Exhaustive Conversion of Inorganic Nitrogen to Nitrogen Gas Based

Subscriber access provided by READING UNIV

Article

Exhaustive conversion of inorganic nitrogen to nitrogen gas based on a photoelectro-chlorine cycle reaction and a highly selective nitrogen gas generation cathode Yan Zhang, Jinhua Li, Jing Bai, Zhaoxi Shen, Linsen Li, Ligang Xia, Shuai Chen, and Baoxue Zhou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04626 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

Environmental Science & Technology

1

Exhaustive conversion of inorganic nitrogen to nitrogen gas based on a

2

photoelectro-chlorine cycle reaction and a highly selective nitrogen gas

3

generation cathode

4

Yan Zhang a, Jinhua Li a, Jing Bai a**, Zhaoxi Shen a, Linsen Li a, Ligang Xia a, Shuai Chen a,

5

Baoxue Zhou a,b∗∗

6

a

7

No. 800 Dongchuan Rd, Shanghai 200240, China.

8

b

9

Education, Shanghai 200240, PR China

School of Environmental Science and Engineering, Shanghai Jiao Tong University

Key Laboratory of Thin Film and Microfabrication Technology, Ministry of

10

∗ Corresponding author. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. ** Corresponding author. Email addresses: [email protected] (B. Zhou) ; [email protected] (J. Bai)

ACS Paragon Plus Environment


11

ABSTRACT:

12

A novel method for the exhaustive conversion of inorganic nitrogen to nitrogen gas

13

was proposed in this paper. The key properties of the system design included an

14

exhaustive photoelectrochemical cycle reaction in the presence of Cl−, in which Cl•

15

generated from oxidation of Cl− by photoholes selectively converted NH4+ to nitrogen

16

gas and some NO3− or NO2−. The NO3− or NO2− was finally reduced to nitrogen gas

17

on a highly selective Pd-Cu-modified Ni foam (Pd-Cu/NF) cathode to achieve

18

exhaustive conversion of inorganic nitrogen to nitrogen gas. The results indicated

19

total nitrogen removal efficiencies of 30 mgL-1 inorganic nitrogen (NO3−, NH4+,

20

NO3−:NH4+=1:1 and NO2−:NO3−:NH4+=1:1:1) in 90 min were 98.2%, 97.4%, 93.1%

21

and 98.4%, respectively, and the remaining nitrogen was completely removed by

22

prolonging the reaction time. The rapid reduction of nitrate was ascribed to the

23

capacitor characteristics of Pd-Cu/NF that promoted nitrate adsorption in the presence

24

of an electric double layer, eliminating repulsion between the cathode and the anion.

25

Nitrate was effectively removed with a rate constant of 0.050 min-1, which was 33

26

times larger than that of Pt cathode. This system shows great potential for inorganic

27

nitrogen treatment due to the high rate, low cost and clean energy source.

28

Keywords: exhaustive cycle process, Pd-Cu/Ni foam, photoelectro-chlorine cycle

29

reaction, WO3 nanoplate array, inorganic nitrogen removal

30


Page 2 of 28

Page 3 of 28


31

1. INTRODUCTION

32

With the rapid development of industrialization processes, a large amount of inorganic

33

nitrogen (ammonia, nitrate and nitrite) wastewater has entered natural water, leading

34

to potential threats to humans and ecosystems, such as methemoglobinemia1 and

35

eutrophication2. Diverse technologies have been developed to treat the nitrogen

36

contamination, such as ion exchange,3,

37

chlorination6 and electrochemical processes.7 In most cases, it is difficult to

38

completely convert inorganic nitrogen to nitrogen gas in one-directional conversion

39

pathway: ammonia is easily oxidized to nitrate, while nitrate is always converted to

40

ammonia, resulting in low removal efficiency of the total nitrogen. The biological

41

method can convert inorganic nitrogen to nitrogen gas by nitrification and

42

denitrification, but it requires precise control of the carbon source to give a certain

43

C/N ratio, which is a bottleneck for the treatment of wastewater with high nitrogen

44

contents.

45

In recent years, the photoelectrocatalytic (PEC) technique has drawn great attention

46

because it offers an eco-friendly route to the degradation of pollutants and generation

47

of electricity by using sunlight8-12. Wang13 developed a PEC system by using a

48

p-GaInP2 photoelectrode for nitrate-N reduction. Xiao14 et al. treated ammonia-N

49

wastewater by using TiO2 nanotube arrays. However, most of the studies only realized

50

one-directional conversion of inorganic nitrogen, and the destruction rate remained

51

low.

52

Herein, we propose the exhaustive PEC cycle system for rapid removal of inorganic

4

biochemical treatments5, breakpoint



Page 4 of 28

53

nitrogen in a solution containing Cl−, achieving the simultaneous reduction of NO3− or

54

NO2− and oxidation of NH4+. Under illumination, Cl− was oxidized to chlorine

55

radicals (Cl•) by photoholes, and Cl• selectively transformed NH4+ into nitrogen gas

56

and small amounts of NO3− or NO2− (Eqs.2-4). Meanwhile, NO3− or NO2− in solution

57

can be reduced to nitrogen gas on cathode (Eqs.5-6). Finally, inorganic nitrogen was

58

completely converted to N2. The introduction of Cl− expanded the radical reaction to

59

the whole solution, and Cl• reacts with NH4+ more rapidly than •OH. In addition, the

60

Cl− underwent a catalytic cycle in the process15, avoiding the addition every run.

61

h++Cl−→Cl

62

2NH4++6Cl→N2+6Cl−+8H+

63

NH4++8Cl+3H2O→NO3−+10H++8Cl−

64

NH4++6Cl+2H2O→NO2−+8H++6Cl−

(minor)

(4)

65

NO3−+3H2O+5e−→2N2+6OH−

(major)

(5)

66

NO3−+7H2O+8e−→NH4++10OH−

(minor)

(6)

67

However, NO3− or NO2− were generated in the process, resulting in incomplete

68

nitrogen removal. The removal rate is also unsatisfactory because NO3− (or NO2−) is

69

difficult to be adsorbed on the cathode surface due to the repulsion between like

70

charges. Moreover, nitrate is easily converted to the byproduct NH4+. To overcome

71

these drawbacks, a Pd-Cu modified Ni foam cathode was designed to effectively

72

reduce the anions. A highly electronically conductive nickel foam with a 3D structure

73

was selected as the cathode substrate16, 17 because of high diffusion efficiency. Cu and

74

Pd

18, 19

(1) (major) (minor)

(2） (3)

are considered to be efficient promoters for nitrate reduction. The Pd-Cu/NF


Page 5 of 28


75

cathode has the characteristics of a capacitor, and nitrate easily adsorbs to the cathode

76

due to the positive charge of its surface, thereby facilitating nitrate reduction.

77

Furthermore, Pd-Cu selectively enhances the reduction of nitrate to nitrogen gas

78

following the adsorption process, inhibiting the formation of NH4+.

79

Based on this idea, we proposed a novel exhaustive cycle using Cl− as the medium

80

and Pd-Cu/NF as the cathode to convert inorganic nitrogen to N2 under solar light.

81

The Pd-Cu/NF cathode was attained by electrodeposition, and a WO3 nanoplate array

82

film was selected as the photoanode for its visible light response and nontoxicity 20.

83

Using this system, inorganic nitrogen removal was carried out by investigating the

84

effects of different operation parameters (pH, applied potential and Cl− concentration),

85

the final products and the degradation pathways. The results showed that the removal

86

efficiency of inorganic nitrogen was greatly promoted and we hope this paper

87

provides an economical and efficient method for sustainable wastewater treatment.

88

2. MATERIALS AND METHODS

89

Chemicals and Materials. Ni foam (3 mm thin) was purchased from Kunshan

90

Jiayisheng Electronics Co., Ltd. NaNO3, NaNO2, (NH4)2SO4 and NaCl were

91

purchased from Sinopharm Chemical Reagent Co., Ltd. and used as the sources of

92

NO3−-N, NO2−-N, NH4+-N and Cl−. Unless otherwise indicated, all chemicals were of

93

analytical reagent, and deionized water was produced by a Milli-Q ultrapure water

94

system.

95

Electrode preparation. Prior to the synthesis, the Ni foam was carefully cleaned

96

through consecutive sonication in H2SO4 (1.0 M), acetone and deionized water to



the

surface

oxidized

layer. The

Pd-Cu/NF

Page 6 of 28

97

remove

98

co-electrodeposition of Pd and Cu using potentiostatic technique. A solution

99

composed of 2 mM PdCl2, 4 mM CuSO4·5H2O and 0.1 M HCl was used as the

100

deposition electrolyte. The Ni foam, Pt foil and a saturated calomel electrode (SCE)

101

were used as the working electrode, counter electrode and reference electrode,

102

respectively. The deposition voltage was controlled at −1.0 V vs. SCE for 30 min by

103

using an electrochemical workstation (CHI 660C, Chenhua Instrument Co., Ltd.,

104

China). Then, the Pd-Cu/NF was cleaned with deionized water and subsequently dried

105

at 60°C under vacuum. For comparison, Pd/NF and Cu/NF electrodes were prepared

106

via a similar method in the absence of CuSO4·5H2O or PdCl2 in the deposition

107

electrolyte.

108

The WO3 photoanode was prepared according to the method in our previous report21,

109

and the detailed procedure is provided in the supporting information.

110

Experimental Setup. The degradation of inorganic nitrogen was carried out in a

111

single 50 mL electrolytic cell. During the reaction, the light source was a 350 W Xe

112

lamps with an AM 1.5 filter (light intensity, 100 mW cm−2). Unless otherwise

113

specifically mentioned, 40 mL of synthetic wastewater containing 0.05 mol L-1

114

Na2SO4 and 0.05 M NaCl was treated at pH 5.0. NaOH or H2SO4 (0.5 M) was used to

115

adjust the pH of the solution. The immersed area of both the Pd-Cu/NF cathode and

116

the WO3 anode was 4 cm2 (2 cm×2 cm). The distance between the two electrodes was

117

maintained at 1.5 cm. An electrochemical workstation was employed as the power

118

source, and a potential of -1.2 V vs SCE was applied.


was

prepared

by

Page 7 of 28


119

Analytical Methods. The surface morphologies of Pd-Cu/NF and WO3/FTO

120

electrodes were studied by a scanning electron microscope (SEM, Zeiss

121

SUPRA55-VP) equipped with an energy-dispersive X-ray spectrometer. The

122

crystallinities of the Cu/NF, Pd/NF and Pd–Cu/NF electrodes were identified by X-ray

123

diffraction (XRD, Rigaku D-Max B). The composition of the prepared electrode was

124

determined by X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD).

125

Electron-spin resonance (ESR) spectra were obtained on a Bruker EMX-8/2.7. Laser

126

Flash Photolysis experiments were performed on a laser flash photolysis spectrometer

127

(LP920) to detect Cl2•−.

128

Linear sweep voltammetry (LSV) was conducted at the Pd-Cu/NF and NF electrodes

129

in solutions containing varying amounts of NaNO3 and 0.05 M NaCl with a sweep

130

rate of 10 mV s−1. Nitrate and nitrite concentrations were measured by ion

131

chromatography (Dionex, USA), and ammonia was determined by Nessler reagent

132

using a UV-visible spectrophotometer (752N, INESA, Shanghai) at 420 nm. The

133

amount of N2 was measured in closed system with a gas chromatograph

134

(GC-2010Plus, SHIMADZU) equipped with a thermal conductivity detector.

135

3. RESULTS AND DISCUSSION

136

Characterization of the Pd-Cu/NF electrode. Figure 1 shows SEM images of the

137

NF and Pd–Cu/NF electrodes, respectively. A cross-linked grid structure with

138

abundant and regular superficial wrinkles was observed for the NF in Figure 1a,

139

providing a high porosity and large specific surface area. After deposition of Pd-Cu,

140

the NF turned from gray to black (Figure S2). The image in Figure 1b showed



141

electrodeposited materials uniformly covered NF, though some agglomeration were

142

observed. The particle size was approximately 150 nm, and this increases the surface

143

area and provides interspaces for penetration of the electrolyte. The elemental

144

distribution of the Pd-Cu/NF electrode was obtained by EDX. The elemental mapping

145

revealed the K-edge signals of Pd, Cu, and Ni (Figure S3). The observed even

146

distribution of Pd, Cu and Ni confirmed the uniform deposition of the Pd-Cu array.

147

Figure 1

148

The XRD patterns of the NF, Pd/NF, Cu/NF and Pd-Cu/NF were investigated, and the

149

results are presented in Figure 2a. Pure NF had strong peaks at 2θ angles of 44.5°,

150

51.8° and 76.4°. A typical Pd peak was observed at 40.1° in the Pd/NF and Pd-Cu/NF

151

cathodes18. However, the characteristic diffraction lines of Cu were not observed. The

152

Cu3Pd phase has two strong peaks at 2θ=42° and 48°, which are close to the Ni

153

peaks22 and may be responsible for the absence of Cu peaks. The low crystallinity of

154

the copper phase may be another reason.23 Moreover, the observed Ni peak of

155

Pd-Cu/NF was in accordance with that of NF, and an obvious peak shift was observed,

156

indicating that the Pd-Cu film synthesized on the NF by electrodeposition was

157

bimetallic and composed of an alloy rather than separate metals.

158

The detailed elemental composition and valence states were characterized by XPS

159

measurements. The spectra of Pd-Cu/NF (Figure 2b) shows the characteristic peaks

160

of Pd and Cu elements. As illustrated in Figure 2c, the binding energies of the

161

spin-orbit coupling (Pd 3d3/2 and Pd 3d5/2) appeared at 340.5 eV and 335.2 eV,

162

respectively, which can be assigned to metallic (Pd0)

24, 25


. Another pair of Pd signals

Page 8 of 28

Page 9 of 28


163

was located at 342.2 eV and 337.0 eV, attributed to the presence of Pd2+. Figure 2d

164

presents the Cu 2p spectrum of the Pd-Cu/NF electrode, which was deconvoluted into

165

three different peaks at 931.8, 934.4, and 942.7 eV, corresponding to Cu0, Cu+, and

166

Cu2+, respectively26.

167

Figure 2

168

Nitrate and nitrite reduction on the Pd-Cu/NF cathode. The performance of

169

Pd-Cu/NF cathode in NO3− removal was compared with that of Pd/NF, Cu/NF, NF

170

and Pt cathodes, and the results are shown in Figure 3a. Obviously, Pd-Cu/NF

171

cathode showed the best performance. The results showed that only 3.3 mgL-1 nitrate

172

was reduced when Pt was used as cathode. This is because Pt is good for H2 evolution

173

in acidic solutions, which suppresses the reduction of NO3−. For the NF cathode,

174

nitrate reduction was slow, with a conversion yield of 6.3 mgL-1 after 90 min, while

175

yields of only 18.2 mg L-1 and 20.6 mg L-1 were obtained for the Cu/NF and Pd/NF

176

cathodes. The rate data were fitted with a pseudo-first-order rate equation as follows:

177

ௗሾேைయష ሿ

178

The apparent rate constants for the Pt, NF, Cu/NF, Pd/NF and Pd-Cu/NF electrodes

179

were 0.0015, 0.002, 0.010, 0.012 and 0.050 min-1, respectively, as shown in Figure S4.

180

These results indicated that the Pd-Cu/NF had the fastest reduction rate, which was 33

181

times greater than that of Pt. Figure S5 shows the ability of the Pd-Cu/NF cathode to

182

perform nitrite reduction. The results showed that 30 mg L-1 of nitrite was removed

183

completely within 60 min. Some nitrate was generated at the beginning and then was

184

removed. This was because some nitrite was oxidized to nitrate by the photoholes27.

ௗ௧

= −‫ܭ‬ሾܱܰଷି ሿ

(7)



185

This result indicated that nitrite can be removed either through direct reduction

186

(NO2−→N2 and NH4+) or indirect reduction (NO2−→NO3−→N2 and NH4+) in the

187

exhaustive cycle process. The produced NH4+ could be further converted to nitrogen

188

gas by chlorine free radicals. To investigate the formation of N2 in the system, we

189

performed the nitrate reduction in a closed-batch reactor (Figure S6). NO3− was

190

completely removed in 90 min, while the NO2− formation was very low throughout

191

the experiment (Figure S6b). The results showed the concentrations of N2 was about

192

28.7 mg L-1 at 90 min, indicating that N2 was the main gaseous product.

193

In view of the reduction characteristics, the LSV of Pd-Cu/NF and NF were measured.

194

The LSV of the NF electrode in a 0.05 M chloride solution showed an obvious rise in

195

the current at 1.1 V (Figure S7a), which was attributed to hydrogen evolution. After

196

the addition of nitrate, a current increase appeared at 1.3 V, resulting from nitrate

197

reduction. A higher current was observed on the Pd-Cu/NF cathode under identical

198

scan operations (Figure S7b), demonstrating its prominent nitrate reduction capability.

199

Meanwhile, a current peak was observed for the Pd-Cu/NF electrode at 0.5 V, which

200

was due to the adsorption of nitrate28. However, defined peaks were not observed for

201

the NF cathode. These results indicated that the Pd-Cu/NF cathode easily captured

202

nitrate for reduction. This was probably because the Pd-Cu/NF cathode had the

203

characteristics of a capacitor and formed an electric double layer, which was

204

conducive to the adsorption of nitrate on the cathode29.

205

Figure 3

206

Ammonia oxidation in the exhaustive cycle process. Figure 3b shows the


Page 10 of 28

Page 11 of 28


207

degradation curves of NH4+-N for different processes. We found NH4+ was not

208

degraded by the PC-chlorine system, and 3.0 mg L-1 of NH4+ was removed by the

209

EC-chlorine process within 90 min. A total of 9.4 mg L-1 of NH4+ was removed in

210

PEC process because bias potential can inhibit the recombination of photogenerated

211

electrons, thereby increasing the efficiency. However, the NH4+ rapidly decreased in

212

the exhaustive cycle process, and about 99% removal efficiency was achieved in 90

213

min. The variations in the NH4+ and NO3− concentrations are shown in Figure S8. In

214

the conversion process, nitrite was hardly detected, and the nitrate first increased and

215

then decreased to zero, proving that NH4+ was converted to N2. To clarify the role of

216

Cl•, experiments were carried out with specific probe scavengers (tert-butyl alcohol

217

reacts with both •OH and Cl•, while nitrobenzene reacts with only •OH)30. Figure S9

218

showed that NH4+ degradation rate significantly decreased after the addition of

219

t-BuOH, revealing that both OH and Cl were responsible for NH4+ oxidation. In

220

addition, the removal efficiency was slightly inhibited by the addition of NB,

221

suggesting that Cl• played a crucial role. To further confirm the conjecture, ESR

222

technique with DMPO as trapping agent was used to determine the radical species. As

223

shown in Figure 3c, quartet peaks of DMPO-OH• with 1:2:2:1 intensity were

224

detected in the absence of Cl−, while an eleven-line ESR spectra were obtained in the

225

PEC-chlorine system. Previous studies demonstrated that a seven-line ESR spectra

226

corresponds to DMPO-X31, which indicated that both •Cl and OH• were generated in

227

the process. The transient absorption spectrum of PEC-chlorine system showed a

228

typical absorption at λ=340 nm (Figure 3d), which corresponding to Cl2•− radical32.



Page 12 of 28

229

Generally, the Cl2•− radical is formed through the reaction between Cl• and Cl−, which

230

also indicating that Cl• was generated.

231

A separated dual-cell PEC-chlorine system with a cation exchange membrane was

232

used to investigate the side-reactions (Figure S10). In cathode chamber, nitrate is

233

reduced to N2 and NH4+ (Figure S10b). However, in anode chamber, there is a slight

234

decrease of nitrate, and it cannot detect ammonium because of the presence of

235

chloride ions (Figure S10c). We then monitored the pH changes in the system and

236

found the pH of the solutions increased in cathode and decreased in anode chamber,

237

which was indicative of the water decomposition (Figure S10d).

238

Mechanism of inorganic nitrogen removal. Based on the above experiment, a

239

reasonable schematic mechanism for the removal of inorganic nitrogen in the

240

PEC-chlorine system was proposed (Scheme 1). Nitrate and nitrite were reduced at

241

the cathode. The adsorption of nitrate to the cathode was the first step and Pd-Cu/NF

242

electrode has the characteristics of a capacitor, which could quickly adsorb nitrate or

243

nitrite (Eq.8 and 11) through the positive charge. After adsorption on Cu site, NO3−

244

was reduced to NO2− and then further reduced to N2 on the Pd sites (Eqs. 9-12). The

245

reduction intermediate was nitrite, and a small amount of nitrite was observed during

246

monitoring of the nitrate reaction. In addition, nitrate could be directly reduced to N2.

247

Nitrate reduction was accelerated at a much higher rate on the Pd-Cu/NF cathode than

248

on the other studied electrodes.

249

NO3−+Cu ↔ Cu-NO3−ads

(8)

250

Cu-NO3−ads +H2O+2e−→NO2−+Cu+2OH−

(9)


Page 13 of 28


251

Cu-NO2−ads+5H2O+6e−→NH3+7OH−

(10)

252

NO2−+Pd ↔ Pd-NO2−ads

(11)

253

2Pd-NO2−ads+4H2O+6e−→N2 (g) +2Pd+8OH−

(12)

254

Next, NH4+ in the solution were oxidized at the photoanode. Under irradiation,

255

photogenerated holes (h+) were generated at the WO3 and could oxidize water to HO•,

256

while Cl• is generated by direct hole oxidation or through a transient adduct of Cl−

257

with the OH• (Eqs. 13-17). The Cl• will react with Cl− that can form Cl2•− and further

258

to HOCl/OCl− (Eqs. 18-19). It is known that HO•, Cl• and HClO are strong oxidants

259

that can react with NH4+ (Eqs. 20-21). Cl• is a selective oxidant and its reactivity is

260

higher than that of HO• in the oxidation of NH4+ (Eqs.22-26). Cl• readily undergoes

261

rapid addition, hydrogen abstraction, and direct electron transfer reactions with NH4+,

262

foaming a series of radicals (NH2• and •NHCl), and chloramines are generated as

263

reaction intermediates. In addition, the chloramines can be further oxidized to produce

264

gaseous nitrogen. It shows that a cyclic reaction between Cl− and Cl• was formed after

265

the complete oxidation of nitrogen to nitrogen gas. It should be noted that small

266

amount of NO3− was formed, indicating NH4+ was excessively oxidized. HO• is

267

theoretically preferable for the oxidation of NH4+ into NO3− and possible pathway

268

were shown in Eqs. 27-30. NH2• can be transformed to NH2OH by HO•. Then,

269

NH2OH can be oxidized to NO2−, and further oxidized to NO3−. However, the

270

generated NO3− could be reduced to nitrogen gas at the Pd-Cu/NF cathode to achieve

271

exhaustive treatment of inorganic nitrogen through cycle reaction.

272

WO3 + hv → h+ + e−

(13)



Page 14 of 28

273

H2O + h+ → HO• + H+

(14)

274

Cl−+ h+ → Cl•

(15)

275

HO• +Cl− → ClOH•−

(16)

276

ClOH•−+H+→ Cl•+H2O

(17)

277

Cl• + Cl−→ Cl2•−

(18)

278

Cl2•− + H2O→HOCl+Cl−+H++e−

(19)

279

NH4+ + Cl• → NH2• + Cl− + 2H+

(20)

280

NH2• + HO-Cl →NH2Cl + HO•

(21)

281

NH2Cl + Cl• → •NHCl+ Cl− + H+

(22)

282

•NHCl + HO-Cl → NHCl2 + HO•

(23)

283

NHCl2+ NH2Cl→N2+3H++3Cl−

(24)

284

NHCl2+H2O→NOH +2H+ +2Cl−

(25)

285

NHCl2 + NOH → N2 + HClO + H+ + Cl−

(26)

286

NH2• + HO•→NH2OH

(27)

287

NH2OH + OH• → NO2−→NO3−

(28)

288

HClO ↔ ClO−+H+

(29)

289

4ClO−+NH4+→NO3−+ H2O+4Cl−+2H+

(30)

290

Scheme 1.

291

Effect of the applied potential. The applied potential is an important parameter for

292

controlling the rate of reactions. Figure S11 shows the variations in the NH4+, NO3−

293

and TN concentrations under different potentials ranging from -0.5 to -1.5 V (vs.

294

SCE). As shown in Fig. S11a, the nitrate reduction rate increased with increasing


Page 15 of 28


295

potential within 0.5-1.2 V. The enhancement of the nitrate reduction was attributed to

296

the occurrence of nitrate reduction at a certain potential. The optimal nitrate removal

297

was identified to occur at -1.2 V. Meanwhile, a decrease in nitrate removal observed at

298

-1.5 V could be attributed to competition with hydrogen ions (Eq. 31). In addition,

299

minimal nitrite was detected in this process. As shown in Figure S11c, the trend in TN

300

removal was similar to that of the nitrate concentration, and the highest efficiency was

301

achieved at -1.2 V.

302

2‫ܪ‬ା + 2݁ ି → ‫ܪ‬ଶ

303

Effect of the initial pH. The solution pH is also a critical factor examining the

304

removal of nitrate by Pd-Cu/NF cathode were conducted at pH 3.0–9.0, and the

305

results are shown in Figure S12. The reduction of nitrate exhibited similar tendencies

306

at the various initial pH values. Complete nitrate reduction was observed at pH 5,

307

while 25.7 mg L-1 of nitrate was degraded by Pd-Cu/NF at pH 3. The decrease in

308

nitrate reduction was attributed to the competition of H+ with nitrate for the active

309

sites of cathode. The removal decreased when the pH was increased to 9, because

310

WO3 is unstable under alkaline conditions. The NH4+ concentration increased sharply

311

from pH 5 to 9, as observed in Figure S12b. One reason was that a high pH value

312

promotes a disproportion reaction to form chlorate, resulting in the loss of chloride

313

ions from solution33. The TN removal also revealed that moderately acidic conditions

314

(pH=5) were favorable for the reduction of nitrate (Figure S12c).

(31)

315

Figure 4

316

Effect of the Cl− concentration. To remove nitrogen completely, Cl− was added to



317

oxidize NH4+ to N2. Figure 4a shows the influence of chloride concentration on the

318

removal of nitrate and the removal efficiency slightly increased in the presence of Cl−.

319

Without the Cl−, the concentration of nitrate declined from 30 to 1.57 mgL-1 within 90

320

min. However, Cl− ions have a strong influence on the NH4+ generation. NH4+

321

generation significantly decreased with increasing Cl− concentration in Figure 4b.

322

There was still 6.93 mg L-1 NH4+-N in solution after 90 min treatment in the absence

323

of Cl−, while the remaining was only 2.58 mg L-1 when 0.02 M Cl− was present, and

324

nearly no NH4+ was detected with further increasing the Cl− concentration to 0.05 M

325

or more. The NH4+ could be effectively oxidized to N2 with the help of Cl, resulting

326

in a great improvement in the TN removal. As shown in Figure S13, the TN removal

327

efficiency increased from 72.9% to 87.4% and 98.2% with the increase in the Cl−

328

concentration from 0 to 0.05 M, respectively. When the concentration was further

329

increased to 0.07 M, no significant improvement was observed. Based on the results

330

above, the optimal Cl− concentration for the removal of nitrate was approximately

331

0.05 M, which gave a high TN degradation efficiency. The exhaustive cycle system

332

was also applied to treat nitrate wastewater with high concentration. Although the

333

efficiencies decrease with increasing initial concentration, the absolute amount of

334

nitrate removal increase. It showed that 90 mgL-1 nitrate can be removed (86.2%) in

335

this system after 90 min treatment (Figure S14).

336

Figure 5

337

Effect of the ratio of NO3−, NO2− and NH4+. The effectiveness of the exhaustive

338

cycle process was proven for NH4+ oxidation and NO3− reduction in the above study;


Page 16 of 28

Page 17 of 28


339

however, its feasibility for the treatment of wastewater with different proportions of

340

inorganic nitrogen remained to be confirmed. As shown in Figure 5a, the TN could be

341

effectively removed under various ammonium and nitrate ratios. This indicates that

342

the system can treat complex nitrogen-containing wastewater. However, the TN

343

removal efficiency within 90 min was slightly reduced when the wastewater

344

contained both nitrate and ammonium, especially in a ratio of 1:1 (93.1%). The

345

variations in the NH4+ and NO3− concentrations are shown in Figure S15a. The results

346

showed that the reduction rate of NO3− decreased when NH4+ was present. On one

347

hand, NH4+ oxidation at the anode contributed a larger amount of NO3−. On the other

348

hand, hydrogen ions were generated during NH4+ oxidation, which competed with

349

nitrate for the active sites on cathode. Upon prolonging reaction time to 105 min,

350

nearly 98% of the total nitrogen was removed in the exhaustive cycle process (Figure

351

S15b). In addition, we found that this system can effectively treat wastewater

352

containing inorganic nitrogen (NO3−, NO2−, and NH4+), and the TN removal

353

efficiency was approximately 98.4% within 90 min (Figure S16).

354

In addition to the inorganic nitrogen removal performance, the stability of the

355

Pd-Cu/NF cathode is also important for its practical application. Five cycle runs of

356

PEC treatments were performed by using the same Pd-Cu/NF cathode with the

357

potential fixed at -1.2 V vs. SCE in a 30 mg L-1 nitrate solution containing 0.05 M

358

chloride ions. Figure 5b showed that the nitrate removal was nearly maintained at the

359

level of the fresh sample after five consecutive runs (97.7%) and the TN removal

360

efficiency also keep at high level (94.2%) in Figure S17.



361

ASSOCIATED CONTENT

362

Additional figures as mentioned in the text.

363

AUTHOR INFORMATION

364

Corresponding Authors

365

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

366

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

367

ACKNOWLEDGEMENTS

368

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

369

China (No. 21776177, 51578332, 21576162) and SJTU-AEMD for support.

370

REFERENCES

371

(1) Kim, J.; Benjamin, M. M., Modeling a novel ion exchange process for arsenic and nitrate

372

removal. Water Res. 2004, 38, (8), 2053-2062.

373

(2) Jung, S.; Bae, S.; Lee, W., Development of Pd-Cu/hematite catalyst for selective nitrate

374

reduction. Environ. Sci. Technol. 2014, 48, (16), 9651-8.

375

(3) Bae, B.-U.; Jung, Y.-H.; Han, W.-W.; Shin, H.-S., Improved brine recycling during nitrate

376

removal using ion exchange. Water Res. 2002, 36, (13), 3330-3340.

377

(4) Kim, Y.-J.; Kim, J.-H.; Choi, J.-H., Selective removal of nitrate ions by controlling the

378

applied current in membrane capacitive deionization (MCDI). J. Membr. Sci. 2013, 429,

379

52-57.

380

(5) Hörold, S.; Tacke, T.; Vorlop, K. D., Catalytical removal of nitrate and nitrite from

381

drinking water: 1. Screening for hydrogenation catalysts and influence of reaction conditions

382

on activity and selectivity, Environ. Technol. 1993, 14, (10), 931-939.

383

(6) Pressley, T. A.; Bishop, D. F.; Roan, S. G., Ammonia-nitrogen removal by breakpoint

384

chlorination. Environ. Sci. Technol. 1973, 6, (7), 622-628.

385

(7) Ding, J.; Li, W.; Zhao, Q.-L.; Wang, K.; Zheng, Z.; Gao, Y.-Z., Electroreduction of

386

nitrate in water: Role of cathode and cell configuration. Chem. Eng. J. 2015, 271, 252-259.


Page 18 of 28

Page 19 of 28


387

(8) Kang, M. J.; Kang, Y. S., Ultrathin insulating under-layer with a hematite thin film for

388

enhanced photoelectrochemical (PEC) water splitting activity. J. Mater. Chem. A. 2015, 3,

389

(30), 15723-15728.

390

(9) Yang, J.; Zhang, X.; Liu, H.; Wang, C.; Liu, S.; Sun, P.; Wang, L.; Liu, Y.,

391

Heterostructured TiO2/WO3 porous microspheres: preparation, characterization and

392

photocatalytic properties. Catal.Today. 2013, 201, 195-202.

393

(10) Lucchetti, R.; Onotri, L.; Clarizia, L.; Natale, F. D.; Somma, I. D.; Andreozzi, R.;

394

Marotta, R., Removal of nitrate and simultaneous hydrogen generation through photocatalytic

395

reforming of glycerol over “in situ” prepared zero-valent nano copper/P25. Appl. Catal. B:

396

Environ. 2017, 202, 539-549.

397

(11) Song, S.; Liu, Z.; He, Z.; Zhang, A.; Chen, J.; Yang, Y.; Xu, X., Impacts of Morphology

398

and Crystallite Phases of Titanium Oxide on the Catalytic Ozonation of Phenol. Environ. Sci.

399

Technol. 2010, 44, (10), 3913-8.

400

(12) Cheng, X.; Liu, H.; Chen, Q.; Li, J.; Wang, P., Preparation and characterization of

401

palladium nano-crystallite decorated TiO2 nano-tubes photoelectrode and its enhanced

402

photocatalytic efficiency for degradation of diclofenac. J. Hazard. Mater. 2013, 254-255,

403

141-8.

404

(13) Wang, H.; Turner, J. A., Photoelectrochemical reduction of nitrates at the illuminated

405

p-GaInP2 photoelectrode. Energy Environ. Sci. 2015, 8, (3), 1046-1046.

406

(14) Xiao, S.; Wan, D.; Zhang, K.; Qu, H.; Peng, J., Enhanced photoelectrocatalytic

407

degradation of ammonia by in situ photoelectrogenerated active chlorine on TiO2 nanotube

408

electrodes. J Environ Sci (China). 2016, 50, 103-108.

409

(15) Xiao, S.; Qu, J.; Zhao, X.; Liu, H.; Wan, D., Electrochemical process combined with UV

410

light irradiation for synergistic degradation of ammonia in chloride-containing solutions.

411

Water Res. 2009, 43, (5), 1432-40.

412

(16) Yuan, C.; Yang, L.; Hou, L.; Shen, L.; Zhang, X.; Lou, X. W. D., Growth of ultrathin

413

mesoporous Co3O4 nanosheet arrays on Ni foam for high-performance electrochemical

414

capacitors. Energy Environ. Sci. 2012, 5, (7), 7883-7887.

415

(17) Yang, G.-W.; Xu, C.-L.; Li, H.-L., Electrodeposited nickel hydroxide on nickel foam

416

with ultrahigh capacitance. Chem. Commun. 2008, (48), 6537-6539.



417

(18) Zhang, Z.; Xu, Y.; Shi, W.; Wang, W.; Zhang, R.; Bao, X.; Zhang, B.; Li, L.; Cui, F.,

418

Electrochemical-catalytic reduction of nitrate over Pd–Cu/γAl2O3 catalyst in cathode chamber:

419

Enhanced removal efficiency and N2 selectivity. Chem. Eng. J. 2016, 290, 201-208.

420

(19) Soares, O. S. G. P.; Pereira, M. F. R.; Órfão, J. J. M.; Faria, J. L.; Silva, C. G.,

421

Photocatalytic nitrate reduction over Pd–Cu/TiO2. Chem. Eng. J. 2014, 251, 123-130.

422

(20) Kalanur, S. S.; Yun, J. H.; Sang, Y. C.; Joo, O. S., Facile growth of aligned WO3

423

nanorods on FTO substrate for enhanced photoanodic water oxidation activity. J. Mater.

424

Chem. A. 2013, 1, (10), 3479-3488.

425

(21) Zeng, Q.; Li, J.; Bai, J.; Li, X.; Xia, L.; Zhou, B., Preparation of vertically aligned WO3

426

nanoplate array films based on peroxotungstate reduction reaction and their excellent

427

photoelectrocatalytic performance. Appl. Catal. B: Environ. 2017, 202, 388-396.

428

(22) Yoshinaga, Y.; Akita, T.; Mikami, I.; Okuhara, T., Hydrogenation of Nitrate in Water to

429

Nitrogen over Pd–Cu Supported on Active Carbon. J. Catal. 2002, 207, (1), 37-45.

430

(23) Yamauchi, M.; Abe, R.; Tsukuda, T.; Kato, K.; Takata, M., Highly selective ammonia

431

synthesis from nitrate with photocatalytically generated hydrogen on CuPd/TiO2. J Am Chem

432

Soc. 2011, 133, (5), 1150-2.

433

(24) Chen, Q.-S.; Xu, Z.-N.; Peng, S.-Y.; Chen, Y.-M.; Lv, D.-M.; Wang, Z.-Q.; Sun, J.; Guo,

434

G.-C., One-step electrochemical synthesis of preferentially oriented (111) Pd nanocrystals

435

supported on graphene nanoplatelets for formic acid electrooxidation. J.Power Sources. 2015,

436

282, 471-478.

437

(25) Li, J.; Liu, H.; Cheng, X.; Xin, Y.; Xu, W.; Ma, Z.; Ma, J.; Ren, N.; Li, Q., Stability of

438

Palladium-Polypyrrole-Foam Nickel Electrode and Its Electrocatalytic Hydrodechlorination

439

for Dichlorophenol Isomers. Ind. Eng. Chem. Res. 2012, 51, (48), 15557-15563.

440

(26) Zhao, H.; Qian, L.; Guan, X.; Wu, D.; Zhao, G., Continuous Bulk FeCuC Aerogel with

441

Ultradispersed Metal Nanoparticles: An Efficient 3D Heterogeneous Electro-Fenton Cathode

442

over a Wide Range of pH 3-9. Environ. Sci. Technol. 2016, 50, (10), 5225-33.

443

(27) Shifu, C.; Gengyu, C., Photocatalytic oxidation of nitrite by sunlight using TiO2

444

supported on hollow glass microbeads. Sol. Energy. 2002, 73, (1), 15-21.

445

(28) El-Deab, M. S., Electrochemical reduction of nitrate to ammonia at modified gold

446

electrodes. Electrochim. Acta. 2004, 49, (9), 1639-1645.


Page 20 of 28

Page 21 of 28


447

(29) Liu, Y.; Pan, L.; Chen, T.; Xu, X.; Lu, T.; Sun, Z.; Chua, D. H. C., Porous carbon spheres

448

via microwave-assisted synthesis for capacitive deionization. Electrochim. Acta. 2015, 151,

449

489-496.

450

(30) Watts, M. J.; Linden, K. G., Chlorine photolysis and subsequent OH radical production

451

during UV treatment of chlorinated water. Water Res. 2007, 41, (13), 2871-2878.

452

(31) Li, T.; Jiang, Y.; An, X.; Liu, H.; Hu, C.; Qu, J., Transformation of humic acid and

453

halogenated byproduct formation in UV-chlorine processes. Water Res. 2016, 102, 421-427.

454

(32) Liu, H.; Zhao, H.; Quan, X.; Zhang, Y.; Chen, S., Formation of Chlorinated Intermediate

455

from Bisphenol A in Surface Saline Water under Simulated Solar Light Irradiation. Environ. Sci.

456

Technol. 2009, 43, (20), 7712-7717.

457

(33) Chen, J.; Shi, H.; Lu, J., Electrochemical treatment of ammonia in wastewater by RuO2–

458

IrO2–TiO2/Ti electrodes. J. Appl. Electrochem. 2007, 37, (10), 1137-1144.

459



Fig.1 (a) SEM images of the surface of Ni foam and (b) Pd-Cu/NF electrode. 110x50mm (300 x 300 DPI)


Page 22 of 28

Page 23 of 28


Fig.2 (a) XRD patterns of the NF, Cu/NF, Pd/NF and Pd-Cu/NF electrodes. (b) The wide region scanning XPS spectrum of Ni foam and Pd-Cu/NF electrodes. XPS spectra for the narrow scan of (c) Pd 3d and (d) Cu 2p on Pd-Cu/NF electrode. 134x99mm (300 x 300 DPI)



Fig.3 (a) NO3–-N removal efficiency on different cathodes. (b) NH4+-N degradation efficiency by electrochemical, photocatalysis, PEC and PEC-chlorine process, respectively. (c) ESR spectra of radicals trapped by DMPO in PEC system and PEC-chlorine system. (d) Transient absorption spectra under laser flash photolysis of aqueous solution in PEC and PEC-chlorine systems. Condition: Potential -1.2 V vs SCE, pH=5 and Cl– 0.05 M. 152x115mm (300 x 300 DPI)


Page 24 of 28

Page 25 of 28


Scheme.1 Illustration of the inorganic nitrogen removal mechanism in the exhaustive photoelectrochemical cycle system. 94x55mm (300 x 300 DPI)



Fig.4 Effect of Cl– concentration on (a) NO3–-N removal and (b) NH4+-N generation. Condition: potential -1.2 V vs SCE, pH=5 and 30 mg L-1 NO3––N. 119x49mm (300 x 300 DPI)


Page 26 of 28

Page 27 of 28


Fig.5 (a) The removal of TN in different ratios of nitrate and ammonium (Cl– 0.05 M, pH=5, potential -1.2V vs SCE, 30 mg L-1 TN, 90 min treatment). (b) Nitrate degradation efficiency in exhaustive photoelectrochemical cycle system during five tests at 90 min intervals. Conditions: Cl– 0.05 M, potential 1.2 V vs SCE, pH=5, and 30 mg L-1 NO3––N. 119x48mm (300 x 300 DPI)



Table of Contents 84x47mm (300 x 300 DPI)


Page 28 of 28

Exhaustive Conversion of Inorganic Nitrogen to Nitrogen Gas Based

Recommend Documents