 110

Electrochemical treatments. Experiments were carried out in a recirculation batch system

111

(see Fig. S1a) constituted of an electrochemical cell with 1.0 L of working solution, a

5

112

jacketed reservoir with 0.7 L of solution and a circulator pump working at 5.0 mL s-1 flow

113

rate. A commercial DSA® of Ti/Ru0,3Ti0,7O2 (purchased from DeNora do Brasil) was used as

114

anode and a Ti plate as cathode, both with 54.4 cm2 and separated by 5 cm in the cell.

115

Solutions of MeP (100 mg L-1, pH = 3.0) + NaCl (0.15 mol L-1) were treated by 2 hours

116

electrolysis using an Autolab PGSTAT 128N (Metrohm B.V.). For UV light irradiation, a

117

germicidal UVC lamp (4 W, λ = 254 nm, Philips) was introduced into a quartz tube and

118

centralized inside the reservoir (see Fig. S1b). For the study of pH using buffer solution,

119

experiments were carried out in a simpler system based on the previous one (see Fig. S1c).

120

The reservoir of the other system was used now as electrochemical cell to treat 0.6 L of

121

solution, without recirculation, using the same electrodes as before. Monochloroacetic

122

acid/chloroacetate (0.1 mol L-1, BMCAc) was added to the solution MeP + NaCl in order to

123

maintain its pH at 3 during the 2 hours electrolysis.

124 125

Determination of MeP, BMCAc and TOC. Concentration of methyl paraben was monitored in a

126

HPLC Shimadzu SPD-10A VP, with a Zorbax SB-C18 (25 cm x 4.6 mm) column and UV

127

detector set in 254 nm. Acetonitrile and water (40:60 v/v) were used as mobile phase, at 30

128

ºC and flow rate of 1 mL min-1. Monochloroacetic acid was also monitored in this system but

129

using a Aminex HPX-87H (Bio-Rad) column (at 25 ºC) and H2SO4 0.005 mol L-1 as mobile

130

phase (1 mL min-1), with detection in 214 nm. Total organic carbon (TOC) was determined in

131

a carbon analyzer Sievers InnovOx, General Electric Company (FAPESP 2014/02739-6).

132 133

Byproducts analysis. For solid product analysis, 1.7 L of treated solution were centrifuged in

134

an Himac CR 2OB2 centrifuge (Hitachi) at 8000 rpm during 15 min. Precipitate was dried for

135

12 hours in a vacuum oven (Precision model 19) at 60 ºC. The solid was analyzed by infrared

136

spectrophotometry (Shimadzu IRAffinity) in KBr pellets. Cyclic voltammetry was performed

6

137

with an Autolab PGSTAT 128N (Metrohm B.V.), using the same anode and cathode as in

138

electrolysis and an Ag/AgCl reference electrode. A cathodic sweep was carried out at 50 mV

139

s-1 ranging from -1.3 to -0.3 V. Aromatic byproducts were extracted and concentrated by

140

solid phase extraction (SPE) using C18ec cartridges (1 mL/ 100 mg, Chromabond®,

141

Macherey-Nagel) and methanol as eluent. These samples were analyzed by HPLC-MS in a

142

Shimadzu Prominence 20A series chromatographer with Zorbax SB-C18 (25 cm x 4.6 mm)

143

column, at 40 ºC, with H2O:ACN (60:40, v/v) acidified by 0.1% formic acid as mobile phase

144

(1.0 mL min-1) coupled to a hybrid mass spectrometer quadrupole/TOF with electrospray

145

ionization (Microtof-QII, Bruker Daltonics) (FAPESP 2004/09498-2).

146 147

3

Results and Discussion

148

3.1

Effect of current density

149

Fig. 1 shows the removals of MeP (MePR) by electrochemical treatment at current densities

150

(japp) ranging from 1.0 to 10.0 mA cm-2, which are equivalent to maximum electrical charges

151

(Q) of 0.06 to 0.64 A h L-1, respectively. As expected, the higher the japp, the higher the

152

removal of the pollutant: MePR of 29 and 76% were obtained for japp = 1.0 and 2.5 mA cm-2,

153

respectively, whereas for higher japp, complete removal was achieved with 80 and 40 min of

154

treatment for 5.0 and 10 mA cm-2 (Tab. 1). In the studied system, chloride is oxidized to

155

chlorine gas (Eq. 1) on the DSA®-Cl2 surface; dissolved Cl2 is hydrolyzed forming

156

hypochlorous acid (Eq. 2), which further dissociates to hypochlorite (Eq. 3) depending on

157

the solution pH (pKa (HClO) = 7.5)[54,55].

158

2 Cl− → Cl2 + 2 e−

(1)

159

Cl2 + H2O ⇌ HClO + Cl− + H+

(2)

160

HClO ⇌ ClO− + H+

(3)

7

161

Hydroxyl radicals are also produced in the system from water electrolysis (Eq. 4), however,

162

they are strongly adsorbed on DSA®-Cl2 surface and will only oxidize species near to this

163

interface [56]. The main species on this region are chloride anions and, thus, the main ●OH

164

reaction might be the formation of Cl● radicals (Eq. 5) [57]. On the other hand, due to its

165

higher stability, active chlorine species (Cl2, HClO, ClO−) will be present in the whole

166

volume of working solution and, therefore, will be the mainly responsible for the indirect

167

oxidation of methyl paraben. Accordingly, as the japp increases, MeP is converted to other

168

compounds at higher rates, due to the higher production of active chlorine species. In the

169

present work, the quantitative determination of free chlorine was not made; however,

170

numerous works report that the degradation of organics increases with the increase of Cl−

171

concentration in the medium and the increase of the applied current density (when using

172

DSA®-Cl2) [58–60].

173

H2O → H+ + ●OH + e−

(4)

174

Cl− + ●OH + H+ → Cl● + H2O

(5)

175

At higher japp values, it is noticeable that for each value of applied electrical charge the same

176

MePR is attained, which indicates that the removal process is limited by mass transfer

177

[61,62]. This limitation can be a result of the transport of Cl- to the anode surface, controlling

178

the oxidants electrogeneration, and the transport of oxidants from the anode to the bulk

179

solution and vice versa, limiting the oxidation of MeP. On the other hand, for japp < 5.0 mA

180

cm-2, charge transfer is facilitated on the electrode surface, nonetheless a small concentration

181

of chlorine is electrogenerated, resulting in low MeP conversion.

182

The elimination of organic matter was evaluated by TOC removal (TOCR), which is

183

presented in Tab. 1. Those removals seem to be enhanced by higher japp values, however for

184

5.0 and 10.0 mA cm-2 similar TOCR is observed, because some products of MeP degradation

185

may be more recalcitrant than MeP itself. Usually, short-chain carboxylic acids and

8

186

organochlorine compounds are found as byproducts of oxidation processes, since they are

187

highly oxidized and recalcitrant molecules [63,64].

188

From TOC results, it is also possible to determine the mineralization current efficiency of the

189

process (MCE), which is indicative of how efficiently the applied electrical charge promotes

190

TOC removal. MCE can be calculated by Eq. 6 [65], where TOCi and TOCf refer to the

191

initial and final concentrations of organic carbon (mg C L-1), respectively, F is the Faraday

192

constant (96,485 C mol-1), V is the volume of working solution (L), I is the applied current

193

(A), t is time (h), n is the number of electrons exchanged in the mineralization process of the

194

organic compound, m is the number of carbon atoms of the molecule under study, and

195

4.32×107 is the conversion factor for units homogenization (3,600 s h-1 × 12,000 mg C mol-1).

196

Tab. 1 shows that MCE sharply decreased for japp > 2.5 mA cm-2, since increasing current

197

densities did not result in increasing TOCR. Consequently, optimal MCE was observed at japp

198

= 2.5 mA cm-2 although this current density did not provide the highest TOCR. This behavior

199

is related to the occurrence of competitive reactions (i.e. oxygen evolution reaction) and other

200

oxidation processes that do not lead to mineralization (i.e. production of highly oxidized,

201

recalcitrant byproducts).

202

 % =

203

It is worth noting that the persistence of byproducts observed for high current densities could

204

also be related to medium composition in terms of oxidizing species. For japp > 5.0 mA cm-2,

205

the final pH of the wastewater was shifted from 3.0 to 7.0 - 8.0. At those final pH values, the

206

ratio [ClO−]/[HClO] is higher than at the initial pH. Since ClO− is a weaker oxidant than

207

HClO (E0 = 0.89 and 1.49 V, respectively [58]), mineralization is possibly impaired in spite

208

of the high concentration of oxidizing species. Nevertheless, the low TOCR and MCE

    ... .× ...

. 100

(6)

9

209

observed at 1.0 mA cm-2 is most likely related to the low production of oxidizing species than

210

to their relative concentration, since the pH increase was too small in this case (0.3 pH units).

211 212

3.2

Effect of UV light irradiation on electrolysis

213 214

A germicide lamp (λ = 254 nm) was coupled to the solution tank in order to improve organic

215

matter removal by means of the UV/Cl2 process. Irradiation of UV light into the solution may

216

contribute to the removal of organic matter by producing highly oxidizing species (such as

217

●

218

Fig. 2 shows the kinetics for the removal of MeP by electrochemical process single (E) and

219

coupled to UV/Cl2 (E-P). Photolysis of MeP in Cl- medium (inset of Fig. 2) was not efficient

220

since MePR was 17% and almost no TOC removal was observed after 2h. On the other hand,

221

when photolysis is coupled to electrolysis, MeP is completely removed from the solution at

222

japp = 5.0-10.0 mA cm-2. However, no improvement was observed for the E-P process in

223

comparison to single electrolysis. It is possible to observe a pseudo-first order kinetic for

224

MePR by the electrolytic processes, which the kinetic constants (k) are presented in Tab. 1 (E

225

process) and Tab. 2 (E-P process). For japp = 5.0 mA cm-2, two different kinetic regions can

226

be observed because indirect oxidation is facilitated by low pollutant concentration. As the

227

process advances, the abatement of MeP changes its behavior because of the lower MeP

228

concentration in the medium. Even though, the reaction still follows pseudo-first order model

229

but with a different kinetic coefficient, thus, it is observed k1 and k2 for each regime,

230

respectively [66]. When lower japp values were applied, this behavior was not observed

231

because the lower removal of MeP. This effect was also observed on previous studies of MeP

232

degradation using boron doped diamond (BDD) anodes [26,67].

OH and Cl●) and degrading photosensitive molecules.

10

233

Differently from MeP removal, Tab. 2 shows that the removal of organic matter was

234

enhanced by the irradiation of UV light. It is possible to observe an increase on TOCR up to

235

55% with relation to single electrolysis (Tab. 1). Active chlorine species are produced by the

236

electrochemical process, and under UV irradiation they can be activated to even more power

237

oxidizing species, such as ●OH and Cl● (Eqs. 7-9). As it was mentioned before, these species

238

will also be formed near to the anode surface. However, the irradiation of UV light promotes

239

the activation effect at the whole volume of solution, which facilitates the mass transport of

240

the system and, thus, the removal of contaminants.

241

HOCl + hν → ●OH + Cl●

(7)

242

ClO− + hν → O−● +Cl●

(8)

243

O−● + H2O → ●OH + OH−

(9)

244

Considering that some byproducts may be organochlorinated compounds, TOCR may also be

245

improved under UV irradiation due to the rupture of C-Cl bonds [68]. Once higher japp

246

represent higher concentrations of byproducts, the UV effect on electrolysis was major for

247

10.0 mA cm-2. Because of better TOC removals, the MCE is also improved, in which 71% of

248

efficiency was achieved for 5.0 mA cm-2.

249

In order to evaluate the coupling of processes from an energetic point of view, the synergistic

250

index (SI) was correlated with the energy consumption (EC) for TOC removal (Fig. 3). The

251

SI is used to evaluate the effect of the combination of different processes and, in this case,

252

was calculated specifically for TOC removal, according to Eq. 10, where TOCR,E-P, TOCR,E

253

and TOCR,P are the removals of TOC (%) for the coupled process (E-P) and the individual

254

processes E and P, respectively. The energy consumption of the processes per unit of TOC

255

mass was estimated by Eq. 11 (adapted from Martínez-Huitle, et al. [69]), where U is the cell

256

voltage (V), I is the applied current (A), t is time (h), Wlamp is the UV lamp consumption (V),

257

V is volume of working solution (L) and ∆TOC is the experimental TOC decay (in mg L-1). 11

 $,&'(

258

"# =

259

 *+ ℎ -./   =

260

According to the correlation presented in Fig. 3, the higher the SI, the lower is the energy

261

required for removing 1 g of TOC from the working solution and, hence, the higher is the

262

efficiency of the process. An antagonistic effect can be observed for low values of japp due to

263

the low TOCR obtained under those conditions. This result can be explained by the greater

264

effect of UV light on the degradation byproducts than on the MeP, as the results of Tab. 2

265

suggest. On the other hand, for higher current values the efficiency of E-P is improved: at 10

266

mA cm-2 the coupling enhances the individual process by almost 40% (SI = 1.38), and the EC

267

is only 1.4 times higher than the single electrolysis. Under this condition, the extra energy

268

required for the use of UV lamp is counterweighed by the better mineralization removal.

269

Depending on the aim of the treatment, the hybrid process might be more interesting for

270

application, even if the EC is slightly higher. The different MCE results obtained for each

271

treatment (Tabs. 1 and 2) indicate that the composition of the treated solutions is different.

272

Hence, chemical oxygen demand (COD) and toxicity might also be different, which are

273

important parameters to consider for an effluent disposal.

(10)

 $,& )  $,( 0.. ) 12345 . .∆

(11)

274 275

3.3

Identification of byproducts

276

3.3.1 Solid product

277

By the end of the degradations, with both E and E-P processes, the initially clear solutions

278

became turbid because of the presence of a yellow, suspended solid; some foam was also

279

formed on the surface of the solution. Higher accumulation of the solid is obtained for lower

280

electrical charges after 2 hours of E process (see Fig. S2). The increase of Q resulted in less

281

turbid solutions, which means that those byproducts can be degraded by the electrochemical

12

282

process. The suspended solid produced during E process (Q = 0.45 A h L-1) was separated

283

from the solution by centrifugation and analyzed by infrared (IR) spectroscopy.

284

IR spectrum of the solid is presented in Fig. 4a and interpreted as follows [70]. The peaks at

285

1462 cm-1 and 849 cm-1, and the shoulder at 1120 cm-1 (all identified by squares) are

286

attributed to 1,4- and 1,2,4-substituted aromatic rings. The peaks between 1740 cm-1 and

287

1540 cm-1 (circles) account for the presence of carboxylic acids (aliphatic and aromatic), and

288

carboxylate salts. The peaks at 3462 cm-1, with shoulders at 3262 cm-1 and 3050 cm-1, and

289

around 1362 cm-1 (triangles) are related to carboxylic and phenolic OH groups. The shoulders

290

at 1060 cm-1 and 1038 cm-1 (diamonds) are assigned to aryl chlorides. According to that, the

291

solid byproduct seems to consist of chlorinated degradation products of MeP.

292

It was further noticed that the solid is formed on the cathode surface since a strong, yellow

293

color is observed around this electrode during the first minutes of the degradations. Hence,

294

the cathode was studied by cyclic voltammetry (Fig. 4b) in the degradation medium, after 0,

295

10 and 20 minutes of applied current (5.0 mA cm-2). A redox process occurs on the cathode

296

and its intensity increases as the degradation proceeds. It is possible that MeP oxidation

297

products, formed at the solution bulk, are reduced on the cathode. When phenolic compounds

298

and their oxidized counterparts are present in the same solution, they can undergo

299

polymerization reactions [71,72]. Therefore, it is possible that the yellow solid observed is a

300

polymerization product, most likely oligomers, of some intermediates of MeP degradation.

301

Also, in this case, the relatively high molecular weight of those oligomers would cause their

302

insolubility in an aqueous medium. In order to diminish this effect studies can be carried out

303

in a divided cell, preventing that the oxidation product of MeP reaches the cathodic region.

304

However, it is important to consider that for some wastewater types divided cells present

305

lower efficiency due to mass transfer problems.

13

306

As mentioned before, pH has an important effect on the active chlorine species and, thus, on

307

the oxidant power of the medium. In this work, when a non-buffered solution was used, it

308

was observed an increase of the solution pH (from 3 to 8 after 4 h), which indicates

309

alkalinizing and loss of oxidant power (because the ratio [ClO-]/[HClO] increases with

310

increasing pH, and ClO- is a weaker oxidizing agent than HClO). Considering that, several

311

experiments were carried out using a buffer (BMCAc) to maintain the pH and the ratio [ClO-

312

]/[HClO] constant during the process. Solution of MeP (100 mg L-1) + NaCl (0.15 mol L-1)

313

was treated by electrochemical process for 2 h and compared to the degradation of the

314

buffered solution (monochloroacetic acid/chloroacetate (0.1 mol L-1) + MeP + NaCl).

315

Initially, the monochloroacetic acid (MCAA) was treated in NaCl medium in order to

316

observe the efficacy of electrochemical process, with DSA®-Cl2, to remove it from water. It

317

can be observed by the inset of Fig. 5a that this process is not efficient to remove MCAA

318

under the studied conditions, resulting in a small variation of 5% on its concentration. After

319

80 min there is a slight increase on the concentration, which can be attributed to intrinsic

320

errors in the method. Therefore, this acid is considered stable and feasible to be used as

321

buffer solution (BMCAc) on MeP degradation, since it will not compete with MeP oxidation

322

under the conditions used.

323

Fig. 5a shows the removal of MeP in non-buffered (NaCl) and buffered (NaCl + BMCAc)

324

media. No difference was observed for the removal of MeP, achieving complete its removal

325

after 80 min of treatment. This means that pH does not affect MeP oxidation. However, in

326

Fig. 5b it is possible to observe that TOC removal is favored by the buffered medium, as well

327

as the values of MCE and the EC. Hence, the removal of byproducts is affected by pH,

328

presenting better efficiency at acidic conditions. Several electro-oxidation studies [18–23]

329

have shown that degradation of organic compounds are facilitated by acidic and oxidant

330

conditions, because reaction steps, as decarboxylation and aromatic ring opening, require

14

331

such conditions. As a result of the constant ratio [ClO-]/[HClO] during the electrolysis, it was

332

observed a improve of 40% in TOCR and of 67% in MCE, with almost half of the energy

333

consumed, because alkalinizing and loss of oxidizing power did not take place.

334

Furthermore, regarding the solid product formation it was observed that in the presence of

335

BMCAc the final solution was more clear than for the non-buffered solution (see Fig. S3) and

336

did not present formation of foam. Also, in comparison with Fig. S2, it can be seen that the

337

aspect of the final solution using BMCA and 0.45 A h L-1 is similar to that obtained with non-

338

buffered medium and 3.63 A h L-1, which is 8 times more applied charge. It is interesting to

339

note that during the experiments in buffered medium, the solution turbidity increased in a

340

first moment followed by its decrease. This effect indicates that the solid product was formed

341

and degraded by the electrochemical process and it reinforces that the buffered medium

342

improves the byproducts oxidation.

343 344

3.3.2 Mechanistical proposal

345

A 4 h electrolysis using japp = 15 mA cm-2 was performed in order to achieve higher organic

346

matter removal. MeP was completely removed after 40 min of treatment; however, the

347

mineralization was not improved. A plateau at 36% of TOCR is reached after the first hour of

348

electrolysis. As a result, low MCE (11%) and high EC for TOC removal (0.64 kW h gTOC-1)

349

were observed.

350

During this electrolysis, samples for liquid chromatography coupled to mass spectrometry

351

(LC-MS) were collected every hour for the investigation of the mechanism of MeP electro-

352

oxidation on DSA®-Cl2. Only MeP was detected in the initial solution (t = 0 h) and no

353

compound was detected for t = 4 h, which indicates that aromatic substances were completely

354

removed after that time. Six main intermediates were detected (those with more intense peaks

355

in the mass spectra) and identified (Fig. 6). The compounds named as 1 to 4 were identified

15

356

from the mass spectrum for t = 1 h. After 2 h of electrolysis, the compound 4 remains in

357

solution and a new compound, 5, was detected. For t = 3 h, only 4 was detected and

358

completely removed within the next hour.

359

Compounds 1 to 4 (m/z = 267, 221, 301 and 255, respectively) have higher molecular weight

360

than MeP (152.15 u), which is only possible if MeP molecules react toward other species by

361

addition or similar reactions. By a retrosynthesis approach, it is possible to suggest that

362

compounds 1 and 2 are formed from compounds A and B, after a sequence of hydrolysis,

363

hydroxylation and decarboxylation steps (see Fig. S3a - S3b). Compounds A and B were not

364

detected, however, retrosynthetic analysis shows that A may be formed by the oxidative

365

coupling of two molecules of C, whereas B may be formed by the oxidative coupling of C

366

and D. Those compounds derive from E and F, respectively, which correspond to hydrolysis

367

and oxidation products of MeP and are very likely to be formed in this medium.

368

It is known that oxidizing conditions, in a wide range of pH values, may lead to oligo- and

369

even polymerization of phenolic compounds [73,74]. Dimers, trimers and tetramers may be

370

the main products in mildly acid and mildly oxidizing conditions [75]. As mentioned before,

371

in this work, when a non-buffered solution was used, it was observed an increase of the

372

solution pH (from 3 to 8 after 4 h), which indicates both alkalinizing and loss of oxidant

373

power. Therefore, oxidative coupling steps involving compounds C and D are likely to

374

happen in the studied conditions.

375

A similar retrosynthesis logic can be used for compounds 3 and 4 (see Fig. S3c - S3d). It can

376

be suggested that compound G is as a common precursor, which is possibly formed by the

377

oxidative coupling of H and C. Compound H can be formed by chlorination of D or derive

378

from I, which is a chlorination product of F. Finally, it is suggested that compound 5 derives

379

from compounds 1 and/or 2, after hydrolysis and decarboxylation steps (see Fig. S3e).

16

380

Compounds C, D, E, F, and other similar products, were detected by Rosales et al. [28] for

381

MeP degradation using BDD anode and an iron-carbonaceous cathode for electro-Fenton

382

process. Steter et al. [18,20] have also detected compounds D, F, and other similar products,

383

during the first stages of MeP oxidation, on BDD anode, in sulfate and chloride media.

384

Moreover, compounds similar to H and I were also found by Steter et al. [20] in their study

385

on the electro-oxidation of MeP on different anodes and different media. However, oxidative

386

coupling products were not observed by those authors [18,20,28]. These studies report that

387

MeP degradation mechanism follows few decarboxylation steps until the formation of

388

hydroquinone and the further rupture of the aromatic ring. In the presence of chloride, a

389

similar mechanism was observed, though with formation of several polychlorinated phenols

390

and chlorinated aliphatic carboxylic acids [20]. On the other hand, Frontistis et al. [15,52]

391

observed the formation of oxidative coupling products on the treatments of ethyl paraben by

392

electrochemical process with a BDD anode and by heat-activated persulfate oxidation. In

393

both studies, it is reported the formation of several oligomers either using persulfate or

394

chloride media. The oligomers were identified by LC-TOF-MS as dimers and trimers of ethyl

395

paraben and its chlorination products.

396

According to the intermediates identified by MS, the retrosynthetic analysis and the

397

literature, a possible mechanistic route for MeP oxidation is proposed as shown in Fig. 6.

398

First, MeP originates its precursor, 4-hydroxybenzoic acid (F) by nucleophilic addition and

399

releasing of a -OCH3 group. F can undergo addition of a -OH or a -Cl group, forming

400

compounds E and I. Those three compounds combine and form more complex molecules,

401

which happen by oxidative coupling at the position 2 of the aromatic ring, and three main

402

pathways are suggested:

403

• Two molecules of C react and form intermediate A, which further reacts to form product 1

404

(m/z = 267);

17

405 406 407 408

• C and D react and form intermediate B, which further reacts to form product 2 (m/z = 221); • H and C react and form intermediate E, which further reacts to form products 3 (m/z = 301) and 4 (m/z = 255).

409

Product 1 is likely formed by hydrolysis of intermediate A, whereas 3 are likely formed by

410

hydroxylation and hydrolysis of G. Products 2 and 4 are possibly formed from intermediates

411

B and G, respectively, by hydrolysis, aromatic ring opening, decarboxylation and cyclizing

412

steps. Product 5 is formed from products 1 and 2 by similar reaction pathways. Therefore,

413

products 3, 4 and 5 are the main last aromatic compounds that remain in the medium before

414

all aromatic rings are open and produce aliphatic structures.

415

It is worth noting that in the present study only aromatic compounds were determined by LC-

416

MS. Compounds 1 to 5 are expected to be relatively stable, even in the presence of oxidants,

417

because of the conjugation between double bonds and non-bonding electron pairs. This is one

418

reason why those compounds would last long enough to be isolated and detected. Even

419

though, all aromatic compounds were completed removed after 4 h of electrochemical

420

treatment. After the opening of aromatic rings, aliphatic compounds are expected to be

421

formed (including chlorinated carboxylic acids) [18,20,64,76,77]. In fact, aromatic pollutants

422

always follow the formation of aromatic intermediates (which can couple and form polymer)

423

and the ring opening to form carboxylic acids [41,44,47,64,78,79], which in case of using

424

active electrodes are more difficult to be removed than aromatic and they accumulate in the

425

electrolyte [21,54]. The nature of the carboxylic acids formed is very well known since the

426

turn of the century, consisting mainly in oxalic acid, which is very slowly mineralized.

427

Despite the mechanistic complications, it was observed that part of the aliphatic byproducts

428

was oxidized and mineralized to CO2, H2O e Cl−, since 36% of the TOC was removed from

429

the medium.

18

430 431

4

Conclusions

432 433

From the results presented, the following conclusions can be drawn:

434

• Methyl paraben can be removed from the aqueous medium containing chloride anion by

435

the electrochemical process using the DSA®-Cl2, mainly by mediated oxidation.

436

• Irradiation of UV light facilitates the removal of byproducts, but not of MeP itself. Thus,

437

the synergism was better for higher japp values. Mineralization can be increased by 40%

438

with low additional energy costs.

439

• The degradation process resulted in the formation of a solid byproduct, which probably

440

derives from the oxidation of MeP products and is of polymeric nature. However, the

441

electrolysis itself seems to be effective to remove this product.

442

• MeP oxidation initially happens through the chlorination and hydroxylation of 4-

443

hydroxybenzoic acid. Those compounds undergo oxidative coupling reactions to form

444

higher molecular weight products before the aromatic ring breakdown. Thereafter, several

445

aliphatic acids (including their chlorination products) are expected to be formed as the last

446

byproducts prior to mineralization.

447 448

Acknowledgements

449 450

Financial support from the Brazilian Federal Agency for the Support and Improvement of

451

Higher Education (CAPES), São Paulo Research Foundation (FAPESP, Brazil) [2016/19662-

452

1, 2016/04825-2, 2017/10118-0 and 2017/20444-1], National Council for Scientific and

453

Technological Development (CNPq, Brazil) [140669/2014-0] and from the Spanish Ministry

454

of Economy, Industry and Competitiveness, European Union through project CTM2016-

19

455

76197-R (AEI/FEDER, UE) are gratefully acknowledged. Authors are also thankful to Prof.

456

Antonio C. B. Burtoloso for the discussion about the final mechanistic proposal.

457 458 459

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28

721

Table 1 Kinetic constant, MeP and TOC removals, and mineralization current efficiency for the

722

single electrochemical process (E) at different current densities. DSA®-Cl2 in NaCl 0.15 mol L-1 at 25

723

°C

MePR (%)

TOCR (%)

MCE (%)

1.0

kE (10 min-1) 2.5

29 (120 min)

3.0

28

2.5

12

76 (120 min)

17

59

5.0

35*

100 (80 min)

29

50

10.0

**

100 (40 min)

27

24

japp (mA cm-2)

-3

724

* Value calculated for the first kinetic region (k1); k2 was not determined due to the lack of values measured.

725

** Kinetic constant could not be determined due to the fast MeP abatement.

726 727

Table 2 Kinetic constant, MeP and TOC removals, and mineralization current efficiency for

728

electrochemical process with UV irradiation (E-P). DSA®-Cl2 in NaCl 0.15 mol L-1 at 25 °C; UVC

729

lamp (λ = 254 nm, 4W)

MePR (%)

TOCR (%)

MCE (%)

1.0

kE-P (10 min-1) 2.9

31

4.4

42

2.5

12

78

16

61

5.0

33*

100

38

71

10.0

**

100

42

38

japp (mA cm-2)

-3

730

* Value calculated for the first kinetic region (k1); k2 was not determined due to the lack of values measured.

731

** Kinetic constant could not be determined due to the fast MeP abatement.

732

29

733

Figure 1 Electrochemical removal of MeP (100 mg L-1) at different current densities as a function of

734

the instant applied charge. japp = (
) 1.0, ( ) 2.5, (∆) 5.0, (◆) 10 mA cm-2. DSA®-Cl2 in NaCl 0.15

735

mol L-1 at 25 °C.

736

30

737

Figure 2 Kinetic of MeP (100 mg L-1) removal by electrochemical processes (black symbols) single

738

and (white symbols) with UV irradiation. japp = (●) 1.0, ( ) 2.5, (▲) 5.0 mA cm-2. Inset: Removal of

739

MeP (100 mg L-1) by photolysis in chloride medium. DSA®-Cl2 in NaCl 0.15 mol L-1 at 25 °C; UVC

740

lamp (λ = 254 nm, 4W).

741

742

31

743

Figure 3 Energy consumption of the electrochemical processes (black bars) single and (gray bars)

744

with UV irradiation, and ( ) synergistic index of the coupled process. DSA®-Cl2, [MeP] = 100 mg L-1

745

in NaCl 0.15 mol L-1 at 25 °C; UVC lamp (λ = 254 nm, 4W).

746

747 748

32

749

Figure 4 (a) IR spectrum of the yellow, solid byproduct, obtained in KBr pellet. (b) Cyclic

750

voltammograms of the Ti cathode in 0.15 mol L-1 NaCl + 100 mg L-1 MeP (100 mg L-1) solution, scan

751

rate: 50 mV s-1, after 0, 10 and 20 min of degradation (japp = 5.0 mA cm-2).

752

753

33

754

Figure 5 Comparison of MeP degradation by electrochemical process in non-buffered (NaCl 0.15 mol

755

L-1) and in buffered (NaCl 0.15 mol L-1 + BMCAc 0.1 mol L-1) media with japp = 2.5 mA cm-2 at 25 ºC.

756

(a) Removal of MeP (black symbols) without and (white symbols) with BMCAc; (b) removal of TOC,

757

MCE and EC obtained (black bars) without and (gray bars) with BMCAc. Inset: Removal of

758

monochloroacetic acid (0.1 mol L-1) by electrochemical process in NaCl medium (0.15 mol L-1).

759

760 761 762

34

763

Figure 6 Mechanistic proposal for the electrochemical degradation of MeP (100 mg L-1, 4 h

764

treatment) on DSA®-Cl2. japp = 15 mA cm-2, NaCl 0.15 mol L-1, 25 ºC.

765

766

35

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: