Undiscovered Mechanism for Pyrogenic Carbonaceous Matter (PCM

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Undiscovered Mechanism for Pyrogenic Carbonaceous Matter (PCM)-Mediated Abiotic Transformation of Azo dyes by Sulfide Han-Qing Zhao, Shi-Qi Huang, WENQING XU, YiRan Wang, Yi-Xuan Wang, Chuan-Shu He, and Yang Mu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 25, 2019

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Environmental Science & Technology

1

Undiscovered Mechanism for Pyrogenic Carbonaceous Matter (PCM)-Mediated

2

Abiotic Transformation of Azo dyes by Sulfide

3 4

Han-Qing Zhao1, Shi-Qi Huang1, Wen-Qing Xu2, Yi-Ran Wang1, Yi-Xuan Wang1,

5

Chuan-Shu He1, Yang Mu1,*

6 7 8

1CAS

Key Laboratory of Urban Pollutant Conversion, Collaborative Innovation

9

Centre of Suzhou Nano Science and Technology, Department of Applied Chemistry,

10

University of Science and Technology of China, Hefei, 230026, China

11 12

2Department

of Civil and Environmental Engineering, Villanova University, Villanova, Pennsylvania, 19085, United States

13 14 15 16 17 18

*Corresponding author:

19

Prof. Yang Mu, Fax: +86 551 63607907; E-mail: [email protected]

1

ACS Paragon Plus Environment


20

ABSTRACT

21

Pyrogenic carbonaceous matter (PCM) catalyzes the transformation of a range of

22

organic pollutants by sulfide in water; however, the mediation mechanisms are not

23

fully understood. In this study, we observed for the first time that the degradation of

24

azo dyes by sulfide initially underwent a lag phase followed by a fast degradation

25

phase. Interestingly, the presence of PCM only reduced the lag phase length of the

26

azo dye decolorization but did not significantly enhance the reaction rate in the fast

27

degradation phase. An analysis of the azo dye reduction and polysulfides formation

28

indicated that PCM facilitated the transformation of sulfide into polysulfides

29

including disulfide and trisulfide, resulting in fast azo dye reduction. Moreover, the

30

oxygen functional groups of the PCM, especially the quinones, may play an important

31

role in the transformation of sulfide into polysulfides by accelerating the electron

32

transfer. The results of this study provide a better understanding of the PCM-mediated

33

abiotic transformation of organic pollutants by sulfide in anaerobic aqueous

34

environments.

35

2


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Table of Contents (TOC) Art

37 38

3



39

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INTRODUCTION

40 41

Sediments are important sinks for hydrophobic organic contaminants such as

42

nitroaromatics, nitrogenous explosives, and azo dye discharged into surface waters.1-3

43

On the other hand, hydrogen sulfide occurs naturally in sediments because of the

44

microbial reduction of sulfate and it was reported that sulfide is present in sediment

45

pore waters at concentrations ranging from tens of micromolar to several millimolar.4

46

As a consequence, sulfide is able to act as both a nucleophile and a reductant in the

47

abiotic transformation of organic pollutants in sediments.3, 5, 6

48

Pyrogenic carbonaceous matter (PCM) includes engineered carbons (biochar,

49

activated carbon), environmental black carbon (fossil fuel soot, biomass char), and

50

related carbon nanomaterials (graphene, carbon nanotubes (CNTs))7 and constitutes

51

up to 10-30% of organic carbon in sediments.8 PCM may come in contact with

52

organic pollutants due to its widespread presence in the aquatic environment or its use

53

in various engineering applications.9-14 Previous studies have demonstrated that black

54

carbon could sequester hydrophobic contaminants in sediments.8,

55

recent studies showed that PCM and sulfide co-existing in sediments were able to

56

foster the degradation of many hydrophobic organic pollutants.7 Related studies

57

suggested that black carbon-mediated reactions occurred with a wide variety of

58

contaminant structures, including nitrated and halogenated aromatic compounds,

59

halogenated heterocyclic aromatic compounds, and halogenated alkanes.16,

60

example, CNTs were used as a catalyst for the reduction of nitroaromatics by sulfide

61

in water18 and black carbon facilitated the abiotic reduction of trifluralin and

62

pendimethalin by sulfides. Very recently, it was found that 99% of trichloroethylene

63

was converted to acetylene after 200 h using a sulfide reductant under the mediation 4


13, 15

Moreover,

17

For

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64

of nitrogen-doped carbon materials.17

65

The mechanisms responsible for the carbon material-mediated transformation of

66

organic pollutants are complex. One hypothesis is that surface oxygen functional

67

groups (especially quinone) promote the electron transfer between the sulfide and

68

contaminants sorbed to the carbon surfaces. For instance, Yu et al. indicated that the

69

reduction rate of nitrobenzene by sulfides was correlated with the oxygenated

70

functional group prevalence on the char surfaces.19 Additionally, Aeschbacher et al.

71

demonstrated that quinone moieties (likely occurring in black carbon) were key

72

participants in the transfer of electrons to/from humic substances.20 A related study

73

proposed that activated carbon fibers enhanced the transformation of reduction of

74

nitroaromatics by sulfides and the results indicated that quinone moieties in the

75

carbon materials accelerated the electron transfer from the sulfides to the target

76

contaminants.21 Another hypothesis is that the excellent electrical conductivity of the

77

graphitic regions in carbon materials promotes the electron transfer from sulfide to the

78

sorbed organic compounds.22 Oh and Chiu suggested that the conductive properties of

79

sheet graphite promoted the contaminant reduction by sulfides.23 Additionally,

80

regarding the degradation of the pollutant hexahydro-1,3,5-trinitro-1,3,5-triazine

81

(RDX), Xu et al. proposed that hydrogen sulfide was partially oxidized by carbon in a

82

reaction enhanced by the carbon’s electrical conductivity to form potent sulfur-based

83

nucleophiles on the carbon surface (detected by energy dispersive spectroscopy),

84

which were capable of degrading the RDX; but the specific sulfur-based nucleophiles

85

have not been identified.24 However, Ding et al. found that sulfide combined with

86

nitrogen-doped carbon materials could produce active intermediate C–S–S–H to

87

facilitate trichloroethylene dechlorination.17

88

In this study, we evaluated the reduction of azo dyes by sulfide in the presence 5



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89

and absence of PCM and discovered a new PCM-mediated reaction mechanism for

90

azo dye removal from water. As an important commercial chemical, azo dyes are

91

widely used in textile, leather, paper, and food industries and the toxicity of azo dyes

92

pose a potential threat to human, aquatic animals, and plants.25,

93

electron potential for the reduction of azo dyes to their constituent aromatic amines by

94

sulfide is -270 mV and the half-wave potential of azo dye reduction ranges between

95

-530 and -180 mV.27 In the absence of PCMs, the azo dye degradation by sulfide

96

suffered from a long lag phase prior to a fast removal phase (named fast degradation

97

phase). Interestingly, the presence of PCMs significantly shortened the lag phase but

98

the degradation rate of the azo dye (the fast degradation phase) remained the same.

99

Furthermore, based on the azo dyes degradation experiments and the polysulfides

100

formation analysis, it was found that PCM facilitated the transformation of sulfide

101

into polysulfides, a process that played an important role during the azo dye

102

degradation.

26

The standard

103 104 105

MATERIALS AND METHODS

106 107

Materials

108

Methyl orange (MO), meta-methyl red, (NH4)2Sn, sulfur, Na2S2O4, Na2S2O3,

109

Na2SO3, and Na2SO4 were obtained from Sinopharm Chemical Reagent Co., Ltd.

110

(China) and the Na2S, Orange I, ortho-methyl red, para-methyl red, dimethyl

111

trisulfide, anthraquinone-2,6-disulfonate (AQDS), and coenzyme Q10 were purchased

112

from Aladdin Industrial Corporation (China). Moreover, dimethyl disulfide was

113

purchased from TCI Development Co., Ltd (Shanghai, China). All chemicals were 6


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used as received without further purification.

115

Multi-walled CNTs (10-20 μm length, 20-30 nm diameter, and higher than 95%

116

purity) were purchased from XFNANO Materials Technology Company (China).

117

Graphene oxide (GO, 0.55-1.2 nm thickness, 0.5-3 μm diameter, higher than 99%

118

purity, and less than 3 layers), reduced graphene oxide (RGO, 1-3 nm thickness,

119

larger than 50 μm diameter, higher than 98% purity, and less than 3 layers), and

120

nitrogen-doped graphene (NG, 1-3 nm thickness, 2-10 μm diameter, higher than 98%

121

purity, and less than 3 layers) were obtained from Chengdu Organic Chemicals

122

(China), while the graphite (5000 mesh diameter and higher than 99.95% purity) was

123

purchased from Aladdin Industrial Corporation (China). The biochar was produced

124

from pinewood according to previous studies.28,

125

for one day to remove moisture, the pinewood samples were cut into 1-2 mm pieces.

126

The pieces were placed into a cylindrical quartz tube in an electric furnace and were

127

pyrolyzed under an Ar2 flow of 1 L min-1. The temperature of the furnace was

128

programmed to increase at a rate of approximately 5 °C min-1 and was held at 400 °C

129

for 3 h.30 After cooling to room temperature, the charred materials were milled to

130

approximately 0.15 mm and were sieved through a 100-mesh sifter. All the carbon

131

materials were washed with nitric acid at 75 °C for 11 h before use in order to remove

132

the residual metal catalysts.31, 32

29

Briefly, after being dried at 80 °C

133

The specific surface functional groups of the PCM were measured using an

134

ESCALAB 250 X-ray photoelectron spectrometer (XPS) (Thermo-VG Scientific Inc.,

135

UK). The curve fitting was performed using a Gaussian-Lorentzian peak shape after

136

subtracting a Shirley background.

137 138

Azo Dye Degradation Experiments 7



139

The batch degradation experiments of the azo dye were conducted in 200-mL

140

serum bottles containing 100 mL phosphate buffer solution (25 mM at pH 7.0);

141

specific amounts of PCM were mixed in serum bottles and were then dispersed using

142

an ultrasonic process for 10 min. The solution was purged with nitrogen for at least 30

143

min to remove oxygen before placing it into the anaerobic operation box. Afterward, a

144

certain amount of Na2S (stored in the anaerobic operation box) was added to the

145

serum bottle. Then, a specific amount of MO was added to the reactor and the

146

solution pH was adjusted to pH 7.2 using an HCl solution. The reaction was

147

conducted using an orbital shaker with 200 rpm at room temperature in an anaerobic

148

glove box with nitrogen atmosphere. The azo dye adsorption of the different types of

149

PCM was also evaluated without the addition of Na2S.

150

The decolorization of the various azo dyes including the MO, orange I,

151

ortho-methyl red, meta-methyl red, and para-methyl red by sulfide was tested with

152

and without the addition of CNTs (series 1). The effect of the CNT dosages on the

153

MO degradation by sulfide was evaluated in series 2. Additionally, the impacts of

154

various kinds of PCM on the MO reduction by sulfide were investigated in series 3.

155

To determine which of the sulfur species affected the reduction of the azo dyes by

156

sulfide, series 4 was performed to investigate the feasibility of different chemical

157

states of sulfur species as a reductant to react with MO. To understand the relationship

158

between the polysulfide concentration and the rate of MO degradation, series 5 was

159

carried out to determine the effect of the polysulfide concentration on the MO

160

reduction without CNTs. To determine whether the quinone groups of the PCM

161

affected the transformation of sulfide to polysulfides, series 6 was used to evaluate the

162

degradation of MO by sulfide by adding AQDS. The conditions of all experiments are

163

summarized in Table S2. 8


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Identification of Polysulfides

166

Voltammetric analyses with an electrochemical workstation (Bio-Logic Science

167

Instruments, France) were carried out to detect the aqueous sulfur species. A

168

three-electrode system consisting of an Ag/AgCl reference electrode, a Pt counter

169

electrode, and an Au/Hg working electrode was used in the voltammetric analyses,

170

which were conducted using the method of Brendel and Luther.33 The Au/Hg

171

electrode (a commercial gold electrode was electroplated with a layer of mercury in

172

0.1 M HgSO4 + 0.03 M H2SO4 electrolyte at a potential of -1.0 V versus a saturated

173

calomel electrode for 200 s) was 2 mm in diameter. Briefly, three electrodes were

174

inserted into the serum bottles reactor that had been pre-purged with O2-free N2 gas.

175

Cyclic voltammetry (CV) was performed between -0.1 and -1.6 V (versus Ag/AgCl)

176

at a scan rate of 1,000 mV s-1 with a 2-s conditioning step.34 The analyses were

177

carried out in sets of at least 12 sequential scans at specific time intervals and the first

178

five scans were discarded (allowing the electrode response to stabilize).

179

The formation of polysulfides was analyzed using gas chromatography-mass

180

spectrometry (GC-MS, Thermo, Trace DSQ II, USA) according to a previous study.35

181

Prior to the analysis, the samples were pretreated with methyl iodide to derivatize the

182

polysulfides to the corresponding dimethyl polysulfides. The GC-MS was equipped

183

with an HP-5 capillary column (30 m × 0.32 mm × 0.25 mm). SPME fiber

184

(DVB/CAR/PDMS) was purchased from the Supelco Company (USA). The

185

following conditions were used: 1 μL sample injection, inlet temperature of 250 °C,

186

mass selective detectors (MSD) transfer line temperature of 250 °C, MSD source

187

temperature of 150 °C, initial column temperature of 40 °C (hold for 8 min) followed

188

by 10 °C/min to 290 °C (hold for 2 min). The MSD was operated in electron impact 9



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mode at 70 eV to generate the mass spectra and was programmed to monitor the

190

selected ion mass/charge (m/z) ratios in selective ion monitoring mode. Additionally,

191

the standard curves of the disulfide and trisulfide were created by using dimethyl

192

disulfide and dimethyl trisulfide standards, respectively.

193 194

Chemical Analysis

195

The liquid samples were collected at appropriate time intervals and were

196

immediately filtered through a 0.22-µm membrane. The MO concentration was

197

measured with a Cary 60 UV-Vis spectrophotometer (Agilent Technologies, USA) at

198

a wavelength of 390 nm, where both the sulfide and polysulfides have insignificant

199

absorption (Figure S1). High-performance liquid chromatography (HPLC) was used

200

to determine the concentrations of the MO reductive products including

201

N,N-dimethyl-p-phenylenediamine

202

(4-ABS). The HPLC analysis was performed on an Agilent 1260 equipped with a 4.6

203

× 250 mm Eclipse Plus C18 column and a diode array detector (Agilent Technologies,

204

USA). The flow rate of the mobile phase was 0.8 mL min-1 and the detection

205

wavelengths were 270 and 254 nm, respectively. Gradient elution was conducted at

206

30 oC and the composition of the mobile phase (A: methanol and B: 0.1% acetic

207

acid/0.1% ammonium acetate) was as follows: 0 min: 5% A and 95% B, 15 min: 20%

208

A and 80% B, 30 min 100% A and 0% B, 38 min: 5% A and 95% B.

(DPD)

and

4-aminobenzenesulfonic

acid

209

The sulfide concentration was determined using the methylene blue method.36 The

210

concentrations of sulfite, sulfate, and thiosulfate were analyzed with an ICSS-1000

211

ion chromatograph (Dionex, USA) with an AS14A column and an electrochemical

212

conductivity detector. A solution of 8 mM of Na2CO3 and 1 mM of NaHCO3 was

213

used as an eluent at a flow rate of 1.0 mL min-1.37 The elemental sulfur from the 10


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214

samples was first extracted by n-hexane (2 mL sample and 1 mL n-hexane, volatile

215

extraction for 20 mins) and then determined by HPLC. The HPLC analysis was

216

performed on an Agilent 1260 equipped with a 4.6 × 250 mm Eclipse Plus C18

217

column and a diode array detector, the mobile phase was 85% methanol and 15%

218

water with 1 mL min-1 flow rate and the detection wavelengths were 240 nm.

219 220 221 222

223

Calculations The Gompertz model (Eq. 1) was used to simulate the degradation of MO by sulfide in the absence or presence of PCM. 𝐶𝑡

―𝑒 𝐶0 = 1 ― 𝐵 ∗ 𝑒

𝑘 ∗ (𝑡0 ― 𝑡) 𝐵

+1

(1)

224

where Ct (mM) is the azo dye concentration at time t (min) and C0 (mM) is the initial

225

azo dye concentration; t0 (min), k (min-1), and B are the three parameters that

226

determine the shape of the model. To be specific, t0 is the parameter that indicates the

227

lag phase of the azo dye degradation. The parameter k indicates the rate of MO

228

degradation in the fast degradation phase. As demonstrated in a previous study, B

229

indicates the potential of azo dye degradation in the experiments and was determined

230

as the ratio of the final degraded MO and the initial MO concentration.38

231 232 233

RESULTS

234 235

Azo Dye Decolorization by Sulfide with and without CNTs.

236

As shown in Figure 1a, the lag phase lasted 64.32±7.04 min for 0.5 mM MO

237

removal by sulfide in the absence of CNTs; subsequently, the MO concentration

238

sharply decreased within 35 min during the fast degradation phase (k of 0.150±0.011 11



239

min-1). Interestingly, the lag phase for the MO decolorization was significantly

240

shortened after 100 mg L-1 CNT addition; the value of t0 decreased from 64.32±7.04

241

min to 13.31±6.05 min in the absence and the presence of CNTs respectively (Table

242

S1). It should be noted that the adsorption of MO on the CNTs was insignificant in

243

the presence of 100 mg L-1 CNTs (± 4.5%, Figure S2). On the other hand, the MO

244

decolorization rate in the fast degradation phase did not change markedly because the

245

k value only varied from 0.150±0.011 min-1 in the absence of CNTs to 0.168±0.019

246

min-1 in the presence of CNTs. Moreover, the major final products were identified as

247

DPD and 4-ABS and their concentrations increased with the decrease in the MO

248

concentration (Figure 1b), suggesting that the reductive cleavage of the MO azo bond

249

by sulfide occurred regardless of the CNT presence. The final concentrations of DPD

250

and 4-ABS were around 90% of the amount of MO degradation (0.5 mM) and less

251

than 10% of the imbalance in the products might be due to the adsorption of CNTs.39

252

As the CNT concentration increased from 0 to 150 mg L-1, the lag phase time for

253

the MO decolorization decreased from 64.32±7.04 to 6.90±0.53 min, whereas the k

254

value did not change significantly (Figure 1c and Table S1). Additionally, a similar

255

phenomenon was also observed in the decolorization of the various azo dyes

256

including Orange I, ortho-methyl red, meta-methyl red, and para-methyl red by

257

sulfide in the absence and presence of CNTs (Figure S3 and Table S1). Overall, the

258

presence of the CNTs significantly shortened the lag phase of the azo dye

259

decolorization, where it did not markedly improve the removal rate in the fast

260

degradation phase.

261 262 263

Effects of Different PCM Types on MO Decolorization by Sulfide. The adsorption of MO on various types of PCM was quantified in the presence 12


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of 100 mg L-1 biochar, 100 mg L-1 graphite, 20 mg L-1 GO, 20 mg L-1 RGO, and 20

265

mg L-1 NG. The amount of MO adsorbed on the PCM was insignificant and varied

266

from 1.5% to 7.3%; the adsorption equilibrium was established in less than 10 min

267

(Figure S2). As shown in Figure 1d, all types of PCM shortened the lag phase of the

268

MO decolorization by sulfide. Compared to the control without PCM addition, the lag

269

phase time was reduced by 13.82%, 68.13%, 87.25%, 88.88%, and 92.24% with the

270

addition of 100 mg L-1 graphite, 100 mg L-1 biochar, 20 mg L-1 GO, 20 mg L-1 RGO,

271

and 20 mg L-1 NG, respectively, suggesting that the NG exhibited the best efficacy to

272

shorten the lag phase of the azo dye MO decolorization by sulfide. Moreover,

273

regardless of the PCM type, the k value did not change significantly compared with

274

the control, which indicated that the addition of different PCM types did not markedly

275

improve the azo dye MO decolorization rate in the fast degradation phase.

276 277

MO Decolorization at Different Chemical States of the Sulfur Species.

278

The results indicate that the azo dye decolorization process with an obvious lag

279

and fast degradation phases may be due to the existence of intermediate products,

280

which gradually formed during the stagnate phase and facilitated the degradation of

281

the azo dyes in the fast degradation phase. A previous study has demonstrated the

282

formation of reactive organic sulfur groups between sulfide and humic acid.40

283

However, the XPS analysis indicated that the sulfur content on the surface of the

284

CNTs was less than 1% (Figure S4 and Table S3). This result implied that the reactive

285

organic sulfur groups of the CNTs did not likely play important roles during azo dye

286

decolorization by sulfide, although their formation could not be completely excluded.

287

In order to determine which sulfur species might act as intermediates of sulfide

288

reacting with the azo dyes, different sulfur species were screened for the MO 13



289

degradation in the absence of PCM; the species included polysulfides, elemental

290

sulfur, thiosulfate, dithionite, sulfite, and sulfate (Figure 2). The MO concentration

291

changed slightly within 2 hr in the presence of elemental sulfur, thiosulfate, sulfite, or

292

sulfate. In contrast, the MO was decolorized in the presence of polysulfides or

293

dithionite. However, the reaction kinetics was too fast for the MO reduction by

294

dithionite, implying that dithionite may not be the intermediate of the sulfide

295

conversion during azo dye decolorization. The MO decolorization by polysulfides did

296

not exhibit a significant lag phase and had the same order of magnitude reaction rate

297

as the MO degradation by sulfide. This result suggested that polysulfides might act as

298

the intermediate product of sulfide for azo dye removal.

299

Interestingly, the addition of 1 mM elemental sulfur to the 8 mM sulfide solution

300

also reduced the lag phase of the MO degradation without a marked change in the

301

decolorization rate in the fast degradation phase (Figure 2), in which the t0 value of

302

8.96±0.88 min was significantly lower than 64.32±7.04 min for the control. Since it

303

was reported that sulfide reacted quickly with elemental sulfur to form polysulfides,41,

304

42

305

product during the azo dye decolorization by sulfide.

our result further supports the speculation that polysulfides were the intermediate

306 307

Identification of Polysulfides and the Critical Concentration for Fast MO

308

Decolorization.

309

In order to identify the intermediate product of sulfide for azo dye decolorization,

310

CV analyses were conducted. As shown in Figure S5, the CV spectrogram of MO

311

showed no oxidation peaks in the potential range between -0.7 and -0.4 V and

312

moreover, the signal intensity of MO was much lower than that of the polysulfides.

313

These results indicated that the existence of MO did not have a significant influence 14


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314

on the determination of the polysulfides by CV. As shown in Figure 3a, two oxidation

315

peaks at -0.55 and -0.59 V with intensities higher than 0.15 mA appeared at the

316

beginning of the fast degradation phase with or without the CNT addition. We further

317

conducted the CV analyses on various sulfur species including sulfide, polysulfides,

318

elemental sulfur, thiosulfate, dithionite, and sulfite. As shown in Figure 3b and 3c,

319

only the CV spectrogram of the polysulfides had two oxidation peaks at -0.55 and

320

-0.59 V respectively, suggesting that polysulfides were the intermediate product of

321

sulfide in the process of azo dye decolorization. In addition, the intensities of the two

322

oxidation peaks at -0.55 and -0.59 V gradually decreased with the reaction time when

323

polysulfides were used as the reductant for the azo dye MO decolorization (Figure 3d),

324

further implying the formation of polysulfides during the azo dye decolorization by

325

sulfide.

326

UV-visible absorption spectra have been commonly used to determine the

327

polysulfide concentration in previous studies but this method was not appropriate in

328

our study because of the absorption interference of azo dyes. Alternatively, the

329

formation of polysulfides was analyzed using GC-MS in this study. As shown in

330

Figure S6, the formation of disulfide, trisulfide, and tetrasulfide was observed and the

331

level of tetrasulfide was relatively low. Therefore, the concentrations of both disulfide

332

and trisulfide were monitored during azo dye decolorization by sulfide in the absence

333

and presence of CNTs. As shown in Figure 1a, in the absence of CNTs, it took about

334

80 min for the polysulfide concentration to increase from 0 to nearly 700 μM-S

335

during the MO decolorization process, in which the disulfide and trisulfide

336

concentrations increased from 0 to 120 μM, respectively (Figure 4a). In the presence

337

of 100 mg L-1 CNTs, the formation of polysulfides was accelerated. It only required

338

about 30 min for the polysulfide concentration to increase from 0 to more than 900 15



339

μM-S (Figure 1a), whereas the concentrations of disulfide and trisulfide increased

340

from 0 to 180 μM, respectively (Figure 4b). Moreover, elemental sulfur also started to

341

accumulate in the fast degradation phase and its final concentration was around 1.5

342

mM, regardless of the addition of CNTs (Figure 4a and b). Additionally, as shown in

343

Figure 4, the calculated sulfur mass balance was about 90% during the MO

344

decolorization process in the absence and presence of CNTs. These results strongly

345

suggested that the intermediate polysulfide products, mainly disulfide and trisulfide,

346

played important roles in the fast degradation phase of MO decolorization and

347

moreover, the CNT addition markedly accelerated the polysulfide accumulation

348

process. In addition, the formation of elemental sulfur, 4-ABS, and DPD during MO

349

decolorization by sulfide indicated that a reduction reaction rather than a nucleophilic

350

one occurred between the polysulfides and azo dye.43 Furthermore, we found that

351

only a small amount of polysulfides, including 14.24±5.43 μM disulfide and

352

6.19±1.05 μM trisulfide, were formed within 30 min in the presence of 8 mM sulfide

353

and 100 mg L-1 CNTs without azo dye addition. This implied that the presence of azo

354

dye was a prerequisite for the enhanced formation of polysulfides in this study.

355

Subsequently, we investigated the MO degradation with different concentrations

356

of polysulfides to determine the critical polysulfide concentration to start the fast

357

degradation of MO. The degradation of MO by polysulfides was fitted by

358

pseudo-first-order kinetics (Figure S7). As shown in Figure 5, the kinetic rate constant

359

of the MO removal only increased from 0.0014 to 0.0028 min-1 as the polysulfide

360

concentration increased from 62 to 248 μM-S, whereas a significant increase in the

361

rate constant of the MO removal was observed when the polysulfide concentration

362

was higher than 248 μM-S. However, the rate constant growth tended to stabilize

363

when the polysulfide concentration was in the range of 600 to 1000 μM-S. 16


Page 16 of 36

Page 17 of 36


364

Coincidentally, it was observed that the concentrations of the produced polysulfides

365

were close to this range in the fast degradation phase of MO decolorization by sulfide

366

with and without the CNT addition (Figure 1a). These findings indicated that the

367

polysulfides accumulated to a critical concentration when the fast degradation phase

368

started in the MO decolorization process.

369

Additionally, we further investigated the influence of the CNTs on the MO

370

reduction by polysulfides. As shown in Figure S8, the MO decolorization rate

371

changed non-significantly with the addition of the CNTs, suggesting that the CNTs

372

did not directly mediate the MO reduction by polysulfides. This further confirmed

373

that the presence of the CNTs did not markedly improve the reaction rate in the fast

374

degradation phase and that polysulfides played important roles in the MO

375

decolorization.

376 377

Effects of AQDS and Q10 Addition on MO Decolorization by Sulfide.

378

Previous studies indicated that the oxygen functional groups of the PCM affected

379

the pollutant conversion by sulfide.19, 20 In order to investigate the influence of the

380

oxygen functional groups on the azo dye degradation, AQDS with a semiquinone

381

structure and Q10 with oxidation state of the quinone were introduced during MO

382

decolorization by sulfide without PCM addition. As shown in Figure S9, the lag phase

383

of the MO decolorization was considerably shortened after the 1 mM AQDS addition

384

and the t0 value decreased from 64.32±7.04 min without AQDS to 24.05±0.65 min

385

with the AQDS. In addition, the k value also did not change significantly. Moreover,

386

the CV results suggested the accumulation of polysulfides in the fast degradation

387

phase of MO decolorization by sulfide with the AQDS addition (Figure S10a).

388

Furthermore, as shown in Figure S10b, the GC-MS quantification results 17



Page 18 of 36

389

demonstrated that the concentrations of disulfide and trisulfide increased with the

390

reaction time from 0 to more than 200 and 150 μM, respectively. Several studies

391

found PCM with abundant quinone-based functional groups by potentiometric

392

titration quantification and it was reported that CNTs with 12 h nitric acid treatment

393

had around 0.161 meq g-1 quinone groups.

394

quinone-based functional groups on the surface of the PCM might play important

395

roles in enhancing the formation of polysulfides during the azo dye decolorization

396

process by sulfide. However, the value of t0 only decreased from 64.32±7.04 min in

397

the absence of Q10 to 51.66±6.36 min in the presence of 0.01 mM Q10 (Figure S9),

398

indicating that Q10 did not markedly accelerate the formation of polysulfides and

399

consequently reduced the time of the stagnant phase in the MO decolorization process.

400

This may be attributed to the much lower solubility of Q10 than AQDS in water.

21,44

These results indicated that the

401 402 403 404

DISCUSSION Previous studies have shown that PCM facilitated the abiotic reduction of

405

organic pollutants, including nitroaromatics,16,

45

organic explosives,22,

406

halogenated organic insecticides47 by sulfide under anaerobic conditions. Moreover, it

407

was found that the degradation of the above-mentioned pollutants by sulfide always

408

corresponded to a pseudo-first-order kinetic reaction, which exhibits a relatively fast

409

degradation in the early stage of the reaction. In this study, for the first time, we found

410

that the degradation of azo dyes by sulfide initially underwent a lag phase followed by

411

a fast degradation phase. Interestingly, the PCM addition only reduced the length of

412

the lag phase of the azo dye decolorization but did not markedly improve the reaction

413

rate in the fast degradation phase. This implies an undiscovered mechanism for azo 18


46

and

Page 19 of 36

414


dye removal by sulfide in the presence of PCM.

415

Based on various experiments and analyses, we found that polysulfides may be a

416

key intermediate product during azo dye decolorization by sulfide. Moreover, the

417

polysulfide concentration reached a critical value prior to the quick reaction with the

418

azo dye, resulting in a long lag phase. As shown in Figure 6, in the absence of PCM,

419

there was a direct reaction between the sulfide and azo dye with the formation of

420

polysulfides, which was a slow process. Therefore, a relatively long period was

421

required for the accumulation of polysulfides to a critical concentration, resulting in a

422

long lag phase of azo dye decolorization. When the polysulfide concentration reached

423

the critical value, the azo dye was quickly decolorized (i.e., fast degradation phase)

424

with the formation of elemental sulfur. Subsequently, the elemental sulfur reacted

425

rapidly with sulfide to form polysulfides again, resulting in a sulfur cycle until the

426

decolorization process stopped. In the presence of PCM, carbon materials accelerated

427

the electron transfer between the sulfide and azo dye through their surface oxygen

428

functional groups (especially quinone) as an electron mediator, leading to the rapid

429

formation of polysulfides up to the critical concentration (similar to that without PCM

430

addition), resulting in a short lag phase for azo dye decolorization. On the other hand,

431

after the polysulfide concentration reached the critical value, the decolorization rate of

432

azo dye was similar in the presence and the absence of PCM in the fast degradation

433

phase, as indicated in Figure 1a. Interestingly, the NG exhibited the best efficacy to

434

shorten the lag phase of the azo dye decolorization by sulfide (Figure 1d). In addition

435

to the large number of oxygen-containing groups (such as quinone) in the NG, it has

436

been reported that nitrogen atoms incorporated into graphene networks remarkably

437

enhanced its conductivity, thereby facilitating the electron transfer.48-50 Therefore,

438

compared to other types of PCM, the presence of NG resulted in better performance 19



439

in terms of the electron transfer between sulfide and azo dye and facilitated the

440

formation of polysulfides, leading to the best efficacy and shortening of the lag phase

441

of azo dye decolorization by sulfide. In a previous study, Xu et al. found that the

442

electrical conductivity rather than the oxygen functional groups played an important

443

role in the sulfide being oxidized to products that served as potent nucleophiles and

444

the oxidized sulfur species promoted the pollutant’s RDX transformation.24 In

445

contrast, many other studies indicated that PCM mainly acted as an electron shuttle to

446

mediate the pollutants’ transformation by sulfide through active oxygen functional

447

groups19,

448

compounds were more likely to act as reductants instead of nucleophiles to form

449

intermediate products.21 These results strongly indicate the existence of different

450

reaction pathways for the transformation of various organic pollutants by sulfide in

451

the presence of PCM, which deserves more study.

21

or conductive graphitic regions.51,

52

Some studies suggested that sulfur

452

The results of this study provide new information on the transformation

453

pathways of organic pollutants in anaerobic aqueous environments; the discovered

454

mechanism may also occur in other organic pollutants, although this requires further

455

investigation. Moreover, the findings of this study provide new insights into the

456

influence of PCM on the transformation of organic pollutants in anaerobic aqueous

457

conditions. Additionally, the results improve our understanding of the roles of sulfur

458

species in the transformation of pollutants in natural environments.

459 460 461

ACKNOWLEDGEMENTS

462

The authors wish to thank the Natural Science Foundation of China (51538012,

463

51821006 and 41728007), and the Program for Changjiang Scholars and Innovative 20


Page 20 of 36

Page 21 of 36

464


Research Team in University, China for financially supporting this study.

465 466 467

SUPPORTING INFORMATION

468

Table S1, Estimated kinetic values for the decolorizaiton of various azo dyes under

469

different conditions; Table S2, Summary of operational conditions for various

470

experiments; Table S3, Contents of different elements on the surface of CNTs that

471

participated in azo dye decolorization process; Figure S1, UV spectrograms of sulfide,

472

polysulfides and MO.; Figure S2, MO adsorption on different PCMs including 100

473

mg L-1 biochar, 100 mg L-1 graphite, 20 mg L-1 GO, 20 mg L-1 RGO, or 20 mg L-1 NG

474

(pH 7.2, and 0.5 mM MO); Figure S3, Different azo dyes degradation by sulfide with

475

and without addition of CNTs (0.5 mM MO, 8 mM sulfide, 100 mg L-1 CNTs if

476

added, and pH 7.2), the lines were fitted by Gompertz model; Figure S4, Full-scale

477

XPS profile of CNTs that participated in MO decolorization process by sulfide;

478

Figure S5, CV spectrograms of 0.5 mM MO (including the insert figure) and

479

polysulfides respectively; Figure S6, GC (a) and MS (b: dimethyl disulfide, c:

480

dimethyl trisulfide and d: dimethyl tetrasulfide) spectrograms of the sample in the fast

481

degradation phase of MO decolorization with 100 mg L-1 CNTs addition; Figure S7,

482

MO degradation with different concentration of polysulfides in water (0.5 mM MO,

483

polysulfides: 62-992 μM and pH 7.2; the point were indicated the experimental data

484

while the lines were fitted by pseudo first order kinetic model); Figure S8, Effects of

485

CNTs on the MO degradation by polysulfides (a: 992 μM, b: 620 μM and c: 310 μM);

486

Figure S9, MO decolorization by sulfide with 1 mM of AQDS, 0.01 mM of

487

Coenzyme Q10, respectively (0.5 mM MO, 8 mM sulfide, and pH 7.2); Figure S10,

488

(a) Cyclic voltammogram, and (b) concentration variation of polysulfides (left 21



489

coordinate axis) and elemental sulfur (right coordinate axis) during the MO

490

decolorization by 8 mM sulfide with the addition of 1 mM AQDS. This material is

491

available free of charge via the Internet at http://pubs.acs.org.

492 493 494

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653 654

28


Page 29 of 36


Figure Captions

655 656 657

Figure 1 (a) MO degradation (left coordinate axis), polysulfide formation (right

658

coordinate axis, C = disulfide×2 + trisulfide×3), and (b) product generation

659

by sulfide in the presence and absence of CNTs (0.5 mM MO, 8 mM

660

sulfide, 100 mg L-1 CNTs if added, and pH 7.2); (c) effect of CNT dosage

661

on MO decolorization by sulfide (0.5 mM MO, 8 mM sulfide, and pH 7.2);

662

the lines were fitted by the Gompertz model; (d) MO degradation by sulfide

663

mediated with different PCM types, including 100 mg L-1 biochar, 100 mg

664

L-1 graphite, 20 mg L-1 GO, 20 mg L-1 RGO, and 20 mg L-1NG (0.5 mM

665

MO, 8 mM sulfide, and pH 7.2); the lines were fitted by the Gompertz

666

model.

667 668 669

Figure 2 MO degradation by different chemical states of sulfur (0.5 mM MO, 8 mM sulfur species, and pH 7.2).

670 671

Figure 3 Cyclic voltammograms of different solutions: (a) MO degradation by sulfide

672

without and with 100 mg L-1 of CNTs; (b) sulfide and polysulfides (8 mM);

673

(c) elemental sulfur, thiosulfate, dithionite, and sulfite (8 mM); (d) MO

674

degradation by 8 mM polysulfides.

675 676

Figure 4 Concentration variation of polysulfides (left coordinate axis, C = disulfide×2

677

+ trisulfide×3), elemental sulfur, sulfide, and total sulfur (right coordinate

678

axis): (a) MO degradation by sulfide without CNTs addition, (b) MO

679

degradation by sulfide with CNTs addition. 29



680 681 682

Figure 5 Effect of polysulfide concentration on the pseudo-first-order kinetic rate constant of the MO degradation (0.5 mM MO and pH 7.2).

683 684 685

Figure 6 Proposed mechanism of azo dye decolorization by sulfide in the presence and absence of PCM.

686 687

30


Page 30 of 36


1.2 (a)

1200

No CNTs (fitted line) CNTs (fitted line) CNTs (polysulfides)

No CNTs (experiment) CNTs (experiment) No CNTs (polysulfides)

1000

1.0

Ct / C0

0.5 (b)

0.8

800

0.6

600

0.4

400

0.4

C (M-S) C (mM)

Page 31 of 36

0.3

No CNTs No CNTs CNTs CNTs

ABS DPD ABS DPD

0.2 0.1

0.2

200

0.0

0

0.0 0

20

40

60

80

0

100

20

40

t (min) 1.0

(c)

0.6 0.4

0.4

0.0

0.0 0

690

0.6

0.2

0.2

688 689

100 Control Graphite Biochar GO RGO NG

0.8

Ct / C0

Ct / C0

0.8

80

(d)

1.0

No CNTs CNTs-10mg L-1 CNTs-50mg L-1 CNTs-100mg L-1 CNTs-150mg L-1

60

t (min)

20

40

60

t (min)

80

100

120

0

20

40

60

t (min)

Figure 1

31


80

100


Page 32 of 36

1.0 Sulfide Polysulfides

0.8

Dithionite

Ct / C0

Elemental sulfur Thiosulfate

0.6

Sulfite Sulfate Sulfide and

0.4

1mM elemental sulfur

0.2

0.0 0 691

20

40

60

80

t (min)

692 693

Figure 2

694

32


100

120

140

Page 33 of 36


0.2

(a)

(b)

0.1 0.0

I (mA)

I (mA)

Sulfide+MO with and without CNTs (0 min) 0.1 Sulfide+MO (60 min) Sulfide+CNTs+MO (10 min) 0.0

-0.1 -0.2

-0.1 -0.2

Sulfide Polysulfides

-0.3 -0.3

-0.4 -1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-1.6

-1.4

-1.2

0.08

Thiosulfate Dithionite Sulfite Elemental sulfur

697

-0.4

-0.2

0.0

Sx-0min Sx-5min Sx-15min Sx-25min

0.08

-0.04

695 696

-0.6

(d)

0.12

0.00

-0.08

-0.8

I (mA)

I (mA)

0.16

(c)

0.04

-1.0

E (V)

E (V)

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

-0.8

-0.6

-0.4

E (V)

E (V)

Figure 3

33


-0.2

0.0


(a)

Sulfide+MO

8 7

Disulfide Trisulfide Elemental sulfur Sulfide Sulfur mass balance

C (M)

200

100

C (mM-S)

300

Page 34 of 36

6 5 1 0

0 0

60

80

t (min)

100

CNTs+Sulfide+MO

(b)

300

40

8

C (M)

Disulfide Trisulfide Elemental sulfur Sulfide Sulfur mass balance

200

7

C (mM-S)

400

20

6 5

100 1 0

0 0 698 699

10

20

t (min)

30

Figure 4

700 701 702

34


40

Page 35 of 36


0.04

k (min-1)

0.03

0.02

0.01

0.00

0

200

400

600

800

Polysulfides (M-S) 703 704

Figure 5

705

35


1000


706 707 708

Figure 6

709

36


Page 36 of 36

Undiscovered Mechanism for Pyrogenic Carbonaceous Matter (PCM

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