Removal of Antibiotic Florfenicol by Sulfide-Modified Nanoscale Zero

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Removal of Antibiotic Florfenicol by SulfideModi#ed Nanoscale Zero-Valent Iron Zhen Cao, Xue Liu, Jiang Xu, Jing Zhang, Yi Yang, Jun Liang Zhou, Xinhua Xu, and Gregory V. Lowry Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02480 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 13, 2017

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Page 1 of 38

Environmental Science & Technology

1

Removal of Antibiotic Florfenicol by Sulfide-Modified

2

Nanoscale Zero-Valent Iron

3

Zhen Caoa, Xue Liua, Jiang Xua, b, *, Jing Zhanga, Yi Yanga, Junliang Zhoua, Xinhua

4

Xuc, Gregory V. Lowryb, d, *

5

a

6

University, Shanghai 200062, China

7

b

8

Pittsburgh 15213, USA

9

c

State Key Laboratory of Estuarine and Coastal Research, East China Normal

Department of Civil and Environmental Engineering, Carnegie Mellon University,

Department of Environmental Engineering, Zhejiang University, Hangzhou,

10

Zhejiang 310058, China

11

d

Center for Environmental Implications of Nanotechnology (CEINT), USA

12

* Corresponding author: Tel/fax: 86-21-62231809

13

E-mail address: [email protected] (J. Xu)

14

[email protected] (G.V. Lowry)

15

S Supporting Information ○

16

Abstract

17

Florfenicol

(FF,

C12H14Cl2FNO4S),

an

emerging

halogenated

organic

18

contaminant of concern was effectively degraded in water by sulfidized nanoscale

19

zero-valent iron (S-nZVI). Sulfidized nZVI (62.5 m2 g−1) that was prepared using a

20

one-step method resulted in small Fe0/Fe-sulfide particles that were more stable

21

against aggregation than unsulfidized nZVI (10.2 m2 g−1). No obvious removal of FF 1

ACS Paragon Plus Environment


22

was observed by unsulfidized nZVI. S-nZVI degraded FF, having a surface area

23

normalized reaction rate constant of 3.1×10−4 L m−2 min−1. The effects of the S/Fe

24

molar ratio, initial FF concentration, initial pH, temperature, and water composition

25

on the removal of FF by S-nZVI, and on the formation of reaction products, were

26

systematically investigated. Both dechlorination and defluorination were observed,

27

resulting in four degradation products (C12H15ClFNO4S, C12H16FNO4S, C12H17NO4S,

28

and C12H17NO5S). High removal efficiencies of FF by S-nZVI were achieved in

29

groundwater, river water, seawater, and wastewater. The reactivity of S-nZVI was

30

relatively unaffected by the presence of both dissolved ions and organic matter in the

31

waters tested.

32

Keywords: nanoscale zero-valent iron, sulfide, modification, florfenicol, removal,

33

degradation mechanism.

34

2


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35

Graphical abstract

36 37

3



38

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INTRODUCTION

39

Antibiotics are a unique category of pharmaceuticals that have been applied to

40

inactivate or kill microbes for over 60 years. Over 250 different antibiotics have been

41

widely used for human health, animal husbandry, and agricultural purposes, which

42

has greatly increased their usage and discharge over the world

43

been found in the effluents of pharmaceutical manufacturing plants, municipal sewage

44

treatment plants, surface water, groundwater, soil, drinking water, and foods

45

presence of antibiotic resistance genes in bacteria has grown due to the extended

46

exposures to antibiotics, and the potential threat of antibiotics pollution to aquatic

47

organisms and human health are receiving significant attention 5. Florfenicol (FF) was

48

selected as the target antibiotic in this study because it is used in many countries for

49

the control of several bacterial diseases in humans, and is one of only a few antibiotics

50

allowed in aquaculture

51

Gram-negative bacteria due to its transpeptidation inhibition in bacterial protein

52

synthesis 8. In our previous investigation, we estimated that the flux of FF into the

53

Yangtze Estuary was 26 tons from June 2013 to May 2014. This represented

54

approximately 20% of the total flux of 24 selected pharmaceuticals, and it was much

55

higher than other antibiotic compounds that are more widely studied, including

56

sulfonamides, tetracycline, macrolides, and non-antibiotic pharmaceuticals

57

a relatively small (MW=358.2 g mol−1) and moderately hydrophilic molecule (log Kow

58

ranges from −0.12 to 0.37) that is not effectively removed by traditional water

1, 2

. Antibiotics have

3, 4

. The

6, 7

. FF is effective against many Gram-positive and

4


9-12

. FF is

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13-17

59

treatment technologies

. Therefore, advanced treatment technologies are needed

60

for addressing this emerging issue.

61

Various physical, chemical, and biological methods have been applied to remove

62

antibiotics from water, such as photodegradation, biodegradation, catalytic oxidation,

63

biocathode reduction, and adsorption

64

reported to remove this specific antibiotic (FF), and either the removal processes are

65

slow (e.g. anaerobic digestion and thermophilic composting) or they do not provide

66

degradation (e.g. adsorption)

67

evaluated for reactivity with aqueous heavy metals and conventional organic

68

contaminants

69

perfluorinated compounds, enrofloxacin, and tetracycline

70

particles are readily oxidized by water, leading to limited reactive lifetime 30. Partial

71

sulfidation of nZVI has been proposed to overcome this limitation by slowing the

72

reaction of nZVI with water (hydrogen formation), improving the selectivity for

73

reduction of contaminants over water, and extending its reactive lifetime

74

Sulfidized nZVI can be made using a one-step reaction where dissolved iron is

75

reduced in the presence of a reduced sulfur species (e.g. dithionite)

76

two-step method where nZVI is partially sulfidized using reduced sulfur species (e.g.

77

sulfide or dithionite)

78

suggested to be a mixture of Fe0 and FexSy

79

well-understood than for the two-step method. These partially sulfidized nZVI have

14, 18-21

. However, few methods have been

16, 17, 22

. Nanoscale zero valent iron (nZVI) has been

23-26

, as well as emerging organic contaminants (EOCs) such as 27-29

. However, nZVI

31-34

.

32, 35

, or using a

34, 36, 37

. The material formed in the one-step method was 38

. However, this material is less

5



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80

been shown to be reactive with a number of contaminants of concern including heavy

81

metals, radionuclides, nitroaromatics, and halogenated organic compounds 36, 38-45.

82

This study measured the reactivity of sulfide-modified nZVI (S-nZVI) with FF in

83

synthetic and natural waters. The S-nZVI was synthesized with the one-step method.

84

The effects of S/Fe molar ratio, initial FF concentration, reaction temperature, and

85

initial solution pH on its properties and reactivity were determined. The degradation

86

pathway of FF by this type of S-nZVI was explored by tracking the reaction

87

byproducts

88

Spectrometry/Mass Spectrometry (UPLC-MS/MS). The performance of S-nZVI in

89

different natural waters was also assessed.

90

EXPERIMENTAL SECTION

91

Materials

using

Ultra

Performance

Liquid

Chromatography-Mass

92

Analytical grade FeSO4·7H2O, NaBH4, Na2S2O4, HCl, and NaOH were

93

purchased from the Sinopharm Group Chemical Reagent Co., Ltd., China. FF in high

94

purity grade was obtained from Shanghai Ruichu Biotechnology Co., Ltd., China.

95

Deschloro FF and dideschloro FF standard samples were obtained from Absin

96

(Shanghai) Bioscience Inc., China. All chemicals were used without further

97

purification, ultrapure water (pH = 7.0) was deoxygenated by 1 h nitrogen purging

98

before use.

99

Preparation

6


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100

S-nZVI was synthesized according to a one-step method reported previously32, 39.

101

Briefly, 0.68 g of NaBH4 was first mixed with a certain amount of Na2S2O4 (0, 0.03 g,

102

0.05 g, 0.11 g, and 0.22 g) in 200 mL ultrapure water in a 1-L three-necked flask

103

under nitrogen purging.

104

solution containing 0.50 g Fe2+. Na2S2O4 decomposes to release H2S, which reacts

105

with soluble Fe2+ to form a FeS precipitate along with nZVI. Approximately 0.5 g

106

nZVI was generated with different S/Fe molar ratios (0, 0.035, 0.07, 0.14, and 0.28) 38,

107

39

108

impurities ions immediately before use. No polymer coating was used.

109

Batch Experiments

Then this solution was added dropwise into 250 mL of a

. The nZVI and S-nZVI were prepared and washed three times to remove the

110

Unless otherwise specified, batch experiments were performed in 1-L

111

three-necked flasks with nitrogen purging and 500 rpm stirring, containing 500 mL

112

solution of 0.28 mM FF and 1.0 g L−1 S-nZVI with a S/Fe molar ratio of 0.14 at pH =

113

7.0 and 298 K. After 2 h of reaction, the solution was moved to a sealed bottle located

114

in a thermostatic shaker to continue the reaction. Samples were collected at specified

115

times and filtered through a 0.22 µm membrane to remove the nanohybrids and

116

quench the reaction. In some experiments, the S/Fe molar ratio, initial FF

117

concentration, reaction temperature, and initial solution pH were changed to

118

determine the effect of these parameters on its reactivity with FF.

7



119

Besides the ultrapure water matrix, similar experiments were also performed in

120

four different natural waters, including groundwater (collected from a well in

121

Shandong Province), river water (collected from Suzhou River in Shanghai), seawater

122

(collected from the Yellow Sea at Shandong Province), and wastewater (collected

123

from secondary effluent of a wastewater treatment plant in Shanghai). In those

124

experiments, 1 g L−1 S-nZVI and 0.28 mM FF were added into each water, and the

125

change in the FF concentrations was measured. In all experiments, the reaction rates

126

for the removal of FF by S-nZVI were calculated using a pseudo-first-order kinetic

127

model assuming an excess of S-nZVI reactive sites 46, 47.

128

Analytical Methods

129

FF in the filtered aqueous samples was analyzed by HPLC (Agilent 1260

130

Infinity). Detector: UV at 210 nm; columm: ZORBAX Eclipse XDB-C18, 250×4.6

131

mm; flow rate: 1.0 mL min−1; mobile phase: MeOH/0.05% formic acid

132

solution=40/60 (v/v); injection volume: 10 µL. The limit of detection (LOD) and limit

133

of quantification (LOQ) of FF by HPLC was about 400 µg L−1 and 700 µg L−1,

134

respectively.

135

The analysis of the products of FF removal by S-nZVI was carried out using a

136

Waters Acquity™ UPLC-MS/MS system (Waters Acquity UPLC with electrospray

137

ionization and Waters Quattro Premier quadrupole tandem mass spectrometer).

138

Column: BEH C18 column, 2.1×100 mm, 1.7 µm particle size, 313 K; flow rate: 0.2

139

mL min−1; mobile phase A: water; mobile phase B: methanol; injection volume: 4 µL 8


Page 8 of 38

Page 9 of 38


140

(after 1000 times dilution). The gradient elution started with 90% A, and gradually

141

decreased to 50% A in 6 min, then further decreased to 5% A in 8 min and held for 1

142

min, and finally back to 90% A in 10 min. The analysis time was 12 min, including 2

143

min for flushing the column and reestablishing the initial conditions. The ionization

144

and MS data acquisition was conducted in negative ion mode [M-H]− (ESI−) and full

145

scan MS mode, respectively. In order to obtain maximum sensitivity for identification

146

and detection, the analysis was performed under a high-pure nitrogen flow rate of 800

147

L h−1 at 773 K, and the capillary voltage was 2.8 kV. The LOD and LOQ of FF by

148

UPLC-MS/MS was about 0.01 ng L−1 and 0.05 ng L−1, respectively.

149

Field emission scanning electron microscopy (SEM, FEI-Quanta 200F), G2

150

transmission

151

Brunauer-Emmett-Teller (BET, Quantachrome 02108-KR-1), energy dispersive X-ray

152

spectroscopy (EDX, FEI-Quanta 200F), X-ray diffraction (XRD, Rigaku-Ultima IV),

153

X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) were applied to

154

characterize the unmodified nZVI and S-nZVI. Samples for SEM and TEM analysis

155

were prepared by sampling from the aqueous suspensions of unmodified nZVI and

156

S-nZVI. Solid samples of nZVI and S-nZVI for BET, EDX, XRD, and XPS analysis

157

were prepared by vacuum drying an aliquot of the suspension. Inductively coupled

158

plasma optical emission spectroscopy (ICP-OES, Thermo iCAP 7000), ion

159

chromatography (IC, Dionex ICP 2000), and a portable multimeter (HACH HQ40d)

electron

microscopy

(TEM,

FEI-Tecnai

9


F20),


Page 10 of 38

160

were used to measure the dissolved Fe concentration, F− concentration, and oxidation

161

reduction potential (ORP), respectively.

162

RESULTS AND DISCUSSION

163

Characterization

164

The morphologies of unmodified nZVI and S-nZVI nanoparticles were

165

characterized by SEM and TEM. By SEM, the freshly prepared nZVI showed

166

chain-like aggregation of spherical particles (Fig. S1), and the sulfide modification

167

did not significantly affect the morphology of the particles. TEM images show some

168

differences between unmodified nZVI and S-nZVI. Spherical core-shell structures

169

were observed in the TEM image of unmodified nZVI (Fig. 1a) with a shell thickness

170

of several nanometers. The S-nZVI exhibited a slightly different morphology, having

171

smaller particles with less aggregation (Fig. 1b). S-nZVI was also surrounded by a

172

separate phase of acicular particles. This was consistent with a previous report using

173

the same synthesis method

174

S-nZVI was 10.2 m2 g−1 and 62.5 m2 g−1, respectively. The higher measured surface

175

area for S-nZVI was consistent with the TEM observations showing less aggregated

176

particles. EDX measurements including analysis and elemental mapping were also

177

carried out in conjunction with SEM. Although the peak of O was observed (Fig. S2)

178

due to the inevitable facile oxidation during the sample preparation

179

amount of Fe was found in the sample, and the presence of S after the sulfide

180

modification was also confirmed by the peak of S. The elemental maps of unmodified

38

. The specific surface area of unmodified nZVI and

10


48

, a substantial

Page 11 of 38


181

nZVI and S-nZVI suggested that the elements are well dispersed.

182

Fig. 1c shows the XRD patterns of unmodified nZVI and S-nZVI. The peak of

183

Fe0 at 2θ=44.9o was observed in the patterns of unmodified nZVI and S-nZVI before

184

reaction with FF, confirming the presence of Fe0 in the nanoparticles

185

peak was more pronounced for the S-nZVI than for unmodified nZVI, indicating that

186

the (one-step) sulfide modification also led to more crystalline Fe0 in the particles

187

given that the overall size did not change much (Fig 1a and 1b). No obvious

188

difference of the peak intensity for Fe0 of S-nZVI was observed after 2 h reaction with

189

FF. However, a weak peak for Fe2O3 appeared, suggesting that only a small portion of

190

Fe0 (approximately 15.0 mM after sulfide modification) reacted with FF (0.28 mM),

191

and the reaction of Fe0 with H2O was slowed by the modification with sulfide 31.

49, 50

. The Fe0

192

XPS measurements were performed to determine the chemical state of Fe and S

193

on the modified nZVI particles before and after 2h of reaction with FF to better

194

understand the surface properties of the materials (Fig. 2a–d). The Fe and S phases

195

present on the particle surface before and after reaction were inferred from the narrow

196

scans of Fe 2p and S 2p XPS spectra of S-nZVI before and after reaction. As shown in

197

the Fe 2p spectra of S-nZVI before reaction (Fig. 2a), peaks forFe2O3 and Fe3O4 were

198

observed without those for Fe0, indicating that the surficial Fe0 was oxidized during

199

the sample preparation. A similar result was observed after reaction for 2h (Fig. 2c),

200

indicating significant oxidation of Fe0 on the surface of S-nZVI during the reaction

201

and preparation despite the Fe0 remaining in the particle cores (according to the XRD 11



Page 12 of 38

202

results). Note that XPS only detects materials on the first several nanometers of the

203

surface

204

reacted S-nZVI are shown in Figs. 2b and 2d. A mixture of sulfur phases was present

205

on the particles, including SO42−, SO32−, and polysulfide (Sn2−).

206

Enhanced Removal of FF by nZVI after Sulfide Modification

39

. The phases of sulfur present on the surfaces of the “as synthesized” and

207

The removal of FF by nZVI after sulfide modification was investigated. A

208

control experiment of 0.28 mM FF without S-nZVI was performed, and almost no FF

209

was removed, indicating that natural photodegradation was negligible over 120 min.

210

As shown in Fig. 3a, almost no FF was removed by unmodified nZVI, while the

211

removal efficiency of FF by S-nZVI after 2 h ranged from 46% to 90% depending on

212

the

213

surface-area-normalized reaction rate constant (kSA) of FF removal by S-nZVI was 48

214

times higher than for unmodified nZVI, and the value of t1/2 decreased from 1730 min

215

to 38 min. The results indicated that the sulfide treatment greatly increased the

216

reactivity with FF.

S/Fe

molar

ratio.

For

the

optimal

S/Fe

molar

ratio

(0.14),

the

217

Note that the Log Kow of FF ranges from −0.12 to 0.37. This is because FF

218

possesses both hydrophilic groups (–CH3SO2 group) and hydrophobic groups (–Cl

219

and –F groups). As shown in Fig. 4a, one reason for the enhanced reactivity could be

220

that the targeted hydrophobic –Cl and –F groups on FF had a greater affinity for the

221

S-nZVI surface than for the nZVI because iron sulfides are more hydrophobic than

222

iron oxides

34, 51

. Moreover, the generation of FexOy would be inhibited by the 12


Page 13 of 38


223

increased sulfide, and the generated FexSy with lower band gap (Eg) (Table S1) could

224

better facilitate the electron transfer 52. However, the removal rate of FF with S-nZVI

225

was slightly decreased at the highest S/Fe ratio used here (0.28). A considerable

226

amount of Fe0 was consumed during the sulfide modification, and the layer of FexSy

227

would likely be thicker. It was reported that excessive iron sulfides, especially a high

228

amount of FeS2 could be generated with the increased dosage of sulfide 34, can slow

229

down the electron transfer due to the higher band gap (0.95 eV) of FeS2. Such a

230

passivating surface layer could block the surface reactive sites and reduce further

231

corrosion of Fe core

232

following experiments.

233

Effects of FF Concentration, Initial pH, and Temperature on FF Reactivity with

234

S-nZVI

39

. Thus, the optimal S/Fe molar ratio of 0.14 was used in the

235

Important factors influencing the reactivity of FF by S-nZVI were investigated,

236

including the initial FF concentration, S-nZVI dosage, initial pH, and reaction

237

temperature. A concentration range of FF (0.11–0.28 mM) was investigated for

238

analytical sensitivity, and to better compare these results with previous studies

239

regarding the removal of antibiotics by nZVI and other methods

240

Fig. 3b, the removal of FF by S-nZVI at low initial FF concentration was slightly

241

faster than those at high initial FF concentrations. At the higher FF concentration,

242

more intermediate products were generated (especially deschloro FF, as shown in Fig.

243

S3). The FF or its reaction products may deactivate the surface active sites, which 13


20, 53-55

. As shown in


244 245

Page 14 of 38

decreased the removal rate of FF. The removal of organic contaminants by nZVI is also dependent on the initial 56-58

246

solution pH and reaction temperature

247

re-dispersed in the solution with different initial pH from 5.0 to 9.0 (Fig. 3c). The

248

removal rate of FF by S-nZVI decreased when the solution pH increased from 7.0 to

249

9.0. This is the opposite of what was reported for the reductive dechlorination rate of

250

hexachloroethane by iron sulfide (FeS) and trichloroethylene by sulfidized nZVI for

251

the same pH range

252

trichloroethylene by sulfidized nZVI at higher pH were explained by assuming a

253

pH-dependent equilibrium between the protonated and deprotonated forms of similar

254

FeS surface species, with the deprotonated species having greater reactivity for the

255

reductive dechlorination. The opposite effect was observed here. This is likely

256

because the surface of S-nZVI was not the same as FeS and sulfidized nZVI used

257

previously,

258

hexachloroethane/trichloroethylene. The optimal pH value for FF removal by S-nZVI

259

was 7.0 in this study. The pKa values of FF are 6.8 and 13.6, and the isoelectric point

260

(IEP) of S-nZVI was around 9.2 (Fig. S4). As shown in Table S2, the FF and S-nZVI

261

would be negatively and positively charged at pH=7.0, respectively, and the

262

electrostatic attraction would favor the removal of FF by S-nZVI. The electrostatic

263

attraction was weaker at pH=9.0 due to the less positively charged surfaces of S-nZVI.

264

More work is needed to fully understand how the surface properties of S-nZVI are

but

. S-nZVI (S/Fe=0.14) after washing was

34, 35, 59

. The greater reactivity of hexachloroethane by FeS and

could

also

be

due

to

differences

14


between

FF

and

Page 15 of 38


265

affecting its reactivity.

266

As shown in Fig. 3d, the removal of FF by S-nZVI was accelerated at higher

267

temperature. The maximum FF removal rate constant of 6.24×10−4 L m−2 min−1 was

268

obtained at 313 K. Additionally, a linear relationship between lnkobs and 1/T was

269

observed with a high correlation coefficient (R2=0.989) in Fig. S5, and the

270

experimental activation energy (Ea) of FF removal by S-nZVI was 48 KJ mol−1.

271

Degradation Products Analysis via UPLC-MS/MS

272

The degradation products were measured by UPLC-MS/MS to identify the

273

reaction pathway. Four typical peaks at retention times around 5.2 min, 4.1 min, 3.2

274

min, and 2.2 min corresponded to molecular weights (MW) of 358, 324, 289/269, and

275

287 in the relative mass spectrums (Fig. S6–S10). The molecular formulas and

276

structures of possible reduction products were identified based on the results of

277

UPLC-MS/MS analysis, product-324 (C12H15ClFNO4S), product-289 (C12H16FNO4S),

278

product-269 (C12H17NO4S), and product-287 (C12H17NO5S) (Fig. 4).

279

Proposed Pathway of FF Removal by S-nZVI

280

Deschloro FF (C12H15ClFNO4S) and dideschloro FF (C12H16FNO4S) standards

281

were purchased, while the other two products could not be obtained. The mass

282

balance based on the concentrations of FF, C12H15ClFNO4S, and C12H16FNO4S are

283

shown in Fig. 4b. The lower than expected mass balance in the first 1 min was

284

probably due to the adsorption of FF onot S-nZVI. The experimental mass balance

285

gradually increased, likely due to weaker adsorption of deschloro FF and dideschloro 15



286

FF than FF on S-nZVI. However, the experimental mass balance was gradually

287

decreasing after 24 h reaction, suggesting the slow formation of other products

288

besides deschloro FF and dideschloro FF. Moreover, it was observed that deschloro

289

FF was the dominant product of FF during the 2 h reaction, and dideschloro FF

290

primarily formed after the parent compound was gone. The dechlorination of FF by

291

S-nZVI was therefore a consecutive reaction, i.e. one chlorine was removed, followed

292

by the second one. This indicates a competitive reaction, i.e. the dechlorination of FF

293

is much easier than the dechlorination of deschloro FF.

294

Fig. 4c presents the schematic pathway of FF dehalogenation by S-nZVI. First,

295

one Cl atom was quickly removed by S-nZVI, generating a reactive intermediate with

296

a retention time of 4.13 min and MW of 324 (C12H15ClFNO4S). This was the

297

dominant degradation product for the first 120 min. The second Cl atom was then

298

removed to generate the C12H16FNO4S, which corresponded to the peak at 3.2 min

299

and 289 MW. As the reaction proceeded, the F atom was removed via dehalogenation

300

to form products C12H17NO4S, which was consistent with the peak at 2.2 min and 269

301

MW. Another product (C12H17NO5S) was also observed at 2.2 min and 287 MW after

302

24 h reaction (Fig. S10). However, the dechlorinated products (C12H15ClFNO4S and

303

C12H16FNO4S) were the main products (Fig. 4a and S11) and the defluorination was

304

very slow. Almost no F− was detected (below LOQ, which is approximately 0.1 mg

305

L−1) by IC after 10 h reaction. However, 1.6 mg L−1 of F− was detected after 4 months

306

of reaction of FF with S-nZVI, which accounted for approximately 30% of the total 16


Page 16 of 38

Page 17 of 38


307

fluorine in FF (5.3 mg L−1). The results of the UPLC-MS/MS spectrum and F−

308

measurements confirmed the partial defluorination of FF. Reductive defluorination by

309

nZVI or S-nZVI at ambient temperature has never been reported and was not

310

expected here. The substitution of the F atom by hydroxyl radical (·OH) could have

311

resulted from Fenton chemistry

312

bottle to continue the reaction without nitrogen purging, a small amount of oxygen

313

would have inevitably dissolved during the transfer. It has been reported that H2O2

314

and hydroxyl radical (·OH) could be generated by Fe0 or Fe2+ in the presence of O2

315

and light

316

strictly anaerobic conditions with nitrogen purging, after 20 days of reaction, no F− or

317

defluorinated products were detected by IC and UPLC-MS/MS, respectively. The

318

results support the premise that the defluorination observed in the previous

319

experiment was attributed to the dissolved oxygen present in the unpurged reactors.

320

Reusability of S-nZVI in Five Consecutive Experiments

54, 60, 61

. Since the solution was moved to a sealed

54, 60, 62, 63

. In addition, a comparative experiment was performed under

321

The reusability of S-nZVI was assessed by adding consecutive doses of 0.28 mM

322

FF and allowing 2 h of reaction time between doses. As shown in Fig. 5a, the removal

323

rate of FF by S-nZVI was 4.1×10−4 L m−2 min−1, 4.5×10−4 L m−2 min−1, 3.1×10−4 L

324

m−2 min−1, 2.6×10−4 L m−2 min−1, and 2.2×10−4 L m−2 min−1 for each run, which were

325

significantly different via one-way ANOVA statistical analysis (P < 0.05). Although

326

the dissolved Fe was gradually increased due to the corrosion of S-nZVI (Fig. 5b), it 17



327

was less than 20 mg L−1 after five runs and accounted for only approximately 2% of

328

the initial addition of Fe. No dissolved F− was detected by IC after the five

329

consecutive runs (10 h). However, approximately 0.4 mg L−1 of F− was determined in

330

the S-nZVI system after continuous 10 days reaction, which further confirmed the

331

defluorination of FF.

332

The ORP and pH value of the solution are two important indicators to evaluate 64

333

the reactivity of nZVI

. The ORP value of S-nZVI suspension before reaction was

334

approximated −690 mV due to the addition of S-nZVI, and this value gradually

335

increased to −180 mV after five runs reaction (Fig. 5c). After the addition of S-nZVI,

336

the solution pH was immediately increased from 7.0 to 8.9, and then became almost

337

constant during each run (Fig. 5d). Besides, the rapid increases of the ORP at the

338

beginning of each run were caused by the addition of FF 65.

339

Application Potential of S-nZVI for FF Removal in Different Water Matrices

340

The impact of dissolved constituents in natural water and effluent from

341

engineered systems was assessed in four real water samples, including groundwater,

342

river water, seawater, and wastewater. Table 1 presents the main properties and

343

compositions of these waters, as well as the relative results of FF removal. After 2 h

344

reaction, FF was largely removed by S-nZVI from each of the four different waters. A

345

one-way ANOVA statistical analysis showed a significant difference between kSA

346

values of FF in these waters, with P < 0.05. Despite the presence of different 18


Page 18 of 38

Page 19 of 38


347

constituents and organic matter in the water, the removal rates of FF by S-nZVI in

348

these waters were not decreased compared to ultrapure water, indicating that the

349

material reactivity was robust.

350

Environmental Implications

351

While many studies have shown the dechlorination of organic contaminants by

352

nZVI, The results here demonstrate that nZVI needs sulfide modification to make it

353

reactive for the removal of the antibiotic FF. Suitable sulfide modification of nZVI

354

not only enhances the affinity between the hydrophobic particles surface and targeted

355

hydrophobic groups, but also facilitates the electron transfer due to the lower band

356

gap of FexSy than FexOy. The pH dependent charge on the particles affected the

357

removal rate, and the higher removal rates were achieved around neutral pH due to

358

the electrostatic attraction between negatively charged FF and positively charged

359

S-nZVI. This suggests that the wastewater pH and sulfidation are important. Besides

360

dechlorination, defluorination was also observed according to the determination of

361

defluorinated products and F− ions. Robust performance of S-nZVI for FF removal

362

was observed in different waters matrices, indicating the feasibility of S-nZVI

363

application for the treatment of real FF contaminated water, regardless of dissolved

364

species present.

365

ACKNOWLEDGEMENTS

366

The authors would like to acknowledge the Shanghai Pujiang Program (No.

367

15PJD014), Chenguang Program of Shanghai Education Development Foundation 19



368

and Shanghai Municipal Education Commission (No. 16CG23), the National Key

369

Research and Development Program of China (2016YFC0402600), and the National

370

Natural Science Foundation of China (Nos. 21477108 and 41522111) for their

371

financial support.

372 373 374 375

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

376

SEM images, EDX spectra, and Zeta potential of unmodified nZVI and S-nZVI;

377

value of band gap of iron sulfides and iron oxides; calculation of the apparent

378

activation energy; UPLC-MS/MS analysis of degraded products of FF by S-nZVI;

379

fitting results of FF removal kinetics in different water matrices.

380 381

20


Page 20 of 38

Page 21 of 38


382

CAPTIONS

383

Fig. 1 TEM images of (a) unmodified nZVI and (b) S-nZVI; and (c) XRD spectra of

384

unmodified nZVI and S-nZVI (1.0 g L−1 nZVI with 0.14 molar ratio of S/Fe).

385

Fig. 2 Fe 2p and S 2p XPS spectra of S-nZVI (a, b) before and (c, d) after 2 h reaction

386

(1.0 g L−1 nZVI with 0.14 molar ratio of S/Fe).

387

Fig. 3 Effects of (a) S/Fe molar ratio, (b) initial FF concentration, (c) initial pH, and

388

(d) temperature on the removal rate of FF by S-nZVI (basic condition: T=298 K,

389

initial pH=7.0, initial FF concentration=0.28 mM, nZVI dosage=1.0 g L−1 with

390

0.14 molar ratio of S/Fe; kSA: L m−2 min−1).

391

Fig. 4 (a) Schematic mechanism of enhanced FF removal by S-nZVI, (b) mass

392

balance of FF and dechlorinated FF during the reaction, and (c) pathway of FF

393

removal by S-nZVI (T=298 K, initial pH=7.0, initial FF concentration=0.28 mM,

394

nZVI dosage=1.0 g L−1 with 0.14 molar ratio of S/Fe).

395

Fig. 5 Reusability of S-nZVI in five consecutive experiments (T=298 K, initial

396

pH=7.0, nZVI dosage=1.0 g L−1 with 0.14 molar ratio of S/Fe, 0.28 mM FF

397

added at the beginning of each run).

398

Table 1 Removal of FF by S-nZVI in different water matrices (T=298 K, initial FF

399

concentration=0.28 mM, nZVI dosage=1.0 g L−1, S/Fe=0.14 molar ratio, 2 h

400

reaction).

401

21



402

Figure 1

403

(a)

404 405 406 407

(b)

408 409 410 411 0

(c)

Fe

unmodified nZVI

412 Intensity

413

0

Fe

S-nZVI before reaction

Fe2O3 0

Fe

414 415

20

30

40

50

S-nZVI after reaction

60

70

Two-Theta, deg

416 417

22


80

Page 22 of 38

Page 23 of 38


418

Figure 2

419 420

100000

(a)

CPS

80000

421 422

800

Fe2O3 Fe3O4

2-

2-

Sn 600

500 172

740 735 730 725 720 715 710 705 700

428

800

100000

425

427

170

168

166

164

162

160

162

160

120000

(c) Fe2O3

CPS

426

2-

SO3

60000

20000

424

SO4

700

40000

423

(b)

2-

SO4

700

80000 FeO

60000

(d)

Fe3O4

600

2-

SO3

40000

500

20000

400

740 735 730 725 720 715 710 705 700

172

170

168

166

2-

Sn

164

Binding energy, eV

Binding energy, eV

429 430 431

23



432

Figure 3

433

0

0

(a)

434

-1 (b)

-1 ×

-6

unmodified nZVI, kSA=6.4 10

-2

-3

×

-4

0.14 S/Fe, kSA=3.1 10

437

0

20

40

60

100

120

-1

40

60

80

100

120

80

100

120

(d)

-3

pH=9.0, kSA=1.5 10

20

40

60

o o

100

120

Time, min

-4

T=25 C, kSA=3.1 10 T=40 C, kSA=6.2 10

-5 80

-4

T=10 C, kSA=8.8 10

-4

-3

-5

×

×

-4

o

×

-4

pH=5.0, kSA=2.1 10

×

-4

×

-2

0

442

20

-2

pH=7.0, kSA=3.1 10

441

0

-1

×

440

(c)

ln(C/C 0)

439

0.28 mM FF, kSA=3.1 10

0

0

438

-4

0.24 mM FF, kSA=3.8 10

-6

80

-4

0.19 mM FF, kSA=5.5 10

-5

0.28 S/Fe, kSA=2.2 10

-4

-4

×

×

-4

0.13 mM FF, kSA=1.0 10

-4

0.07 S/Fe, kSA=2.2 10

-3

×

×

-4

×

0.035 S/Fe, kSA=7.8 10

×

-5

×

ln(C/C 0)

Control, kSA=3.5 10

-3

436

-2

-6

×

435

Page 24 of 38

0

20

40

60

Time, min

443 444 445

24


Page 25 of 38


446

Figure 4

447 448 449 450 451 452 FF Deschloro FF Experimental balance

453 454 455 456 457

Concentration, mM

0.30 (b)

Dideschloro FF Theoretical balance Other products

0.25 0.20 0.15 0.10 0.05 0.00 0.0

0.5

1.0

1.5

2.0 40

80 120 160

Time, h 458 459 460 461 462 463 464 465 25



466

468

Run 1

Run 2

Run 3

Run 4

Run 5

0 -1 (a) -2 -3 -4

-1

469

Figure 5 ORP, mV [Fe], mg L ln(C/C0) of FF

467

Page 26 of 38

470 471 472

20 (b) 10 0

-200 (c) -400 -600

pH

473 474 475

9

(d)

8 0

100

200

300

400

Time, min

476 477 478

26


500

600

Page 27 of 38


479

Table 1 Water pH

Ultrapure

Ground

River

Sea

Waste

water

water

water

water

water

7.6

7.2

7.0

8.1

7.8

9.0×10

1.2×10

2

7.8×10

1.2×102

1.1

1.1×103

5.4×102

1.5×104

4.6×102

Salinity (‰)

n.d.

5.7×10−1

2.8×10−1

9.2

2.3×10−1

TOC (mg L−1)

n.d.

1.3

A254nm

n.d.

1.6×10−2

1.4×10−2

1.3×10−2

6.8×10−2

Na+ (mg L−1)

n.d.

5.2×10

3.9×10

1.5×103

2.8×10

K+ (mg L−1)

n.d.

2.9

7.4

1.5×102

9.5

ORP (mV) Conductivity −1

(µS cm )

1.4×10

2

4.0

2+

−1

2+

−1

Mg (mg L )

n.d.

3.0×10

Cl− (mg L−1)

n.d.

NO3− (mg L−1)

Ca (mg L )

2−

−1

3−

−1

SO4 (mg L ) PO4 (mg L )

4.1

1.4×10

2

5.1×10

1.0×10

2.8×10

2

9.8

2.3×102

5.7×10

5.2×103

4.0×10

n.d.

3.6×10

1.2×10

7.8

3.4×10

n.d.

2

n.d.

1.3×10

1.2×10

2

3.6

5.2×10

9.5×10

−1

5.6×10

−1

5.1×10−1

1.1×10

n.d.

4.0×10

90%

100%

96%

100%

100%

kobs

1.9×10−2

4.7×10−2

2.6×10−2

6.0×10−2

3.8×10−2

kSA

3.1×10−4

7.5×10−4

4.2×10−4

9.6×10−4

6.0×10−4

t1/2

3.8

1.5

2.7

1.2

1.9

0.995

0.984

0.995

0.998

0.916

Removal efficiency

R

2

4.7×10

−1

2

4.3×10

480

n.d.: not detected; kobs (min−1): the observed reaction rate; kSA (L m−2 min−1): the

481

surface-area-normalized reaction rate; t1/2 (min): the half-lives time that calculated

482

from the first-order decay fitting using kobs.

483

27



484

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Removal of Antibiotic Florfenicol by Sulfide-Modified Nanoscale Zero

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