Cathode-Introduced Atomic H* for Fe(II)-Complex Regeneration to

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Cathode-Introduced Atomic H* for Fe(II)-Complex Regeneration to Effective Electro-Fenton Process at a Natural pH Xiaocheng Liu, Wen-Qiang Li, Yi-Ran Wang, Guan-Nan Zhou, YiXuan Wang, Chuan-Shu He, Gongming Wang, and Yang Mu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00345 • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 31, 2019

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Environmental Science & Technology

1

Cathode-Introduced Atomic H* for Fe(II)-Complex Regeneration to Effective

2

Electro-Fenton Process at a Natural pH

3 4

Xiao-Cheng Liu,† Wen-Qiang Li,† Yi-Ran Wang,† Guan-Nan Zhou,† Yi-Xuan Wang,†

5

Chuan-Shu He,† Gong-Ming Wang,‡ Yang Mu,†,*

6 7

†

8

Chemistry, University of Science & Technology of China, Hefei 230026, China

9

‡

10

CAS Key Laboratory of Urban Pollutant Conversion, Department of Applied

Department of Applied Chemistry, University of Science & Technology of China,

Hefei 230026, China

11 12 13 14 15 16

* Corresponding author:

17

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

18

1

ACS Paragon Plus Environment


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19

ABSTRACT

20

Promotion of iron solubility using ligands is the preliminary step in the homogeneous

21

electro-Fenton (EF) process at a mild pH, but the chelate efficiencies of most organic

22

ligands are unsatisfactory, resulting in insufficient Fe(II) availability. In this study,

23

atomic H* was, for the first time, introduced to the EF process to accelerate the

24

regeneration of the Fe(II)-complex at a mild pH using Ni-deposited carbon felt

25

(Ni-CF) cathode. The introduction of atomic H* significantly elevated total organic

26

carbon (TOC) abatement of ciprofloxacin (CIP) from 42% (CF) to 81% (Ni-CF) at a

27

natural pH. In the presence of humic acids (HAs), atomic H* introduced via Ni-CF

28

enhanced the CIP degradation rate to 10 times that of the CF at a mild pH. The

29

electron spin resonance (ESR), density functional theory (DFT) calculations,

30

electrochemical characterization and in situ electrochemical Raman study clearly

31

demonstrated that the atomic H* generated from the Ni-CF cathode was highly

32

efficient at reducing Fe(III)-complexes at a natural pH. Additionally, the Ni-CF could

33

generate atomic H* without significant nickel leaching. Thus, the atomic H* could

34

continuously

35

mineralization via the homogeneous EF process at a mild pH in an environmentally

36

friendly manner.

facilitate

iron

cycling

and,

consequently,

37

2


enhance

pollutant

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TOC/Abstract art:

39 40

3



41

INTRODUCTION

42

The electro-Fenton (EF) process is an effective approach for the complete

43

mineralization of bio-recalcitrant wastewater, but its practical application has always

44

been impeded by the iron cycling with the raise of natural pHs.1, 2 Continuous Fe(II)

45

regeneration for the EF system has been accomplished by introducing inorganic

46

ligands,3 Fe@Fe2O3 core-shell structures,4 or bimetallic catalysts.5, 6 Importantly, the

47

reductive pathway for Fe(III) species is necessary for a continuous Fenton reaction at

48

a mild pH, since Fe(II) species are critical for the generation of hydroxyl radicals

49

(•OH).7

50

Solid Fe(OH)3 is the most unexpected speciation of Fe(III) species for iron

51

cycling, and the increased solubility of iron species using ligands would be the

52

preliminary step of a homogeneous process at a mild pH. Nevertheless, the iron

53

chelates efficiency of most organic ligands is insufficient to dissolve enough solid

54

Fe(OH)3,8 and this is in addition to the negative effect on the consumption of •OH.9

55

Although a few pollutants, such as ciprofloxacin (CIP), could possess electro-activity

56

for a high chelates efficiency, total organic carbon (TOC) abatement would still be

57

hindered by the final conversion of less coordinated ligands.10 Moreover, the electron

58

transfer of natural organic matter (NOM) is even worse in high ionic strength systems,

59

such as that required for the EF process,11 increasing the barrier to reduce a stable

60

complex of Fe(III)-NOM.12 The photo-reduction of low chelate Fe(III)-complexes has

61

been accomplished,13,

62

homogeneous EF process by utilizing a local electric current has not been presented in

63

the literature.

14

whereas the promotion of Fe(III)-complex reduction in the

64

The most common cathodes for electrochemical degradation are carbon felt (CF)

65

and nickel foam (NF), and the electro-reduction rate of Fe3+ on the CF cathode is 4


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66

faster than that on the NF cathode.15, 16 Unfortunately, both cathodes can not ensure

67

continuously active reductive species for Fe(III)-complex reduction under natural

68

conditions.16 The present modification methods for CF cathodes in the homogeneous

69

system require an acidic pH, ensuring that the available Fe(III) is in accord with the

70

basic needs of the EF process.17,

71

atomic H* could be used in the EF process.19 Atomic H* has been successfully

72

adopted to reduce chemisorbed Fe(III) and, thus, might be useful for effective

73

reduction of Fe(III)-complexes under natural conditions. The active H* site for the

74

carbon-supported Pt material has been shown to be available, but multiplex pollutants

75

in wastewater could poison the Pt.20 As nickel would not be poisoned in such a

76

manner, and the carbon base could manipulate the activity of the nickel, a reductive

77

species of atomic H* might be generated with a similar carbon-nickel structure.21

78

Importantly, the carbon-supported nickel structure has been found to improve the

79

dissociation of water during the hydrogen evolution reaction (HER) by providing H*

80

adsorption sites; therefore, a Ni coating might be an excellent candidate for

81

introducing atomic H*.22 Additionally, carbon tailoring could facilitate the turn-off

82

between faster generation of atomic H* and slower H2 desorption, according to the

83

d-band center theory,23 as the H2 might not be as active as atomic H* in reducing the

84

Fe(III)-complexes.

18

Interestingly, our recent research proposed that

85

Herein, the generation of atomic H* was introduced by coating nickel onto the

86

surface of a CF cathode using a fast, easily scalable, and affordable electro-deposition

87

method.24 The influence of atomic H* on the homogeneous EF at a natural pH was

88

evaluated using widespread and bio-recalcitrant CIP as the pollutant. The

89

carbon-coated nickel foam (C-NF), CF, and NF were also adopted at the cathode for

90

comparison. The results suggest that the Ni coating on the CF (Ni-CF) was the most 5



91

favorable structure for generating atomic H* compared to the C-NF, CF and NF,

92

indicating the significant promotion of the EF process at a natural pH in terms of the

93

CIP degradation, particularly in the presence of humic acids (HAs). Electron spin

94

resonance (ESR) analysis directly confirmed that the presence of atomic H* was only

95

found in the Ni-CF system, serving to reduce the low chelate Fe(III)-complexes and

96

Fe(III)-HA. The in situ electrochemical Raman study proved the interaction between

97

the Fe(III)-complexes and atomic H*. The HER results suggest that the water

98

dissociation step was promoted after Ni coating. The density functional theory (DFT)

99

calculations reveal that the tailoring of the carbon base might prolong the presence of

100

H*, which was confirmed by cyclic voltammetry (CV) analysis. Moreover, the

101

promotion of iron cycling by atomic H* was confirmed as the dominant mechanism

102

for the enhanced CIP mineralization at natural pHs, which occurred by excluding the

103

impact of excess H2O2 generation by the oxygen reduction reaction (ORR). Finally,

104

the evaluation of Ni leaching and recyclability indicated that the atomic H* generated

105

through the Ni coating on the CF cathodes could serve as an efficient, durable, and

106

environmentally friendly approach for pollutant removal at mild pHs via the EF

107

process.

108 109

EXPERIMENTAL SECTION

110

Chemicals and Materials

111

CIP (C17H18FN3O3, > 98% purity) and HA were obtained from Aladdin Reagent Co.

112

(Shanghai, China). Na2SO4, absolute ethanol, p-benzoquinone, tertiary butanol,

113

glucose, NiSO4·7H2O, NiCl2·6H2O, FeSO4·7H2O, Fe2(SO4)3·xH2O, H3BO3, H2SO4,

114

HCl, HNO3, hydroxylamine hydrochloride (HONH2∙HCl), 1,10-phenanthroline, and

115

dimethyl pyridine N-oxide (DMPO, 97%) were bought from Sigma-Aldrich (USA) at 6


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analytic grade. High-purity water for all solutions was prepared using a Millipore

117

(USA) Milli-Q water purification system.

118 119

Preparation of Various Cathodes

120

The electrochemical deposition of Ni on the CF (Qingdao Baofeng Graphite Material

121

Co., China; 10 cm × 5 cm × 0.5 cm) was processed with optimization according to

122

Döner et al.24 Briefly, the CF was activated and deposited using DC power

123

(WYK-303S; Shenzhen Nolepower Technology Co., China) and denoted as Ni-CF in

124

this study. For comparison, the C-NF was processed according to Song et al.25

125

Briefly, the commercial NF (Hunan Corun New Energy Co., China; 10 cm × 5 cm ×

126

0.5 cm) was pretreated and carbonized, and the NF was treated for a total of four

127

cycle times and was denoted as C-NF in this study. The preparation methods for

128

Ni-CF and C-NF are presented in detail in Text SI-1. In addition, the CF and NF were

129

cut to the same size at the cathode without further treatment for comparison.

130 131

Experimental Setup

132

All the EF trials were performed in an undivided and cylindrical glass cell with a

133

500-mL capacity. The DC power supply was set at a constant current for the

134

degradation of CIP containing synthetically prepared solutions. The dimensionally

135

stable anode (DSA) (Ti/RuO2-IrO2), from Baoji Ruicheng Titanium Co. (China), was

136

immobilized at the center as the anode and cooperated with the CF, Ni-CF, NF, and

137

C-NF cathodes. Electrolysis was initiated by addition of compressed air, supplied at

138

0.5 L/min for 30 min, into the solution containing 0.05-M Na2SO4, 0.2-mM Fe2+, and

139

50-mg/L CIP. The solution was treated as prepared without pH adjustment; if

140

indicated, the initial pH was adjusted to 3.0 with 0.1 M H2SO4 using a Hanna HI9025 7



141

pH meter. A constant current of 200 mA was set to the 2-electrode system using a DC

142

power supply, ensuring that the differentiation efficiency was entirely contributed by

143

the cathode property. Repeated runs were conducted by collecting, washing, and

144

drying the used cathodes for subsequent 90-min electrolysis reactions under the same

145

experimental conditions.

146 147

Characterization and Analytical Methods

148

The morphological characterization of the cathodes and elemental mapping of C and

149

Ni were conducted using a scanning electron microscopy-energy dispersive X-ray

150

spectrometer (SEM-EDS; JSM-7001F; JEOL, Japan). The crystalline phase and

151

identification of the elemental composition was assessed based on X-ray diffraction

152

patterns (XRD; TTR-III; Rigaku, Japan), XPS (ESCALAB 250Xi; ThermoFisher,

153

USA) using monochromatic Al Kα radiation, and a micro-Raman spectrometer at 633

154

nm from an argon ion laser excitation source (LabRam HR; Horiba, France).26 All the

155

details for the in situ electrochemical Raman spectra and electrochemical

156

characterization are provided in the Supplementary Information (Text SI-2-3).

157

The time-consuming of aqueous Fe(II) was followed using a 1,10-phenanthroline

158

colorimetric method, employing UV-vis spectrometry at a wavelength of 510 nm.12

159

The Fe(III)-HA could be removed using 0.2 mm pore size filters, and the large-sized

160

Fe(III) micelles were eliminated at this point as well.27 In addition, the Fe2(SO4)3

161

solutions for the determination of total iron were prepared using 0.1 M H2SO4,

162

eliminating the interference from both the precipitation of Fe(III) and the increase in

163

volume.28 The Ni leaching of the cathode was analyzed using ICP-AES (Perkin

164

Elmer, USA). The accumulation of H2O2 was determined using the titanium

165

oxysulfate method, and the wavelength for UV-vis spectrometry was set at 409 nm.29 8


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The F- concentration was determined by Dionex ICS-1100 ion chromatography

167

equipped with a 4 × 250 mm Dionex AS-19 analytical column and 4 × 50 mm Dionex

168

AG-19 guard column. The mobile phase for the F- analysis was 10 mM KOH at a

169

flow rate of 1.25 mL/min.30 The concentration of CIP was followed using HPLC

170

(Agilent Technologies, USA) equipped with a diode array detector (DAD) and an

171

Eclipse Plus C18 column (5-μm particle size, 4.6 × 250 mm). The mobile phase for

172

the CIP was a 0.3% formic acid/acetonitrile (75:25, v/v) mixture at flow rate of 1

173

mL/min, and the wavelength used was 278 nm.10 The thermal catalytic oxidation

174

method for the Shimadzu TOC-VCSH analyzer was employed to assess the TOC

175

abatement, whereas the specific energy consumption (SEC) for each cathode was

176

computed using eq 1:31

177

SEC = (TOC0 ― TOCt) × V

178

where Uc, I, V, and t re the average voltage, applied electric current, volume of the

179

treated wastewater, and reaction time during the electrolysis, respectively. TOC0 and

180

TOCt are the TOC values at the initial time and time t.

181

Uc × I × t

(1)

The atomic H*, O2•-, and •OH were trapped by DMPO and detected using a Bruke

182

200A-9.5 ESR spectrometer (JEOL, Japan). The d-band center tailoring of the Ni{111}

183

facet on the carbon substrate was conducted by DFT calculations, as described in the

184

Supplementary Information (Text SI-4).

185 186

RESULTS and DISCUSSION

187

Characterization of the Various Cathodes

188

The diameter of the CF was ca. 28 μm (Fig. 1a), and the Ni coating made little

189

difference to the size or morphology of whole fiber (Fig. 1b). The NF showed a much

190

rougher Ni framework (Fig. 1c), whereas the diameter of the numerous carbon 9



191

microspheres introduced for the C-NF was ca. 1 μm (Fig. 1d). The EDS mapping of

192

Ni-CF and C-NF gave evidence to the loading of Ni and C on the CF and NF,

193

respectively (Fig. 1e, f).

194

The metallic state of Ni and element composition of the Ni-CF and C-NF are

195

shown in the full XPS spectra (Fig. SI-1a). The high-resolution XPS spectra of Ni2p

196

indicated the valence states of Ni2+ and Ni0 (Fig. SI-1b). The Ni2+2p3/2 and Ni2+2p1/2

197

spectra for the peaks centered at 855.8 and 873.9 eV in the Ni-CF were much higher

198

than the C-NF, whereas the Ni02p3/2 and Ni02p1/2 spectra for the peaks centered at

199

852.9 and 870.6 eV were only observed in the C-NF.32 Nevertheless, the superficial

200

information provided by the XPS spectra might not have revealed the entire property

201

of the carbon and nickel interface of the cathodes. The content of C1s for C-NF

202

detected in the XPS spectra was high (Fig. SI-1c), but the XRD pattern of the C-NF

203

peak {002} at 23.52o showed no response (Fig. SI-1d). Obviously, the carbon

204

microspheres in the SEM image of the C-NF were denoted as the presence of

205

amorphous carbon.26 Furthermore, the peak of the Ni{111} facet centered at 44.68o

206

(Fig. SI-1d) was hindered by the amorphous carbon on the C-NF surface, which was

207

not hindered by the high oxidation state of Ni on the Ni-CF surface (Fig. SI-1a). The

208

XPS spectra and the XRD pattern comprehensively reveal that the superficial NiO

209

would cover the Ni0 on the Ni-CF surface, while the facet of Ni{111} was still

210

available.

211 212

CIP Removal Kinetics and Mineralization at a Natural pH

213

The removal of CIP was evaluated in the EF process with the different cathodes,

214

including CF, Ni-CF, NF, and C-NF, at a natural pH (Fig. 2a). The Ni-CF showed no

215

obvious superiority in the CIP removal kinetics at a mild pH within 90 min among 10


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four cathodes. This could have been due to the generation of Fe(III)-CIP chelate,10

217

ensuring that adequate Fe(II) was available at the initial stage of the EF process (Fig.

218

2b). Moreover, the presence of a Fe(III)-CIP chelate might conceal the function of the

219

additional reduction pathway, indicating the reduction of Fe(III)-CIP would not limit

220

the degradation process.

221

Interestingly, the CIP degradation rate constant (0.028 min-1) was the lowest for

222

the CF compared to other Ni-containing cathodes at a mild pH (Fig. 2a), although the

223

highest concentration of Fe(II) was available in the CF system (Fig. 2b). The high CIP

224

removal rate of the Ni-containing cathodes might be attributed to the additional

225

generation of H2O2 via one-electron oxygen reduction of O2•- from the chemical

226

corrosion of Ni0 (eq 2).33 The DMPO trapping experiments suggested the coexistence

227

of •OH and O2•- in the system with the Ni-containing cathodes (Fig. SI-2). After

228

quenching O2•- using benzoquinone (BQ), the CIP degradation rate became highest for

229

the CF (Fig. SI-3), suggesting the high performance of the Ni-containing cathodes

230

was likely due to O2•- generation. Obviously, the promotion of H2O2 production

231

appeared to be the rate-limiting step if the reduction of Fe(III) was not hindered.

232

Ni + 2O2→Ni2 + +2·O2―

(2)

233

As shown in Fig. SI-4, under a constant current of 200 mA, the stable

234

Ti/RuO2-IrO2 anode potential was almost same with various cathodes (around 1.3 V),

235

whereas the stable potential of the CF, Ni-CF, NF, and C-NF cathodes was obviously

236

different with a highest one of -1.6 V for the Ni-CF cathode. Accordingly, the effect

237

of cathode potential on CIP degradation in the EF process was investigated in a

238

three-electrode system. As shown in Fig. SI-5, the more negative cathode potential

239

resulted in higher CIP degradation efficiency in the EF process with each cathode.

240

Moreover, the variation of cathode potential had more remarkable effect on CIP 11



241

degradation in the EF process with CF, NF and C-NF cathodes compared to the

242

Ni-CF one. Meanwhile, as shown in Fig. SI-5, the potentials far exceeded the

243

decomposition voltage of water, highly suggesting that oxygen and hydrogen would

244

be generated on the anode and cathode in the EF process, respectively. As shown in

245

Fig. SI-6, the pH reduced from 7.0 to 5.0 during CIP degradation in the EF process

246

with both CF and Ni-CF cathodes in the absence of NOM, while it changed slightly in

247

the presence of NOM. In addition, we found that the pH changed insignificantly

248

without Fe species (Fig. SI-6). These results suggest that the reduction of solution pH

249

during CIP degradation in the EF process without NOM would likely result from the

250

hydrolysis of Fe(III) rather than H2 and O2 production on the electrodes. The pH

251

decrease may affect the CIP degradation efficiency in the EF process, but this impact

252

was not resulted from water splitting reaction.

253

The TOC decay for the evaluation of the various cathodes could prove more

254

valuable since the Fe(III)-CIP chelate would finally convert into low chelate

255

Fe(III)-carboxylate complexes.13 As shown in Fig. 2c, the TOC removal at a natural

256

pH was significantly improved by introducing the Ni coating onto the CF. The natural

257

conditions greatly limited the 8-h TOC abatement in the EF process for CF (42%) as

258

compared to the Ni-CF (81%). Interestingly, the TOC removal of NF (60%) and C-NF

259

(71%) were lower compared to the Ni-CF (Fig. 2c), even though they had comparable

260

CIP removal kinetics at the initial stage (Fig. 2a). The difference of the CIP

261

degradation kinetics and TOC removal for various cathodes might be due to that the

262

fractures from CIP degradation had lost the Fe(III) chelating efficiency, as suggested

263

by the decreased available Fe(II) concentration for the CF, NF, and C-NF from 4 to 8

264

h as compared to the Ni-CF (Fig. SI-7). The decrease of available Fe(II) might be

265

caused by the following two reasons. On one hand, the Fe(II)-complexes could be 12


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oxidized into Fe(III)-complexes by •OH, and then the Fe(III)-complexes would finally

267

be degraded into oxidized products.34 On the other hand, the degradation of ligands by

268

•OH

269

the Ni-CF (1.80 kWh/g TOC) was the lowest among the cathodes at 8 h at natural

270

pHs, demonstrating the advantage of the Ni-CF in terms of energy elimination.

could also eliminate the Fe chelating species.35 Additionally, the SEC value of

271

To determine the actual reductive spices, the ESR spectra were adopted to verify

272

the presence of atomic H* (Fig. 2d). The signal of the nine characteristic peaks for

273

DMPO-H was only observed in the Ni-CF system, whereas the signal in the other

274

cathodes was negligible.36 As mentioned above, atomic H* is able to promote the

275

available Fe(II) on condition that the chelate efficiency of the ligand to Fe(OH)3 is

276

limited. Therefore, it seems that the atomic H* might not be the predominant pathway

277

for the adequate reduction of aqueous Fe(III) at an acidic pH. Actually, the electron

278

always conducts the reduction of aqueous Fe(III) at the optimal pH of 3.0.1

279

Fortunately, the TOC removal efficiency of the CF, NF, and C-NF greatly improved

280

at pH 3.0 as compared to that of the Ni-CF (Fig. SI-8), indicating the available

281

electron was more efficient than atomic H* under acidic conditions. The results of the

282

removal kinetics and TOC evolution suggested that the atomic H* accelerated the

283

reduction of low chelate Fe(III)-complexes and indirectly promoted the aqueous

284

Fe(III) by dissolving more solid Fe(OH)3.

285

The CIP defluorination efficiencies in the EF process with various cathodes are

286

shown in Fig. SI-9. The EF process with the Ni-CF cathode achieved the highest

287

defluorination efficiency compared to other three cathodes. Since the atomic H*

288

majorly presented in the Ni-CF system, this result suggests that atomic H* would

289

have a positive impact on CIP degradation in the EF process. On one hand, atomic H*

290

could react with CIP by abstracting F atoms from CIP, contributing to the CIP 13



291

degradation.30 On the other hand, the atomic H* facilitated the regeneration of Fe(II)

292

in the Fenton reaction to produce •OH particularly after 120 min (Fig. SI-7), which

293

could also result in the degradation of CIP.

294 295

Effect of NOM

296

NOM is widespread in wastewater, and numerous carboxyls in the NOM can result in

297

the generation of Fe(III)-NOM at a mild pH, which might interfere the chelation of

298

CIP during the EF process. Herein, the Fe2(SO4)3 solution was adopted to evaluate the

299

chelation effects of CIP and HA as typical NOMs, avoiding the unpredictable factor

300

caused by Fe(II).37 The solutions were stirred to complete the chelation without pH

301

adjustment, and the available Fe(III) concentration was measured.8 In order to prove

302

the chelating of HA/CIP with Fe(III), the ESR spectra were performed for different

303

solutions containing Fe(III). As shown in Fig. SI-10, the introduction of both CIP and

304

HA increased the spin state of Fe(III), indicating the chelating of Fe(III)-CIP and

305

Fe(III)-HA.2 As shown in Fig. 3a, the introduction of CIP for generation of Fe(III)

306

chelate promoted the availability of aqueous Fe(III). Interestingly, the increase in

307

Fe(III)-HA concentration suggested that the HA would compete with CIP to chelate

308

Fe(III), which might block the electro-activity of Fe(III)-CIP.11 It was reported that

309

the major active reductive species would be dependent on the chelate ligand of the

310

Fe(III) complexes in the EF system.38 Generally, the reduction of Fe(III)-HA relies on

311

unidirectional thermal reduction2 and the poor electron reduction of Fe(III)-HA,39

312

both of which are not effective in the EF process, particularly under conditions of high

313

salinity.11 In contrast, atomic H* might serve as a convincing reduction species in this

314

situation.

315

Actually, the EF process for CIP degradation was inhibited for all of the cathodes 14


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in the presence of humic acids (HAs), where the Ni-CF cathode resulted in lowest

317

inhibition compared to other three cathodes. This is because that more atomic H* was

318

produced for the Ni-CF cathode, leading to continuous reduction of Fe(III)-HA to

319

Fe(II)-HA and consequently higher CIP degradation efficiency than other cathodes.

320

As shown in Fig. 3b, the degradation rate constant of CIP for Ni-CF (0.0375 min-1)

321

was the highest compared with the other cathodes, and even 9.87 times higher than

322

that of CF (0.0038 min-1) at a natural pH. In the Ni-CF system, the Fe(III)-HA can be

323

reduced to Fe(II)-HA by atomic H* (eq 3), and the Fe(II)-HA can easily carry on the

324

subsequent decomposition of H2O2 (eq 4).2 As shown in Fig. 3c, the available Fe(II)

325

concentration was highest for the Ni-CF in the presence of 50-mg/L HA, suggesting

326

the Ni-CF cathode was more efficient in the reduction of Fe(III)-HA at a natural pH.

327

The degradation efficiencies of all the cathodes were promoted in the acidic pH,

328

especially for the CF, NF and C-NF (Fig. SI-11). As the carboxylate is going to

329

change into carboxyl at acidic pH, the Fe(III)-HA would be dissociated as Fe(III) and

330

HA due to the lower coordination efficiency of carboxyl.40 Therefore, the Fe(III)

331

could be directly reduced to Fe(II) on all the cathode, and the atomic H* was not

332

necessary to reduce the Fe(III)-HA for the degradation process. It should be noted that

333

the detected Fe(II) concentration in the presence of HA was lower than that in the

334

absence of HA at a natural pH, and this was due to the completion of HA with

335

1,10-phenanthroline for chelating Fe(II) during the Fe(II) measurements.12 Overall,

336

above results clearly indicated that the atomic H* directly reduced the Fe(III)-HA,

337

ensuring sufficient Fe(II)-HA to conduct eq 4.

338

H ∗ +Fe(III) ―HA→H + +F(II) ―HA

339

H2O2 +Fe(II) ―HA→·OH + OH ― +Fe(III) ―HA

340

(3) (4)

Furthermore, the in situ electrochemical Raman spectroscopy was conducted 15



341

using the Ni-CF during CIP degradation in the absence and presence of NOM,

342

respectively. As shown in Fig. SI-12, the new peak located at 1106 cm-1 was observed

343

at 180 min for CIP degradation without NOM, whereas the peak of 1083 cm-1 was

344

found in the presence of NOM, indicating the formation of v(Fe-H) during CIP

345

degradation in the EF process with Ni-CF cathode.41 This result verified the

346

interaction between adsorbed H* and the low chelate Fe(III)-complexes, since the

347

Fe(III)-CIP completely transferred into low chelate Fe(III)-complexes after 180 min

348

in the EF process. In addition, the presence of δ(H-Ni-Fe) bond located at 1437 and

349

1528 cm-1 suggests indirectly interaction between the adsorbed H* and

350

Fe(III)-complexes.41 Moreover, there was an oxidation peak of atomic H* in the CV

351

curves for the Ni-CF in the absence of Fe(III)-HA (Fig. SI-13c), while this peak was

352

not found for the Ni-CF in the presence of Fe(III)-HA (Fig. SI-14), strongly

353

suggesting most of the atomic H* would be utilized for the reduction of chelate

354

Fe(III)-complexes.

355 356

Catalytic Mechanisms of the Ni-CF Cathode

357

In order to understand the catalytic mechanisms of the Ni-CF, both the HER and ORR

358

were introduced to investigate if the enabled removal efficiency was contributed by

359

the additional H2O2 or atomic H*. The overpotential of Ni-CF (226 mV) was much

360

lower than that of CF (744 mV) at 5 mA/cm2 (Fig. 4a). Moreover, the Tafel study

361

indicated a slope of 590.2 and 350.6 mV dec-1 for the CF and Ni-CF, respectively

362

(Fig. 4a).42 The Tafel slope provides an insight into the HER kinetics according to the

363

kinetic analysis in previous studies,22, 43 involving the control step of the Volmer (eq

364

5, 120 mV dec-1), Heyrovsky (eq 6, 40 mV dec-1), and Tafel (eq 7, 30 mV dec-1)

365

pathways. Apparently, the HER of the tested cathodes was mainly limited by water 16


Page 16 of 35

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366

dissociation, as the Tafel slope was much higher than 120 mV dec-1.44 This result also

367

suggests that the Ni coating facilitated the Volmer step of the CF cathode to generate

368

H*.22, 45 in particularly the surface coverage of atomic H* was promoted.43

369

H2O + e ― →H ∗ + OH ― (Volmer)

(5)

370

H2O + H ∗ + e ― ↔H2(g) + OH ― (Heyrovsky)

(6)

371

2H ∗ ↔H2(g)(Tafel)

(7)

372

Although the Ni coating promoted the production of atomic H*, the presence of

373

active H* still required the tailoring of the carbon. The partial exposure of Ni on the

374

surface of CF could be a favorable structure for the spillover of atomic H*, whereas

375

the carbon could stabilize the atomic H* for the electric field.20 The DFT calculation

376

for the superlattice of the Ni{111}-graphite and bare Ni{111} facet was performed,

377

and the values of the d-band center were calculated as -1.30 and -1.45 eV,

378

respectively (Fig. 4b). The ascension of the d-band center (shifting up to an

379

antibonding state) after graphite tailoring provided evidence for the enhancement of

380

spillover of atomic H* on the Ni-CF.23 In other words, the transformation from atomic

381

H* to H2 was slower on the Ni-CF as compared to the NF.46 Furthermore, the relative

382

content of carbon was adjusted by changing the Ni coverage to simulate the loss of

383

nickel, and the d-band center of the different Ni coverages on the CF was also

384

calculated (Fig. SI-15). A gradually higher center for the d-band was observed with

385

the drop of Ni coverage, indicating the loss of Ni species enabled the spillover of the

386

adsorbed H* and dissociation of H2.46 The combinations of HER kinetics and

387

calculation strongly indicate that the atomic H* possessed a high catalytic activity in

388

the Ni-CF system, and the spillover of atomic H* to the carbon could improve the

389

reduction of the Fe(III)-complexes. In order to further support this conclusion, the CV

390

curves with varying start potentials were adopted to identify the reductive species on 17



391

each cathode according to previous studies.47,48 The oxidation peak was not found in

392

the CF system (Fig. SI-13a), and the H2 oxidation peak was observed without notable

393

peak of H* oxidation for the NF (Fig. SI-13b). These results suggest both of the CF

394

and NF were not efficient to provide H* as the cathode during the electrocatalysis. On

395

the contrary, it was observed the oxidation peak of H* for the Ni-CF cathode (Fig.

396

SI-13c), further verifying the enhancement of spillover of the adsorbed H* and

397

dissociation of H2 on the Ni-CF.

398

On the other hand, the activity and selectivity of the ORR evaluation were

399

required to distinguish the dominant mechanism. Thus, the RDE tests for the

400

mentioned cathodes were systematically conducted (Fig. SI-16 to Fig. SI-19). As

401

shown in Text SI-5, the Koutecky-Levich (K-L) equations were adopted to determine

402

the apparent electron transfer number,49 and the electroactive surface area was

403

corrected using the Randles-Sevcik equation.50 The CV measurements were directly

404

conducted on the RDE, and the electroactive surface area of all cathodes was

405

normalized (Fig. SI-20). The final electron transfer numbers obtained for the CF,

406

Ni-CF, NF, and C-NF cathodes were 2.61, 3.29, 1.28, and 2.37, whereas the

407

corresponding H2O2 production concentrations were 32, 44, 58, and 41 mg/L,

408

respectively (Fig. 4c), indicating that the selectivity of the two-electron pathway was

409

decreased after Ni coating onto the CF. The estimated electron transfer number for the

410

NF was below 2.0, and this was attributed to the single electron reduction per O2 from

411

the chemical corrosion of Ni0 (eq 2). As a consequence, the increased production of

412

H2O2 on the Ni-CF would also be caused by the Ni0.

413

In order to exclude the unstable Ni0 species for H2O2 production on the Ni-CF

414

cathode, the experiments were designed using either HCl etching or 16 repeated runs.

415

The XPS spectra of the Ni-CF after a 5-min etching confirm the presence of Ni0 under 18


Page 18 of 35

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416

the surface oxide layer, while the samples after a 10-min etching and 16 repeated runs

417

verified the stable presence of Ni2+ on the CF (Fig. SI-21). The results of XPS suggest

418

that most of Ni atoms coordinated at the nickel and carbon interfaces were Ni2+,

419

demonstrating a greater stability than both the unstable Ni0 and the surface nickel

420

oxide.51 As illustrated in Fig. 4c, the selectivity of 2 e- reduction for the Ni-CF

421

cathode was not better than the CF one, but higher H2O2 accumulation was observed

422

on the Ni-CF cathode, which might be attributed to the Ni0 on the Ni-CF. In order to

423

further prove this, we also measured the H2O2 accumulation of the CF, Ni-CF, and

424

Ni-CF with Ni0 removal using 10-min HCl etching (Fig. SI-22). The H2O2 production

425

of the Ni-CF was higher than that of the CF, while the lower H2O2 accumulation was

426

found for the Ni-CF with Ni0 removal compared to the CF. These results indicate that

427

the active cites for the O2 adsorption and reduction to produce O2•- could be Ni0 on the

428

Ni-CF, since O2 is reported to be reduced by Ni0 to form O2•-.33 On the other hand, the

429

CV curves with varying start potentials were adopted to identify the atomic H* for

430

various cathodes. The oxidation peak of atomic H* was observed for the Ni-CF

431

cathode with and without 10-min HCl etching for Ni0 removal (Fig. SI-13c and Fig.

432

SI-23), indicating the Ni0 would not be the active cites for the H2O adsorption and

433

reduction to produce atomic H*. Moreover, the oxidation peak of atomic H* was

434

found in the CV curves for the CF cathode (Fig. SI-13a). These results imply that the

435

coordinated Ni atom at the nickel and carbon interface determined the atomic H*

436

generation, since the 10-min HCl etching was unable to remove the coordinated Ni

437

atom on the Ni-CF (Fig. SI-21b). Correspondingly, the coordinated Ni atom on the

438

Ni-CF could be the active cites for H2O adsorption and reduction to generate the

439

atomic H*. It’s reported that the O2•- is tend to combine with H+ to generate the

440

H2O2,52 while the reactions of atomic H* with H2O2 (2H* + H2O2 → 2H2O) and •OH 19



441

(H* + •OH → H2O) to produce H2O are thermodynamically favorable.53 Therefore,

442

these latter two reactions would likely happen in the EF process with the Ni-CF

443

cathode for CIP degradation, resulting in competition of atomic H* with

444

Fe(III)-complexes reduction. Additionally, as shown in Fig. SI-12, the v(Fe-H) bond

445

wasn’t formed at 30 min in the absence of HA, indicating an insignificant contribution

446

of atomic H* to reduce Fe(III)-complex to initiate the E-Fenton process at the initial

447

reaction stage. Under such conditions, the atomic H* would have a remarkable

448

contribution after the iron species changed from Fe(III)-CIP to low chelate

449

Fe(III)-complex (i.e., between 30 and 180 min). However, the formation of the

450

v(Fe-H) bond was found at the initial reaction stage (i.e., 30 min) in the presence of

451

HA, suggesting an important role of atomic H* for Fe(III)-complex reduction to

452

initiate the E-Fenton mineralization of CIP during the whole process.

453

Furthermore, the removal of CIP in the presence of HA was evaluated using the

454

Ni-CF after a 10-min and 120-min HCl etching or 16 repeated runs, where the

455

cathodes’ remaining coordinated Ni atom showed comparable efficiency for CIP

456

removal to that of the untreated Ni-CF (Fig. 4d). In other words, the generation of

457

H2O2 caused by the unstable Ni0 species did not significantly contribute to CIP

458

removal, which implied that the regeneration of Fe(II) species would be the

459

rate-limiting step in the presence of Fe(III)-HA. Moreover, complete damage of the

460

Ni attachment after a 120-min etching caused a sharp drop in the generation of the

461

reducing species, confirming the critical effect of the coordinated Ni atom on atomic

462

H* production. Overall, the dominant mechanism for the high TOC abatement with

463

the Ni-CF at a natural pH, particularly in the presence of NOM, would result from the

464

regeneration of Fe(II) species by inducing atomic H*.

465 20


Page 20 of 35

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466

Leaching and Recyclability Evaluation

467

The leaching of nickel is of sufficient environmental concern for the application of

468

Ni-containing cathodes, as excess nickel ions can be removed by calcium hydroxide.33

469

The evolution of nickel leaching at a natural pH was compared with freshly prepared

470

Ni-CF, C-NF, NF, and Ni-CF after a 10-min etching or 16 repeated runs (Fig. 5a).

471

The nickel leaching of Ni-CF (24.1 ppm) resembled that of NF (27.9 ppm) at a natural

472

pH after a 60-min reaction, whereas the leaching of Ni-CF after HCl etching (3.1

473

ppm) and 16 repeated runs (1.4 ppm) was much lower within 60 min, suggesting Ni

474

leaching can be eliminated via the removal of unstable Ni0. It should be noted that the

475

production of atomic H* was still maintained with relatively low Ni leaching (

Cathode-Introduced Atomic H* for Fe(II)-Complex Regeneration to

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