Degradation of indigo carmine in water induced by non-thermal plasma, ozone and hydrogen peroxide: A comparative study and by-product identification

Journal Pre-proof Degradation of indigo carmine in water induced by non-thermal plasma, ozone and hydrogen peroxide: A comparative study and by-product identification Anna Paula Safenraider Crema, Lucas Diamantaras Piazza Borges, Gustavo Amadeu Micke, Nito Angelo Debacher PII:

S0045-6535(19)32742-0

DOI:

https://doi.org/10.1016/j.chemosphere.2019.125502

Reference:

CHEM 125502

To appear in:

ECSN

Received Date: 30 July 2019 Revised Date:

1 November 2019

Accepted Date: 27 November 2019

Please cite this article as: Crema, A.P.S., Piazza Borges, L.D., Micke, G.A., Debacher, N.A., Degradation of indigo carmine in water induced by non-thermal plasma, ozone and hydrogen peroxide: A comparative study and by-product identification, Chemosphere (2020), doi: https://doi.org/10.1016/ j.chemosphere.2019.125502. 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. © 2019 Published by Elsevier Ltd.

1

Degradation of indigo carmine in water induced by non-thermal

2

plasma, ozone and hydrogen peroxide: a comparative study and by-

3

product identification

4 5

Anna Paula Safenraider Crema1, Lucas Diamantaras Piazza Borges1, Gustavo

6

Amadeu Micke1 Nito Angelo Debacher1

7

1. Chemistry Department University Federal of Santa Catarina, Brazil

8

E-mail address. [email protected]

9 10

Highlights

11 12

•

The species OH• and NO• formed with the NTP discharge were identified

13

•

Degradation rate as follows: O2-NTP > O3 >, N2-NTP > H2O2

14

•

Post-discharge effect was significant for N2-NTP

15

•

IC degradation by-products identified were similar for O2-NTP, O3 and

16

N2-NTP

17 18 19 20 21 22 23 24 25 1

26

Abstract

27 28 The non-thermal plasma (NTP) technique is an advanced oxidation technology 29

(AOT) applied to the degradation of organic compounds in water. In this study,

30

the degradation kinetics of indigo carmine was investigated systematically,

31

applying N2-NTP, O2-NTP, ozonolysis and hydrogen peroxide and the results

32

were compared. The transient species (OH, O and NO radicals) formed with the

33

NTP discharge at the gas-liquid interface and their products (NO3−, NO2−, H2O2)

34

stabilized by the water, were identified and quantified. These species contribute

35

to the effects on the chemical characteristics of the water, such as a decrease in

36

the pH and increase in the conductivity and redox potential. Additionally, the

37

stabilization of the oxidative species was estimated from the degradation

38

reactions induced by the post-discharge effect, which was significant in the case

39

of N2-NTP, due to the presence of long-lived species, such as nitrite and nitrate.

40

The kinetics study revealed first-order kinetics for IC color removal and the rate

41

constant values followed the order: O2-NTP (3.0x10-1 min-1) > O3 (1.4x10-1 min-1)

42

> N2-NTP (2.2x10-2 min-1) > H2O2 (negligible). Also the main by-products of N2-

43

NTP, O2-NTP and ozonolysis degradation reaction were identified by ultra-fast

44

liquid chromatography coupled with mass spectrometry. The route fragmentation

45

showed the formation of indole intermediates, such as isatin, which is an

46

important precursor in organic synthesis.

47 48 KEYWORDS: non-thermal plasma, plasma activated water, ozonolysis, Indigo 49

carmine, color removal, by-products.

50

2

51

Introduction

52 53

Dye and pigments, such as indigo carmine, are used in several sectors

54

including the food, pharmaceutical and textile industries. According to Wang et

55

al. (2015), 33 million kg of indigo dyes are consumed annually, and this amount

56

is continually increasing. This extensive use has led to environmental

57

contamination, causing serious damage to the health human and aquatic

58

organisms (Othaman et al., 2012).

59

Due to their chemical structural stability, these compounds are refractory

60

and resistant to traditional treatment procedures, such as physicochemical and

61

biological methods. These methods also have limited application due to

62

drawbacks including sludge generation and a phase change of the pollutants,

63

leading to secondary pollution (Meiqiang et al., 2012). Given that most dyes are

64

not biodegradable, biological methods are not applicable (Subrahmayam et al.,

65

2013; Dhiraj et al., 2010).

66

The use of advanced oxidation technologies AOTs is an attractive

67

approach to wastewater treatment, owing to the in-situ formation of the hydroxyl

68

radical (OH●), an excellent oxidant (E° = 2.85 V) able to degra ded hazardous

69

compounds in water (Benetolli et al., 2011; Jiang et al., 2014). Hence, AOTs,

70

such as ozonolysis (Wang et al., 2015; Hashim et al., 2016), photocatalysis

71

(TiO2/UV photo-Fenton reaction) (Hu et al., 2019; Ahmed et al., 2010),

72

photochemistry (e.g., UV/O3, UV/H2O2; Vauthey et al., 2016; Kanakaraju et al.,

73

2018) and, more recently, non-thermal plasma NTP (Krishna et al., 2016,

74

Magureanu et al., 2015), offer several alternatives for water treatment (Hao et

75

al., 2017) .

3

76

In this context, NTP is among the most notable ATOs, considering the

77

amount of reactive species formed, in addition to the physical events that

78

characterize it. NTP is an ionized gas, and the gas used to produce the NTP

79

governs the identity and characteristics of the reactive species formed

80

(Fridman, 2008).

81

During plasma discharge at the gas–liquid interface, several chemical

82

and physical processes occur. For instance, active species, such as hydrogen

83

peroxide (H2O2), ozone (O3) and hydroxyl radicals (OH•) (Guo, et al., 2019), are

84

produced. Also the physical effects of ultraviolet light, obtained during the

85

process, improve the pollutant removal (Wang et al., 2015; Chauvin et al.,

86

2017). Consequently, with the application of this process, the initial

87

characteristics of the liquid, such as pH, conductivity, oxy-reduction potential

88

(ORP), are altered during the treatment.

89

NTP provides a promising alternative for the rapid and environmentally-

90

friendly degradation of dyes, such as indigo carmine (IC), since there is no need

91

to add a catalyst or other chemicals, and the process adheres to the principles

92

of green chemistry (Jiang et al., 2014).

93

In this study, a comparative investigation of the degradation of IC

94

induced by NTP, ozone and hydrogen peroxide was performed. The formation

95

of some reactive species was accompanied in the gas phase and liquid

96

medium. Also, the stability of some species was elucidated by examining the

97

post-discharge effect. Lastly, a detailed characterization of the by-products of IC

98

degradation via NTP and the ozonolysis reaction was obtained.

99 100

4

101

2 Experimental

102 103

2.1

Materials

104 Indigo

105

carmine

(IC),

5.5′-indigodisulfonic

acid

sodium

salt

106

(C16H8N2Na2O8S2), of analytical grade, was used as the model pollutant for the

107

degradation study. The solutions were prepared using deionized water obtained

108

from a Millipore system. Nitrogen and oxygen gases of commercial grade

109

(99.5%) were purchased from White Martins.

110 111

2.2

NTP reactor

112 113

The NTP reactor used in this study was of the cylindrical type (Fig. 1),

114

with a point-to-plate gas discharge (5 mm) over the water phase, operated at

115

atmospheric pressure at 20 oC (Benetolli et al., 2012). The working plasma

116

gases were O2 and N2, with a flow rate of 0.5 L min-1, and the electrical

117

parameters applied in all experiments were a DC pulsed power supply,

118

frequency pulse of 100 Hz and energy pulse of 50 mJ. All experiments were

119

performed at an initial pH of 5.8 and conductivity of 1.0 µS cm−1 using a volume

120

of 100 mL of IC solution (20 mg L−1). The temperature, conductivity and pH

121

were controlled during the experiments.

5

122 123

Figure 1. Schematic drawing of the non-thermal plasma reactor setup.

124 125

2.3

Sampling

126 127

A solution volume of 100 mL was used in the NTP reactor. Aliquots of 2

128

mL were withdrawn from the NTP reactor at time zero (no plasma action) and

129

after 1, 3, 5, 7, 10, 15, 20, 25, 30, 40, 50 and 60 min of NTP discharge. After the

130

analysis, the aliquot was returned to the reactor. All experimental sampling

131

procedures were the same unless otherwise specified. The IC concentration

132

was determined according to the Beer-Lambert law at the maximum

133

absorbance (λmax = 610 nm) using a UV-Vis spectrophotometer (HP, model

134

8452A) with a spectral range of 190 to 820 nm.

135 136

2.4

Ozonolysis 6

137 138

The ozone was produced in an O2-NTP reactor and injected directly into

139

the aqueous solution of IC (20 mg L-1). The color removal was monitored for 60

140

min by UV-Vis analysis at a wavelength of 610 nm to observe the decay in the

141

IC absorbance.

142 143

2.5

Optical emission spectrometry analysis

144 145

The optical emission spectra to identify the ionized and radical species

146

originating from the NTP reactor were obtained at the gas/water interface using

147

an Ocean Optics emission spectrometer, operating in the wavelength range of

148

177 nm to 900 nm. The optical fiber of the spectrometer was fixed to the NTP

149

quartz reactor. The ionized species were analyzed by adding 100 mL of the IC

150

solution to the NTP reactor and observing all of the optical emission signals

151

obtained for the N2 gas, O2 gas and water vapor at the applied frequencies (60,

152

100, 200, 300 and 500 Hz).

153 154

2.6

Chemical analysis

155 156

2.6.1 Hydrogen peroxide

157 158

The amount of hydrogen peroxide produced during the NTP treatment

159

was determined using the vanadate method (Benetolli et al., 2012) with UV-Vis

160

spectroscopy analysis, at a wavelength of 254 nm. A volume of 100 mL of

161

distilled water was added to the NTP reactor. Subsequently, 2 mL aliquots were

7

162

withdrawn from the reactor at different times as described above and 1 mL of

163

vanadate was added prior to the UV-Vis spectroscopy analysis.

164 165

2.6.2 Nitrite and nitrate

166 167

The formation of nitrite and nitrate in the water treated with N2-NTP was

168

monitored by capillary electrophoresis (CE) analysis, using a Hewlett Packard

169

(HP) system. The capillary had a diameter of 75 µm and total length of 32 cm

170

and it was conditioned with a 1 mol L-1 NaOH solution. The choice of electrolyte

171

was based on previous simulations using the free software Peakmaster. The

172

electrolyte used was β - alanine (20 mM) at pH 4.20, adjusted with 1 mol L-1

173

HCl, and BrO3- was used as the internal standard. The analysis was performed

174

using UV absorption at the wavelength of 210 nm.

175 176

2.7

Post-discharge effect

177 178

A volume of 100 mL of the IC solution (20 mg L-1) was added to the NTP

179

reactor. The aliquots taken at predetermined times were stored in test tubes and

180

left to stand for 24 h prior to UV-Vis analysis at a wavelength of 610 nm.

181 182

2.8

Kinetics studies

183 184

The kinetics of the IC color removal were followed by UV-Vis

185

spectroscopy analysis at a wavelength of 610 nm. The rate constant for the IC

186

degradation was determined considering first-order kinetics, using Eq. 1:

8

187




 

= −

(1)

188

Where C0 is the initial concentration (mg L-1) at time zero, Ct is the

189

concentration at a given reaction time and k is the first-order rate constant (min-

190

1

).

191

2.9

Identification of by-products

192 193

The by-products formed in the treatments with O2-NTP, N2-NTP and the

194

ozonolysis reaction were identified after 30 min of treatment using an ultra-fast

195

liquid chromatograph (UFLC; Prominence Shimadzu, model 2020), coupled to a

196

high resolution mass spectrometer (Bruker micrOTOF-Q II).

197

The ionization source was an electrospray (electrospray ionization - ESI)

198

and the system was operated with negative ion polarity. A Perkin Elmer C18

199

column (250 x 2.0 mm) was used, with a particle diameter of 3 µm, temperature

200

of 27.6 °C and solvent mixture comprised of acetoni trile 60% and water 40%

201

(0.1% formic acid), in the isocratic mode of separation. The peaks were

202

detected at a wavelength of 260 nm.

203 204

3.

Results and discussion

3.1

Optical emission

205 206 207 208

Figure 2 shows the emission spectra for the reactive species formed by

209

the application of the NTP discharge over the IC solution, at the gas/liquid

210

interface. 9

211 212

Figure 2. Optical emission spectra. (A) O2-NTP and (B) N2-NTP applying

213

different frequency values.

214 215

Figure 2 (A) shows the optical emission signals identified that are typical

216

of reactive oxygen species (ROS). The signal for OH• (A–X) can be observed at

217

310.6 nm and the signals for atomic oxygen O at 777.7 nm and 844.0 nm are

218

related to the transition states of 5s to 5p and 3s to 3p, respectively. The signal

219

at 656.3 nm is associated with the Balmer α line of hydrogen.

220

Figure 2 (B) shows the signals typical of reactive nitrogen species (RNS).

221

The signals in the region of 320 nm to 350 nm are related to N2(C–B) species

222

and that at 379.8 nm is related to the N2+(B–X) system. The signals at 656.3 nm

223

and 485.1 nm are associated with the Balmer α and β lines of hydrogen,

224

respectively, while that at 589.0 nm is related to sodium from the IC salt. The

225

insert in Fig. 2 (B) shows signals at 297 nm, related to NO●, and 310 nm,

226

related to the OH● from the water.

227

Therefore, the species identified in Fig. 2, produced at the NTP gas-liquid

228

interface, are the primary species (Eq. 2-9) that migrate by diffusion to the liquid

229

phase and they are stabilized by the water (Lukes et al., 2014, Cadorin et al.,

230

2015) forming the secondary species. Equations 10-21 show the secondary 10

231

species produced in the bulk solution, which can induce the chemical

232

degradation reactions of organic compounds in aqueous media.

233 234

H2O + e- → HO•+ H•

(2)

235

O2 + e- → 2O

(3)

236

O2 + O→ O3

(4)

237

N2 + e- → N•+ N•

(5)

238

O + N2 → NO•+ N•

(6)

239

NO• + O• → NO2

(7)

240

N• + O2 → NO• + O

(8)

241

NO• + N• → N2 + O

(9)

242

H2O + e- → H2O+ + 2e-

(10)

243

H2O+ + H2O → H3O+ +OH●

(11)

244

OH● + OH●

(12)

245

2O3 + H2O2 → 3O2 + HO•

(13)

246

O3 + HO2•- → O2•- + HO• + O2

(14)

247

2NO• + O2 → 2NO2

(15)

248

2NO2– + 2H+

NO• + NO2 + H2O

(16)

249

2NO2 + H2O

2NO2– + NO3– + 2H+

(17)

250

NO2– + H2O2 + H3O+ → ONOOH + 2H2O

251

ONOOH

252

ONOOH → HNO3

253

ONOOH + H2O

H2O2

NO2 + HO•

(18) (19) (20)

ONOO– + H3O+

(21)

254

11

255

The impact of the high-energy electrons resulting from the application of

256

the NTP on the surface of the water induces hydrolysis, ionization by electron

257

impact, as shown in Eq. 2 (OH•) and Eq. 10 (H2O+), the recombination reaction

258

seen in Eq. 12 (H2O2), and the appearance of other species, such as those

259

observed in Eq. 4 (O3) and Eqs. 18-21 (HNOx) (Bruggeman et al., 2016).

260

The diffusion of the species produced by NTP at the interface obeys

261

Henry's law (M atm-1), which is related to the equilibrium between species in the

262

gas phase and in the liquid phase at the interface. The H2O2 or HNOx species

263

have a high Henry coefficient of around 105 M/atm, and easily diffuse to the

264

liquid medium, while short-lived transient species, such as OH•, O2•– and HO2•,

265

are rapidly converted into H2O2 (Eq. 12). Gaseous species, such as O2, O3 and

266

NO•, have low Henry coefficients (10-3; 10-2 and 2×10-3 M atm-1, respectively)

267

and less easily diffuse into the liquid.

268

In fact, the diffusion of ozone in water is quite slow due to its

269

hydrophobicity (Bruggeman et al., 2016). However, the rate of transfer may be

270

accelerated if the ozone reacts with other species present in the liquid medium

271

and, in this case, the consumption of ozone (Eq. 13, 14) in the liquid medium

272

increases and the rate of diffusion is enhanced.

273

Furthermore, reactive oxygen and nitrogen species (RONS) interact

274

strongly with the liquid medium, forming acid intermediates, such as nitrite and

275

nitrate (Eqs. 16, 17) (Sun et al., 2017), and changing the oxidation-reduction

276

potential (ORP), pH and conductivity of the water.

277 278 279

12

280

3.2

Measurement of pH, conductivity and redox potential

281 282

Figure 3 shows the effect of 60 min of NTP discharge on the water, with

283

regard to pH, conductivity and oxidation reduction potential (ORP). The O2-NTP

284

(Fig. 3A) decreased the pH from 5.7 to 4.2, increased the conductivity from 2.0

285

to 74.3 µS cm-1 and increased the ORP from zero to 400 mV. The results

286

observed for N2-NTP (Fig. 3B) were pH 3.5, conductivity 95.2 µS cm-1 and ORP

287

530 mV, which are higher compared to those obtained for O2-NTP.

288 289

Figure 3. Effect of 60 min of NTP discharge on the pH, conductivity and ORP of

290

the water. (A) O2-NTP and (B) N2-NTP.

291 292

Figure 3 (A) shows that the ORP species produced by O2-NTP increased up to

293

80%, showing a plateau at approximately 10 min, which continuously grew

294

slowly. Similar behavior can be seen for N2-NTP in Fig. 3 (B), reaching 80% of

295

the ORP in around 20 min and continuously increasing slowly during the plasma

296

treatment.

297

Equations 15 to 17 show the formation of nitrite and nitrate in the water

298

medium with the application of NTP, decreasing the pH and increasing the

13

299

conductivity. Equation 18 shows the formation of peroxynitrous acid and its

300

isomerization to HNO3 (Eq. 20) or decomposition to NO2 and OH• (Eq. 19).

301

The ORP measurement is related to the oxidation potential of the main

302

species formed with the application of NTP to the water environment and these

303

include H2O2 (E0 = 1.77 V), OH (E0 = 2.85 V), O3 (E0 = 2.07 V), NO2- (E0 = 1.10

304

V), NO3- (E0 = 0.98 V), HONOO (E0 = 2.04 V) and ONOO-, (E0 = 2.44 V)

305

(Chauvin et al., 2016; Lukes et al., 2014). These species increase the ORP of

306

the reaction medium during the NTP treatment.

307 308

3.3

Nitrite, nitrate and hydrogen peroxide profiles

309 310

The NO3– and NO2– (Eq. 16 and 17) were determined and quantified in

311

solution using the CE technique and H2O2 (Eq. 12) by UV–Vis spectroscopy

312

analysis. Figure 4 shows the profiles for the NO2–, NO3– and H2O2 formation and

313

consumption during the N2-NTP treatment.

314 315

Figure 4. Profiles showing the formation and consumption of (A) nitrate, nitrite

316

and (B) hydrogen peroxide in the aqueous medium during N2-NTP treatment.

317

14

318

The ratio between the NO2– and NO3– ions in solution was 1:10, the

319

maximum nitrate concentration was 0.8 mM after 15 min of N2-NTP treatment.

320

Nitrite, which is unstable in acid medium (Eq. 16, 17) is produced and quickly

321

consumed due to NO2– formation (Lukes et al., 2014; Cadorin et al., 2015).

322

The profile for H2O2 (Fig. 4 B), produced mainly by the OH• recombination

323

reaction (Eq. 12), applying N2-NTP to the water, shows that the concentration

324

increased up to 1.6 x 10-5 M at around 10 min and then decreased, remaining

325

constant at around 1.0 x 10-6 M. This decrease in the H2O2 concentration is

326

related to the homolytic cleavage and the consumption of ozone (Eqs. 12 and

327

13, respectively). Both of these reactions produce OH•, and thus H2O2 is an

328

important source of OH•. In addition, several chemical reactions between H2O2

329

and other reactive species present in the N2-NTP environment generate

330

important products, such as HONOO acid and peroxynitrite. ONOOH, for

331

instance, is produced from NO2– and H2O2 (Eq. 18) (Lukes et al., 2014; Cadorin

332

et al., 2015).

333

The ONOOH and its conjugate base, ONOO-, are transients and

334

oxidative species. They are studied in relation to RNS, due to their high

335

oxidation potentials of 2.04 V and 2.44 V, respectively, Eq. 21 (Mc Leen et al.,

336

2015, Jorolan et al., 2015; Pfeiffer et al., 1997, Kovacevic, et al., 2018).

337

In fact, studies on the reaction between H2O2 and NO2– have been widely

338

reported in the literature and antibacterial activity has been observed as a post-

339

discharge effect (Lukes et al., 2014).

340

The main degradation routes of ONOOH involve deprotonation to form

341

ONOO- (Eq. 21) and, in an acidic environment, ONOOH can form NO2 (30%),

342

according to Eq. 19. In the isomerization reaction, NO3- is formed as the main 15

343

product (70%) as seen in Eq. 20 (Jorolan et al., 2015; Pfeiffer et al., 1997,

344

Kovacevic, et al., 2018).

345 346

3.4.

Oxidation Reaction

347 348

Figure 5 shows the UV–visible spectra for IC with absorption peaks at

349

285 nm in the UV and 610 nm in the visible region. The decrease in the

350

absorption peaks at 610 nm is related to the homolytic cleavage of IC, and the

351

increase in the absorption peaks at 240 nm is related to the formation of isatin

352

5-sulfonic acid with the O2-NTP discharge.

353 354

Figure 5. UV-visible spectra for IC with decreasing peaks related to color

355

removal during treatment using O2-NTP.

356 357 358

The degradation reaction was monitored by UFLC-MS analysis and Fig. 6 shows the by-products and the pathway of IC degradation.

16

359 360

Figure 6. The by-products of the IC degradation reaction using N2-NTP, O2-NTP

361

and O3.

362 363

The mass spectra showing the by-products identified for N2-NTP, O2-

364

NTP and O3, after 30 min of treatment, are shown in the supplementary material

365

(Fig. 1S). The by-products identified were similar for all three systems studied,

366

with a slight increase in the number of secondary species in the following order:

367

N2-NTP > O2-NTP > O3.

368

LC-MS analysis showed homolytic cleavage at the C=C bond of indigo

369

carmine (m/z 420), resulting in isatin 5-sulfonic acid (m/z 226) as the main

370

aromatic product. The main routes were then identified through the by-products

371

formed, which included m/z 209.98, m/z 197.98 (dehydration reaction), m/z

372

146.98 (desulfonation reaction) and m/z 241.97 (hydroxylation), leading to the

373

formation of important indole intermediates. 17

374

Hao et al., 2017 studied IC dye degradation by “saturated resin ectopic

375

regeneration by non-thermal dielectric barrier discharge plasma and the

376

fragmentation route observed from the obtained by-products of IC dye

377

degradation analysis was manly via OH• a radical. While, our IC dye

378

degradation reaction result showed also secondary pathway reactions such as

379

dehydration (m/z 197.98) and desulfonation (m/z 146.98), besides the action of

380

the OH• radicals. The byproducts obtained by Hao et al., 2017 are similar to

381

those obtained in this study.

382

The identification of indole intermediates formed during the oxidation of

383

the IC dye could lead to an alternative approach for the synthesis of these

384

important precursors used in the production of bioactive drugs, such as isatin

385

derivatives (Davidovich et al., 2014).

386

Isatin (1H-indole-2,3-diones) is an extremely versatile molecule, an

387

important precursor molecule in organic synthesis. Isatin derivatives have

388

diversified

389

anticonvulsant, antiviral, antimicrobial, anti-tubercular and antitumor activity,

390

among others (Davidovich et al., 2014).

drug

applications,

exhibiting

anti-inflammatory,

analgesic,

391 392

3.5

Color Removal Reaction

393 394

Figure 7 (A) shows a comparative study on the kinetic profile for IC

395

cleavage and the color removal reaction in water during 60 min of treatment

396

using four different approaches: N2-NTP, O2-NTP, the addition of commercial

397

H2O2 and the injection of O3 into the IC solution. Figure 7 (B) shows the

398

percentage color removal for N2-NTP, O2-NTP, and O3. 18

399

400 401

Figure 7. Kinetic profiles for (A) H2O2, N2 - NTP, O3 and O2 - NTP and (B) color

402

removal during IC reaction.

403 404

As can be seen from Fig. 7 A, when commercial hydrogen peroxide

405

without NTP discharge was added to the IC solution, the color removal

406

efficiency was very low after 60 min and the rate constant was negligible.

407

Although commercial H2O2 is unable to degrade the IC, when the H2O2 is

408

formed in the NTP environment, it can react directly or, more frequently,

409

indirectly in the degradation reactions, since the H2O2 provides a source of OH

410

radicals (Eqs. 12–13), the main reactive species in AOTs, besides promoting

411

the formation of peroxynitrite (Eq. 21), a stronger reactive species (Lukes et al.,

412

2014, Cadorin et al., 2015).

413

With the application of N2-NTP, the kinetic profile for the color removal

414

(Fig. 7 A) shows an induction period from 0 to 15 min, when the reaction is

415

slow, and then gradually the reaction speed increases. The induction time effect

416

is related to the formation of OH radicals in solution under N2-NTP. The OH

417

radical formation and consumption reaction takes around 15 min to reach the

418

steady state, as can be seen in Fig 4 B. H2O2 consumption and the induction

419

period indicate the formation of other reactive species, such as ONOOH (Eq. 19

420

18) and ONOO- (Eq. 21), which contribute significantly to the degradation

421

reaction in this NTP atmosphere (Cadorin et al., 2015).

422

The induction time is related to the diffusion of species from the gas

423

phase to the liquid phase, initiating the chemical reactions. After the diffusion of

424

the species, the rate of reaction will be dependent on the RNS and ROS

425

diffused in the liquid (Bruggmann et al., 2016).

426

This period was not observed for treatments with O2-NTP and ozone

427

injected into the water solution, (Fig. 7 A), because in these cases the main

428

reactive species is ozone. Although ozone has a low Henry's diffusion

429

coefficient (10-2 M atm-1), it reacts with the IC molecule and is rapidly consumed

430

in solution. Since the O3 rate of consumption in the liquid medium is high, the

431

gas liquid diffusion process is favored.

432

The results obtained in the study on the percentage of color removal

433

show that the treatments with O2-NTP and ozone injected into the aqueous

434

solution provided similar profiles, with 100% of color removal at around 10 min,

435

as seen in Fig. 7 B. This similarity is attributed to ozone being the main

436

oxidative species produced in both systems (Eqs. 3 and 4), that is, injected

437

directly into the water solution or produced in situ by O2-NTP.

438

The reaction rate was slightly higher for the O2-NTP (3.6 x 10-1 min-1)

439

than in the case when O3 was injected and no NTP discharge was used (2.0 x

440

10-1 min-1). Also, with the use of O2-NTP, radicals such as O and OH● are

441

produced. These strong oxidative species are stable in water, according to Eqs.

442

13 - 14, which aids the color removal reaction.

443 444

20

445

3.6

Post-discharge effect

446 447

Figure 8 and Table 1 show a comparison of the data used to obtain the

448

kinetic profiles for the IC color removal reaction applying N2-NTP and O2-NTP,

449

showing the direct application of the plasma and the post-discharge effect on

450

aqueous medium.

451 452

Figure 8. Post-discharge effect on the IC color removal from the aqueous

453

medium applying: (A) N2-NTP and (B) O2-NTP.

454 455

Table 1. Comparison of rate constants at 20 °C for the color removal reaction

456

applying: N2-NTP; N2-PDE; O2-NTP; O2-PDE IC solution

K (min-1)

t1/2

N2 - NTP

2.2 x 10-2

30.8 min

N2 – PDE

7.30 x 10-2

9.5 min

O2 – NTP

3.0 x 10-1

1.9 min

O2 – PDE

1.88 x 10-1

3.7 min

457 458

As seen in Fig. 8 (A), the application of the N2-NTP discharge before

459

adding the IC overcomes the induction time effect due to the oxidation species 21

460

produced and stabilized by the water. Also, an induction time is not observed

461

when O2-NTP is used, as shown in Fig. 8 B. Regarding the profile for color

462

removal against time, in Fig. 8 B and Table 1 the rate constants are similar, with

463

a slightly faster process for the color removal applying O2-NTP.

464

The experiments with plasma-activated water can aid an understand of

465

the NTP mechanism involved in the formation of species like OH●, O2●–, NO●,

466

since from these radicals more stable species are obtained, such as H2O2, O3.

467

The long-lived species in water are the most effective in color removal

468

reactions, because they remain in contact with the target molecule longer

469

(Parvuluscu et al., 2012; Hsieh et al., 2016).

470 471

3.7

Comparative study

472 473

The results of a comparative study of the color removal are shown in

474

Table 2, where the rate constants of the IC homolytic cleavage using different

475

advanced oxidative processes are reported.

476 477

Table 2. Comparative study of IC color removal from aqueous medium. Procedure Photocatalysis ZnO-Bi2O3-2C3N4/H2O2/Vis Micellar catalysis Surfactant/BAP

Electrocoagulation

Photocatalysis ZnFe2O4/ZnO + UV-Vis Photocatalysis

Reagents, organic solvents required and analysis method Índigo carmine, hydrogen peroxide, UV-Vis analysis. Índigo carmine, hydrogen peroxide, bicarbonate sodium, dodecyl sulphate, hexadecyl-pyridinium chloride monohydrate, Triton. LC–MS/UVVIS analysis. Índigo carmine, chloride, sodium. Zeta potencial Índigo carmine, zinc chloride, sodium hydroxide, sodium borohydride, Iron (III) chloride hexahydrate, Ethylene diamine tetra acetic acid disodium salt dihydrate, ethylene glycol. UV-Vis analysis. Índigo carmine, zinc chloride, sodium

% Color removal

Time (min)

Ref.

93.0

180

22

60.0

6-15

35

80.0

240

38

82.0

90

16

99.0

90

16

22

Tannin/ZnFe2O4/ZnO + UVVis

Electrochemical ACFF anode/50mM NaCl Bio - electrochemical ACFF MANAE – lcc anode/TW Electrochemical doped-Sb2O5 Ti/IrO2-SnO2 O2-NTP N2-NTP ozonolysis

hydroxide, sodium borohydride, Iron (III) chloride hexahydrate, Ethylene diamine tetra acetic acid disodium salt dihydrate, ethylene glycol and commercial tannin extract. UV-Vis analysis. Índigo carmine, monoaminoethyl-Naminoethyl, sodium periodate, ethylenediamine, buffer. UV-Vis analysis. Índigo carmine, ACFF anode, buffer, Lcc crude extract. UV-Vis analysis. Índigo carmine, ethyleneglycol sodium sulfate, sodium chloride, potassium iodide, ammonium heptamolybdate. Chronopotentiometry analysis. Indigo carmine, solvent free. UV-Vis analysis. Indigo carmine, solvent free. UV-Vis analysis. Indigo carmine, solvent free. UV-Vis analysis.

62.7

60

11

83.6

60

11

75.0

420

12

~100.0

10

This work

96.7

45

This work

97.3

10

This work

478 479

As can be seen from Table 2 all the techniques listed are useful to

480

remove the IC dye from wastewater. Although in some cases a catalyst is

481

needed, and several steps are involved before the final treatment are achieved.

482

Among others NTP applications gets its attention regarding to its high efficiency,

483

one step process and also easy coupling with hybrid degradation techniques

484

inducing synergistic effect (Guo et al, 2019).

485 486

4. Conclusions

487 488

The main reactive species produced with the application of N2-NTP and

489

O2-NTP discharge to water were identified. The physical-chemical analysis

490

shows an increase in the conductivity and ORP and a decrease in the pH, and

491

these changes were slightly enhanced in the case of N2-NTP.

492

The kinetic study revealed that the reaction rate was highest for O2-NTP,

493

followed by O3, N2-NTP and H2O2. In the rate constants of color removal with

494

N2-NTP an induction time was observed due to the time needed for the

23

495

production of the reactive nitrogen species in water. However, this effect was

496

reduced by using plasma-activated water and the induction time was not

497

observed when O2-NTP was used.

498

The main IC degradation by-products identified by UFLC-MS were similar

499

for N2-NTP, O2-NTP and O3, with a slight increase in the number of secondary

500

species in the following order: N2-NTP > O2-NTP > O3. The IC homolytic

501

cleavage by-product identified was sulfonate isatin followed by isatin (1H-indole-

502

2.3-diones).

503 504

Acknowledgements

505 506

The authors are grateful for technical support of Dr. Morgana Frena, Prof. Dr. T.

507

Maranhão and CEBIME. A.P.S. Crema gratefully acknowledges the support of

508

the Brazilian agency CNPq for the scholarship and CAPES for financial support.

509 510

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Declaration of Interest Statement

The manuscript entitled “Degradation of indigo carmine in water induced by non-thermal plasma, ozone and hydrogen peroxide: a comparative study and by-product identification ” by Anna P. S. Crema, Lucas Diamantaras, Gustavo A. Micke and Nito A. Debacher to be considered for publication as academic research article in CHEMOSPHERE.

We declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We wish to confirm that there are no known conflicts of interest associated with this publication. The research paper has been read and approved by all named authors for submission.

We hope you find our manuscript suitable for publication and look forward to hearing from you.

Sincerely yours,

Anna P. S. Crema