Electrochemically Catalytic Degradation of Phenol with Hydrogen

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Electrochemically Catalytic Degradation of Phenol with Hydrogen Peroxide in-situ Generated and Activated by a Municipal Sludge-derived Catalyst Bao-Cheng Huang, Jun Jiang, Wei-Kang Wang, Wen-Wei Li, Feng Zhang, Hong Jiang, and Han-Qing Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00416 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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ACS Sustainable Chemistry & Engineering

Electrochemically Catalytic Degradation of Phenol with Hydrogen Peroxide in-situ Generated and Activated by a Municipal Sludge-derived Catalyst

Bao-Cheng Huang†, Jun Jiang†, Wei-Kang Wang, Wen-Wei Li, Feng Zhang, Hong Jiang, Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, No. 96, Jinzhai Road, Baohe District, Hefei 230026, P. R. China

†

These authors contributed equally to this work.

*Corresponding Author: Prof. Han-Qing Yu, Fax: +86-551-63601592. E-mail: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT

2

With the rapid global urbanization, today’s cities are facing an increasing pressure

3

for the treatment of both domestic and industrial wastewaters. In this work, a

4

proof-of-concept of “treating industrial wastewater using the sludge originating from

5

domestic wastewater treatment for urban pollution control” was proposed. After

6

one-step pyrolysis of the excess sludge from domestic wastewater treatment, a

7

metal-carbon composite catalyst with a high H2O2-producting capacity (432 mg/h/g)

8

was successfully synthesized. By applying the prepared material as a cathode catalyst

9

in an electro-Fenton system, phenol (40 mg/L), a model pollutant in industrial

10

wastewaters, was completely degraded within 40 min at a potential of 0.15-0.35 V (vs.

11

reversible

12

approximately 60% of total organic carbon was efficiently removed by the

13

electro-Fenton system within 4 h at 0.25 V, and the hydroxyl radicals were found to be

14

the main oxidation agent for the phenol degradation. More importantly, the phenol

15

removal efficiency remained at a high level (87%) and the released iron was low (0.8

16

mg/L) even after 10 cycles of reuse. Thus, an efficient and cost-effective integrated

17

system for the treatment of both domestic and industrial wastewaters was successfully

18

developed and validated. The results from this work are useful to establish a new

19

sustainable pollution control scenario.

hydrogen

electrode)

without

dosing

external

iron.

Meanwhile,

20 21

Keywords: Catalyst; electro-Fenton; H2O2 production; municipal sludge; pollutant

22

degradation

23

2


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24

INTRODUCTION

25 26

A large amount of municipal wastewater, i.e., domestic and industrial wastewaters, is

27

increasingly produced in today’s urban cities. Taking China as an example,

28

approximately 73.5 billion m3 wastewater was produced in 2015,1 which places

29

tremendous pressure on the environment and calls for efficient and cost-effective

30

municipal wastewater pollutant control approaches. Electro-Fenton (EF) process, in

31

which H2O2 can be continually generated on cathode via a two-electron oxygen

32

reduction reaction (ORR), has recently received great interests because of its

33

numerous merits.2-4 The efficiency of the EF for pollutant degradation depends

34

heavily on the generation rate and cumulative concentration of H2O2.5,6 Among the

35

studied

36

H2O2-generating electrocatalyst due to its nontoxic, high overpotential for H2

37

evolution and good stability.2 Up to date now graphite felt,7 carbon sponge,8 and

38

activated carbon fiber9 have been widely tested for their feasibilities of using as

39

cathode catalysts. However, the catalytic activity of ORR by pure carbon materials is

40

low.10 Thus, doping of heteroatoms such as nitrogen and sulfur10,11 and synthesis of

41

metal-carbon composites6,12 have also been used to improve the catalytic performance.

42

Nevertheless, the fabrication of a composite H2O2-generating electrocatalyst is a

43

multi-step process and external chemical reagents are generally required, which often

44

makes it environmentally unfriendly. Thus, green synthesis of electrochemical

45

catalysts with high H2O2-generating capacity at a low cost is crucial to facilitate the

46

practical application of EF for wastewater treatment.

cathodes,

carbonaceous

material

is

recognized

3


as

a

promising


47

In fact, domestic sewage is embodied with a high content of organics and such an

48

organic complex could be used as low-cost precursor to synthesize electro catalysts

49

for H2O2 generation after appropriate treatment. Previous studies have shown that

50

sewage sludge, which is formed during wastewater treatment, could exhibit an

51

electrochemical activity for 4-e- ORR via pyrolysis.13,14 However, how to separate and

52

obtain organic precursor from domestic wastewater in a cost-effective way is

53

challenging. Coincidently, iron-based coagulant is widely used for capturing organic

54

carbon in domestic wastewater treatment process and its application is found to be

55

essential in upcoming wastewater treatment plants.15 Fe-enriched sludge would be

56

produced accordingly from domestic wastewater treatment process with a pursuit of

57

utmost recovery of energy and resource.16 Therefore, it is hypothesized that the

58

metal-carbon composite (MCC) originating from the domestic wastewater treatment

59

process exhibits an ORR activity to produce H2O2 after appropriate treatment.

60

Furthermore, the produced H2O2 could be in-situ activated by MCC to degrade

61

industrial pollutants. Thus, a new sustainable scenario for the treatment of both

62

domestic and industrial wastewaters can be developed. In such a scenario, the sludge

63

originating from the coagulation treatment of domestic wastewater is reused as

64

electro-catalysts in an EF system to further treat industrial wastewaters.

65

Therefore, to demonstrate the above concept, a composition-directed pyrolysis

66

strategy was designed to prepare the MCC with a high electrochemical activity. The

67

impact of doping Fe and N, which are the two elements found to be effective in

68

improving electrochemical activity of the material,11,17 on the H2O2 production 4


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69

performance by MCC was explored. Also, an MCC-based EF system was established

70

and its applied potential, mineralization efficiency, degradation kinetics, and cycling

71

use stability for the degradation of phenol, a model pollutant in industrial wastewaters,

72

were evaluated. Furthermore, the mechanism for the phenol degradation in the EF

73

system was elucidated. In this way, an alternative sludge waste reuse approach was

74

proposed and an efficient EF system to degrade phenol was developed. Moreover, a

75

concept “treating industrial wastewater using the sludge originating from domestic

76

wastewater treatment for urban pollution control” could be architected.

77 78

MATERIALS AND METHODS

79 80

Chemicals. Analytical grade phenol, sodium sulfate, iron (III) chloride (FeCl3•6H2O)

81

were purchased from Sinopharm Chemical Reagent Co., China. Nafion 117 Proton

82

exchange membrane was purchased from DuPont Co., USA. Nafion solution and

83

5,5-dimethylpyrroline-N-oxide (DMPO) were obtained from Sigma Co., USA.

84

Carbon papers (Toray Co., Japan) used in this study were sequentially rinsed with

85

acetone, HCl (1 M), and ethanol for grease and other impurities removal. Milli-Q

86

water was used to remove the residual chemicals from each rinsing step.

87

MCC Preparation and Cathode Fabrication. MCC was obtained by

88

carbonation of the coagulated sludge under a NH3/Ar atmosphere (V%=1:9) (Fig. S1).

89

In brief, domestic wastewater after grit settling pretreatment was taken from the

90

Wangtang Wastewater Treatment Plant (Hefei, China). FeCl3•6H2O of 0.5 mM was 5



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91

added to the wastewater, followed by 130-rpm rapid agitation for 3 min and 40-rpm

92

slow agitation for 25 min. Then, the precipitates were freeze-dried for 24 h to obtain

93

the precursor. Afterwards, MCCs were prepared by carbonization of the precursor at

94

800 °C for 4 h (MCC800-4), 6 h (MCC800-6), and 8 h (MCC800-8) under a NH3/Ar

95

atmosphere (detailed information in SI). Moreover, the MCCs at the other two

96

carbonization temperatures, 600 °C (MCC600-6) and 1000 °C (MCC1000-6), were

97

additionally prepared. All of the prepared MCCs were immersed in 1 M HCl solution

98

three times to remove soluble ash, washed with milli-Q water to pH=7.0, and then

99

dried at 105 °C overnight. As a comparison, the precursor was also annealed under Ar

100

for 6 h, and the resulting product was named as MCC800-6Ar. In addition, the

101

precursor with HCl pre-rinsing was carbonized under NH3 and named as MCCH800-6.

102

The main preparation conditions and the characteristics of the different catalysts are

103

summarized in Table S1.

104

To prepare an MCC cathode, 10 mg of MCC was mixed with 0.5 ml of 75%

105

isopropanol and 10 µL of a Nafion solution. Then, the mixture was sonicated for 30

106

min and dropped onto a 3×3 cm carbon paper. The MCC electrode was obtained after

107

fully drying at room temperature (25 °C).

108

Electrochemical Experiment Setup. All of the electrochemical experiments

109

were conducted on a CHI 760E potentiostat (Chenhua Instrument Co., China) via the

110

potentiostatic method. The potentials reported in this study were referenced to the

111

reversible hydrogen electrode (EVS RHE) according to the following equation: EVS RHE=EVS Ag/AgCl+Eθ Ag/AgCl+0.059pH

(1) 6


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112

where EVS Ag/AgCl (V) is the applied potential referenced to the saturated Ag/AgCl

113

electrode, Eθ Ag/AgCl (V) is the potential of Ag/AgCl under the standard conditions.

114

The H2O2 generation performance via ORR was firstly evaluated in a

115

two-compartment cell with a Nafion 117 membrane as separator. Pt wire and Ag/AgCl

116

(in 3.0 M KCl) were respectively used as the counter and reference electrodes. Both

117

the anode and cathode cells were filled with 75 ml of electrolyte (0.05 M H2SO4 +

118

0.05 M Na2SO4, pH=1.0), and oxygen was continuously flushed with a flowrate of 50

119

ml/min.

120

The ORR activity was measured with a rotating ring-disk electrode (5.5 mm

121

diameter; Pine Research Instrumentation, Inc., USA) using a three-electrode mode. To

122

prepare the working electrode, MCC was dispersed on the 75% isopropanol solution

123

with a final concentration of 10 g/L. Nafion solution with 2% content was supplied to

124

the above dispersion and sonication was then applied to form a homogeneous ink.

125

Then, 10 µL of ink was loaded onto a rotating ring-disk electrode and dried at room

126

temperature. Linear sweep voltammograms (LSV) were tested by sweeping the

127

potential from 0.8 to -0.1 V (vs. RHE) at a rate of 5 mV/s with a rotating speed of

128

1600 rpm. To detect H2O2, the ring potential was kept constant at 1.2 V, and the H2O2

129

selectivity (H2O2%) was calculated as follows:17

‫ܪ‬ଶ ܱଶ % =

200݅௥ ݅௥ + ܰ݅ௗ

(2)

130

where ir and id are the ring current and disk current, respectively, and N is the current

131

collection efficiency of the Pt ring in the rotating ring-disk electrode (N=0.4). The

132

electrochemical impedance spectroscopy measurements were conducted at a potential 7



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133

of 0.25 V with a frequency ranging from 100 kHz to 0.1 Hz and an amplitude of 10

134

mV.

135

In order to examine the feasibility of MCC for the treatment of industrial

136

wastewater, the EF system was established to degrade phenol as a representative

137

organic pollutant. The degradation of phenol (40 mg/L) was conducted in a single

138

compartment cell with the MCC electrode as the working electrode and Pt wire as the

139

counter electrode. Phenol was dissolved in 0.1 M Na2SO4, and the pH was adjusted to

140

3.0 with 0.1 M H2SO4. A constant potential of 0.05~0.45 V was applied to the

141

working electrode in the EF system. Moreover, an electrochemical cell with an open

142

circuit was used to eliminate the adsorption impact. As a comparison, O2 was replaced

143

with N2 to exclude electrical oxidation. In the EF degradation process, methanol (20%,

144

v/v) was used as a quenching agent to determine the radical species.

145 146

The mineralization current efficiency (MCE, %) at a given time was calculated as follows to evaluate the current utilization efficiency of the EF system:2 MCE=[△(TOC)tnFV/(4.32×107mIt)]×100

(3)

147

where △(TOC)t is the TOC decay (mg C/L), F is faraday constant (96485 C/mol), n is

148

the number of electrons exchanged, V is the electrolyte volume (L), m is the number

149

of the carbon atoms of the pollutants, I is the applied current (A), t is the electrolysis

150

time (h), and 4.32×107 is the conversion factor for units homogenization (= 3600

151

s/h×12000 mg C/mol).

152

Analysis. The H2O2 concentration was measured using a colorimetric method.18

153

Phenol was detected by high performance liquid chromatography (HPLC, 1260 8


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154

Infinity, Agilent, Inc., USA) with 50% methanol as the mobile phase. Samples were

155

taken at set intervals and mixed with methanol immediately to stop the reaction.

156

However, in order to avoid the interference of methanol to the total organic carbon

157

(TOC) detection, sodium nitrite was dosed to terminate the reaction. TOC was

158

detected with a TOC analyzer (Muti N/C 2100, Analytik AG, Germany). The product

159

morphology was observed with a field emission scanning electron microscopy (SEM,

160

Zeiss Co., Germany). The chemical compositions and valences of the elements on the

161

MCC surface were analyzed by X-ray photoelectron spectroscopy (XPS,

162

EXCALAB250, Thermo Fisher, Inc., USA). The surface areas of the samples were

163

measured by the Brunauer-Emmett-Teller (BET) method with a Builder 4200

164

instrument (Tristar II 3020 M, Micromeritics Co., USA). Raman spectra were excited

165

by radiation at 514.5 nm from a confocal laser micro-Raman spectrometer

166

(LABRAM-HR, Jobin-Yvon Co., France). The radicals formed in the EF process were

167

examined by electron spin resonance (ESR, JES-FA200, JEOL Co., Japan). Before the

168

ESR detection, DMPO was immediately mixed with the sample to form DMPO-•OH.

169

The metal element was detected by atomic absorption spectroscopy (AA800, Perkin

170

Elmer Co., USA).

171 172

RESULTS AND DISCUSSION

173 174

Optimized H2O2 Production with MCC. The cumulative H2O2 concentration is vital

175

to the overall EF performance, thus, the MCC preparation conditions were optimized 9



176

to improve the H2O2 yields. It was found that the H2O2 concentration increased almost

177

linearly over time, and the highest H2O2 concentration (116 mg/L) was achieved by

178

MCC800-6 after 120-min electrolysis (Fig. 1a). Further increase in pyrolysis

179

temperature or prolonging the carbonation time would cause the deterioration of

180

catalytic activity. The average H2O2 production efficiency of the MCC reached 432

181

mg/h/g, which is superior to most of the other carbon-based non-noble metal

182

electrocatalysts under similar conditions (Table S2). Since the material obtained at

183

800 °C for 6 h exhibited the best performance, such a carbonation condition was

184

selected to further prepare MCC800-6Ar and MCCH800-6 to investigate the pyrolysis

185

atmosphere and precursor composition influences.

186

The electro-catalytic H2O2-producing abilities of MCC800-6, MCC800-6Ar, and

187

MCCH800-6 were further evaluated. It was observed that Ar gas atmosphere

188

pyrolysis strategy could greatly prevent the 2 e- oxygen reduction and no obvious

189

H2O2 accumulated for the obtained product (Fig. 1b). In comparison, acid pickling of

190

the precursor reduced the H2O2 level with accumulation concentration of 58 mg/L

191

after 120-min electrolysis, which was only half of that for the MCC800-6. A

192

comparison of the XRD patterns and SEM images between the three MCCs shows

193

that no obvious differences were observed (Fig. S2 and S3), implying that the

194

material’s morphology is not the key factor governing the performance.

195

Electrochemical Properties of MCC. The electrochemical properties of MCC

196

were further investigated by rotating ring-disk electrode measurements. Carbonization

197

under Ar gas atmosphere greatly decreased the electrochemical activity, as both the 10


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198

ring and disk currents were the lowest among the three studied materials (Fig. 2a).

199

Although the calculated H2O2 selectivity of MCC800-Ar was close to 100%, the high

200

impedance resulted in a low current intensity (Fig. 2b and 2c), which further inhibited

201

the oxygen reduction and induced the decline of H2O2 accumulation level. In

202

comparison, the precursor with acid pretreatment changed the primary ORR on the

203

surface from 2 e- to 4 e-, as evidenced by the decline of selectivity (10-18%) for H2O2

204

production at 0.2-0.4 V.

205

To further understand the electro-activity of the materials, the Raman spectrum of

206

the as-obtained MCC was recorded (Fig. 3a). The obtained materials showed two

207

main Raman peaks at ~1360 cm-1 (D band) and 1600 cm-1 (G band). D bands are

208

known to be characteristic of disordered graphite with structural defects, while the G

209

band is associated with graphitic carbon and the D/G band intensity ratio (ID/IG) is

210

commonly used to characterize the graphited degree (the degree of graphitization) of

211

carbonaceous materials.19,20 In the current case, the ID/IG of MCC800-6 (0.89) was

212

clearly lower than those of MCC800-6Ar (1.02) and MCCH800-6 (0.98), which

213

indicates a higher graphitization degree and results in better electrical conductivity.

214

XPS analysis was further performed to explore the differences in the electronic

215

structure and surface chemical composition among the three materials. The signals of

216

C, N, O, and Fe were clearly observed on the MCC800-6 (Fig. 3b). Interestingly, the

217

N1s signal vanished on MCC800-6Ar, implying that a NH3 atmosphere is essential for

218

N doping. The high-resolution N 1s spectra of the MCC800-6 and MCCH800-6

219

catalysts suggest that pyridinic N, pyrrolic N, graphitic N, and quaternary N21,22 11



220

existed (Fig. S4). It has been widely reported that pyridinic N and pyrrolic N can form

221

Fe-Nx moieties with Fe due to their long-pair electrons,17,23 and the graphitic N

222

combined with Fe-Nx moieties was the efficient active sites for ORR.17,24 For

223

comparison, since the N content of the MCC800-Ar was quite low (0.3 %, Table S3),

224

the lack of ORR sites resulted in a negligible H2O2 accumulation. Although the N

225

content of MCCH800-6 was comparable to that of MCC800-6, the low Fe content

226

(Fig. 3b) might reduce the activity and induce a reduction in the H2O2 level due to the

227

less Fe-Nx active sites. Therefore, N and Fe might play an important role in the

228

electrosynthesis of H2O2.

229

Phenol Degradation by MCC in EF. To examine the application potential of the

230

prepared MCC catalysts, phenol degradation by these catalysts in EF was investigated.

231

It was found that the phenol was efficiently removed within 40 min in EF system by

232

loading MCC as cathode catalyst and no external supply of iron was required (Fig. 4a).

233

Moreover, the contribution of the pure carbon paper, which was not coated with

234

catalyst, to the overall phenol degradation was low. The phenol degradation kinetics

235

in the EF with the MCC electrode was further analyzed. Although the initial

236

degradation rate was slow, which might be due to the insufficient accumulation of

237

H2O2, the overall phenol degradation was found to follow the pseudo-first order (Fig.

238

S5). The apparent rate constant (k) for the system with the carbon paper was only

239

0.001 min-1. The degradation rate was greatly improved with an increase in k to 0.078

240

min-1 when MCC was used as the working electrode.

241

To optimize the phenol removal performance of the system, the impacts of 12


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242

several operational parameters on the system were investigated. Since the potential

243

applied to the cathode affected the H2O2 accumulation level, the impact of the

244

potential on the phenol removal without the addition of iron was initially explored.

245

The phenol concentration rapidly declined within 40 min when the potential was

246

increased from 0.15 V to 0.35 V (Fig. 4b). However, a lower (0.015 V) or higher

247

potential (0.45 V) resulted in a poorer performance. A lower potential might further

248

reduce H2O2 to H2O and 4 e- ORR process became predominated. The TOC removal

249

efficiency increased continuously over time for each investigated potential (Fig. 4c).

250

With the same tendency as the phenol removal efficiency, the highest TOC removal

251

efficiency was achieved at 0.25 V. Approximately 60% of TOC was removed after 4-h

252

electrolysis at 0.25 V. Such results imply a high efficiency of the EF system. In

253

addition to the applied potential, pH is another crucial factor to greatly affect overall

254

Fenton reaction performance. In this work, the phenol removal performances of the

255

EF system at raised pHs were further investigated. Phenol was found to be completely

256

removed at pH 4.0, while only 30% removal efficiency was achieved by raising the

257

pH to 5.0 (Fig. S6a). The poor performance at pH 5.0 might be due to the low H2O2

258

concentration (Fig. S6b). As discussed above, Fe might play a key role in catalyst’s

259

activity. Hence, FeCl3•6H2O at reduced concentrations (0.2 mM and nil) was dosed in

260

the coagulation process to explore the impact of Fe content on MCC’s performance. It

261

was observed that by reducing the Fe dosage to 0.2 mM, only 20% of phenol could be

262

removed within 60-min (Fig. S7a). If the domestic wastewater precipitate was directly

263

used as a precursor to carbonize without dose of Fe, the obtained product showed no 13



264

H2O2 producing ability (Fig. S7b). The above results again proved that Fe was

265

essential to achieve a high performance of H2O2 synthesis for MCC.

266

To exclude the influences of anode oxidation and cathode material adsorption,

267

electrical oxidation (aeration N2 to inhibit the H2O2 generation) and electrode

268

adsorption (with open-circuit) experiments were conducted as references at 0.25 V. As

269

shown in Fig. 5d, 97% phenol was degraded in the EF system after 40-min treatment.

270

In comparison, only 6% phenol was adsorbed by the electrode and 12% was removed

271

via electric oxidation. These results clearly demonstrate that H2O2 was in-situ

272

electro-synthesized and activated by the MCC electrode effectively to mineralize

273

phenol.

274

In the Fenton reaction, intermediate free radical formation via H2O2 activation is

275

the key step to achieve effective pollutant degradation. Thus, the presence of hydroxyl

276

radicals in the EF was verified using the DMPO spin-trapping method. Fig. 5a shows

277

the typical ESR spectrum obtained after a 30-min reaction. A spectrum consisting of

278

quartet-lines with a peak height ratio of 1:2:2:1 was clearly observed. Such an ESR

279

spectrum is characterized as a hydroxyl radical.25 Moreover, methanol was used as the

280

radical scavenger to quench the hydroxyl radicals. The results indicate that phenol

281

degradation was greatly inhibited by the dose of methanol (Fig. 5b). Therefore, the

282

hydroxyl radicals formed in the EF system was the main reactive species to

283

mineralize phenol.

284

From an economic perspective, the cycling reuse stability is another issue for the

285

practical application of catalysts. In our work, the recycling capability of the catalysts 14


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286

was evaluated by using a reused MCC electrode in the EF system. The MCC

287

electrode could achieve a high phenol removal efficiency (91%) after 7 repeated uses

288

(Fig. 5c). The phenol removal was slightly reduced to 87% even after 10 cycles. Since

289

the MCC contained iron, its release into the solution needs to be evaluated. Although

290

the released iron concentration in solution was slightly high after the third cycle (4

291

mg/L), this value was below 1 mg/L after seven-cycle use. H2O2 may be activated by

292

the released Fe for phenol degradation. As a result, H2O2 with a cumulative

293

concentration (135 mg/L) was mixed with ferrous iron at the released concentration

294

level for phenol degradation by the conventional Fenton reaction. It was observed that

295

4 mg/L iron resulted in excellent phenol removal (Fig. 5d). Reduction in iron dose to

296

1.9 mg/L decreased the degradation efficiency to 76%. When the iron dose was

297

continually reduced to 0.8 mg/L, the overall phenol removal sharply declined to 10%

298

within 60 min. However, the above experimental result shows that 87% of phenol was

299

removed by EF in the tenth cycle. Such a comparison indicates that MCC might be

300

able to activate the generated H2O2 in-situ, resulting in a better pollutant removal

301

performance compared to the conventional Fenton process.

302

The morphology and structure of reused material were further examined to prove

303

its stability. The SEM images indicate that the morphology of material remained

304

unchanged after recycling use (Fig. S8). In addition, the pyridinic N, pyrrolic N,

305

graphitic N, and quaternary N were observed in the XPS spectra (Fig. S9a). By

306

comparing to the pristine material, additional C-F structure (292 eV) was observed at

307

the reused catalyst’s surface (Fig. S9b), which is ascribed to the added Nafion 15



308

substance. In all, both the morphology and chemical composition of the catalysts were

309

proven to be stable.

310

Other Pollutants Degradation in MCC Fabricated EF System. In addition to

311

phenol, the feasibility of the EF system on other industrial wastewater treatment

312

would of great importance to the proposed concept. As a result, the degradation of

313

various typical refractory pollutants, e.g., dye (rhodamine B), pesticide (atrazine), and

314

bisphenol-A by the fabricated EF system was evaluated. All of these three types of

315

pollutants were efficiently removed within 40 min at 0.25 V applied potential (Fig. 6).

316

These results further validate the universality of our proposed strategy. Although the

317

domestic wastewater quality is site-specific and the activity of the obtained MCC

318

might vary, the high activity of the material can be achieved by post chemical

319

modifications.

320

One of the major concerns for EF degradation of pollutants is the electric energy

321

consumption. Here, the mineralization current efficiency of EF system for phenol

322

degradation was only 5.9%, which is low. However, the current density (1.67 mA/cm2)

323

is much lower than those reported in other studies (10-30 mA/cm2).26,27 In this work,

324

only two factors, i.e., carbonation atmosphere and pyrolysis time, which affect the

325

product performance, were taken into account. Considering that the BET surface area

326

of the material (89 m2/g) was low (Fig. S10), other strategies such as tailoring pore

327

structure and tuning surface hydrophilicity,28 could be adopted to further improve the

328

MCC performance and current efficiency.

329 16


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330

CONCLUSIONS

331 332

In this work, a green waste pollution control strategy for urban cities’ sustainable

333

development, i.e., treating industrial wastewater with the sludge originating from

334

domestic wastewater treatment process was proposed. After one-step pyrolysis of

335

excess sludge formed in the domestic wastewater treatment process, an efficient H2O2

336

electrochemical synthesis material was successfully fabricated. Electrochemical tests

337

revealed that the obtained MCC was able to catalyze O2 reduction to H2O2 with an

338

accumulation rate of 432 mg/h/g under acidic condition. Benefitting from its high

339

activity and selectivity for H2O2 production, the MCC-fabricated EF system was

340

efficient in complete removal of 40 mg/L phenol within 40 min at a potential of

341

0.15-0.35 V without the need of dosing external iron. The hydroxyl radicals were

342

found to be the main reactive species and approximate 60% of total organic carbon

343

removal efficiency was achieved within 4 h at 0.25 V. This work shows a great

344

potential of such an integrated approach for efficient and cost-effectively urban water

345

pollution control. Further optimization of the MCC material and operation system

346

could also improve its performance.

347 348

ACKNOWLEDGEMENTS

349

We thank the National Natural Science Foundation of China (51538011), the

350

Collaborative Innovation Center of Suzhou Nano Science and Technology of the

351

Ministry of Education of China, and the Anhui S&T Key Project (1501041118) for the 17



352

support of this work.

353 354

ASSOCIATED CONTENT

355

Supporting Information Available. The detail description of pyrolysis

356

procedure; comparison of catalytic activities of various catalysts; elemental analysis

357

of the precursor and MCC800-6; MCC preparation process; XRD patterns; SEM

358

images; XPS spectra of C1s, Fe 2p, and N1s; kinetic analysis of the phenol

359

degradation; impacts of pH and Fe content on phenol degradation and BET surface

360

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

361 362

REFERENCES

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Figure Captions Figure 1. Optimization of the MCC fabrication for the improved H2O2 generation performance. Impacts of pyrolysis temperature and time on cumulative H2O2 production by MCCs (a). Comparison of carbonation atmosphere and acid pickling pretreatment of precursor for H2O2 production (b). The applied potential was 0.25 V and the experiment was conducted in triple trials (n=3). Figure 2. Electrochemical performance of the as-prepared catalyst. Rotating ring-disk electrode measurements (a) show the oxygen reduction currents. The calculated H2O2 selectivity (b) at different potentials and EIS analysis (c) of MCCs under different carbonation atmospheres. Figure 3. Raman spectra (a) and XPS surveys (b) of MCCs obtained under different conditions. Figure 4. Phenol degradation performance by the loading MCC as cathode catalyst in an EF system. Phenol removal in the EF system with MCC as working electrode at 0.25 V (a). The impacts of applied cathode potential on phenol degradation (b) and TOC removal efficiency (c) in the EF system without the external assistant of iron. Phenol removal efficiency over time for the EF, electrode adsorption, and electrical oxidation at 0.25 V condition (d). Figure 5. Reactive species responsible for phenol degradation in EF system and the cycling reuse stability of MCC. ESR spectrum of the DMPO-trapped hydroxyl radical after 30-min reaction (a), phenol removal as a function of time by using methanol as radical scavenger (b), phenol degradation in different batch runs in the EF system (c) and conventional Fenton system by dosing with 135 mg/L H2O2 (d). Figure 6. Performance of the MCC-based EF system for the degradation of other pollutants. Inert is the color change of rhodamine B after EF treatment.

23



H2O2 concentration (mg/L)

a

MCC800-4 MCC800-6 MCC800-8 MCC600-6 MCC1000-6 C paper

120 100 80 60 40 20 0 0

b H2O2 concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

40

60

80

100

120

100

120

Time (min) 120

MCC800-6 MCC800-6Ar MCCH800-6

100 80 60 40 20 0 0

20

40

60

80

Time (min)

Figure 1

24


Page 24 of 30

Page 25 of 30

a

0.06

Current (mA)

0.04 0.02

Ring

0.00 0.0 -0.2 Disk -0.4 MCC800-6 MCC800-6Ar MCCH800-6

-0.6 -0.8 -1.0 0.0

0.2

0.4

0.6

0.8

Potential vs. RHE (V)

b

MCC800-6

H2O2 selectivity (%)

100

MCC800-6Ar

MCCH800-6

80 60 40 20 0

0.2

0.3

0.4

Potential vs. RHE (V)

c

MCC800-6 MCC800-6Ar

1600

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1200 800 400 0 0

2000

4000

6000

Z' (ohm)

Figure 2

25


8000


a

1.0

1.03

d b

0.98

0.89

O 1s

D band G band

0.4 0.2 MCC800-6

MCC800-6Ar MCCH800-6

MCCH800-6

MCC800-6Ar

MCC800-6Ar MCC800-6 500

750

MCCH800-6

N 1s

Intensity (a.u.)

ID /I G

0.6

0.0

Fe 2p

C 1s

0.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 30

MCC800-6

1000

1250

1500

1750

0

200

-1

400

600

800

Binding energy (eV)

Raman shift (cm )

Figure 3

26


1000

Page 27 of 30

a

b

1.0

0.6

C paper MCC

0.4 0.2

0.6 0.4 0.2

0.0

0.0

0

10

0.05 V 0.15 V 0.25 V

60 50

20

30

40

50

60

0

Time (min) 0.35 V 0.45 V

d

10

20

30

40

50

60

Time (min) 1.0 0.8

40

C/C 0

c

0.25 V 0.35 V 0.45 V

0.05 V 0.15 V

1.0 0.8

C/C 0

C/C 0

0.8

TOC removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

Electrode adsorption Electrical oxidation Electrical Fenton

0.6 0.4

20 0.2

10

0.0

0 1

2

3

0

4

10

20

30

40

Time (min)

Time (h)

Figure 4

27


50

60


a

b 0 min

1.0

C/C0

Intensity

0.8

30 min

0.6 0.4 0.2

320

322

324

326

0.0

328

0

10

20

Magnetic intensity (mT) Cycle 3

1.0

Cycle 5

Cycle 10

d

4

3

0.6 2 0.4 1 0.2 0

0.0 30

60

90

40

50

60

Relased Fe concentration Cycle 7

0.8

0

30

Time (min)

Leaching Fe (mg/L) C/C0

Phenol concentration

c

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

1.0 0.8 0.6 0.4 4.0 mg/L Fe 1.9 mg/L Fe 1.0 mg/L Fe 0.8 mg/L Fe

0.2 0.0

120 150 180 210 240

0

30

60

Time (min)

90

120 150 180 210 240

Time (min)

Figure 5

28


Page 29 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6

29



For Table of Contents Use Only

Electro-Fenton cathode catalyst has been synthesized by using sludge originating from domestic wastewater treatment in a green way and applied for the industrial pollution control.

30


Page 30 of 30

Electrochemically Catalytic Degradation of Phenol with Hydrogen

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