Removal of triphenyl phosphate by nanoscale zerovalent iron (nZVI) activated bisulfite: Performance, surface reaction mechanism and sulfate radical-mediated degradation pathway

Journal Pre-proof Removal of triphenyl phosphate by nanoscale zerovalent iron (nZVI) activated bisulfite: Performance, surface reaction mechanism and sulfate radical-mediated degradation pathway Ruxia Chen, Hua Yin, Hui Peng, Xipeng Wei, Xiaolong Yu, Danping Xie, Guining Lu, Zhi Dang PII:

S0269-7491(19)36861-7

DOI:

https://doi.org/10.1016/j.envpol.2020.113983

Reference:

ENPO 113983

To appear in:

Environmental Pollution

Received Date: 19 November 2019 Revised Date:

10 January 2020

Accepted Date: 13 January 2020

Please cite this article as: Chen, R., Yin, H., Peng, H., Wei, X., Yu, X., Xie, D., Lu, G., Dang, Z., Removal of triphenyl phosphate by nanoscale zerovalent iron (nZVI) activated bisulfite: Performance, surface reaction mechanism and sulfate radical-mediated degradation pathway, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.113983. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Removal of triphenyl phosphate by nanoscale zerovalent iron

1 2

(nZVI) activated bisulfite: Performance, surface reaction mechanism

3

and sulfate radical­mediated degradation pathway

4

Ruxia Chena, Hua Yina*, Hui Pengb, Xipeng Weia, Xiaolong Yua, Danping Xiec, Guining Lua, Zhi Danga

5 6

a

7

Restoration in Industry Clusters, School of Environment and Energy, South China

8

University of Technology, Guangzhou 510006, Guangdong, China

9

b

Department of Chemistry, Jinan University, Guangzhou 510632, Guangdong, China

10

c

South China Institute of Environmental Sciences, Ministry of Ecology and

11

Environment, Guangzhou 510655, Guangdong, China

12

*Corresponding author.

13

E-mail address: [email protected] (H. Yin)

14

Key Laboratory of Ministry of Education on Pollution Control and Ecosystem

15

Abstract

16

Recently, sulfate radical-based advanced oxidation processes (SR-AOPs) have

17

been studied extensively for the removal of pollutants, however, few researches

18

focused on the activation of bisulfite by nanoscale zerovalent iron (nZVI), especially,

19

surface reaction mechanism and sulfate radical-mediated degradation pathway have

20

not been elucidated in detail. In this study, influencing factors, the kinetics,

21

transformation pathway and mechanism of triphenyl phosphate (TPHP) degradation in

22

the nZVI/bisulfite system were systematically discussed. Compared with Fe2+, nZVI

23

was found to be a more efficient and long-lasting activator of bisulfite via gradual

24

generation of iron ions. The optimal degradation efficiency of TPHP (98.2%) and

25

pseudo-first-order kinetics rate constant (kobs = 0.2784 min-1) were obtained by using

26

0.5 mM nZVI and 2.0 mM bisulfite at the initial pH 3.0. Both Cl− and NO3− inhibited

27

the degradation of TPHP and the inhibitory effect of Cl− was stronger than that of

28

NO3− due to the higher reaction rate of Cl− with •SO4−. Furthermore, SEM, XRD and

29

XPS characterization revealed that a thin passivation layer (Fe2O3, Fe3O4, FeOOH)

30

deposited on the surface of fresh nZVI and a few iron corrosion products generated

31

and assembled on the surface of reacted nZVI. Radical quenching tests identified that

32

•SO4− was the dominant reactive oxidative species (ROS) for TPHP removal. Based

33

on HRMS analysis, six degradation products were determined and a sulfate

34

radical-mediated degradation pathway was proposed. In a word, this study revealed

35

that the nZVI/bisulfite system had a great potential for the TPHP elimination in

36

waterbody.

37

Key words

Triphenyl phosphate, nZVI, Bisulfite, Surface reaction, Sulfate radical

38

(•SO4−)

39

1. Introduction

40

Flame retardants (FRs) are widely used as additives of consumer products to

41

retard combustion. Brominated flame retardants (BFRs) had been banned in Europe

42

and America due to their persistence, bioaccumulation, and toxicity. With the phasing

43

out of BFRs, organophosphorus flame retardants (OPFRs) have gradually gained a

44

widespread attention recently (He et al., 2018a; He et al., 2018b). According to the

45

survey results by China Industry Information Network (CNII), the global total

46

production of OPFRs has increased from 1.97×106 in 2012 to 2.60×106 tons in 2018.

47

Triphenyl phosphate (TPHP), one of the primary OPFRs, has been applied as a flame

48

retardant, plasticizer and antifoaming agent in various industries (Hou et al., 2016;

49

Wei et al., 2019). TPHP can easily leak to the environment due to forming no

50

chemical bond with the polymer material (Marklund et al., 2003; Shi et al., 2019; Wei

51

et al., 2019), resulting in widespread detection in various biological (fish, birds and

52

poultry) and non-biological samples (air, water and sediments) (Cristale et al., 2013;

53

Ding et al., 2016). As was reported, the measured concentration of TPHP reached to

54

1.0 ng/L in seawater from coastal seas of China (Song et al., 2019b). In addition,

55

TPHP has a variety of toxic effects which can reduce the body’s immunity, including

56

neurotoxicity (Jarema et al., 2015; Sun et al., 2016; Shi et al., 2019), thyroid

57

endocrine disruption (Kim et al., 2015), reproductive and developmental toxicity (Liu

58

et al., 2013; Chen et al., 2015). Therefore, it is an urgent task to develop an

59

economical and effective technology to remove TPHP from waterbody.

60

In recent years, advanced oxidation processes (AOPs) have been universally

61

applied to degrade organic pollutants through •SO4− or OH oxidation attack (Matzek

62

et al., 2016; Cao et al., 2019; Xie et al., 2019). Compared with •OH (E0 = 1.8-2.7 V),

63

•SO4− has a higher redox potential (E0 = 2.5-3.1 V), higher selectivity and longer

64

half-life (30-40 µs) (Zhang et al., 2018; Zhou et al., 2019), leading to more advantages

65

than •OH in the degradation of contaminants by capturing and adding hydrogen atoms

66

over a wide pH range (Liu et al., 2017; Du et al., 2018; Cao et al., 2019). •SO4− can be

67

induced by various activation pathways of persulfate (PS) or peroxymonosulfate

68

(PMS), such as light radiation (An et al., 2015; Xie et al., 2015; Xie et al., 2019), heat

69

(Chen et al., 2017), alkali (Oh et al., 2016), transition metals (Li et al., 2014),

70

ultrasound (Hao et al., 2014) or organic matter (Fang et al., 2013). However, the high

71

price and the residual nature of PS and PMS limit the further application of this

72

technology (Xie et al., 2019). Thus, bisulfite (S(IV)) is receiving more and more

73

attention due to its advantages of economy, low toxicity and high reactivity (Zhou et

74

al., 2014; Du et al., 2018). Iron species have been widely applied as an activator of PS

75

or PMS as they are economical, accessible, eco-friendly and high-efficient. Song et al.

76

(2019a) had reported that Fe(II)/PS system exhibited an excellent TPHP removal

77

efficiency compared with Fe(II)/H2O2 system. However, a small amount of Fe2+ is

78

easily oxidized to Fe3+ quickly, thus losing catalytic activity. In addition, excessive

79

Fe2+ can quench •SO4− and even compete with contaminants for electrons, inhibiting

80

the degradation of pollutants (Xie et al., 2019). Unlike Fe2+, nZVI can gradually

81

release Fe2+ in waterbody to maintain a relatively stable Fe2+ concentration, which

82

shows excellent potential for catalyzing bisulfite.

83

Various studies have focused on the degradation aspects of free radical oxidation,

84

however, the surface reaction mechanism and sulfate radical-mediated degradation

85

pathway in nZVI/bisulfite system have not been elucidated.

86

In this work, the TPHP degradation kinetics, influencing factors (e.g., initial

87

solution pH, the dosage of nZVI/bisulfite and inorganic anions) and degradation

88

pathway in TPHP/nZVI/bisulfite system were systematically explored, focusing on

89

investigating the surface reaction process and sulfate radical-mediated TPHP

90

degradation mechanism. The current study expected to a) determine the excellent

91

synergistic effect of nZVI and bisulfite during the TPHP degradation process via

92

comparative experiments; b) investigate the interference of some possible influencing

93

factors ( pH, nZVI dosage, bisulfite concentration, inorganic anions) in the reaction; c)

94

interpret the reaction mechanism between nZVI and solution by SEM, XRD and XPS

95

characterization methods; and d) reveal the main free radical and the degradation

96

pathway of TPHP in the nZVI/bisulfite system.

97

2. Materials and methods

98

2.1. Chemicals and materials

99

All chemicals of high purity were used without further purification. Triphenyl

100

phosphate (TPHP, ≥99.8%, CAS: 115-86-6) was supplied from Sigma-Aldrich. TPHP

101

stock solution of 500 mg/L was prepared in HPLC grade methanol and stored in a

102

4 °C refrigerator for the follow-up experiments. This study used deionized water

103

produced by a Millipore Milli-Q system (EPED-20TS). Detailed information of other

104

reagents was provided in the supplementary Materials (Text S1).

105

2.2. Experimental procedure

106

2.2.1. Preparation of nZVI

107

nZVI was synthesized by borohydride reduction of ferrous chloride with some

108

modification according to the previous reports (Wang et al., 2017). Briefly, 6 g

109

FeCl2•4H2O was dissolved in 70 mL deionized water in a 500 mL conical flask. Under

110

vigorous stirring, 160 mL solution containing 7 g NaBH4 was added dropwise to

111

reduce Fe2+. After the mixture completely turned black, the conical flask was cooled

112

under vigorous stirring. Subsequently, the mixture was aged for 3 h and washed 3

113

times with absolute ethyl alcohol and deionized water to remove the impurity ions.

114

Finally, nZVI so produced was stored in the -20 °C refrigerator.

115

2.2.2. Batch experiments

116

All the batch experiments were conducted in a 100 mL conical flask at 30 ± 2 °C.

117

The 50 mL reaction system containing desired concentrations of TPHP and bisulfite

118

with 5% acetone as a cosolvent was completely mixed by magnetic stirrer. The initial

119

solution pH was adjusted by 0.1 M H2SO4 and 0.1 M NaOH. Then the reaction was

120

initiated after adding nZVI to the reactor. A volume of 2 mL sample was withdrawn at

121

the specific time and quenched with 40 µL methanol. Subsequently, 0.3 mL of 3 M

122

HCl was injected into the 2 mL sample to dissolve iron hydroxide, facilitating the

123

release of TPHP and its degradation intermediates. The sample was firstly extracted

124

with ethyl acetate for three times, then purged with nitrogen, fixed volume with

125

chromatographic grade n-hexane, filtrated by a 0.22 µm polyether sulfone (Nylon)

126

filter. The residual concentration of TPHP in the solutions was measured by a Gas

127

chromatography mass spectrometry (GCMS-QP2020, Shimadzu).

128

2.3. Analytical methods

129

The residual concentration of TPHP in the system was determined through a

130

GCMS (QP2020, Shimadzu) with an electron impact ion source, equipped with the

131

column of SH-Rxi-5Sil MS (30.0 m × 0.25 mm × 0.25 µm). The sample of 1 µL was

132

injected into the autosampler and splitless injection was conducted in this work. The

133

procedure was as follows: 50 °C for 1 min, then 15.0 °C/min to 200 °C for 1 min,

134

finally 1.0 °C/min to 250 °C for 2 min. Other detailed information of the operational

135

condition parameters was presented in Text S2. Analysis of TPHP degradation

136

products was determined by a HPLC-TOF-MS system (Shimadzu Nexera Prominence

137

liquid chromatogram, Applied Biosystems SCIEX, USA) equipped with a Turbolon

138

Spray ion source. Detailed information of the operational procedure and condition

139

parameters were presented in Text S3 and Table S1. The solution pH was monitored

140

by a pH meter (PHS-3C). The concentrations of ferrous ions and total iron ions in the

141

solution were detected via O-phenanthroline spectrophotometry. Fresh and reacted

142

nZVI were characterized through SEM (Zeiss Sigma 500), XRD (BRUKER D8

143

Advance) and XPS (Thermo Fisher Scientific K-Alpha), respectively.

144

3. Results and discussion

145

3.1. Comparison of different processes on the degradation of TPHP

146

3.1.1. The removal efficiency of TPHP

147

In this study, five comparative experiments (I. nZVI, II. bisulfite, III. Fe2+, IV.

148

nZVI/bisulfite, V. Fe2+/bisulfite) for the removal of TPHP were firstly performed

149

under the same condition. As shown in Fig. 1a, only 7.0% and 18.7% of TPHP were

150

removed in Fe2+ and nZVI alone system, demonstrating that the reaction ability was

151

low between TPHP and the two compounds. Similarly, a slight TPHP removal (16.0%)

152

was obtained in bisulfite system, which could be attributed to a relative stability of

153

bisulfite in waterbody (Rayaroth et al., 2017; Cao et al., 2019). Compared with

154

Fe2+/bisulfite system (84.5%), a significant increase of TPHP removal (98.2%) was

155

acquired when nZVI was brought in bisulfite activation system. These results revealed

156

that nZVI could activate bisulfite adequately. In the nZVI/bisulfite system, reactions

157

could be induced by the following series of Eqs. (1)-(9) (Du et al., 2018):

158

Fe0 → Fe2+ + 2e-

(1)

159

Fe2+ + HSO3− → FeHSO3+

(2)

160

FeHSO3+ + 1/4O2 → FeSO3+ + 1/2H2O

(3)

161

Fe3+ + HSO3−

(4)

162

FeSO3+ → Fe2+ + •SO3−

(5)

163

O2 + •SO3− → •SO5−

(6)

164

•SO5− + •SO3− → •SO4− + SO42- + H+

(7)

165

•SO5− + •SO5− → 2•SO4− + O2

(8)

166

HSO3− + •SO4− → •SO3− + SO42- + H+

(9)

FeSO3+ + H+

167

It was worth noting that TPHP was continuously degraded throughout the whole

168

nZVI/bisulfite reaction process, while the degradation mainly occurred in the first 10

169

min and then remained stable in the follow-up reaction in the Fe2+/bisulfite system.

170

The result could be explained by the following reasons: (i) nZVI was gradually

171

corroded into Fe2+, which could permanently activate bisulfite in the nZVI/bisulfite

172

system, as shown in Fig. 1b; (ii) excessive Fe2+ would quickly consume bisulfite,

173

leading to the shortage of oxidants in the follow-up reaction (Eq. (2)); (iii) excessive

174

Fe2+ could quench the generated free radicals and inhibit the reaction (Eq. (10)) (Cao

175

et al., 2019).

176

Fe2+ + •SO4− → SO42- + Fe3+

177

(10)

In order to further clarify the role of dissolved iron ions during the degradation

178

reaction, the Fe2+ and total iron ions concentration were monitored subsequently.

179

3.1.2. Iron ions in the reaction system

180

As Fig. 1b depicted, Fe2+ concentration increased gradually to a maximum of

181

7.90 mg/L and remained constant with time in the nZVI alone system, suggesting that

182

nZVI was slowly corroded into Fe2+ in the acidic solution. Compared with the nZVI

183

alone, Fe2+ concentration increased rapidly to the peak (8.87 mg/L) at the first 4 min

184

and then dropped quickly to 0.82 mg/L in the nZVI/bisulfite system. The results could

185

be explained that nZVI was firstly corroded into Fe2+ at acidic condition, which was

186

supported by the decrease of solution pH (Fig. 1c) (Cao et al., 2017; Cao et al., 2019).

187

Generated Fe2+ would react with HSO3− to form •SO4− subsequently, resulting in a

188

significant decrease of Fe2+ from 4 min to 60 min. Similarly, the Fe2+ level in the

189

Fe2+/bisulfite system was instantaneous to reach the maximum value (11.02 mg/L)

190

and then declined to the minimum (0.57 mg/L). It was worth noting that the

191

concentration of Fe2+ remained basically unchanged (2nd-8th mins), which was

192

attributed to the regeneration of Fe2+ through the decomposition of FeSO3+ at high

193

concentration of bisulfite in the Fe2+/bisulfite system (Eqs. (2-5)). The total iron ions

194

concentration (Fig. 1d) quickly reached the maximum value (nZVI: 9.28 mg/L,

195

nZVI/bisulfite: 11.27 mg/L, Fe2+/bisulfite: 12.94 mg/L) within 20 min and then

196

remained basically stable in the three systems. As pH of the solution was lower than

197

4.0 (Fig. 1c), no large amounts of Fe2+ and Fe3+ complexes (Fe(OH)2, FeOH2+,

198

Fe2(OH)24+, Fe(OH)2+, Fe(OH)3 and Fe(OH)4−) would form, resulting in the relative

199

stability of total iron ions concentration in the three reaction systems (Wang et al.,

200

2017; Tan et al., 2018). In addition, a continuous increasing trend of Fe3+

201

concentration appeared in both nZVI/bisulfite and Fe2+/bisulfite systems and the

202

content kept constant at 1.0 mg/L in the nZVI alone system. It could also be attributed

203

to that a small amount of Fe2+ and Fe3+ in the solution formed an iron complex by

204

hydration and deposited on the nZVI surface which was further confirmed by XRD

205

and XPS in section 3.3.1, to prevent the inner Fe0 dissolution and achieve a balance of

206

hydration and dissociation, thereby maintaining the stability of the total iron ions

207

concentration on the solid-liquid interface. Yet, Fe2+ in the solution was constantly

208

oxidized to Fe3+ under aerobic conditions, causing a Fe2+ concentration decreasing

209

and Fe3+ concentration increasing (Fig. 1d).

210

In summary, higher concentrations of Fe2+ in the system would rapidly activate

211

bisulfite but cause a quenching reaction subsequently. Additionally, a large amount of

212

iron ions could lead to secondary pollution. Therefore, nZVI, as an ideal iron catalyst,

213

was selected in current work based on its important practical significance. As the

214

operating conditions played a key role in the degradation of TPHP by the

215

nZVI/bisulfite system, the kinetics of TPHP degradation need to be further explored.

216 217

Fig. 1. The change of relevant parameters with time in different systems. (a) removal

218

efficiency of TPHP; (b) ferrous ions concentration; (c) pH value; and (d) total

219

dissolved iron ions and ferric ions concentration. Experimental conditions: [TPHP]0 =

220

2 mg/L, [nZVI]0 = [Fe2+]0 = 0.5 mM, [bisulfite]0 = 2 mM, [pH]0 = 3.0 ± 0.1,

221

temperature = 30 ± 2 °C. The error bars represent the standard deviations.

222

3.2. Kinetics of TPHP degradation

223

3.2.1. Effect of initial pH

224

The solution pH may play a significant role in degradation of organic pollutants

225

via affecting the formation of free radical in the nZVI/bisulfite system. The effect of

226

initial pH on TPHP degradation was explored over the pH ranged from 2.0 to 6.0. As

227

illustrated in Fig. 2a, the elimination of TPHP at pH 3.0 and 4.0 was 98.2% and 76.0%,

228

respectively. However, poor degradation efficiencies were observed when initial pH

229

was 2.0, 5.0 and 6.0. It could be explained as follows: (i) the conversion from nZVI to

230

Fe2+ was facilitated under the acidic condition (pH = 3.0 and 4.0), which was

231

contributed to the activation of bisulfite; however, the activation efficiency of bisulfite

232

by Fe2+ dropped with the increase of pH (Tan et al., 2018); (ii) decomposition of

233

bisulfite was accelerated under the strong acidic condition (pH = 2.0), resulting in the

234

absence of oxidants for TPHP removal; (iii) a series of iron complexes (FeOH2+,

235

Fe(OH)2+, Fe2(OH)24+) could be generated on the nZVI surface due to water hydration

236

at pH > 4.0 (Wang et al., 2017), which had a low capability for inducing bisulfite to

237

generate •SO4−. More importantly, such iron complexes would restrain the corrosion

238

process of inner nZVI, leading to the reduction of nZVI reaction site and inhibiting

239

the electron transfer significantly. The following equations demonstrates the formation

240

of Fe3+ complexes (Zhang et al., 2017).

241

Fe3+ + H2O

FeOH2+ + H+

242

Fe3+ + 2H2O → Fe(OH)2+ + 2H+

(12)

243

2Fe3+ + 2H2O → Fe2(OH)24+ + 2H+

(13)

(11)

244

From another perspective, the increase of pH promoted the conversion of Fe2+ to

245

Fe3+ by phosphate ions, leading to the generation of iron phosphate complex which

246

could decrease the reactivity of free radicals (Liu et al., 2019). As such, pH = 3.0 was

247

selected as the optimal pH condition parameter.

248

3.2.2. Effect of initial nZVI concentration

249

The effect of nZVI dosage (0.1-0.75 mM) on the TPHP degradation efficiency

250

was investigated comprehensively. As shown in Fig. 2b, when nZVI dosage increased

251

from 0.1 mM to 0.5 mM, the TPHP removal efficiency increased rapidly from 68.0%

252

to 98.2% after 60 min. Simultaneously, the whole reaction followed pseudo-first-order

253

kinetics in the first 10 min. As presented in Fig. S1a, the kobs at 0.5 mM nZVI (0.2784

254

min-1) was almost 8.29 times higher than that at 0.1 mM nZVI (0.0336 min-1). The

255

improvement of TPHP removal efficiency could be attributed to the increased

256

available active sites and generated more Fe2+, which could effectively activate

257

bisulfite to produce •SO4− (Eqs. (1-8)). In addition, •OH might also be produced

258

through the corrosion of nZVI in the presence of O2 (Eqs. (14-16)) (Du et al., 2018).

259

Both •SO4− and •OH could contribute to the removal of TPHP. Yet, the lower TPHP

260

removal efficiency (90.0%) and kobs (0.1954 min-1) were obtained at higher nZVI

261

dosage (0.75 mM). The reasons could be elucidated as follows: (1) excessive Fe2+ was

262

generated to consume •SO4− (Eq. (10)); (2) the bisulfite decomposition was

263

accelerated, resulting in the sulfate radical recombination (Eq. (17)) (Cao et al., 2019).

264

Given that the higher reactivity of free radicals, 0.5 mM was chosen as the optimal

265

nZVI concentration.

266

Fe0

267

(14)

268

Fe0

269

(15)

+

O2

+

H2O2

2H+

+

+

2H+

→

→

Fe2+

+

H2O2

Fe2+

+

2H2O

270

Fe2+

271

(16)

272

•SO4− + •SO4− → S2O82−

273

3.2.3. Effect of initial bisulfite concentration

274

+

H2O2

→

Fe3+

+

OH-

+

•OH

(17)

As depicted in Fig. 2c, TPHP removal efficiency rose rapidly from 55.0% to 98.2%

275

and the kobs increased from 0.0320 min-1 to 0.2784 min-1 (Fig. S1b) with increasing

276

bisulfite concentration (0.5 mM to 2.0 mM) after 60 min. These results could be

277

interpreted that bisulfite was a source of ROS and the proportion of ROS elevated

278

with increasing bisulfite dosage. Similar to the negative impact of nZVI dosage, there

279

was a little decrease (80.1%) at 2.5 mM bisulfite concentration. It could be explained

280

that additional bisulfite would compete •SO4− and •OH with TPHP or consume •SO4−

281

to form a less oxidative •SO3− and •SO5− (Eqs. (9) and (6)), inhibiting the degradation

282

of TPHP. These results suggested that initial bisulfite dosage of 2.0 mM was the

283

optimum concentration.

284

3.2.4. Effect of coexisting ions

285

Coexisting ions in waterbody can quench free radicals and create new active

286

species to affect the removal process (Xie et al., 2017; Cao et al., 2019). This study

287

systematically investigated the effects of Cl− and NO3− as relative common anions in

288

waterbody on the degradation of TPHP. As shown in Fig. 2d, a significant inhibition

289

of TPHP removal (98.2% to 55.0%) was presented and the kobs declined dramatically

290

from 0.2784 min-1 to 0.0682 min-1 (Fig. S2a) with Cl− concentration enhancing from 0

291

to 5.0 mM, indicating that Cl− could significantly inhibit the degradation of TPHP.

292

According to Eqs. (18)-(20) (Xie et al., 2019), Cl− could react with •SO4− and •OH to

293

produce other chlorine active species such as •Cl and •Cl2− whose oxidation ability

294

were lower than that of •SO4− and •OH (Rao et al., 2014; Liu et al., 2019). Therefore,

295

the stronger inhibitory effect was observed as increasing Cl− concentration. Similarly,

296

a slightly declined removal efficiency (from 98.2% to 85.2%) of TPHP occurred in the

297

presence of NO3− (Fig. 2d) due to the lower oxidative capacity species formation

298

(•NO3−). Yet, NO3− had a lower reaction rate with •SO4− and •OH than Cl− (Eqs.

299

(21)-(22)) (Cao et al., 2019), therefore, it had almost no obvious inhibition on the

300

degradation of TPHP.

301

•SO4− + Cl− → SO42− + •Cl

k = (1.3-3.1) × 108 M-1S-1

(18)

302

•OH + Cl− → OH− + •Cl

k = 4.3 × 109 M-1S-1

(19)

303

•Cl + Cl− → •Cl2−

k = (0.65-2.1) × 1010 M-1S-1

(20)

304

NO3− + •SO4− → SO42− + •NO3

k = 5.5 × 105 M-1S-1

(21)

305

NO3− + •OH → OH− + •NO3

k < 5.5 × 105 M-1S-1

(22)

306

In a word, the kinetics of TPHP degradation was preliminarily discussed in the

307

nZVI /bisulfite system. Compared with other systems, an excellent TPHP degradation

308

efficiency was obtained in nZVI /bisulfite system which was attributed to the efficient

309

activation of bisulfite by nZVI.

310 311

Fig. 2. Effects of reaction conditions on kinetics of TPHP degradation. (a) initial pH;

312

(b) initial nZVI concentration; (c) initial bisulfite concentration; and (d) coexisting

313

inorganic anions (Cl− and NO3−). Experimental conditions: [nZVI]0 = 0.5 mM,

314

[bisulfite]0 = 2.0 mM, [TPHP]0 = 2.0 mg/L, [pH]0 = 3.0 ± 0.1, temperature = 30 ±

315

2 °C. The error bars represent the standard deviations.

316

3.3 Mechanism of nZVI/bisulfite system on TPHP degradation

317

3.3.1 Characteristics of nZVI particles

318

To elucidate the surface reaction of nZVI with bisulfite, spectroscopic analysis

319

was performed including SEM, XRD and XPS. As shown in Fig. 3a, the fresh nZVI

320

was present as chain-like aggregated spherical particles and uniformly dispersed with

321

each other, resulting in a large amount of voids generated (Ji et al., 2017). However,

322

the surface of nZVI was seriously damaged after reaction and numerous fragments

323

were formed according to Fig. 3b, which implied that the surface reaction was

324

involved in the degradation of TPHP (Cao et al., 2019).

325

To further reveal the phase composition of nZVI before and after reaction, XRD

326

analysis was conducted in this study under the premise of SEM results. The wide

327

angle XRD patterns of nZVI before and after reaction were presented in Fig. 3c.

328

Three prominent diffraction peaks at 2θ angles of 44.8°, 65.4° and 82.5° were in

329

accord with standard XRD structure of nZVI (Stefánsson et al., 2007; Rao et al., 2014;

330

Wu et al., 2017). Nevertheless, iron (hydroxy) oxides might not be detected by XRD

331

on account of too little content on the surface of nZVI (discussed subsequently by

332

XPS) (Cao et al., 2019). The XRD diffraction peak after the reaction indicated that the

333

peak intensity of Fe0 reduced or even disappeared with the progress of oxidation,

334

which was consistent with previous studies (Rao et al., 2014). Furthermore, it was

335

also observed that FeSO4 was assembled onto the surface of nZVI, suggesting that

336

sulfate was generated in the nZVI/bisulfite system (Cao et al., 2019; Tan et al., 2018).

337

The XPS analysis was conducted to better comprehend the surface elemental

338

composition and (hydroxyl) oxidation state of the nanomaterial (Fig. 4a-d). As shown

339

in Fig. 4a, for the fresh nZVI, a small peak at 706.5 eV indicated the existence of Fe0

340

(Rao et al., 2014), while the peaks at 710.0 eV, 710.8 eV and 712.0 eV were assigned

341

to Fe2+ peak of Fe3O4, Fe3+ peak of Fe2O3 and peak of FeOOH, respectively (Xu et al.,

342

2004; Ghauch et al., 2013). In addition, two oxygen peaks (O 1s: 529.7 eV, 531.1 eV)

343

ascertained in Fig. 4c represented lattice oxygen in metal oxides (O2−) and hydroxides

344

in surface hydroxides (OH−), respectively (Liang et al., 2009). It could be interpreted

345

that oxygen was still adsorbed onto the nZVI surface, even if protective measures

346

were taken (Li et al., 2019). These results indicated that the surface of fresh nZVI

347

might be covered by Fe3O4, Fe2O3 and FeOOH.

348

For nZVI after reaction, the Fe 2p3/2 and O 1s spectra were described in Fig. 4b

349

and Fig. 4d, respectively. The peak intensities of the three Fe(hydroxy)oxides had

350

been transformed based on the Fe 2p3/2 envelop (Fig. 4b). Moreover, the peak of Fe0

351

vanished and the peak (713.5 eV) of FeSO4 formed after reaction, indicating that the

352

electron transfer occurred on the nZVI surface during the degradation reaction. It was

353

revealed from Fig. 4d that typical spectrums of O 1s region were fitted into three

354

spectral bands at 529.7, 531.0 and 532.0 eV, corresponding to the lattice oxygen (O2−),

355

surface-adsorbed oxygen (OH−) and sulfate (SO42−), respectively. The appearance of

356

SO42− suggested that OH− on the nZVI surface took part in the reaction (Wu et al.,

357

2017). As shown in Fig. 4e and Fig. 4f, the fresh nZVI did not contain S element,

358

however, a distinct peak intensity was centered at 168.8 eV (S 2p) after reaction,

359

which stood for the presence of SO42− rather than other sulfur compounds (S2−, Sn2−).

360

This was consistent with the XRD results (Xie et al., 2017).

361

In brief, nZVI corrosion process involved nZVI oxidation and oxygen reduction

362

reaction under aerobic condition. Fe2+ was firstly generated via loss of electron on the

363

nZVI surface (Eq. (1)) and then hydrolyzed to form unstable Fe(OH)2 (Eq. (23)) (Guo

364

et al., 2015) . However, Fe(OH)2 was easy to be oxidized by O2, leading to various

365

iron (hydroxy)oxides generation such as Fe3O4, Fe2O3 and FeOOH. Simultaneously,

366

bisulfite played an oxidant role in the reaction and facilitated FeSO4 generation

367

according to Eq. (24) (Li et al., 2019). In addition, Fe3+ in the solution was also easily

368

hydrolyzed to form Fe(OH)3. On the basis of the above analysis, several iron (hydroxy)

369

oxides were produced and coated on the surface of nZVI.

370

Fe2+ + 2H2O → Fe(OH)2 + 2H+

(23)

371

FeOOH + SO42− + H2O → FeSO4 + 2OH− + 1/2H2 + 1/2O2

(24)

(a)

(b)

(c) 372 373

Fig. 3. Characterization of nZVI particles before and after reaction. (a) SEM images

374

of fresh nZVI; (b) SEM images of reacted nZVI; and (c) XRD observation.

(a)

(b)

(c)

(d)

(e)

(f)

375 376

Fig. 4. XPS characterization of nZVI particles. (a)-(b) Fe 2p core level of nZVI

377

before/after reaction; (c)-(d) O 1s core level of nZVI before/after reaction; (e)-(f) S 2p

378

core level of nZVI before/after reaction.

379

3.3.2. Identification of the oxidation species

380

Some reactive oxygen species (•SO4− and •OH) responsible for the degradation

381

of TPHP might be generated in nZVI/bisulfite system. Scavenging experiments were

382

performed to evaluate contribution level of these free radicals. MeOH (with the

383

α-hydrogen) could react with •SO4− and •OH at 2.5×107 and 9.7×108 M-1S-1,

384

respectively, while the reaction rate of TBA (without the α-hydrogen) with •OH

385

((3.8-7.6) ×108 M-1S-1) was much higher than that with •SO4− ((4.0-9.1)×105 M-1S-1).

386

Hence, MeOH was employed to quench both •SO4− and •OH, and TBA was

387

considered to scavenge •OH (Qi et al., 2014; Du et al., 2018). As shown in Fig. 5, the

388

removal efficiency of TPHP achieved 98.2% at 60 min, whereas it declined to 89.7%

389

and 24.0% with addition of 100 mM TBA and MeOH, respectively, indicating that

390

•SO4− was major oxidants responsible for TPHP degradation. The contributions of

391

•SO4− and •OH to the degradation of TPHP were also calculated according to the

392

method reported by Song et al. (2019b) with the result of 66.9% and 8.7%,

393

respectively (Table S1). Therefore, •SO4− was the predominant radical for the

394

degradation of TPHP in the nZVI/bisulfite system. Insight into the effects of •SO4− for

395

TPHP degradation would be further provided in the further study.

396

397

398

Fig. 5. Effects of two quenchers (TBA and MeOH) on kinetics of TPHP degradation.

399

Experimental conditions: [nZVI]0 = 0.5 mM, [bisulfite]0 = 2.0 mM, [pH]0 = 3.0 ± 0.1,

400

[TPHP]0 = 2 mg/L, temperature = 30 ± 2 °C. The error bars represent the standard

401

deviations.

402

3.3.3. Possible degradation pathways of TPHP

403

Based on HRMS analysis, six stable products were screened and their molecular

404

formulas were also displayed: C12H11O4P (product A, m/z 251.047), C18H15O5P

405

(product B, m/z 343.073), C18H14O10PS (product C, m/z 454.012), C12H11O5P

406

(product D, m/z 267.042), C12H11O6P (product E, m/z 283.037), and C6H7O4P

407

(product F, m/z 175.015). The MS2 spectrum and detailed information of degradation

408

products were presented in Figs. S3-S8 and Table 1. Due to a series of addition,

409

substitution and rupture reaction of TPHP induced by •SO4−, the evolution pattern of

410

degradation products was shown in Fig. 6a and was further proved through the peak

411

intensity alteration of various degradation intermediates (Fig. 6b). The molecular

412

structure of TPHP consists of a central phosphate and three benzene rings which are

413

highly sensitive to be attacked by oxidants, resulting in generation of various

414

degradation products.

415

The generating pathway of product A may involve two conversion steps. Firstly,

416

the phosphate center was attacked by •SO4−, leading to an addition on TPHP molecule

417

and subsequent P-O bond cleavage. Secondly, •SO4− was ruptured via a chain of

418

electron transfer, and then product A was left through H2O molecules addition

419

reaction (Fig. S9). Product F was produced through the further conversion of product

420

A following the same pattern. The generating pathway of product B was similar to

421

that of product A and the different sites of TPHP molecule attacked by •SO4− were

422

observed. Hence, the generating pathway of product B may also relate to an addition,

423

substitution and cleavage (Fig. S10). Of note, there might be two conversion pathways

424

for the generation of product D. One might be formed via •SO4− attacking the benzene

425

ring of product A, and the other via •SO4− attacking the phosphate center of product B.

426

Product E might be formed via the further conversion of product D following a

427

similar route like product B. An addition of •SO5− on benzene ring of TPHP resulted

428

in the formation of product C.

429

Table 1 Identification of degradation products in the nZVI/bisulfite system Products labels

Molecular formula

[M+H]+

Retention time

A

C12H11O4P

251.047

20.44

B

C18H15O5P

343.073

28.64

C

C18H14O10PS

454.012

19.38

D

C12H11O5P

267.042

20.02

E

C12H11O6P

283.037

19.97

F

C6H7O4P

175.015

5.14

430

The relative intensity curves of degradation intermediates were presented in Fig.

431

6b. The relative intensity of product B rapidly reached to 1.327×107 at the 2nd min

432

and then presented a reducing tendency in the 8th min. Subsequently, the intensity of

433

product B remained stable until the end of the reaction. Similarly, the maximum

434

intensity of product A (2.234×106) was detected at the 4th min and then appeared a

435

moderate decreasing trend. The relative intensities of product D and E rapidly

436

increased to the maximum value 2.054×105 and 2.966×104, respectively, and then

437

exhibited a sharp downtrend. However, for the relative intensity of product C, no

438

obvious fluctuation occurred. The relative intensity of product F peaked at the 10th

439

min with the maximum value of 1.987×102 and then gradually declined within 60 min.

440

Of note, products A, B and D were the primary degradation intermediates in terms of

441

the intermediate abundance (Fig. S11). Besides, these results also indirectly elucidated

442

the potential transformation pathways between the intermediate products (Govindan et

443

al., 2014).

444 445

Fig. 6. The potential transformation pathways of TPHP (a) and peak evolution of

446

degradation products (b) using nZVI driving bisulfite. Experimental conditions:

447

[nZVI]0 = 0.5 mM, [bisulfite]0 = 2.0 mM, [pH]0 = 3.0 ± 0.1, [TPHP]0 = 2 mg/L,

448

temperature = 30 ± 2 °C.

449

In a word, reduction of TPHP by nZVI/bisulfite system was a coupling reaction

450

between nZVI surface reaction and sulfate radical-mediated oxidative degradation,

451

hence, possible degradation mechanism of TPHP was proposed based on above results

452

as follows: (I) H+ in strong acid solution dissolved the passivating oxide layer on

453

nZVI surface and accelerated the dissolution of inner nZVI, promoting Fe2+ formation

454

and activating a continuous electron transfer chain at the solid-liquid interface; (II) the

455

generated Fe2+ reacted with HSO3− rapidly to form unstable FeHSO3+, following a

456

series of decomposition and oxidation reaction to produce •SO4− and SO42−; (III) the

457

reactive oxide species attacked TPHP immediately through addition, substitution and

458

cleavage reaction (Guo et al., 2015; Wang et al., 2019). In addition, a small part of

459

nZVI reacted with H2O in the presence of O2 to generate •OH which might also be

460

contributed to degrading TPHP.

461

4. Conclusion

462

In this work, the kinetics, evolution pathway and surface reaction mechanism of

463

TPHP degradation were systematically investigated by a novel AOPs (nZVI/bisulfite

464

system). Five comparative experiments demonstrated the excellent synergistic effect

465

of nZVI and bisulfite, indicating the superiority of the material selection in this work.

466

Furthermore, increasing nZVI dosage (0.1 mM to 0.75 mM) or bisulfite concentration

467

(0.5 mM to 2.5 mM) accelerated the degradation of TPHP in the nZVI/bisulfite

468

system while an obvious inhibitory effect was achieved with the further increasing of

469

nZVI and bisulfite concentration. Both Cl− and NO3− inhibited the degradation of

470

TPHP, and Cl− presented a more pronounced inhibition. Based on the results of

471

characterization (SEM, XRD, XPS), a chain-like aggregated spherical particles,

472

wrapped with a passivation layer (Fe3O4, Fe2O3, FeOOH) of nanomaterial was

473

synthesized and FeSO4 was assembled on the surface of nZVI after reaction. In

474

addition, •SO4− was proved to be the main ROS responsible for the degradation of

475

TPHP in the nZVI/bisulfite system. According to LC-QTOF/MS analysis, six stable

476

intermediate products and a possible sulfate radical-mediated degradation pathway of

477

TPHP were proposed which was also demonstrated by the relative intensity curves of

478

intermediate products. In summary, the nZVI/bisulfite system was a promising

479

advanced oxidation process for the removal of TPHP.

480

Acknowledgments

481

This work was financially supported by the National Natural Science Foundation

482

of China (Nos. 41673091, U1501234, 41573091) and the National Key Research and

483

Development Program of China (No. 2018YFC1802800).

484

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HIGHLIGHTS: nZVI could permanently activate bisulfite so as to degrade TPHP efficiently. •SO4− was a key free radical for TPHP degradation. The surface reaction mechanism of nZVI was emphatically elucidated. Six degradation products and an evolution pathway were proposed.

Author Contribution Statement Contributor

Statement of contribution Conceptualization, Methodology, Validation, Data Curation,

Ruxia Chen Writing - Original Draft Conceptualization, Writing - Review & Editing, Hua Yin Funding acquisition Hui Peng

Writing - Review & Editing, Supervision

Xipeng Wei

Writing - Review & Editing

Xiaolong Yu

Writing - Review & Editing

Danping Xie

Supervision

Guining Lu

Supervision

Zhi Dang

Supervision

Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

 The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: