Novel Cu(II)âEDTA Decomplexation by Discharge Plasma Oxidation

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Remediation and Control Technologies

Novel Cu(II)-EDTA decomplexation by discharge plasma oxidation and coupled Cu removal by alkaline precipitation: Underneath mechanisms Tiecheng Wang, Yang Cao, Guangzhou Qu, Qiuhong Sun, Tianjiao Xia, Xuetao Guo, Hanzhong Jia, and Lingyan Zhu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02039 • Publication Date (Web): 21 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

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Environmental Science & Technology

1

Novel Cu(II)-EDTA decomplexation by discharge plasma oxidation and coupled

2

Cu removal by alkaline precipitation: Underneath mechanisms

3 4

Tiecheng Wang1,2, Yang Cao1,2, Guangzhou Qu1,2, Qiuhong Sun3, Tianjiao Xia1,2,

5

Xuetao Guo1,2, Hanzhong Jia1,2,*, Lingyan Zhu1,2 College of Natural Resources and Environment, Northwest A&F University,

6

1

7

Yangling, Shaanxi Province 712100, PR China

8

2

9

Ministry of Agriculture, Yangling, Shaanxi 712100, PR China

Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China,

Institute of Soil and Water Conservation, Northwest A&F University, Yangling,

10

3

11

Shaanxi Province 712100, PR China

12

*Corresponding author: Hanzhong Jia

13

E-mail: [email protected]

14 15

ABSTRACT

16

Strong complexation between heavy metals and organic complexing agents

17

makes the heavy metals difficult to be removed by classical chemical precipitation. In

18

this study, a novel decomplexation method was developed using discharge plasma

19

oxidation, which was followed by alkaline precipitation to treat water containing

20

heavy metal-organic complex, i.e., Cu-ethylenediaminetetraacetic acid (Cu-EDTA).

21

The decomplexation efficiency of Cu complex reached up to nearly 100% after 60

22

min’s oxidation by discharge plasma, which was accompanied by 82.1% of total 1 ACS Paragon Plus Environment


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organic carbon removal and energy efficiency of 0.62 g kWh-1. Presence of free Cu2+

24

favored Cu-EDTA decomplexation, whereas the presence of excessive EDTA

25

depressed this process. Cu-EDTA decomplexation was mainly driven by the produced

26

1

27

characterized by UV-Vis, ATR-FTIR, total organic carbon, and three-dimensional

28

fluorescence diagnosis. The main intermediates including Cu-EDDA, Cu-IDA,

29

Cu-NTA, small organic acids, NH4+ and NO3- were identified, accompanied by Cu2+

30

releasing. The followed precipitation process removed 78.1% of Cu2+, and

31

Cu-associated precipitates included CuCO3, Cu2CO3(OH)2, CuO, and Cu(OH)2. A

32

possible pathway of Cu complex decomplexation and Cu2+ removal in such a system

33

was proposed.

34

TOC Art

O2, O2•-, O3 and •OH by discharge plasma. Cu-EDTA decomplexation process was

35 36

Table of Contents

37

The present research opens a possible way to control Cu(II)-organic complexes

38

pollution in the water environment.

39

Introduction

40

Organic chelating agents, including diethylenetriaminepentaacetic acid, citrate,

41

tartrate, and ethylenediaminetetraacetic acid (EDTA), have been widely utilized in

42

electroplating and mining industries (1). They are potentially discharged in sewage 2 ACS Paragon Plus Environment

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and readily form extremely stable chelating complexes with heavy metal ions

44

co-present in wastewater (1-3). Conventional wastewater treatment methods, such as

45

coagulation-flocculation,

46

ion-exchange, are ineffective in removal of the chelated heavy metal from the

47

effluents (2-5). Moreover, the chelated metals display toxicities to microorganisms

48

and then restrain the removal efficiency of chemical oxygen demand (COD) in the

49

biological treatment process (6). Therefore, it is urgent to develop new methods to

50

eliminate the chelating complexes from the wastewater.

chemical

precipitation,

membrane

filtration

and

51

Decomplexation is proved to be essential to achieve high-efficient elimination of

52

the chelating complexes, which could be achieved by strong oxidation techniques

53

(7-12). Decomplexation would release free heavy metal ions into solutions, allowing

54

further removal of free metal ions using post-precipitation method. Some classical

55

oxidation methods such as ultraviolet (UV)/H2O2 (7, 8), Fenton oxidation (9),

56

ozonation (10), TiO2 photocatalysis (11) and photoelectrocatalytic oxidation (12),

57

have been used for decomplexation. However, one single oxidation technique usually

58

did not give high decomplexation efficiency. Jiraroj (7) and Lan (8) reported that only

59

H2O2 displayed low decomplexation efficiency to Pb-EDTA and Cu-EDTA, whereas

60

the combination of H2O2 and UV could significantly promote the decomplexation

61

efficiency. Fenton oxidation only exhibited high decomplexation performance to

62

Ni-EDTA at low pH conditions (9), and further processing for the iron sludge resulted

63

from iron residual was required. There was high selectivity for ozone to react with

64

organic compounds, the oxidative potential of direct ozone oxidation was relatively 3 ACS Paragon Plus Environment


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poor, and thus it was usually combined with other methods or materials. Malinen (10)

66

reported that it was necessary to combine ozone and UV to realize high-efficient

67

decomplexation for Co-EDTA. Photo-catalysis was also applied to decompose the

68

chelating complexes; however, light absorptivity of TiO2 restrained its utilization in

69

practical application (11). In addition, ecological risk brought by nano-TiO2 residual

70

should be taken into consideration.

71

Recently, as a novel advanced oxidation technology, electrical discharge plasma

72

has attracted great attention on pollution control due to its advantages of

73

high-efficiency, simple equipment, and environmental friendliness. When the

74

electrical discharge plasma was triggered, electrons with high energy would generate

75

and then oxidative active species such as •OH, •O, H2O2 and O3 would form. These

76

active species could excite, ionize, and dissociate organic compounds, leading to the

77

decomposition of the organic compounds. Simultaneously, several physical actions

78

including UV radiation, shock waves, and cavitation effects would also generate in

79

the electrical discharge plasma process. Under the synergistic effects of these active

80

species and physical actions, chemical bonds of the organic compounds would be

81

ruptured or valence state of the heavy metal ions would be changed (13-18). Previous

82

works indicated that organic chelating agents such as EDTA could be efficiently

83

decomposed by discharge plasma (13). It was reported discharge plasma could

84

achieve simultaneous dyes degradation and Cr6+ conversion in a solution where the

85

dyes and Cr6+ co-existed without complexation between them (15,16). It was worth

86

noting that (CH3COO)2Pb, which was a non-complex and simple compound, was 4 ACS Paragon Plus Environment

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destructed by discharge plasma and then Pb2+ was released, which was advantageous

88

for the removal of Pb2+ by traditional chemical precipitation (17). In our previous

89

study, a surface discharge plasma (SDP) was developed to remove organic matter in

90

natural water, and the molecular structure of the organic matter was significantly

91

destroyed by active species attacks (18). In the SDP system, the discharge plasma

92

occurred in gas phase and it was not affected by solution conductivity; the generated

93

active species could be rapidly injected into liquids via small bubbles, which

94

enhanced their utilization, especially for short-lived active species. More importantly,

95

any exogenous chemicals were not required in the SDP process. Hence, it is presumed

96

that the SDP may be an efficient technique for chelating complex decomplexation.

97

However, there is a big knowledge gap on the performance and internal mechanisms

98

of the SDP in decomplexation of chelating complexes.

99

In this study, simultaneous decomplexation of chelating complex and removal of

100

heavy metal were explored using a novel coupling process, namely surface discharge

101

plasma oxidation/alkaline precipitation. Cu-EDTA, widely detected in the effluents

102

from mining and electroplating industries, was selected as the model chelating

103

complex. The aim of this research was to probe the performance and mechanism of

104

Cu-EDTA decomplexation by the SDP oxidation. Influences of operation conditions

105

including discharge voltage and the molar ratio of Cu and EDTA on Cu-EDTA

106

decomplexation were particularly investigated. The types and scope of reactive

107

species for Cu-EDTA decomplexation were qualitatively and quantitatively explored.

108

The decomplexation process and generated intermediates were also diagnosed via 5 ACS Paragon Plus Environment


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ultraviolet-visible (UV-Vis) spectrum, attenuated total reflectance-Fourier transform

110

infrared spectroscopy (ATR-FTIR), total organic carbon (TOC), three-dimensional

111

fluorescence, ion chromatography,

112

morphology and chemical composition of precipitates were also analyzed.

113

Experimental Methods and Analysis

114

Materials. Copper sulfate (CuSO4, purity > 99%) and Edetate disodium (EDTA-2Na,

115

purity > 99%) were analytical reagents, which were purposed from Sinopharm

116

Chemical Reagent Co., Ltd, China. Cu-EDTA stock solution with an initial

117

concentration of 1000 mg L-1 was prepared by dissolving CuSO4 and Na2EDTA in 1 L

118

of deionized water (Cu2+/EDTA molar ratio was 1:1) unless specifically stated. The

119

obtained stock solution was diluted with deionized water prior to each treatment.

120

5,5-Dimethyl-1-pyrroline N-oxide (DMPO), 2,2,6,6-Tetramethylpiperidine (TEMP),

121

dimethylsulfoxide (DMSO), iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), and

122

EDDA were obtained from Sigma Chemical Co.; and Cu-IDA, Cu-NTA, and

123

Cu-EDDA were prepared by dissolving CuSO4 and corresponding ligand acids with

124

molar ratio of 1:1. All other chemicals were analytical grade and used without further

125

purification.

126

Surface Discharge Plasma System. The experimental setup for Cu-EDTA

127

decomplexation is depicted in Figure S1, and the details are illustrated in S1 in the

128

Supporting Information (SI). In each batch experiment, Cu-EDTA concentration was

129

0.3 mmol L-1 and solution pH was 4.0 unless specifically stated, and the solution

130

volume was 500 mL.

and capillary

6 ACS Paragon Plus Environment

electrophoresis. Moreover,

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To evaluate the effect of molar ratio of Cu2+ and EDTA on Cu-EDTA

132

decomplexation, serial concentration gradients were set as illustrated in S1 in the SI.

133

Methods and Analyses. Voltage and current signals were recorded by a digital

134

oscilloscope (Tektronix TDS2014), and their representative waveforms are depicted

135

in Figure S2 (see Supporting Information). Calculation methods on discharge power

136

and energy efficiency are presented in S2. Calculation method on decomplexation

137

efficiency is depicted in S3.

138

•OH radical was diagnosed by fluorescent spectrometry (F-4600, Hitachi) as

139

reported by Kanazawa (19). H2O2 concentration was monitored using potassium

140

oxalate colorimetric method as described by Sellers (20). Ammonia nitrogen was

141

measured by salicylate spectrophotometry (21). Total organic carbon analyzer

142

(LIQUIC TOCII, Germany) was chosen to detect the TOC. ATR-FTIR spectrum was

143

obtained by Fourier Transform Infrared Spectroscopy (Excalibur3100, USA).

144

Absorption

145

spectrophotometer (U2800, Shimadzu). Three-dimensional fluorescence spectrum of

146

Cu-EDTA was diagnosed by fluorescent spectrometry (F-4600, Hitachi). Electron

147

Paramagnetic Resonance spectrometer (EPR, Bruker E500, Germany) was used to

148

examine the formation of reactive species (22). Cu-EDTA concentration was

149

monitored by High Performance Liquid Chromatography (SCL-10ACP, Shimadzu).

150

The generated organic acids were monitored by ion chromatography (ICS-90,

151

DIONEX). Capillary Electrophoresis (P/ACE MDQ, Beckmann) was used to detect

152

the intermediates. Copper ion in the filtrate was analyzed by Atomic Absorption

spectrum

of

Cu-EDTA

solutions

was


obtained

by

UV-Vis


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Spectrophotometer (AA-7000, Shimadzu). The detailed procedures were depicted in

154

S4.

155

The

precipitates

were

collected

and

analyzed

using

Fourier

156

Transform Infrared Spectroscopy (FTIR, Vetex 70, Bruker), X-ray Diffraction (XRD,

157

XTRA, Switzerland), X-ray Photoelectron Spectroscopy (XPS, PHI-5300/ESCA,

158

ULVAC-PHI), and Energy Dispersive X-ray Spectrometer (EDX, NORAN system),

159

as described in S5.

160

Results and Discussion

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Decomplexation Performance. The influence of discharge voltage on the

162

decomplexation efficiency was firstly evaluated. Figure 1a depicts the evolution of

163

residual of Cu complex concentration as a function of time at different discharge

164

voltages. The decomplexation efficiency increased gradually with the discharge

165

voltage. The Cu complex concentration decreased to 0.001 mmol L-1 at discharge

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voltage of 19 kV in 60 min treatment (the decomplexation efficiency was 99.7%);

167

whereas it only decreased to 0.14 mmol L-1 at 10 kV (the decomplexation efficiency

168

was 53.3%). The decomplexation process could be fitted well with the first-order

169

kinetic model, as shown in the inset of Figure 1a. The reaction rate constant

170

exponentially increased from 0.012 min-1 to 0.091 min-1 when the discharge voltage

171

was enhanced from 10 kV to 19 kV (see Figure S3). Previous study reported that

172

production of reactive species such as ozone, •OH radical, and O2•- were enhanced at

173

relatively higher discharge voltage, which were effective in organic contaminant

174

decomposition (18, 23). Thus, the higher decomplexation efficiency at relatively 8 ACS Paragon Plus Environment

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higher discharge voltage might be due to the promoted generation of reactive species,

176

and the detailed roles of these species would be discussed later. The energy efficiency

177

for Cu complex decomplexation was 9.76 and 7.55 mmol kWh-1 at 16 kV and 19 kV,

178

respectively (see Figure S4); while the corresponding decomplexation efficiency was

179

93% and 99.7%. Comprehensively considering decomplexation efficiency and energy

180

efficiency, the following experiments were carried out at 16 kV.

181

Decomplexation of Cu-EDTA was reported previously using several oxidation

182

techniques, such as UV/persulfate, TiO2 photocatalysis, and Fe(III) displacement/UV

183

degradation. Table S1 compares the performances, energy efficiency and other

184

parameters

185

ion-exchanger/zero-valent iron methods could induce decomplexation of Cu-EDTA,

186

but relatively longer treatment time (180-240 min) was necessary to achieve

187

satisfactory efficacy (12, 24). Other techniques, such as UV/persulfate, TiO2

188

photocatalysis, and Fe(III) displacement/UV degradation, displayed high-efficient

189

decomplexation of Cu-EDTA in a relatively short treatment time, but the energy

190

efficiencies were relatively low (11, 25, 26). Cu-EDTA decomplexation efficiency

191

reached 95% within 60 min treatment by interior microelectrolysis; whereas stringent

192

reaction conditions, such as acidic and aerobic conditions, were required (27). In the

193

present study, Cu complex decomplexation efficiency reached up to nearly 100% after

194

60 min oxidation with the energy efficiency of 0.62 g kWh-1; this energy efficiency

195

was higher than that of UV/persulfate (0.58 g kWh-1) and TiO2 photocatalysis (0.034 g

196

kWh-1), and lower than that of photoelectrocatalytic oxidation (1.2 g kWh-1). The

of

these

techniques.

Both

photoelectrocatalytic


oxidation

and


197

results indicated that the SDP provided very efficient performance to decomplex

198

Cu-EDTA with reasonable energy efficiency.

199

Figure 1b displays the effect of the molar ratio of Cu and EDTA on Cu complex

200

decomplexation. The decomplexation efficiency was only 13.6% as the molar ratio of

201

Cu: EDTA was 1:4 in 45 min oxidation treatment. It increased to 97.9% as the molar

202

ratio was 4:1. This suggested that excess EDTA depressed Cu complex

203

decomplexation while excess Cu dosage favored its decomplexation. It was well

204

known that EDTA generally chelates with metal ions at equal molar ratio of 1:1.

205

Excess EDTA dosage would result in presence of free EDTA molecules in the

206

solutions. It was reported that EDTA could be efficiently decomposed by electrical

207

discharge plasma (13). Thus, the free EDTA molecules could compete with Cu-EDTA

208

for the produced reactive species, and thus depressed Cu-EDTA decomplexation.

209

Previous researches reported that Cu-peroxide could be generated via reaction of

210

H2O2 and Cu2+, leading to enhanced generation of •OH radicals, as shown in reactions

211

1 and 2, which then promoted organic pollutant degradation (28, 29). Wang (30)

212

reported that addition of Cu2+ facilitated degradation of polyvinyl alcohol in an

213

electrical discharge plasma system. The fluorescence intensities of •OH radicals in

214

deionized water containing different amounts of Cu2+ after 15 min discharge plasma

215

treatment are depicted in Figure S5a, which increased with Cu2+ concentration in the

216

range of 0.3-0.9 mmol L-1. H2O2 concentration also decreased with the increase of

217

Cu2+ concentration in the range of 0.3-0.9 mmol L-1, as shown in Figure S5b.

218

Therefore, the excess free Cu2+ in the solution could act as catalysts to promote 10 ACS Paragon Plus Environment

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formation of •OH radicals, which acted as one of the species in decomplexation of Cu

220

complexes.

221

Cu 2+ + H 2 O 2 → (Cu 2+ OOH - ) + + H +

(1)

222

(Cu 2+ OOH - ) + → Cu + +1/2O 2 OH

(2)

223

Figure 1c displays the effect of solution pH on Cu complex decomplexation. The

224

decomplexation efficiency reached 99.6% in 30 min treatment at solution pH 2.0, and

225

it was reduced to 65.3%, 36.5%, 27.9%, and 19.6% at the solution pH 4.0, 6.0, 8.0,

226

and 10.0, respectively. Solution pH could affect the behavior of copper ions and

227

EDTA. CuH2EDTA and CuHEDTA- were the main species at pH 2.0, CuHEDTA- and

228

fully deprotonated CuEDTA2- were the dominant species at pH 4.0, CuEDTA2- was

229

the only species at pH 6-9, and parts of CuOHEDTA3- occurred in addition to

230

CuEDTA2- at pH 10 (31). Previous study reported that protonated species of Cu

231

complexes was oxidized by •OH radical faster than the deprotonated species (7, 12).

232

Therefore, acidic conditions favored Cu complex decomplexation in this study. Huang

233

et al (31) reported that better performance of Cu-EDTA degradation by ozonation was

234

obtained at acidic conditions. In interior microelectrolysis system, higher Cu-EDTA

235

degradation efficiency was also observed at relatively lower solution pH value (27).

236

Zhao (12, 32) also reported that Cu-EDTA and Ni-EDTA degradation efficiency both

237

decreased in photoelectrocatalytic oxidation process with the increase of solution pH.

238

Involved Reactive Species for Complex Decomplexation. Isopropanol (IPA),

239

benzoquinone (BQ), and 1,4-Diazabicyclooctane triethylenediamine (DABCO) were

240

frequently-used scavengers of •OH, superoxide radical (O2•-), and 1O2, respectively 11 ACS Paragon Plus Environment


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(18, 23, 33). Figure 2a depicts their influences on decomplexation process. Clearly

242

inhibiting effects in Cu complex decomplexation were all observed in the presence of

243

these scavengers. Approximately 37.4%, 58.9%, and 48.0% of decline in the

244

decomplexation efficiency were observed in 60 min treatment with addition of 0.5

245

mmol L-1 IPA, BQ, and DABCO, respectively. These results suggested that •OH

246

radical, O2•-, and

247

decomplexation process, especially for O2•-. Previous studies also reported the

248

significant roles of ·OH radicals, O2•-, and 1O2 in dimethyl phthalate degradation in

249

aqueous using sodium percarbonate activated by discharge plasma (34).

O2 in the SDP system all displayed vital roles in the

1

250

Figures 2b-2d depict the changes of EPR signals of ·OH radicals, 1O2, and O2•-.

251

A four-line signal with an intensity ratio of 1:2:2:1 was detected using DMPO as the

252

spin-trapping reagent, a triplet-line signal with an intensity ratio of 1:1:1 was detected

253

using TEMP as spin-trapping reagent, and a six-line signal with an intensity ratio of

254

1:1:1:1:1:1 was detected using DMPO-DMSO as spin-trapping reagent after 5 min

255

reaction. These provided strong evidences for the formation of ·OH radicals, 1O2, and

256

O2•- in the SDP system. The intensities of these signals were lower in the solutions

257

containing Cu-EDTA than that without Cu-EDTA. On the other hand, their intensities

258

were strengthened gradually with the discharge voltage. These findings further

259

confirmed that ·OH radicals, 1O2, and O2•- took part in Cu complex decomplexation

260

process.

261

In addition, Cu complex decomplexation was conducted by pure ozone, in which

262

ozone concentration was equal to that in the SDP process. There was approximately 12 ACS Paragon Plus Environment

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30.9% decline in decomplexation efficiency in pure ozonation treatment, compared

264

with that in SDP process (see Figure S6). These results indicated that ozone also

265

played significant roles in the decomplexation process.

266

Decomplexation

267

mechanisms were investigated via UV-Vis spectrum, ATR-FTIR spectrum, TOC,

268

three-dimensional fluorescence, and intermediates identification.

269

the UV-Vis spectra of Cu-EDTA solution as a function of treatment time. Cu-EDTA

270

solution displayed a strong absorption peak at 239 nm before the SDP treatment,

271

while the peak intensity decreased as the reaction went on. A new peak at around 221

272

nm appeared after the treatment. Figure 3b depicts the ATR-FTIR spectra of

273

Cu-EDTA solution as a function of treatment time. Cu-EDTA complex displayed two

274

absorption peaks at 580 cm-1 and 546 cm-1 before the discharge plasma treatment,

275

which could be assigned to Cu-N and Cu-O vibrations (35, 36). The intensities of

276

these two peaks decreased after the treatment, suggesting that the Cu-N and Cu-O

277

bonds were gradually reduced.

Process

and

Mechanisms.

Decomplexation

process

and

Figure 3a depicts

278

Approximately 82.1% of TOC was removed within 60 min treatment, as depicted

279

in Figure S7, which was lower than the decomplexation efficiency. This further

280

indicated that Cu-EDTA was destroyed and some byproducts were formed during the

281

SDP process. In addition, the TOC removal in the SDP system was relatively higher

282

than

283

displacement/UV degradation system (Table S1).

284

those

in

interior

Three-dimensional

microelectrolysis,

fluorescence

was

TiO2 photocatalysis,

commonly


selected

and

Fe(III)

to

analyze


285

transformation of organic compounds. Figure 4 depicts the fluorescence spectra of

286

Cu-EDTA solution at different treatment times. Only one peak with the EX/EM

287

around 450/450 (marked as peak A) appeared in the fluorescence spectrum before the

288

SDP treatment, as illustrated in Figure 4a. Peak A was still observed and the intensity

289

gradually decreased after the SDP treatment (Figure 4b-4e). Three new fluorescence

290

peaks with the EX/EM around 320/460, 250/460, and 210/450 (marked as peak B, C,

291

and D, respectively) emerged after 15 min reaction (Figure 4b), which was

292

respectively assigned to humic acid-like fluorescence region, fulvic-like fluorescence

293

region, and fulvic-like fluorescence region (37). The fluorescence intensities of these

294

three peaks increased after 30 min treatment, and thereafter, they all decreased until

295

disappearance, as shown in Figure 4d and Figure 4e. The changes supported that

296

Cu-EDTA was decomposed gradually, accompanied by production of some organic

297

substances. Previous study also reported that the fluorescence intensity of tetracycline

298

decreased gradually during its degradation by photocatalysis, following by organic

299

substances generation in the humic acid-like and fulvic-like fluorescence regions (37).

300

Similar phenomenon was also observed by Yuan during nitroaromatic pollutant

301

degradation using Fenton and Fe0, who attributed the appearance and changes of

302

humic acid-like and fulvic-like fluorescence groups to the elimination of some

303

functional groups such as carbonyl and hydroxyl (38). Furthermore, the SDP

304

treatment led to position shift of the fluorescence group of peak A. Around 30 nm

305

blue-shift was observed in the excitation axis after 60 min treatment, as well as in the

306

emission axis. The blue-shift phenomenon could be associated with the decrease of 14 ACS Paragon Plus Environment

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conjugated bonds or/and removal of carbonyl and hydroxyl groups (39). The

308

blue-shift phenomenon also occurred during nitroaromatic pollutant degradation by

309

Fenton and Fe0, which was accompanied by some functional groups destruction (38).

310

This again demonstrated that Cu-EDTA molecular structure in the SDP system was

311

destructed, followed by the decrease of carbonyl and hydroxyl groups.

312

Decomplexation byproducts were analyzed using capillary electrophoresis and

313

ion chromatography. Figure S8a depicts the evolution of decomplexation

314

intermediates with treatment time using the capillary electrophoresis analysis.

315

Cu-EDDA, Cu-NTA, and Cu-IDA were identified as the main intermediates. The

316

peak intensity of Cu-EDTA decreased gradually as the reaction went on, and

317

disappeared after 60 min treatment (Figure S8a), which was in consistent with its

318

decomplexation efficiency. The peak intensity of Cu-EDDA increased gradually in

319

the first 30 min and decreased afterwards. The peak intensities of Cu-NTA and

320

Cu-IDA were also strengthened within 30 min treatment, while decreased afterwards

321

until disappearance.

322

As depicted in Figure S8b, CH3COOH, HCOOH, oxalate acid, and NO3- were

323

detected by ion chromatography, and their peak intensities all increased gradually

324

with treatment time. These results manifested that some organic acids with small

325

molecular weight and NO3- were accumulated in the decomplexation process. NH4+-N

326

was also found to be accumulated within 45 min treatment, and then its concentration

327

decreased, as depicted in Figure S9. Here, it must be noted that these NH4+-N and

328

NO3- concentrations were calculated by subtracting their concentrations formed in 15 ACS Paragon Plus Environment


329

SDP in deionized water from those in Cu-EDTA simulated wastewater, respectively;

330

because the SDP in air would also generate NOx-. These results demonstrated that

331

nitrogen atoms in the Cu-EDTA molecules were gradually converted to NH4+-N

332

during decomplexation process, and it could further be oxidized to NO3--N as the final

333

state.

334

Cu-NTA, Cu-IDA, and Cu-EDDA were detected as the predominant

335

intermediates during Cu-EDTA degradation by photo-assisted electrolysis (40).

336

Ni-IDA, Ni-EDDA, and Ni-NTA were also found as the intermediates during

337

Ni-EDTA decomplexation by photoelectric oxidation, accompanied by accumulation

338

of CH3COOH, HCOOH, and oxalate acid (32). Oviedo (41) detected CH3COOH and

339

oxalate acid as the byproducts during Fe-EDTA decomplexation by Fenton oxidation.

340

In addition, Huang (31) observed CH3COOH, HCOOH, NH4+-N, and NO3--N during

341

Cu-EDTA abatement by ozonation, and a few parts of NH4+-N were also converted to

342

NO3--N. Based on the above analyses, the possible pathways for Cu complex

343

decomplexation are proposed in Figure 5. On the one hand, reactive species such as

344

O3, •OH, and O2•- could attack C-N bonds of Cu-EDTA molecules at position I,

345

resulting in generation of Cu-EDDA and CH3COOH. Further attacks on the C-N

346

bonds in the Cu-EDDA molecules would lead to the generation of Cu-IDA and

347

Cu-NTA. On the other hand, the reactive species could also attack C-N bonds of the

348

Cu-EDTA molecules at position II, generating Cu-IDA and Cu-NTA. These generated

349

Cu-containing intermediates could be further oxidized into small organic acids and

350

inorganic ions such as NH4+, which was then oxidized as NO3- finally. Simultaneously, 16 ACS Paragon Plus Environment

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351

Cu2+ would be released during the decomplexation process, enabling it to be

352

eliminated by chemical precipitation.

353

Cu Removal via Chemical Precipitation. Cu removal as a function of time is

354

depicted in Figure S7. Approximately 78.1% of Cu was removed within 60 min,

355

which was lower than the decomplexation efficiency. This result suggested that some

356

Cu-containing intermediates still existed at the end of the decomplexation treatment.

357

The elementary compositions of the precipitates via EDX analysis is depicted in

358

Figure S10a. Cu, O, and C were clearly observed, suggesting that the precipitates

359

were mainly composed of Cu, O, and C chemical elements. The typical FTIR of the

360

precipitates is shown in Figure S10b. The absorption band around 3437 cm-1 was

361

corresponded to -OH group of stretching mode, the peak at 1601 cm-1 represented the

362

-OH group of bending mode, and the two absorption bands around 880 and 780 cm-1

363

could be associated with bidentate carbonates. These bands suggested that

364

Cu2CO3(OH)2 might be generated in the precipitates (42-44). The two absorption

365

bands around 1441 and 1390 cm-1 could be associated with Cu(II)-basic carbonates

366

(45), absorption bands around 1337 and 1128 cm-1 could be due to C=O and C-O

367

vibrations coordinating to metal cations (46, 47), and the two bands at 619 and 519

368

cm-1 were assigned to Cu(II)-O vibrations. These further confirmed that some

369

Cu(II)-basic carbonates were produced in the precipitates. Huang et al (31) also

370

reported that Cu2CO3(OH)2 and Cu(OH)2 were the predominant components in

371

precipitates during Cu-EDTA decomplexation by ozonation and Cu removal by

372

alkaline precipitation. 17 ACS Paragon Plus Environment


373

Figure S10c depicts the XRD pattern of the precipitates. The diffraction peaks at

374

32.6°, 35.6°, 38.7°, 48.7°, 53.4°, 58.3°, and 61.6° could be associated to the

375

monoclinic structure of CuO, the peaks at 23.5°, 42.2°, and 62.9° could be

376

corresponded to a structure of Cu-basic carbonates, and the peaks at 34.1° and 39.0°

377

could be due to a structure of Cu(OH)2 (42, 48, 49). Previous studies reported that the

378

monoclinic structure of CuO was one of the main components in precipitates during

379

Cu-EDTA decomplexation by photocatalysis, as well as by UV/H2O2 (11, 26).

380

Figure S10d depicts the XPS pattern of the precipitates, in which the

381

photoelectron lines of binding energies at 934.08, 531.08, and 285.08 eV were

382

associated with Cu2p, O1s, and C1s, respectively. To further explore the chemical

383

status of Cu2p and O1s in the precipitates, their resolution spectra are depicted in

384

Figure S11. Two peaks at 954.28 and 934.28 eV were allotted to Cu2p1/2 and Cu2p3/2

385

(Figure S11a), respectively, with the energy gap of 20 eV between them, suggesting

386

that the main species of Cu in the precipitates was Cu(II). Two shake-up peaks at

387

962.0 and 943.1 eV were also observed, which further ruled out the presence of Cu(I)

388

phase (31, 32). For the resolution spectrum of Cu2p3/2, four peaks at 935.0, 934.6,

389

934.0, and 932.7 eV were obtained, which represented the presence of CuCO3,

390

Cu2CO3(OH)2, CuO, and Cu(OH)2, respectively (42, 50), as depicted in Figure S11b.

391

The resolution spectrum of O1s is depicted in Figure S11c, where three peaks at 530.9,

392

531.5, and 529.6 eV were associated with –OH, –C=O, and Cu(II)-O groups,

393

respectively (42).

394

Based on the analysis on the chemical components of Cu in the precipitates, we 18 ACS Paragon Plus Environment

Page 18 of 33

Page 19 of 33


395

assumed that the predominant species might be CuCO3, Cu2CO3(OH)2, CuO, and

396

Cu(OH)2. The C-containing precipitates could be attributed to CO32- which was partly

397

derived from complex mineralization.

398

Summarily, high-efficient and rapid decomplexation of Cu-EDTA and

399

elimination of copper ions was realized in the SDP oxidation/alkaline precipitation

400

processes, and it may be employed as an alternative for controlling Cu(II)-EDTA

401

complex pollution in the water environment. It should be noted that both

402

decomplexation efficiency and mineralization efficiency of Cu-EDTA were

403

satisfactory in the discharge plasma process, and any exogenous chemicals were not

404

required in the treatment. However, more study should be carried out to further reduce

405

its energy consumption in future; as discussed above, the energy efficiency was still

406

lower than that in photoelectrocatalytic oxidation process. In addition, its efficacy on

407

other Cu-organic complexes (in addition to Cu-EDTA) decomplexation should be

408

further explored, especially in actual effluent.

409

Acknowledgments

410

The National Natural Science Foundation of China (51608448, 21737003), Young

411

Talent Cultivation Scheme Funding of Northwest A&F University (Z109021802), and

412

Fundamental Research Fund for the Central Universities (Z109021617) supported this

413

research.

414

Supporting Information Available

415

Texts S1-S5 include SDP system introduction, discharge power and energy efficiency

416

calculation, decomplexation efficiency calculation, aqueous component analysis, and 19 ACS Paragon Plus Environment


417

precipitate analysis. Figures S1-S11 include reaction system, voltage and current

418

waveforms, reaction rate constant, energy efficiency, active species formation,

419

Cu-EDTA decomplexation by ozonation, TOC and Cu removal, chromatogram of the

420

generated intermediates, evolution of NH4+-N, and analysis on precipitates. Table S1

421

includes Cu-EDTA decomplexation by different methods.

422

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580 581

Figure captions

582

Figure 1. Residual of Cu complexes as a function of time at various conditions (a.

583

discharge voltage; b. molar ratio of Cu and EDTA; c. pH value)

584

Figure 2. Analysis on reactive species roles (a. decomplexation efficiency under

585

various scavengers; b. DMPO spin-trapping EPR spectra; c. TEMP spin-trapping EPR

586

spectra; d. DMPO-DMSO spin-trapping EPR spectra)

587

Figure 3. Evolution of UV-Vis and ATR-FTIR spectra of Cu-EDTA with treatment

588

time (a. UV-Vis; b. ATR-FTIR)

589

Figure 4. Three-dimensional fluorescence spectra of Cu-EDTA as a function of time

590

Figure 5. Schematic diagram of Cu-EDTA decomplexation pathway in this study

591 592



593

594

595 596

Figure 1

597


Page 28 of 33

Page 29 of 33


598

599

600



Page 30 of 33

601

Figure 2

602 603

1.5

221 nm

Absorbance

1.2

0 min 30 min 60 min

239 nm

(a)

0.9 0.6 0.3 0.0

210

240

604

45

270 300 330 Wavelength (nm)

0 min

30 min

360

390

60 min

Intensity (a.u.)

40 35

-1

580 cm Cu-N

30 -1

546 cm Cu-O

(b)

25 600 605 606

580 560 540 -1 Wavenumber (cm )

Figure 3 30 ACS Paragon Plus Environment

520

Page 31 of 33


607

600

EX (nm)

500

(a) 0 min peak A

0 375.0 750.0 1125 1500 1875 2250 2625 3000

400 300 200 200

608

300

400 EM (nm)

500

600

600

EX (nm)

500

(b) 15 min 0 275.0 550.0 825.0 1100 1375 1650 1925 2200

peak A

400

peak B

300

peak C peak D

200 200 609

300

400 EM (nm)

600

600

EX (nm)

500

(c) 30 min

peak A

400

200 200

0 325.0 650.0 975.0 1300 1625 1950 2275 2600

peak B

300

610

500

peak C peak D

300

400 EM (nm)

500


600


611

612 613

Figure 4

614


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Figure 5

617 618 619


Novel Cu(II)âEDTA Decomplexation by Discharge Plasma Oxidation

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