Extremely efficient decomposition of ammonia-N to N2 using ClOâ¢ from

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Remediation and Control Technologies

Extremely efficient decomposition of ammoniaN to N using ClO• from reactions of HO• and HOCl generated in situ on a novel bifacial photo-electroanode 2

Yan Zhang, Jinhua Li, Jing Bai, Linsen Li, Shuai Chen, Tingsheng Zhou, Jiachen Wang, Ligang Xia, Qunjie Xu, and Baoxue Zhou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00959 • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on June 2, 2019

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Environmental Science & Technology

1

Extremely efficient decomposition of ammonia-N to N2 using ClO•

2

from reactions of HO• and HOCl generated in situ on a novel bifacial

3

photo-electroanode

4

Yan Zhang a, Jinhua Li a, Jing Bai a, Linsen Li a, Shuai Chen a, Tingsheng Zhou a, Jiachen

5

Wanga, Ligang Xiab, Qunjie Xub,c, Baoxue Zhou a,c,d

6

a School

7

No. 800, Dongchuan Rd, Shanghai 200240, PR China.

8

bCollege

9

Power, No.2588 Changyang Road, Shanghai, 200090, PR China.

of Environmental Science and Engineering, Shanghai Jiao Tong University,

of Environmental and Chemical Engineering, Shanghai University of Electric

10

cShanghai

11

China.

12

dKey Laboratory of Thin Film and Microfabrication Technology, Ministry of Education,

13

Shanghai 200240, PR China.

Institute of Pollution Control and Ecological Security, Shanghai, 200092,PR

14



Corresponding authors at: School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China E-mail addresses: [email protected] (B.X. Zhou); [email protected](J. Bai)

ACS Paragon Plus Environment


15

ABSTRACT

16

The conversion of excess ammonia-N into harmless N2 is a primary challenge for

17

wastewater treatment. We present here a method to generate ClO• directionally for

18

quick and efficient decomposition of NH4+-N to N2. The ClO• was produced and

19

enhanced by a bifacial anode: a front WO3 photoanode and a rear Sb-SnO2 anode, in

20

which HO• generated on WO3 reacts with HClO generated on Sb-SnO2 to form ClO•.

21

Results show that ammonia decomposition rate of Sb-SnO2/WO3 is 4.4 times than that

22

of WO3 and 3.3 times than that of Sb-SnO2, achieving the removal of NH4+-N on Sb-

23

SnO2/WO3 and WO3 are 99.2% and 58.3% in 90 min, respectively. This enhancement

24

is attributed to the high rate constants of ClO• with NH4+-N, which is 2.8 and 34.8 times

25

than those of Cl• and HO•, respectively. The steady-state concentration of ClO•

26

(2.5×10-13 M) is 102 times those of HO• and Cl•, and this is further confirmed by kinetic

27

simulations. Combining with Pd-Cu/NF cathode to form denitrification exhaustion

28

system, Sb-SnO2/WO3 shows excellent total nitrogen removal (98.4%), which is more

29

effective than WO3 (47.1%) in 90 min. This study provides new insight on the directed

30

ClO• generation and its application on ammonia wastewater treatment.

31 32 33 34 35 36


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37

TOC ART

38 39



40

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INTRODUCTION

41

The removal of ammonia nitrogen has been a primary challenge in wastewater

42

treatment due to its characteristics of offensive smell, aquatic biota toxicity, conducive

43

to eutrophication, and oxygen-consuming in nitrification process1, 2. Hence, focuses on

44

the enhancement of nitrogen removal has intensified and high standard is set for

45

wastewater discharge. Various technologies have been developed to remove NH4+-N,

46

including reverse osmosis3, breakpoint chlorination4, photocatalytic oxidation5, and

47

biological method6. However, their drawbacks have been reported frequently.

48

Biological treatment is greatly affected by the C/N ratio7 and reverse osmosis processes

49

require a secondary treatment8. Breakpoint chlorination is impeded by their high cost

50

and operational complexity9. Photocatalytic/Photoelectrochemical treatment only

51

converts NH4+-N to NO3‾-N, resulting in slow and ineffective removal of total nitrogen

52

from water 10-12.

53

To overcome above drawbacks, our group proposes a strategy for transforming

54

NH4+ to N2 based on Cl• radical reactions11. However, this method cannot achieve

55

complete denitrification, still 22% of NH4+ is converted to NO3‾ and remains in the

56

water. We then designed an exhaustive cycle system using Pd-Cu/NF as cathode to

57

convert inorganic nitrogen to N212. Unfortunately, the oxidation of NH4+ to N2 is

58

relatively low, limited by the low concentration of Cl• in the system. As known, the

59

generation of OH• and O2 are competitive processes,13

60

chloride ions to ClOH•‾ and ClO• instead of Cl• (Eqs.1-2).

61

HO• + Cl‾ → ClOH•‾

14

and OH• tends to convert

k=4.30×109 M-1s-1


(1)

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62

ClOH•‾→ HO•+ Cl‾

k=6.10×109 M-1s-1

(2)

63

HO• + HOCl→ ClO• + H2O

k=2.0×109 M-1 s-1

(3)

64

HO• + OCl‾→ ClO• + OH‾

k=8.8 ×109 M-1 s-1

(4)

65

In this study, we designed a method for the first time to generate ClO• directionally

66

for rapid decomposition of NH4+ to N2, which is more efficient than previous method

67

of Cl• reaction.11,12 Since ClO• has a high redox potential of 1.8 V 15, it reacts rapidly

68

with electron-rich moieties.16, 17 According to literature, the concentration of ClO• was

69

102-103 times those of HO• and Cl• in the UV/chlorine process18, and the reactivity can

70

be higher than that of Cl• in some instances19. Then ClO• usually produced easily during

71

the scavenging of •OH by free chlorine (HClO or ClO‾)20 with a reaction rate of 109

72

M−1 s−1 (Eq.3-4)21.

73

Based on this, we propose an idea to explore ClO• for rapid removal of NH4+-N.

74

In order to realize this idea, ClO• was produced directionally and enhanced by a

75

uniquely designed bifacial anode: a front WO3 photoanode and a rear Sb-SnO2 anode,

76

in which HO• generated on WO3 reacts with HClO generated on Sb-SnO2 to form ClO•.

77

In this hybrid anode, WO3 photoanode is selected for the generation of OH• because of

78

visible light response, great hole mobility and moderate hole diffusion length22 (Eq.5-

79

6). The tin oxide doped with antimony (Sb-SnO2) is selected as a candidate for HOCl

80

production due to excellent chlorine evolution activity23 and considerable electronic

81

conductivity24, 25.

82

WO3 + hv → h+ + e−

(5)

83

H2O + h+ → HO• + H+

(6)



84

Thus, we report a novel double-sided structure for modifying WO3 with Sb-SnO2,

85

which is used to generate ClO• in situ. The direct coating is avoided because Sb-SnO2

86

may fill the pores of WO3, thereby decreasing the photoelectrocatalytic performance.

87

The as-fabricated Sb-SnO2/WO3 shows synergetic effects on NH4+-N decomposition

88

by coupling electrocatalysts and photoelectrocatalysts, and their performance is much

89

superior to those of single WO3 and Sb-SnO2. By constructing an exhaustive

90

denitrification system with Pd-Cu/NF cathode, Sb-SnO2/WO3 can completely convert

91

total nitrogen into N2 in a short time. A combination of experimental and kinetic model

92

are used to investigate ClO• generation mechanisms and model their steady-state

93

concentrations in the system. This paper provides an economical and efficient method

94

for sustainable wastewater treatment.

95

MATERIALS AND METHODS

96

Materials. Fluorine-doped tin oxide (FTO) was purchased from Nippon Sheet

97

Glass Co., Ltd. Analytical grade Na2SO4, (NH4)2SO4 and NaCl were bought from

98

Sinopharm Chemical Reagent Co., Ltd. Benzoic acid (BA), nitrobenzene (NB) and 1,4-

99

dimethoxybenzene (DMOB) were obtained from Aladdin Industrial Corporation.

100

Electrode preparation. WO3 nanoplate arrays were fabricated on FTO via a

101

hydrothermal reaction22, and the detailed procedure is shown in Supporting Information.

102

To prepare the double-sided electrode, another side of FTO was protected by parafilm

103

and all edges were sealed with adhesive tape. The Sb-SnO2 films were deposited on

104

FTO by spin-coating method. 1.402g SnCl4·5H2O, 0.406g SbCl3 and 1g PEG 6000

105

were dissolved in 20 mL isopropanol and sonicated for 30 min to disperse evenly. The


Page 6 of 30

Page 7 of 30


106

SnO2 precursor sol was spread on the FTO and then spin-coated at 1000 rpm for 15s,

107

followed by drying at 350 °C for 5 min. This above cycle was repeated 6 times. Finally,

108

the electrode was annealed at 500 °C for 2 h under atmospheric air (heating rate 1°C

109

min-1). Figure.S1 represents the schematic illustration for fabricating Sb-SnO2/WO3.

110

Analytical Methods. The X-ray photoelectron spectra (XPS) was obtained using

111

an Omicron EA125. Morphologies and elemental composition were characterized by a

112

Zeiss SUPRA55-VP field emission scanning electron microscope (SEM) equipped with

113

an energy dispersion spectrometer (EDS). The crystalline structure was examined by

114

X-ray diffraction (XRD). Electron spin resonance (ESR) spectra were obtained with

115

Bruker Electron Paramagnetic Resonance Spectrometer (ESP 300E)26.

116

The DPD method was used to determine the concentrations of free chlorine in the

117

system26. NO3‾ and NO2‾ were measured by ion chromatography (Dionex, USA), and

118

NH4+ was measured by Nessler reagent using a UV-visible spectrophotometer at 420

119

nm. The photoelectrochemical performance of Sb-SnO2/WO3 was investigated by

120

linear sweep voltammetry. The concentration of chlorate and perchlorate were

121

identified by ion chromatograph (Shimadzu HIC-10A, Japan) using an anion exchange

122

column (Shin-Pack IC-SA2). The TN removal was monitored by a multi N/C 3100

123

TOC/TN analyzer (Analytikjena, Germany). NB, BA and DMOB were quantified by

124

HPLC (LC-20AT, Japan) coupled with C-18 column at 266 nm, 227 nm and 230 nm,

125

respectively.

126

Degradation experiment. The radical degradation of NH4+-N was performed in

127

a single quartz reactor and the WO3 side of the electrode were irradiated from a 150 W



Page 8 of 30

128

xenon lamp (Perfect, China). Sb-SnO2/WO3 was used as the anode, and a Pt sheet was

129

used as the cathode. 30 mg L−1 of NH4+-N solution was added into the cell. The initial

130

pH was 5 and constant potential of 1.7 V vs Ag/AgCl was applied. About 1 mL of

131

solution was taken out at predetermined time intervals for the analysis of ammonia,

132

nitrite and nitrate concentration.

133

Determination of the concentration of radical species. NB, BA and DMOB

134

were used as a probe to quantify the steady-state concentrations of HO•, Cl• and ClO•

135

in WO3/Sb-SnO2 system (Eqs.7-9). As known, NB only reacts with HO• (kHO•,

136

9 NB=3.9×10

137

S-1; kCl•, BA=1.8×1010 M-1 S-1). In addition, DMOB can react with HO•, Cl• and ClO• (k

138

HO•, DMOB=

139

KNB = k OH•, NB[HO•]

(7)

140

KBA = kOH•,BA[HO•] + kCl•,BA[Cl•]

(8)

141

KDMOB = k OH•, DMOF[HO•] + k Cl•, DMOF[Cl•] + k ClO•, DMOF [ClO•]

(9)

142

where KNB, KBA and KDMOB are the rate constants for the degradation of NB, BA and

143

DMOB in WO3/Sb-SnO2, respectively. [HO•], [Cl•] and [ClO•] are the steady-state

144

concentrations of HO•, Cl• and ClO•, respectively.

M-1 S-1), while BA reacts with both HO• and Cl•19 (kHO•, BA=5.9×109 M-1

7.0×109 M-1 S-1; kCl•, DMOB=1.8×1010 M-1 S-1; kClO•, DMOB= 2.1×109 M-1 S-1).

145

Determination of the rate constants between ClO• and NH4+. To exam the

146

second-order rate constants of reaction between ClO• and NH4+, we use DMOB as the

147

reference compound

148

2.1×109 M−1 S−1. DMOB and NH4+ were added into the system at concentrations of 2

149

mM. NaHCO3 (100 mM) was used to quench HO• and Cl• in the system.

27.

DMOB can react with ClO• rapidly with rate constants of


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[𝑁𝐻4+ ]𝑂

𝐾𝐶𝑙𝑂•,𝑁𝐻 + 4

[𝐷𝑀𝑂𝐵]𝑂

150

ln ( [𝑁𝐻 + ] ) = 𝐾𝐶𝑙𝑂•,𝐷𝑀𝑂𝐵 × ln ( [𝐷𝑀𝑂𝐵]

151

Where 𝐾𝐶𝑙𝑂•,𝑁𝐻4+ and 𝐾𝐶𝑙𝑂•,𝐷𝑀𝑂𝐵 represent the second-order rate constants of ClO•

152

reacting with NH4+ and DMOB, respectively. Furthermore, the rate constants of OH•

153

and Cl• reacting with NH4+ were also measured and the detail was shown in Supporting

154

Information.

4

(10)

155

Kinetic Model Simulation. Kinetic modeling of the radical production during the

156

experiment was performed using Kintecus 6.5 chemical kinetic modeling software

157

equipped with Bader-Deuflhard integrator28. The model established in this study

158

contained 24 reactions (Table.S1).

159

RESULTS AND DISCUSSION

160

Characterization of the bifacial anode. The morphologies of Sb-SnO2 was

161

characterized by SEM. Morphologies of the Sb-SnO2 layer prepared by spin coating

162

were dense and smooth (Figure 1A). In Figure 1B, it presented a uniform and less

163

‘‘crackmud” structure, which could favor the improvement of stability29. From the

164

cross-sectional SEM in Figure 1C, Sb-SnO2 was sponge-type film with the thickness

165

of 500 nm and the film tightly adhered to the FTO. The formation of Sb-SnO2 film was

166

then studied by XRD in Figure S2. The XRD pattern of Sb-SnO2 film has three peaks

167

at 2θ=26.6°, 33.9° and 53.0°, which were referenced to the (110), (101), and (211)

168

planes of SnO230. In fact, no significant change was observed in XRD spectrums after

169

coating of Sb-SnO2, where only little peaks appear at 2θ=19.1° and 29.2°. These peaks

170

may be responsible for the presence of Sb2O4 peaks. To confirm the Sb-doping, the

171

compositional analysis for Sb-SnO2 were performed by EDS. The distribution of Sn,



172

Sb and O confirmed the uniform deposition of Sb-SnO2 layer (Figure 1D). The surface

173

morphology of WO3 side was also investigated and it appeared to be plate-like

174

morphology (Figure S3A). From the cross-sectional SEM images in Figure S3B, the

175

thickness of the WO3 film remained almost the same at 900 nm.

176

In terms of the chemical bonding state of Sb-SnO2, the XPS measurement was

177

recorded. The survey spectrum (Figure 2A) showed several strong peaks assigned to

178

Sn 3d, O 1s and Sb 3d, indicating that the layer was composed of SnO2 and Sb. In the

179

Sn 3d region (Figure 2B), the spectrum had two peaks at 486.1 eV and 494.5 eV, and

180

the interval is 8.4 eV, which agreed with the standard spectrum of SnO2. In Figure 2C,

181

it exhibited two peaks at 539.2 and 540.1 eV, which correspond to Sb3+ and Sb5+,

182

respectively. In Figure 2D, the spectra was difficult to assign due to the binding

183

energies for O 1s overlap with Sb 3d 31. The O 1s spectra was deconvoluted into two

184

independent peaks at 530.7 and 532.0 eV, which associated with metallic oxides and

185

hydroxides, respectively.

186

Radical degradation of NH4+-N on Sb-SnO2/WO3. Figure 3A showed the

187

removal of NH4+-N by WO3/Sb-SnO2, WO3 and Sb-SnO2. The degradation processes

188

were found to obey first-order kinetics. For WO3, about 12.5 mg L-1 of NH4+-N was

189

removed in 90 min and the rate constant was 0.009 min−1 (Figure 3B). For Sb-SnO2,

190

10.3 mg L-1 of NH4+-N was degraded in 90 min and the rate constant was 0.012 min−1.

191

When WO3/Sb-SnO2 was used, about 99.2% of NH4+-N was decomposed and the rate

192

constant was 0.040 min−1, which was about 4.4 times that in WO3 electrode, 3.3 times

193

that in Sb-SnO2 electrode and even 1.9 times greater than the sum of individual WO3


Page 10 of 30

Page 11 of 30

194 195


and Sb-SnO2 electrode. Subsequently, we use synergy index to study the synergy effects of WO3/Sb-SnO2

196

on the degradation of NH4+-N according to the previous studies32.

197

S = 100% ×

𝐾𝑆𝑏 ― 𝑆𝑛𝑂2/𝑊𝑂3 ― 𝐾𝑆𝑏 ― 𝑆𝑛𝑂2 ― 𝐾𝑊𝑂3 𝐾𝑆𝑏 ― 𝑆𝑛𝑂2/𝑊𝑂3

(11)

198

S was calculated as 47.5%, which indicated that there was a significant synergistic

199

effect between photoelectrocatalysis and electrocatalysis. This synergistic effect may

200

be due to enhanced chorine evolution. We evaluated the electrocatalytic activities of

201

WO3/Sb-SnO2 in two different electrolyte solutions: a saturated NaCl solution for

202

chorine evolution reaction (CER), and a 0.2 M NaH2PO4 solution for oxygen evolution

203

reaction (OER). As shown in Figure S4, the currents for WO3 in NaCl and NaH2PO4

204

solutions were weaker than those for WO3/Sb-SnO2. The WO3/Sb-SnO2 displayed a

205

higher activity in NaCl than in NaH2PO4, so WO3/Sb-SnO2 were more efficient for

206

CER than for OER.

207

We monitored the LSV of Sb-SnO2, WO3, and Sb-SnO2/WO3 in 50 mM NaCl

208

under dark and irradiation condition, respectively. In Figure S5A, Sb-SnO2 showed an

209

abrupt increase of current at 1.25 V, which was due to the reactive chlorine species

210

(RCS) generation. In contrast, WO3 did not generate current up at 1.25 V. A slight

211

increase was observed when potential above 1.6 V. The behavior of Sb-SnO2/WO3 is

212

unique. It showed Sb-SnO2-like behavior in the absence of light. The current generated

213

on the bifacial electrodes is lower than that of Sb-SnO2 under the same potential, due

214

to the positive shift of chorine evolution potential. Under irradiation, Sb-SnO2/WO3

215

became similar to that of WO3. In Figure S5B, Sb-SnO2/WO3 has the largest current,



Page 12 of 30

216

which is consistent with LSV results. This indicates that Sb-SnO2/WO3 possess both

217

conductive and semiconductive characters.

218

Reactive chlorine generation. As known, electrochemical NH4+-N oxidation

219

process commences with the oxidation of Cl− to HClO 33, 34. We therefore investigated

220

the variations of free chlorine in Sb-SnO2, WO3, and Sb-SnO2/WO3. As shown in

221

Figure 4A, free chlorine gradually increased to 254 mg L-1 in Sb-SnO2/WO3, which

222

was lower than the sum of individual WO3 and Sb-SnO2. It indicated that free chlorine

223

was not primarily responsible for degradation of NH4+-N in Sb-SnO2/WO3.

224

Subsequently, ESR technique was performed using DMPO as a spin-trap reagent to

225

identify the oxidative radicals. The signals of DMPO-OH• were detected34, indicating

226

the generation of OH (Figure 4B). The signal of DMPO-ClO• was also observed 35and

227

other peaks of low intensity may be correspond to Cl•

228

OH• and ClO• in Sb-SnO2/WO3 were stronger than that in WO3. Therefore, radical-

229

dominated oxidation took responsibility in the Sb-SnO2/WO3. To clarify the roles of

230

•OH on the generation •ClO in this system, we compared the NH4+-N degradation by

231

HClO in the absence or presence of WO3 photoanode, and found that the degradation

232

rates significantly increased when WO3 was used, revealing that HClO may be

233

converted to more active ClO• (Figure S6A). Then the ESR test was carried out to

234

confirm the •ClO generation in this WO3/HClO process (Figure S6B).

36, 37.The

signals intensities of

235

To confirm the contributions of radicals, experiments are carried out with adding

236

different kinds of scavengers. NB can only react with OH• rapidly, while TBA can

237

scavenge HO•, Cl• and ClO•38. In addition, HCO3‾ is used to scavenge HO• and Cl•,


Page 13 of 30


238

while its reaction with ClO• was negligible 27. The rate constants of scavengers reacting

239

with different radicals are listed in Table S2. In Figure 4C, TBA could completely

240

inhibit NH4+-N degradation, indicating that radical reaction played a dominant role. By

241

contrast, only 13% of degradation was inhibited in the presence of NB, implying that

242

OH• are not the major radicals for the direct conversion of NH4+-N to N2. We also found

243

that NH4+-N degradation was not completely inhibited with the addition of NaHCO3,

244

suggesting that ClO• was the predominant oxidant. As displayed in Figure S7, the

245

contribution of CO3•– is excluded as NH4+-N was hardly degraded in 50 mM NaHCO3

246

electrolyte. The scavenge experiments were also carried out in WO3 and Sb-SnO2

247

electrodes (Figure 4D). Different from Sb-SnO2/WO3, NH4+-N degradation rates

248

significantly decreased with adding NaHCO3, revealing that ClO• are not involved WO3

249

and Sb-SnO2 electrodes.

250

To identify the concentration of HO•, Cl• and ClO• in Sb-SnO2/WO3 system,

251

different probes are used. As shown in Figure S8, KNB, KBA, and KDMOB are 0.0054 min-

252

1,

253

[ClO•] are then examined to be 4.03 ×10-15, 4.14×10-16, and 2.53×10-13 M, respectively.

254

It shows that the concentration of ClO• is 102-103 times higher than those of HO• and

255

Cl•, which means that ClO• is the dominant radical. The steady-state concentrations of

256

HO•, Cl• and ClO• were then estimated by using the kinetic model. Even though the

257

generation of OH• and Cl• were the initial radical formation steps, the modeling results

258

also showed that ClO• was the dominant radical (Figure S9), which was consistent with

259

the experimental results.

0.0094 min-1, and 0.038 min-1, respectively, According to Eqs.7-9, [HO•], [Cl•] and



Page 14 of 30

260

To better explain the reactivity of ClO• with NH4+, we thus investigat the rate

261

constants in the Sb-SnO2/WO3 system (Figure S10). The second-order rate constants

262

of ClO• with NH4+ are calculated to be 3.1×109 M-1S-1, which is 2.8 times that of Cl•

263

(1.1×109 M-1S-1) and 34.8 times that of HO• (8.9×107 M-1S-1). It demonstrats that ClO•

264

plays important roles in the transformations of NH4+-N to N2.

265

ClO• generation mechanisms of bifacial electrode. Based on the experimental

266

and modelling results, a reasonable schematic mechanism for treating NH4+-N was

267

illustrated (Figure 5). Under irradiation, photogenerated hole yield at WO3 surface can

268

oxidize water to OH• and chlorine ion to Cl•, respectively. On the Sb-SnO2 side,

269

chloride oxidation occurs efficiently in a low potential, creating the free chlorine, such

270

as HClO and ClO¯ (Eq.12-15). Free chlorine would rapidly quench the OH• and Cl•

271

generated on the WO3 surface, forming a large number of ClO• (Eq.16-18). The

272

reaction of NH4+-N with •ClO may proceed via the addition of Cl to H-abstraction39.

273

•ClO radical will take the H on the NH4+ molecule to form the N-centered radicals

274

(•NH2 and •NHCl), and N-centered radicals are unstable and can be gradually convert

275

to N240 (Eq.19-26). The experimental results indicate that N2 is the primary product

276

during NH4+-N oxidation. NH4+ degradation may also generate some nitrate via a series

277

of radical reactions (Eq.27-29).

278

Cl−+ h+ → Cl•

(12)

279

2Cl−→ Cl2+2e–

(13)

280

Cl2+H2O→HClO+HCl

(14)

281

Cl‾+2OH‾→OCl‾+ H2O+ 2e‾

(15)


Page 15 of 30


282

•Cl+HOCl→ClO•+ H+ + Cl−

(16)

283

•OH+ HOCl→ClO•+H2O

(17)

284

•OH+OCl‾→ClO•+OH–

(18)

285

NH4++•ClO→•NH2+HClO + H+

(19)

286

•NH2+HClO→NH2Cl+OH•

(20)

287

NH2Cl+•ClO→•NHCl+HClO

(21)

288

•NHCl+HClO→ NHCl2+OH•

(22)

289

NHCl2+H2O→NOH+2H+ +2Cl−

(23)

290

NH2Cl+NOH → N2 +H+ +Cl−+H2O

(24)

291

NHCl2+NOH → N2 + 2H++2Cl−

(25)

292

NHCl2+ NH2Cl→N2+3H++3Cl−

(26)

293

•NH2+ •OH→NH2OH

(27)

294

NH2OH+•OH→NO2−→NO3−

(28)

295

4ClO−+NH4+→NO3-+H2O+Cl−+2H+

(29)

296

Effect of the anodic potential. Figure S11A showed the effect of potential on the

297

degradation of NH4+-N. When the potential increased from 1.1 to 2.0 V, the removal

298

efficiency of ammonia-N increased from 51.3% to 99.8%. This result indicated that a

299

higher potential was beneficial for ammonia-N degradation. When potential was low,

300

the generation of HClO on Sb-SnO2 decreased and it only inhibited the recombination

301

of electron and hole, which would reduce the synergy of bifacial electrode. As shown

302

in Figure S11B, excessive potential would convert ammonia into more nitrate, which

303

may due to direct oxidation of NH4+ by surface OH• on anode41. Taking the two aspects



304

into consideration, the potential of 1.7 V was chosen for the subsequent experiments.

305

Effect of chloride ion concentration. To study the effect of chlorine ion on NH4+-

306

N degradation, different chlorine concentrations were used. In Figure 6A, when the

307

chlorine concentration was 0 mM, the NH4+-N concentration declined from 30 to 19.1

308

mg L-1 in 90 min. The degradation may derived from photoelectrocatalysis of WO3

309

anode. The NH4+-N removal increased sharply from 36.3% to 99.7% with increasing

310

chlorine ions from 0 to 75 mM. The results suggested that the presence of chloride ions

311

were beneficial for the degradation of NH4+-N in WO3/Sb-SnO2 system. In this study,

312

the degradation of NH4+-N by WO3/Sb-SnO2 may proceed through the formation of

313

ClO•. In addition to accelerating the degradation of NH4+-N, chloride ion also affects

314

degradation products. In this study, N2 and NO3‾ were the main products, while NO2‾

315

was not detected throughout the reaction. In Figure 6B, the concentration of NO3‾-N

316

increase to 7.87 mg L-1 without chloride ion, which mean most NH4+ was converted to

317

NO3‾. The nitrate concentration significantly decreased with increasing chlorine ion

318

concentration. However, nitrate production would increase with further increasing

319

chlorine concentration to 75 mM. As known, it is reasonable to convert NH4+ to N2

320

directly without other byproducts such as NO3‾. Therefore, 50 mM chloride ion was the

321

most active catalyst for NH4+ oxidation to N2.

322

Effect of pH value. Figure S12A shows the degradation of NH4+-N at different

323

pH values. It shows that NH4+-N degradation is enhanced with decreasing the pH. As

324

for acid condition, the reduction of H+ to H2 would increase, which then improve the

325

electron transfer from anode to cathode. When solution is strongly alkaline, NH4+-N


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326

removal is significantly retarded. WO3 may be damaged by corrosion in strong alkali

327

condition, which reduced the oxidation ability of WO3/Sb-SnO2. In fact, the pH can

328

remarkably influence the products. Although NH4+-N removal increases in lower pH

329

values, we find that NH4+-N is easily converted to nitrate. Along with decreasing pH

330

from 5 to 3, the yield of NO3‾ increased from 2.7 to 3.5 mg L-1 (Figure S12B).

331

Therefore, we should control the pH to inhibit the NO3‾-N yield and the optimum pH is

332

5.

333

The effects of Sb-SnO2 dosages were also examined. In Figure S13, an inadequate

334

dose of Sb-SnO2 induced cracks and faults on the electrode surfaces. For excessive Sb-

335

SnO2 dosages, polymerized particles appeard. The electrochemical performance were

336

then tested. In Figure S14A, 6 layers Sb-SnO2 film had the highest current, suggesting

337

that the generation of RCS more efficiently. The degradation of NH4+-N also verified

338

that the optimal activity was obtained with Sb-SnO2 of coating 6 layers (Figure S14B).

339

The Sb-SnO2/WO3 system was applied to treat ammonia wastewater with high

340

concentration. Although the efficiencies decreased as initial concentration increased,

341

the absolute amount of NH4+-N removal increased. It showed that 100 mg L-1 NH4+-N

342

could be removed (88%) in this system after 120 min treatment (Figure S15).

343

The generation of byproducts such as chlorate and perchlorate were investigated.

344

In this system, chlorate was the main byproducts and the concentration of perchlorate

345

below detection limit. In Figure S16, the concentration of ClO3– increased with the

346

reaction time and the final concentrations was 0.32 mM, which was less than 1% of the

347

initial Cl− concentration. It may indicated that the byproducts generation in our system



348

was suppressed.

349

The stability of electrode. The stability of WO3/Sb-SnO2 anode is important for

350

practical application. In Figure S17, the removal of NH4+-N is nearly maintained at the

351

level of fresh sample after five consecutive runs. Furthermore, the SEM patterns of the

352

used anode are found to be similar to that recorded before reaction (Figure S18), which

353

has a potential for long-term application. The current efficiency and energy

354

consumption were also evaluated. As shown in Figure S19, current efficiency first

355

increased with the increase in potential and then decreased, with the maximum value

356

(64.3%) achieved at 1.7 V. In addition, this system consumed the lowest energy (16.4

357

kW·h·kg N−1) at 1.7 V, confirming that it is a low-cost system for ammonia wastewater

358

treatment.

359

The Sb-SnO2/WO3 system was used to treat real wastewater after filtration and the

360

main characteristics were shown in Table S3. The deficiency of chloride ions was made

361

up by adding NaCl. As displayed in Figure S20A, about 43.4 mg L-1 of NH4+-N was

362

removed after 90 min treatment and the TN removal was 81.2%. The effect of anions

363

in water was also evaluated on the degradation of NH4+-N and the results are shown in

364

Figure S20B. We found that NH4+-N could be efficiently removed in the presence of

365

different anions (bicarbonate, nitrate and phosphate), although its efficiency slightly

366

decreased, indicating that the main water anions do not significantly affect the

367

performance of this system.

368

Sb-SnO2/WO3 has been shown to rapidly convert NH4+-N into N2 via ClO•.

369

However, about 14% of NH4+ is converted to NO3‾ in 90 min. To complete remove


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370

nitrogen from the water, the Pt electrode is replaced with a Pd-Cu modified Ni foam

371

cathode, in which NO3− can be reduced to N2 (the mechanism shown in Figure S21).

372

In Figure S22, we found that this system can effectively treat NH4+ wastewater, and

373

the TN removal efficiency was increased to 98.4 % within 90 min, which is 2.1 times

374

higher than that of WO3 (47.1%).

375

ASSOCIATED CONTENT

376

Supporting Information. A description of the preparation of WO3 photoanode and

377

analytical method of NB, BA and DMOB. Figures provided in supporting information

378

include the SEM, XRD and LSV measurements of Sb-SnO2/WO3 photoanode; model

379

simulation results; the second-order rate constants of NH4+ reacting with ClO•, Cl• and

380

OH•; the degradation of NH4+-N by HClO in the presence of WO3; the removal of

381

NH4+-N under different pH, applied potential, Sb-SnO2 layers and NH4+-N initial

382

concentration; time profiles of ClO3– ions; current efficiency and energy consumption;

383

the stability test of Sb-SnO2/WO3; the performance of Sb-SnO2/WO3 for degrading real

384

wastewater; the total nitrogen removal mechanism and performance in the exhaustive

385

denitrification system. Tables include the details of kinetic model, rate constants of

386

scavengers reacting with different radicals and actual wastewater composition.

387

AUTHOR INFORMATION

388

Corresponding Authors

389

Email addresses: [email protected] (B. Zhou); [email protected] (J. Bai)

390

Phone: (+86)21-54747351. Fax: (+86)21-54747351,

391

ACKNOWLEDGMENTS



392

The authors would like to acknowledge the National Natural Science Foundation of

393

China (No. 21875139, 21776177) and SJTU-AEMD for support.

394

Reference

395

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Figure 1 SEM images of top views of Sb-SnO2 layers (A, B); cross-sectional views of the Sb-SnO2 layers (C); elemental mapping of Sb-SnO2 layers (D). 156x108mm (300 x 300 DPI)



Figure 2 (A) survey spectra and (B−D) high-resolution spectra on Sn 3d, Sb 3d and O 1s of Sb-SnO2 electrode. 182x137mm (300 x 300 DPI)


Page 26 of 30

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Figure 3 (A) NH4+-N removal and (B) the plots of ln (C/Co) versus time for the NH4+-N degradation at different anodes. Condition: pH=5, NaCl 50 mM, 1.7 V vs Ag/AgCl. 242x94mm (300 x 300 DPI)



Figure 4 (A) Variation of free chlorine concentration during the reaction. (B) ESR spectra in the SbSnO2/WO3 and WO3 system. (C) The NH4+-N degradation in Sb-SnO2/WO3 with adding different

scavengers. (D) The NH4+-N degradation rate constant in the Sb-SnO2/WO3, Sb-SnO2 and WO3 with adding different scavengers. 234x177mm (300 x 300 DPI)


Page 28 of 30

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Figure 5 ClO• generation mechanisms on the Sb-SnO2/WO3 bifacial electrode for NH4+-N degradation. 169x102mm (300 x 300 DPI)



Figure 6 Effects of chloride concentration on (A) NH4+-N removal and (B) NO3−-N generation. Condition: pH=5, 1.7 V vs Ag/AgCl, 30 mg L-1 NH4+-N. 244x94mm (300 x 300 DPI)


Page 30 of 30

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