Catalytic degradation of p-nitrophenol by magnetically recoverable Fe3O4 as a persulfate activator under microwave irradiation

Journal Pre-proof Catalytic degradation of p-nitrophenol by magnetically recoverable Fe3O4 as a persulfate activator under microwave irradiation Limin Hu, Peng Wang, Guoshuai Liu, Guangshan Zhang PII:

S0045-6535(19)32216-7

DOI:

https://doi.org/10.1016/j.chemosphere.2019.124977

Reference:

CHEM 124977

To appear in:

ECSN

Received Date: 26 June 2019 Revised Date:

13 September 2019

Accepted Date: 25 September 2019

Please cite this article as: Hu, L., Wang, P., Liu, G., Zhang, G., Catalytic degradation of p-nitrophenol by magnetically recoverable Fe3O4 as a persulfate activator under microwave irradiation, Chemosphere (2019), doi: https://doi.org/10.1016/j.chemosphere.2019.124977. 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. © 2019 Published by Elsevier Ltd.

P-nitrophenol can be degraded in Fe3O4/microwave/persulfate system with the generating of SO4•-.

1

Catalytic degradation of p-nitrophenol by magnetically

2

recoverable Fe3O4 as a persulfate activator under microwave

3

irradiation

4

Limin Hu, Peng Wang, Guoshuai Liu, and Guangshan Zhang*

5

State Key Laboratory of Urban Water Resource and Environment, School of

6

Environment, Harbin Institute of Technology, No. 73 Huanghe Street, Nangang

7

District, Harbin, Heilongjiang, 150090, China

8 9 10 11 12 13 14 15 16 17 18

*Corresponding author.

19

Tel.: 86-451-86283557; Fax: 86-451-86283557

20

E-mail address: [email protected] (G.S. Zhang)

1

21

Abstract

22

In this study, Fe3O4 and microwave (MW) were combined to activate persulfate

23

(PS) for the removal of organic matter, resulting in the enhanced degradation of

24

p-nitrophenol (PNP) in solution. During the preparation of Fe3O4, the effect of sodium

25

acetate was examined, and the results showed that the concentration of sodium acetate

26

had little effect on the catalytic activity of the Fe3O4/PS/MW system but did have an

27

effect on the Fe3O4 yield. In addition, with regards to the representative environmental

28

factors, the degradation experiment showed that humic acid and the co-existing anions

29

of chloride, sulfate, nitrate, and phosphate had little effects on p-nitrophenol removal;

30

however, carbonate had a negative effect. In addition, the Fe3O4/PS/MW system

31

performed well in the initial pH range of 3.0 to 9.0. According to the quenching

32

experiment and electron paramagnetic resonance (EPR) detection, sulfate radicals and

33

a minority of hydroxyl radicals play dominant roles in the degradation process. In

34

addition, the role of Fe3O4 was confirmed to take part in the degradation process by

35

X-ray photoelectron spectroscopy (XPS) analysis. Because of the good performance

36

observed in the water matrices of tap water and the Songhua River, these results

37

demonstrate the potential application of the Fe3O4/PS/MW system for wastewater

38

treatment.

39

Keywords: magnetic Fe3O4; persulfate; microwave; p-nitrophenol; sulfate radical;

40

wastewater treatment.

41 42

1. Introduction 2

43

An increasing amount of p-nitrophenol can be detected in aquatic environments

44

because of the dependence on p-nitrophenol during industrial production,

45

agrochemical production and daily necessities (Errampalli et al., 1999; Wei et al.,

46

2017). In addition, p-nitrophenol is considered to be too stable to be removed from

47

water by self-decomposition, which is concerning because p-nitrophenol is highly

48

toxic, highly pathogenic, highly teratogenic and only weakly biodegradable (Howe et

49

al., 1994; Hatzinger and Alexander, 1995; Bright and Healey, 2003). To date, many

50

methods have been developed to address p-nitrophenol-contaminated wastewater,

51

such as adsorption (Tang et al., 2007; Guzman et al., 2019), extraction (Caro et al.,

52

2002), photocatalysis (Abazari et al., 2019; Yin et al., 2019), electrochemistry

53

(Canizares et al., 2004; Quiroz et al., 2005), and sonolysis (Tauber et al., 2000). Of

54

these abovementioned methods, the first two methods involve physical treatments,

55

which achieve the removal of p-nitrophenol from water through phase transfers. The

56

last several methods are regarded as chemical treatment, which are dependent of extra

57

energy input and chemical reagents.

58

Sulfate radical-based advanced oxidation processes (SR-AOPs) represent the

59

most effective methods for the treatment of refractory organic matter, due to their

60

outstanding advantages of high redox potential, long life-time, and broad scope of

61

application in a large pH range (Tsitonaki et al., 2010; Matzek and Carter, 2016;

62

Ghanbari and Moradi, 2017), especially for the application of p-nitrophenol

63

degradation. For example, ultraviolet (UV) (Mohammadzadeh et al., 2016), heat 3

64

(Chen et al., 2016), heterogeneous activators (Yan et al., 2013), UV coupled with heat

65

(Zarei et al., 2015), and heat coupled with metal ions (Zhang et al., 2015) have been

66

used to activate persulfate and degrade p-nitrophenol in wastewater. However,

67

homogeneous systems (UV or heat) required large amounts of energy output, and the

68

activators used in heterogeneous systems were difficult to separate from solution for

69

the next cycle. Herein, we aimed to develop an energy-saving heterogeneous system

70

with easy recovery by coupling magnetic Fe3O4 and a microwave system to activate

71

persulfate.

72

Microwave irradiation is an alternative method that can be applied to chemical

73

reactions and chemical synthesis because of its thermal and nonthermal effects. For

74

example, several groups have investigated that microwave-induced persulfate could

75

be efficiently generated sulfate radicals and performed well in the treatment of

76

organic matters and landfill leachate (Qi et al., 2014; Chou et al., 2015; Qi et al.,

77

2015). In our previous work, microwave treatment was applied to the synthesis of

78

adsorbents (Deng et al., 2016a; Deng et al., 2016b), Fenton activator (Li et al., 2017),

79

and peroxymonosulfate activators (Hu et al., 2018a, b), where superior effects were

80

observed compared with conventional heating treatments. This result might be caused

81

by microwave-responding media that promote the interaction among the dipole

82

moments of molecules with high-frequency electromagnetic radiation to heat the

83

solution rapidly, without a temperature gradient (Wang and Wang, 2016). Both the

84

solvent, water, and the oxidant, persulfate, have polar structures (Costa et al., 2009), 4

85

which suggest that microwave irradiation could play an important role in the present

86

SR-AOPs.

87

Ferrous ions have been shown to be superior persulfate activators, with reduced

88

toxicity (Rao et al., 2014; Rodriguez et al., 2014). However, small amounts of ferrous

89

ions display poor catalytic activity because of their weak ability to activate persulfate.

90

In contrast, large amounts of ferrous ions also perform poorly because of the

91

self-decomposition of the generated radicals. Fe3O4 is a preferred activator that is able

92

to constantly produce ferrous ions to react with persulfate and generate sulfate

93

radicals. In addition, the outstanding ferromagnetic behavior of Fe3O4 makes it a

94

bifunctional material that can easily be separated from solution. Herein, Fe3O4 was

95

introduced to the PS/MW system to reduce the microwave temperature with high

96

catalytic activity.

97

In this study, Fe3O4 was synthesized using a solvothermal method, and its

98

physicochemical properties were analyzed, including phase structure, morphology,

99

ferromagnetic behavior, surface area, etc. In addition, the performance of the

100

Fe3O4/PS/MW system on the degradation of p-nitrophenol was evaluated. The

101

possible effects of environmental factors were discussed in detail. The role of active

102

radical species and Fe3O4 were also explored using quenching experiments, EPR

103

detection and XPS analysis. Finally, water matrices, including tap water and the

104

Songhua River, were used to assess the practical application of the Fe3O4/PS/MW

105

system in the treatment of organic matter. 5

106

2. Materials and methods

107

2.1 Chemicals

108

Ethylene glycol, sodium acetate anhydrous and ferric (III) chloride hexahydrate

109

(FeCl3 6H2O) were obtained from the Yongtai (Tianjin, China), Bodi (Tianjin,

110

China), and Kermel Chemical Reagent Companies (Tianjin, China), respectively.

111

Ethanol was obtained from Xilong Scientific (Guangdong, China). Sodium persulfate

112

was purchased from Fuchen Chemical Reagent Factory (Tianjin, China). H2SO4 and

113

NaOH were from Sinopharm Chemical Reagent Co. (Shanghai, China). NaCl,

114

Na2SO4, Na2CO3, NaNO3, and Na3PO4 were obtained from Sinopharm Chemical

115

Reagent Co. Humic acid (HA) was obtained from Aladdin (Shanghai, China).

116

Tert-Butanol (TBA) and methanol (MeOH) were obtained from Tianli Chemical

117

Reagent (Tianjin, China). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) from Aladdin

118

was used in the quenching experiment.

119

2.2. Preparation of magnetic Fe3O4 nanoparticles

120

Fe3O4 nanoparticles were synthesized using the modified solvothermal method

121

described by Deng (Deng et al., 2005). Specifically, as shown in Fig. 1, 1.35 g

122

FeCl3 6H2O were added to a beaker containing 40 mL ethylene glycol under

123

constant stirring until the ferric salt was completely melted. Then, 1.2 g sodium

124

acetate anhydrous was slowly added to the solution, a nontransparent, khaki-colored

125

solution was generated. After 30 min, the mixture was sealed in a Teflon-lined

126

stainless-steel autoclave, heated, and maintained at 200 °C for 8 h, followed by 6

127

cooling to room temperature. The black products were washed five times with ethanol

128

and dried at 60 °C overnight.

129 130

(Fig. 1) 2.3. Characterization of magnetic Fe3O4 nanoparticles

131

The phase structures of the synthesized magnetic Fe3O4 samples were analyzed

132

by X-ray diffraction (XRD). The patterns were detected in the 2θ range of 10°-90°,

133

using a Bruker D8 Advance system equipped with Cu Kα radiation (Bruker,

134

Germany). The morphologies of the samples were inspected with a Zeiss (Sigma 500)

135

scanning electron microscope (SEM). Low-temperature nitrogen adsorption was

136

measured using a Micromeritics ASAP 2020 apparatus operated at -196 °C, and the

137

samples were pretreated by degassing at 100 °C for 420 min. Magnetization

138

measurements were performed on a vibrating sample magnetometer (VSM,

139

LakeShore7404) at room temperature. XPS measurements were obtained on an

140

AXIS-Ultra instrument from Kratos Analytical with Al Kα radiation.

141

2.4. Experimental setup and procedures

142

The reaction process was performed in a microwave reactor (COOLPEX-E,

143

PreeKem, Shanghai), using the apparatus shown in Fig. S1 in the supporting

144

information. During each run, 100 mL of PNP solution (20 mg L-1), and different

145

doses of synthesized magnetic Fe3O4 and persulfate were added to the reactor and

146

mixed with mechanical stirring. The degradation processes started when the

147

temperature reached the designated level, which generally occurred within several 7

148

minutes (approximately 2 min). At given intervals, samples were withdrawn using an

149

injection syringe and filtered through a 0.45-µm syringe filter into a color comparison

150

tube for further analyses.

151

2.5. Analytical methods

152

The residual concentration of PNP was detected using an ultraviolet-visible

153

spectrophotometer (UV-Vis, T6 new century, Pgeneral) with a maximum absorption

154

wavelength at 400 nm after the sample was alkalized to pH ~11 with one drop of

155

NaOH solution (Tang et al., 2012; Saien and Fallah Vahed Bazkiaei, 2017). The

156

residual persulfate concentration was monitored using a rapid spectrophotometric

157

determination method, as reported by Liang (Liang et al., 2008), with the aid of

158

UV-Vis. The pH of the solution was measured using a pH meter (PHS-3F, INESA)

159

equipped with an E-301-C model pH combination electrode. The total iron ion was

160

quantified by inductively coupled plasma-atomic emission spectrometry (ICP-AES,

161

Optima 5300 DV, Perkin Elmer). The total organic carbon (TOC) of samples was

162

detected by a TOC analyzer (5000A, Shimadzu). All measurements were conducted in

163

triplicate to confirm reproducibility, and the data are shown as the mean values.

164

3. Results and discussion

165

3.1. Characterization of magnetic Fe3O4 nanoparticles

166

Prior to analyze the optimal Fe3O4 nanoparticles, the effect of sodium acetate

167

was discussed. In general, sodium acetate can be used as a dispersant to prevent

168

particle agglomeration because of its electrostatic stabilization and plays an important 8

169

role during the reduction of ferric salt to Fe3O4 in ethylene glycol solution (Deng et al.,

170

2005). Herein, the dose of sodium acetate used was examined. During the degradation

171

processes, samples prepared with different sodium acetate concentrations (0.6 g, 1.2 g,

172

2.4 g and 3.6 g) showed no distinctly different effects on the degradation efficiencies

173

of PNP (Fig. S2a), indicating that sodium acetate had little effect on the catalytic

174

performance of the Fe3O4/PS/MW system. In addition, the XRD patterns of different

175

samples are shown in Fig. S3, and the results showed that all samples had similar

176

specific diffraction peaks for Fe3O4. However, the differences in yield could not be

177

ignored. Interestingly, when a sample was prepared without the addition of sodium

178

acetate, no product was obtained, illustrating the promotion effect of sodium acetate

179

on the reduction of ferric salt to Fe3O4. As shown in Fig. S2b, increasing the dose of

180

sodium acetate from 0.6 g to 1.2 g during sample preparation increased the sample

181

yield from 0.0815 g to 0.3744 g, and the corresponding stoichiometric yield rates

182

increased from 21.1% to 97.1%. Furthermore, increasing the dose of sodium acetate

183

to 3.6 g (refer to the data from previous report (Deng et al., 2005)) resulted in a

184

similar yield rates of 98.5%. The optimal dose of sodium acetate was determined to be

185

1.2 g in the present study.

186

As shown in Fig. 2a, the XRD pattern showed the highly crystalline structure of

187

synthesized Fe3O4 nanoparticles. The characteristic diffraction peaks could be

188

assigned to the cubic crystal phase of Fe3O4, with the standard card of 11-0614, and

189

its lattice constants were a=b=c=8.40 Å. The hysteresis loops (Fig. 2b) of the 9

190

synthesized Fe3O4 nanoparticles, with a high-saturation magnetization value of 77.0

191

emu g-1, implied the ability of these nanoparticles to be easily recovered from

192

reaction solutions due to outstanding ferromagnetic behavior. In addition, as shown in

193

the inset photographs in Fig. 2b, the synthesized Fe3O4 nanoparticles displayed good

194

dispersive behavior under gravity and good ferromagnetic behavior under magnetic

195

conditions within several seconds. According to the SEM images, using different

196

magnifications (Figs. 2c and 2d), the morphology of the samples exhibited regular,

197

sphere-like shapes and uneven surfaces, which were favored to enhance the surface

198

area of the sample. Based on the N2 adsorption-desorption isotherms and the BJH

199

desorption pore diameter distribution measurement (Fig. 2e and 2f), the surface area

200

of sample was 9.41 m2 g-1, and the pore volume of the sample was 0.045 cm3 g-1.

201

These results indicated that these nanoparticles were much larger than natural

202

magnetite (0.63 m2 g-1 for surface area and 0.00124 cm3 g-1 for pore volume) that

203

purchased from Henan Longcheng Group Xuchang Mining Limited Company (Henan,

204

China) (as shown in Fig. S4), demonstrating that the larger surface area and pore

205

volume of the sample could offer a larger solid-liquid interface for the degradation

206

reaction.

207 208

(Fig. 2) 3.2. The degradation performance of the Fe3O4/PS/MW system

209

The degradation efficiencies for PNP using various oxidation processes were

210

examined, and the results are shown in Fig. 3a. During the degradation process, a 10

211

negligible effect on PNP degradation was observed using the Fe3O4/MW and

212

PS/Fe3O4 systems, excluding the effects of PNP self-degradation caused by Fe3O4

213

adsorption under microwave irradiation and the oxidation of persulfate activated by

214

Fe3O4 under ambient conditions, respectively. Based on our previous study, we found

215

that microwave irradiation tended to activate persulfate when the microwave

216

temperature was set to 80 °C, with a degradation efficiency of 73% within 28 min.

217

Although a higher temperature (90°C) resulted in increased catalytic activity (93.5%),

218

it also required more energy consumption. Using this system with the addition of

219

Fe3O4, approximately 98.2% of PNP could be removed from the solution. This

220

demonstrated the acceleration effect of Fe3O4 on persulfate activation under

221

conditions of microwave irradiation. In addition, the reaction rates for Fe3O4/PS and

222

Fe3O4/PS/MW were also calculated (Fig. 3b), and the results showed that the two

223

systems could be well-fitted with a pseudo first-order kinetics model. Clearly, the

224

reaction rate increased to 0.139 min-1 in Fe3O4/PS/MW system, compared to the

225

PS/MW system with 0.0436 min-1. In addition, the synergetic effects (SE) of

226

Fe3O4/PS/MW and PS/MW systems were evaluated based on the kinetics study using

227

equations (1) and (2). Since all of the k(Fe3O4), k(PS) and k(MW) were close on zero,

228

the precise synergetic effect value did not calculate but the order was exhibited with

229

SEFe3O4/PS/MW>SEPS/MW>>1, indicating that both Fe3O4 and microwave irradiation

230

played critical roles in the present system.

11

231

SEFe3O 4/ PS/ MW =

232

SEPS/ MW =

k (Fe3O 4 / PS / MW) k (Fe3O 4 ) + k (PS) + k (MW)

k (PS / MW) k (PS) + k (MW)

(1)

(2)

233

(Fig. 3)

234

To further analyze the effect of Fe3O4 on the Fe3O4/PS/MW system, the

235

degradation efficiencies of PNP under various microwave temperatures, ranging from

236

60 °C to 80 °C, were examined (Fig. 3c). Notably, in the absence of Fe3O4, PNP could

237

barely be degraded at 60 °C, which implied that this level of microwave irradiation

238

was not sufficient to activate persulfate to generate active species for organic pollutant

239

removal. However, when Fe3O4 was added under the same conditions, the degradation

240

efficiency of PNP reached 37.6% within 28 min. This result was similar to the

241

degradation capacity of the MW/PS system at 70 °C, which illustrated that the

242

presence of Fe3O4 could reduce the reaction temperature. Interestingly, the PNP

243

degradation efficiency of the Fe3O4/MW/PS system at 70°C was similar to that of the

244

MW/PS system at 80°C, which confirmed the acceleration effect of Fe3O4 in the

245

present system.

246

The dependence of PNP removal on the dose of Fe3O4 was examined, as shown

247

in Fig. 3d. The degradation efficiency of PNP increased from 93.0% to 98.2% as the

248

dose of Fe3O4 increased from 0.01 g L-1 to 0.1 g L-1. Then, the degradation

249

efficiency decreased to 93.5% when the dose of Fe3O4 increased to 0.5 g L-1. This

250

result confirmed that Fe3O4 plays an important role in the degradation process. On one

251

hand, the ferrous ions from the surface of Fe3O4 acted as a persulfate activator to 12

252

generate sulfate radicals. On the other hand, the large surface area supported adequate

253

active sites for the oxidation to occur between activated radicals and absorbed-PNP

254

molecules. However, too high a dose of Fe3O4 led to the excessive generation of

255

sulfate radicals during a short period of time, resulting in the self-decomposition of

256

sulfate radicals, causing an inferior PNP degradation efficiency. Here, the optimal

257

dose of Fe3O4 was confirmed to be 0.1 g L-1.

258

3.3. Identification of active radicals in the Fe3O4/MW/PS system

259

A quenching experiment was carried out to verify the possible active radicals in

260

the Fe3O4/MW/PS system, using MeOH and TBA as specific probe scavengers. Since

261

the reaction rate constants of MeOH for hydroxyl (9.7×108 M-1 s-1) and sulfate

262

radicals (1.0×107 M-1 s-1) were similar (Liu et al., 2017a), we utilized MeOH as the

263

probe scavenger for both radicals. In addition, the reaction rate constant of TBA for

264

hydroxyls (3.8-7.6×108 M-1 s-1) is three orders of magnitude greater than that for

265

sulfate radicals (4-9.1×105 M-1 s-1) (Liu et al., 2017a), indicating that TBA favors

266

quenching with hydroxyl radicals and was considered to be a hydroxyl scavenger. As

267

shown in Fig. 4a, the degradation efficiencies were distinctly reduced in the presence

268

of MeOH compared with those in the presence of TBA, and the degradation

269

efficiencies were also reduced with the increased the dose of scavengers, with 29.3%

270

and 76.2% of PNP removal occurring when the molar ratio of scavenger to persulfate

271

was 100 to 1 for MeOH and TBA, respectively. This result demonstrated that the

272

majority of active species were sulfate radicals, whereas a minority of hydroxyl 13

273

radicals was generated in the Fe3O4/MW/PS system, which greatly improved the

274

degradation of PNP during the degradation process. The corresponding persulfate

275

decomposition was monitored, as shown in Fig. 4b. The control experiment showed

276

that 72.0% of persulfate was consumed within 28 min. Interestingly, the consumed

277

number of persulfate molecules increased to approximately 90% in the presence of

278

MeOH, which implied a rapid reaction rate between MeOH and sulfate radicals. A

279

similar amount of persulfate was consumed in the presence of TBA, indicating that

280

this was not the limiting factor for TBA addition.

281

In addition, EPR detection was performed using DMPO as a trapping agent (Fig.

282

4c). As expected, no confirmable characteristic peaks could be observed for the

283

Fe3O4/MW (blue curve) and Fe3O4/PS systems (magenta curve), indicating that no

284

available active species were produced, resulting in the poor degradation efficiency of

285

PNP (Fig. 3a). However, the DMPO-SO4 and DMPO-OH adducts emerged in the

286

Fe3O4/MW/PS system (red curve), which showed higher intensities than those in the

287

MW/PS system (black curve). These results implied that the oxidative capacity of the

288

Fe3O4/MW/PS system was stronger than that of the MW/PS system. Therefore, we

289

inferred that a majority of sulfate radicals and a minority of hydroxyl radicals played

290

valuable roles during PNP degradation in the Fe3O4/MW/PS system.

291 292 293

(Fig. 4) 3.4. The role of Fe3O4 The role of Fe3O4 during the degradation process has been detected using XPS. 14

294

The high-resolution XPS of Fe 2p before and after reaction was shown in Fig. 5. The

295

fitted peaks with binding energies at 708.7 eV and 722.0 eV were assigned to bivalent

296

of Fe, while the binding energies at 710.3 eV and 724.8 eV belonged to trivalent of Fe

297

(Yamashita and Hayes, 2008). In addition, the area ratio of Fe2+/Fe3+ has reduced from

298

3.7 to 0.9, which demonstrated that a part of bivalent Fe on the surface of Fe3O4 took

299

part in the degradation process with transformation to trivalent Fe. To identify the

300

synergism effect of microwave and Fe3O4, the controlling experiment with

301

conventional heating mode was introduced as shown in Fig. S5. Under the same

302

reaction temperature, the degradation efficiency of PNP was reduced to 17.0% with

303

conventional heating mode, compared to the system with microwave mode with

304

98.2% PNP removal. In addition, the leaching Fe after the degradation process has

305

been detected by ICP-AES, with the concentration of total iron of 0.474 mg L-1 and

306

0.895 mg L-1 for microwave mode and conventional heating mode, respectively.

307

Summing up the above results, it showed that the activation of persulfate mainly took

308

place on the surface of Fe3O4 with electron transfer between Fe2+/Fe3+ and

309

S2O82-/SO4•- by reducing the leaching iron element (eq. 3).

310

S2 O82− + Fe 2+ ↔ Fe3+ + SO 24− + SO 4−

311

(Fig. 5)

312

(3)

3.5. Effects of environmental factors

313

The nature of aquatic environments is always complicated and diverse. To

314

investigate the practical applicability of the Fe3O4/MW/PS system, several parameters 15

315

were discussed in detail. First, the effects of initial pH on PNP removal are shown in

316

Figs. 6a and 6b. Specifically, the degradation efficiencies of PNP reached greater than

317

90.4% when the initial pH ranged from acidic (pH of 3.0) to alkaline conditions (pH

318

of 9.0), which covers the scope of pH levels in general water bodies. Under strong

319

alkaline condition (pH of 11.0), an obvious decrease in PNP degradation was

320

observed, decreasing to 74.0%. These results demonstrated that PNP-containing

321

pollutants can be degraded under most naturally occurring initial pH ranges, except

322

for strong alkaline conditions, using the Fe3O4/MW/PS system. In addition, the

323

changes in pH values during the reaction time course were monitored, as shown in

324

Fig. 6b. As expected, the final pH of the solution was maintained at approximately

325

2.58-2.96 with the buffering ability of the Fe3O4/MW/PS system. However, strong

326

alkaline conditions were beyond the buffering capacity of the system, resulting in a

327

final pH of 6.21. This phenomenon was similar with to previously reported results

328

(Lei et al., 2015; Yan et al., 2015). In addition, a positive correlation was observed

329

between the degradation efficiency of PNP and persulfate consumption. Persulfate

330

consumption reached as high as 70.2% to 75.4% when the pH ranged from 3.0-9.0

331

and reduced to 38.3% under strong alkaline conditions, which indicated that the pH

332

might affect the persulfate consumption and cause differences in PNP degradation

333

efficiency.

334

The representative anions were examined, and the results are shown in Figs. 6c

335

and 6d. Clearly, a weak effect on PNP removal was observed in the presence of 16

336

anions, including Cl-, SO42-, NO3-, and H2PO4-, with more than 90.5% of PNP being

337

eliminated in the Fe3O4/MW/PS system. However, a distinct negative effect was

338

observed on PNP degradation in the presence of CO32-. Notably, a dependence on the

339

final pH of the system was observed, where the negligible effect of anions on PNP

340

degradation result in a well-behaved buffering capacity compared with the control

341

experiment. Instead, the system where CO32- was added resulted in a corrupted

342

buffering capacity, with a final pH of 9.89. This result was consistent with the results

343

observed when examining the effects of pH. In addition, the possible formation of

344

subradicals with different redox potentials may explain the fine distinctions observed

345

for PNP degradation, where the subradicals could oxidize PNP molecules by replacing

346

sulfate radicals, as described in the following previously reported equations

347

(Anipsitakis et al., 2008; Wang and Chu, 2011; Wang et al., 2011; Yuan et al., 2011;

348

Oh et al., 2016):

349

SO 4− + Cl − ↔ SO 42− + Cl , k=2.47×108 M-1 s-1

(4)

350

Cl− + Cl ↔ Cl2− , k=8.0×109 M-1 s-1

(5)

351

H 2 CO3 ↔ H + + HCO3− , pKa1=6.38

(6)

352

HCO3− ↔ H + + CO32− , pKa2=10.38

(7)

353

SO 4− + CO32− ↔ SO 24 − + CO3− , k= 6.1×106 M-1 s-1

(8)

354

SO 4− + HCO3− ↔ SO42 − + HCO3 , k= 9.1×106 M-1 s-1

(9)

355 356

SO 4− + H 2 PO −4 ↔ SO 24− + H 2 PO 4 , k<7.0×104 M-1 s-1

(10)

Humic acid, a common component of natural organic matter, always play a vital 17

357

role during wastewater treatment. Herein, the effect of humic acid on PNP

358

degradation was considered, using concentrations of 2.5 mg L-1 and 5 mg L-1. As

359

illustrated in Figs. 6e and 6f, the presence of humic acid had negligible effects on

360

PNP degradation, demonstrating the selectivity of the Fe3O4/MW/PS system. In

361

addition, the corresponding persulfate consumption was monitored, and as described,

362

the increased consumption of persulfate compared with control trials might be the

363

result of the decomposition of humic acid by using persulfate. These experiments

364

demonstrated the potential for the practical application of the Fe3O4/MW/PS system.

365

(Fig. 6)

366

Different water matrices, including tap water and the Songhua River (sampling

367

coordinate: 45°45’43.5’’N, 126°35’25.8’’E), were examined to determine the

368

degradation capacity of the Fe3O4/MW/PS system. As depicted in Fig. 7a, the tap

369

water matrix had a slight promotion effect on PNP degradation, which might be

370

ascribed to the low concentration of Cl- in tap water. This result is consistent with the

371

results from the examination of the effects of co-existing anions. However, when the

372

Songhua River was examined, an obvious decreasing tendency was observed, with

373

only 72.5% of PNP degradation occurring. This phenomenon might be ascribed to the

374

complicated water environment. First, some organic matter in the Songhua River

375

could consume the limited persulfate oxidant. Second, the various anions in the matrix

376

could deplete the generated radicals. Third, the pH of the matrix might affect the

377

degradation capacity of the Fe3O4/MW/PS system, as the initial pH was 7.12 and the 18

378

final pH was 3.18 (Fig. 7b). Although the pH was within the scope of application,

379

between 3.0 to 9.0, the combination of other parameters, such as co-existing ions and

380

organic matter, should be considered. Therefore, one solution to enhance the oxidative

381

capacity of the system would be to provide more persulfate when using this system in

382

this matrix. As shown in Fig. 7c, the degradation efficiency of PNP in the Songhua

383

River could match the distilled water matrix efficiency, with 99.6% PNP degradation,

384

when [persulfate]/[PNP] increased from 15/1 to 25/1. In addition, the corresponding

385

TOC removal was also monitored, as shown in Fig. 7d. As expected, when increasing

386

the [persulfate]/[PNP] from 15/1 to 25/1, the TOC removal reached a superior level,

387

increasing from 27.8% to 83.3%, which was much larger than the control trial

388

(53.2%). This result proved that sulfate radicals possess an excellent oxidative

389

capacity.

390 391

(Fig. 7) 3.6. Stability

392

As a magnetic material, the reusability of Fe3O4 is one of the most important

393

indicators for practical application. As shown in Fig. S6a, the degradation efficiency

394

of PNP reached as high as 93.4% after recycling Fe3O4 three times, which indicated

395

the reusability of Fe3O4, which could be easily recovered from solution using a

396

magnet. The slight decrease in the degradation efficiency might be due to the

397

residence of PNP molecules on the surface of Fe3O4. In addition, the steady,

398

high-saturation magnetization of used Fe3O4 implied great ferromagnetic behavior 19

399

(Fig. S6b), and the lack of changes to the morphology of Fe3O4 indicated the rugged

400

structure of the sample (Fig. S6c). All of these parameters indicated the good

401

reusability and stability of Fe3O4 in the present work, which showed the potential for

402

the practical application of this compound during wastewater treatment.

403

3.7. Comparison with other results

404

Fe3O4 has been considered to be a classical persulfate activator in recent years,

405

and several representative systems are shown in Table 1. The undecorated Fe3O4/PS

406

system could be used to treat landfill leachates, which contain toxic, hazardous and

407

nonbiodegradable substances, with 61% of TOC removal after 24 h. Moreover, the

408

coupling systems performed well when coupled with ultrasound, visible light, and

409

electrochemistry for the elimination of organic pollutant. However, our present

410

Fe3O4/MW/PS system exhibits distinct advantages with regards to the dose of Fe3O4

411

and persulfate, reaction times and reaction rates. Herein, we confirmed that the

412

Fe3O4/MW/PS system might represent a new possibility for refractory organic

413

pollutant removal during wastewater treatment.

414

(Table 1)

415

4. Conclusion

416

In conclusion, the Fe3O4/PS/MW system has been proven to be an efficient

417

method for the removal of p-nitrophenol in solution. Under the conditions of [Fe3O4]

418

= 0.1 g‧L-1, [persulfate]/[p-nitrophenol] = 15, and microwave temperature = 80°C,

419

approximately 98.2% of p-nitrophenol could be degraded within 28 min. The 20

420

degradation capacity was maintained above 90% when the pH ranged from 3.0 to 9.0,

421

which matched well with the general parameters of actual water. The effects of

422

inorganic and organic matter on the catalytic activity showed that most of the tested

423

materials had little influence on the Fe3O4/PS/MW system, except for carbonate. In

424

addition, sulfate radicals and a minority of hydroxyl radicals were generated by

425

activation PS during the degradation process along with the transformation between

426

Fe2+ and Fe3+. The analysis of performance using the water matrices of tap water and

427

Songhua River suggested that the present Fe3O4/PS/MW system may be utilized

428

broadly for SR-AOPs during wastewater treatment.

429

Conflicts of interest

430

There are no conflicts to declare.

431

Acknowledgements

432

The authors sincerely appreciate the National Natural Science Foundation of China

433

(51678185 and 51779066) and the Open Project of State Key Laboratory of Urban

434

Water Resource and Environment, Harbin Institute of Technology (QA201924) for

435

their financial support. The authors thank the China Scholarship Council for

436

supporting our work (201806120348).

437

References

438 439 440 441 442 443

Abazari, R., Mahjoub, A.R., Salehi, G., 2019. Preparation of amine functionalized g-C3N4@H/SMOF NCs with visible light photocatalytic characteristic for 4-nitrophenol degradation from aqueous solution. J Hazard Mater 365, 921-931. Anipsitakis, G.P., Tufano, T.P., Dionysiou, D.D., 2008. Chemical and microbial decontamination of pool water using activated potassium peroxymonosulfate. Water res 42, 2899-2910. Bright, D.A., Healey, N., 2003. Contaminant risks from biosolids land application. Environ Pollut 126, 21

444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485

39-49. Canizares, P., Saez, C., Lobato, J., Rodrigo, M.A., 2004. Electrochemical treatment of 4-nitrophenol-containing aqueous wastes using boron-doped diamond anodes. Ind Eng Chem Res 43, 1944-1951. Caro, E., Masque, N., arce, R.M.M., Borrull, F., Cormack, P.A.G., herrington, D.C.S., 2002. Non-covalent and semi-covalent molecularly imprinted polymers for selective on-line solid-phase extraction of 4-nitrophenol from water samples. J Chromatogr A 963, 169-178. Chen, X., Murugananthan, M., Zhang, Y., 2016. Degradation of p-Nitrophenol by thermally activated persulfate in soil system. Chem Eng J 283, 1357-1365. Chou, Y. C., Lo, S. L., Kuo, J., Yeh, C. J., 2015. Microwave-enhanced persulfate oxidation to treat mature landfill leachate. J Hazard Mater 284, 83-91. Costa, C., Santos, V.H.S., Araujo, P.H.H., Sayer, C., Santos, A.F., Fortuny, M., 2009. Microwave-assisted rapid decomposition of persulfate. Eur Polym J 45, 2011-2016. Deng, H., Li, X., Peng, Q., Wang, X., Chen, J., Li, Y., 2005. Monodisperse Magnetic Single-Crystal Ferrite Microspheres. Angew Chem Int Edit 44, 2782-2785. Deng, S., Wang, P., Zhang, G., Dou, Y., 2016a. Polyacrylonitrile-based fiber modified with thiosemicarbazide by microwave irradiation and its adsorption behavior for Cd(II) and Pb(II). J Hazard Mater 307, 64-72. Deng, S., Zhang, G., Chen, S., Xue, Y., Du, Z., Wang, P., 2016b. Rapid and effective preparation of HPEI modified biosorbent based on cellulose fiber with microwave irradiation method for enhanced arsenic removal in water. J Mater Chem A 4, 15851-15860. Errampalli, D., Tresse, O., Lee, H., Trevors, J.T., 1999. Bactrial survival and mineralization of p-nitrophenol in soil by green fluorescent protein-marked Moraxella sp. G21 encapsulated cells. FEMS Microbiol Ecol 30, 229-236. Ghanbari, F., Moradi, M., 2017. Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants: Review. Chem Eng J 310, 41-62. Guzman, M., Estrada, M., Miridonov, S., Simakov, A., 2019. Synthesis of cerium oxide (IV) hollow nanospheres with tunable structure and their performance in the 4-nitrophenol adsorption. Micropor Mesopor Mat 278, 241-250. Hatzinger, P.B., Alexander, M., 1995. Effect of aging of chemicals in soil on their biodegradability and extractability. Environ Sci Technol 29, 537-545. Howe, G.E., Marking, L.L., Bills, T.D., Rach, J.J., Jr, F.L.M., 1994. Effects of water temperature and pH on toxicity of terbufos, trichlorfon, 4-nitrophenol and 2,4-dinitrophenol to the amphipod gammarus pseudolimnaeus and rainbow trout (oncorhynchus mykiss). Environ Toxicol Chem 13, 51-66. Hu, L., Zhang, G., Liu, M., Wang, Q., Wang, P., 2018a. Enhanced degradation of Bisphenol A (BPA) by peroxymonosulfate with Co3O4-Bi2O3 catalyst activation: Effects of pH, inorganic anions, and water matrix. Chem Eng J 338, 300-310. Hu, L., Zhang, G., Liu, M., Wang, Q., Wang, P., 2018b. Optimization of the catalytic activity of a ZnCo2O4 catalyst in peroxymonosulfate activation for bisphenol A removal using response surface methodology. Chemosphere 212, 152-161. Lei, Y., Chen, C.-S., Tu, Y.-J., Huang, Y.-H., Zhang, H., 2015. Heterogeneous Degradation of Organic Pollutants by Persulfate Activated by CuO-Fe3O4: Mechanism, Stability, and Effects of pH and 22

486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527

Bicarbonate Ions. Environ Sci Technol 49, 6838-6845. Li, S., Zhang, G., Zhang, W., Zheng, H., Zhu, W., Sun, N., Zheng, Y., Wang, P., 2017. Microwave enhanced Fenton-like process for degradation of perfluorooctanoic acid (PFOA) using Pb-BiFeO3/rGO as heterogeneous catalyst. Chem Eng J 326, 756-764. Liang, C., Huang, C.-F., Mohanty, N., Kurakalva, R.M., 2008. A rapid spectrophotometric determination of persulfate anion in ISCO. Chemosphere 73, 1540-1543. Liu, G., You, S., Tan, Y., Ren, N., 2017a. In Situ Photochemical Activation of Sulfate for Enhanced Degradation of Organic Pollutants in Water. Environ Sci Technol 51, 2339-2346. Liu, J., Zhou, J., Ding, Z., Zhao, Z., Xu, X., Fang, Z., 2017b. Ultrasound irritation enhanced heterogeneous activation of peroxymonosulfate with Fe3O4 for degradation of azo dye. Ultrason Sonochem 34, 953-959. Liu, Y., Guo, H., Zhang, Y., Cheng, X., Zhou, P., Zhang, G., Wang, J., Tang, P., Ke, T., Li, W., 2018a. Heterogeneous activation of persulfate for Rhodamine B degradation with 3D flower sphere-like BiOI/Fe3O4 microspheres under visible light irradiation. Sep Purif Technol 192, 88-98. Liu, Z., Li, X., Rao, Z., Hu, F., 2018b. Treatment of landfill leachate biochemical effluent using the nano-Fe3O4/Na2S2O8 system: Oxidation performance, wastewater spectral analysis, and activator characterization. J Environ Manage 208, 159-168. Matzek, L.W., Carter, K.E., 2016. Activated persulfate for organic chemical degradation: A review. Chemosphere 151, 178-188. Mohammadzadeh, M., Behnajady, M.A., Eskandarloo, H., 2016. Hybridized advanced oxidation 2-

processes involving UV/H2O2/S2O8

for photooxidative removal of p-nitrophenol in an annular

continuous-flow photoreactor. Kinet Catal 57, 768-775. Oh, W.-D., Dong, Z., Lim, T.-T., 2016. Generation of sulfate radical through heterogeneous catalysis for organic contaminants removal: Current development, challenges and prospects. Appl Catal B-Environ 194, 169-201. Qi, C., Liu, X., Lin, C., Zhang, X., Ma, J., Tan, H., Ye, W., 2014. Degradation of sulfamethoxazole by microwave-activated persulfate: Kinetics, mechanism and acute toxicity. Chem Eng J 249, 6-14. Qi, C., Liu, X., Zhao, W., Lin, C., Ma, J., Shi, W., Sun, Q., Xiao, H., 2015. Degradation and dechlorination of pentachlorophenol by microwave-activated persulfate. Environ Sci Pollut R 22, 4670-4679. Quiroz, M.A., Reyna, S., Martínez-Huitle, C.A., Ferro, S., De Battisti, A., 2005. Electrocatalytic oxidation of p-nitrophenol from aqueous solutions at Pb/PbO2 anodes. Appl Catal B-Environ 59, 259-266. Rao, Y.F., Qu, L., Yang, H., Chu, W., 2014. Degradation of carbamazepine by Fe(II)-activated persulfate process. J Hazard Mater 268, 23-32. Rodriguez, S., Vasquez, L., Costa, D., Romero, A., Santos, A., 2014. Oxidation of Orange G by persulfate activated by Fe(II), Fe(III) and zero valent iron (ZVI). Chemosphere 101, 86-92. Saien, J., Fallah Vahed Bazkiaei, M., 2017. Homogenous UV/periodate process in treatment of p-nitrophenol aqueous solutions under mild operating conditions. Environ Technol 39, 1823-1832. Sepyani, F., Darvishi Cheshmeh Soltani, R., Jorfi, S., Godini, H., Safari, M., 2018. Implementation of continuously electro-generated Fe3O4 nanoparticles for activation of persulfate to decompose amoxicillin antibiotic in aquatic media: UV254 and ultrasound intensification. J Environ Manage 224, 315-326. Tang, D., Zheng, Z., Lin, K., Luan, J., Zhang, J., 2007. Adsorption of p-nitrophenol from aqueous 23

528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563

solutions onto activated carbon fiber. J Hazard Mater 143, 49-56. Tang, Y., Liu, Y., Cao, A., 2012. Strategy for sensor based on fluorescence emission red shift of conjugated polymers: Applications in pH response and enzyme activity detection. Anal Chem 85, 825-830. Tauber, A., Schuchmann, H.-P., Sonntag, C.v., 2000. Sonolysis of aqueous 4-nitrophenol at low and high pH. Ultrason Sonochem 7, 45-52. Tsitonaki, A., Petri, B., Crimi, M., MosbÆK, H., Siegrist, R.L., Bjerg, P.L., 2010. In situ chemical oxidation of contaminated soil and groundwater using persulfate: A review. Crit Rev Env Sci Tec 40, 55-91. Wang, N., Wang, P., 2016. Study and application status of microwave in organic wastewater treatment – A review. Chem Eng J 283, 193-214. Wang, Y.R., Chu, W., 2011. Degradation of a xanthene dye by Fe(II)-mediated activation of Oxone process. J Hazard Mater 186, 1455-1461. Wang, Z., Yuan, R., Guo, Y., Xu, L., Liu, J., 2011. Effects of chloride ions on bleaching of azo dyes by 2+

Co /oxone reagent: kinetic analysis. J Hazard Mater 190, 1083-1087. Wei, X., Wu, H., Sun, F., 2017. Magnetite/Fe-Al-montmorillonite as a Fenton catalyst with efficient degradation of phenol. J colloid interf sci 504, 611-619. 2+

3+

Yamashita, T., Hayes, P., 2008. Analysis of XPS spectra of Fe and Fe ions in oxide materials. Appl Surf Sci 254, 2441-2449. Yan, J., Han, L., Gao, W., Xue, S., Chen, M., 2015. Biochar supported nanoscale zerovalent iron composite used as persulfate activator for removing trichloroethylene. Bioresour Technol 175, 269-274. Yan, J., Zhu, L., Luo, Z., Huang, Y., Tang, H., Chen, M., 2013. Oxidative decomposition of organic pollutants by using persulfate with ferrous hydroxide colloids as efficient heterogeneous activator. Sep Purif Technol 106, 8-14. Yin, H., Kuwahara, Y., Mori, K., Che, M., Yamashita, H., 2019. Plasmonic Ru/hydrogen molybdenum bronzes with tunable oxygen vacancies for light-driven reduction of p-nitrophenol. J Mater Chem A 7, 3783. Yuan, R., Ramjaun, S.N., Wang, Z., Liu, J., 2011. Effects of chloride ion on degradation of Acid Orange 7 by sulfate radical-based advanced oxidation process: implications for formation of chlorinated aromatic compounds. J Hazard Mater 196, 173-179. Zarei, A.R., Rezaeivahidian, H., Soleymani, A.R., 2015. Investigation on removal of p-nitrophenol using a hybridized photo-thermal activated persulfate process: Central composite design modeling. Process Saf Environ 98, 109-115. Zhang, M., Chen, X., Zhou, H., Murugananthan, M., Zhang, Y., 2015. Degradation of p-nitrophenol by heat and metal ions co-activated persulfate. Chem Eng J 264, 39-47.

24

Table caption: Table 1. Results of different Fe3O4/PS systems for the degradation of various organic pollutants.

25

Table 1. Results of different Fe3O4/PS systems for the degradation of various organic pollutants. Case

Target pollutant

Reaction conditions

η

TOC removal

Kinetic (min-1)

Refs

Fe3O4+PS

Landfill leachate

COD=780-1160 mg L-1, [PS]=3.5 g L-1, [Fe3O4]=1.5 g L-1, t=24 h.

-

61%

-

(Liu et al., 2018b)

Fe3O4+PMS+US

Acid orange 7 (AO7)

[AO7]=0.06 mM, [PMS/AO7]=50, [Fe3O4]=0.4 g L-1, US power=200 W, t=30 min.

90%*

54.1%

0.078

(Liu et al., 2017b)

BiOI/Fe3O4+PS+Vis

Rhodamine B (RhB)

[RhB]=20 mg L-1, [PS/RhB]=24, [BiOI/Fe3O4]=0.5 g L-1, t=30 min.

98.4%

44.9%

0.13

(Liu et al., 2018a)

CEMNPs+PS

Amoxicillin (AMX)

[AMX]=50 mg L-1, [PS/AMX]=585, [electrolyte]=50 mM， t=60 min.

72.6%

23.0%

0.022

(Sepyani et al., 2018)

Fe3O4+MW+PS

PNP

[PNP]=20 mg L-1, [PS/PNP]=15,

98.2%

53.2%

0.139

Present work

[Fe3O4]=0.1 g L-1, t=28 min. Note: * The data were evaluated from the references; η-degradation efficiency of target pollutant; PMS-peroxymonosulfate; US-ultrasound; CEMNPs-continuously electro-generated magnetite (Fe3O4) nanoparticles.

26

Figures captions: Fig. 1. Diagram of the preparation process for magnetic Fe3O4. Fig. 2. XRD (a), magnetic hysteresis loops (b), SEM with ×10K (c) and ×22K magnification (d), N2 adsorption-desorption isotherms (e), and pore diameter distributions (f) of synthesized magnetic Fe3O4. Fig. 3. The degradation efficiencies of PNP (a) and reaction rates (b) using various systems. The degradation abilities under different microwave temperatures were examined with and without the addition of Fe3O4 (c). The effect of Fe3O4 dose on PNP degradation (d). Fig. 4. Degradation efficiencies of PNP (a) and persulfate decomposition (b) in the quenching experiment. EPR detection spectra of different systems (c). Fig. 5. High-resolution XPS of Fe 2p before and after reaction. Fig. 6. Effects of initial pH (a, b), co-existing anions ([anions] = 5 mM) (c, d), and HA (e, f) on PNP degradation in the Fe3O4/MW/PS system. Reaction conditions: [PNP] = 20 mg L-1, [PS]/[PNP] = 15, [Fe3O4] = 0.1 g L-1. Fig. 7. Degradation efficiencies of PNP (a) and the evaluation of pH (b) using different water matrices in the Fe3O4/MW/PS system. The degradation efficiencies of PNP (c) and TOC removal (d) using various [PS]/[PNP] in the Songhua River.

27

Fig. 1. Diagram of the preparation process for magnetic Fe3O4.

28

100

(a)

(b)

Magnetization (emu/g)

Relative intensity (a.u.)

80

magnetite Fe3O4

PDF 11-0614

60 40 20 0 -20 -40 -60 -80

-100

10

20

30

40

50

o

60

70

80

-15000 -10000 -5000

90

0

5000 10000 15000

Applied magnetic field (Oe)

2 Theta ( )

(c)

(d)

-4

(e)

Å

(f)

-4

1.0x10

3

25

1.2x10

Pore volume (cm /g⋅ )

3

Quantity Adsorbed (cm /g STP)

30

20 15 10 5 0

-5

8.0x10

-5

6.0x10

-5

4.0x10

-5

2.0x10

0.0 0.2

0.4

0.6

0.8

1.0

0

Relative Pressure (P/P0)

500

1000

1500

Å

0.0

2000

2500

Pore diameter ( )

Fig. 2. XRD (a), magnetic hysteresis loops (b), SEM with ×10K (c) and ×22K magnification (d), N2 adsorption-desorption isotherms (e), and pore diameter distributions (f) of synthesized magnetic Fe3O4.

29

(a)

0.139

0.14

Reaction rate (min )

80

-1

PNP Removal (%)

100

60 Fe3O4+MW

40

Fe3O4+PS PS+MW Fe3O4+PS+MW

20

(b)

0.12 0.10

(1) Fe3O4+PS

0.08

(2) Fe3O4+PS+MW

0.06 0.0436 0.04 0.02

0 0

5

10

15

20

25

0.00

30

1

Time (min) 100 60

blank blank blank

catalyst catalyst catalyst

100

(c)

80

PNP Removal (%)

PNP Removal (%)

70 80

2

Different systems

60 40 20 0

(d)

80 60

-1

40

0.01 g⋅L -1 0.05 g⋅L -1 0.1 g⋅L -1 0.25 g⋅L -1 0.5 g⋅L Control

dosage of Fe3O4

20 0

0

5

10

15

20

25

30

Time (min)

0

5

10

15

20

25

30

Time (min)

Fig. 3. The degradation efficiencies of PNP (a) and reaction rates (b) using various systems. The degradation abilities under different microwave temperatures were examined with and without the addition of Fe3O4 (c). The effect of Fe3O4 dose on PNP degradation (d).

30

100

(a)

[TBA]/[PS]=10 [TBA]/[PS]=50 [TBA]/[PS]=100 Control

80

PS Decomposition (%)

PNP Removal (%)

100

60 40 20

[MeOH]/[PS]=10 [MeOH]/[PS]=50 [MeOH]/[PS]=100

0 5

10

15

20

25

30

MeOH

TBA

(b)

80 60 40 20 0

0

Blank

Control 10/1

50/1 100/1 10/1

50/1 100/1

System with differnet quenchers

Time (min)

(c) Relative Intensity (a.u.)

Fe3O4/MW/PS

MW/PS

Fe3O4/MW Fe3O4/PS 3440 3450 3460 3470 3480 3490 3500 3510 3520

Magnetic field (G) Fig. 4. Degradation efficiencies of PNP (a) and persulfate decomposition (b) in the quenching experiment. EPR detection spectra of different systems (c).

31

Relative intensity (a. u.)

Fe2p

2+

2+

Fe

3+

Fe

Fe 3+

Fe

before reaction

after reaction

740

735

730

725

720

715

710

Binding energy (eV)

705

700

Fig. 5. High-resolution XPS of Fe 2p before and after reaction.

32

(a)

6

PNP Removal (%)

80

(b)

5

pH fin

60 pH 3.0 pH 5.0 pH 7.0 pH 9.0 pH 11.0

40 20 0

70

4

60

pHfin

3

50

2

40

1

30

0

0

5

10

15

20

25

20 3.0

30

Time (min) 100

80

PS Decomposition (%)

(c)

10

PNP Removal (%)

80

5.0

40

pH

Control Cl 2SO4

7.0

9.0

System with different pHini

11.0

(d)

8

60

PS Decomposition (%)

100

pHini pHfin

6 4

2-

CO3

20

-

2

NO3 -

H2PO4

0

0 0

5

10

15

20

25

30

Time (min)

Cl

2-

SO4

-

2-

NO3

CO3

-

H2PO4

Systems with different co-existing ions

(e)

80

PS Decomposition (%)

PNP Removal (%)

100

-

Control

80 60 Control -1 2.5 mg⋅L -1 5.0 mg⋅L

40 20

(f)

60

40

20

0 0

0

5

10

15

20

25

30

Time (min)

Control

-1

2.5 mg⋅L

-1

5.0 mg⋅L

Systems with differnet HA addition

Fig. 6. Effects of initial pH (a, b), co-existing anions ([anions] = 5 mM) (c, d), and HA (e, f) on PNP degradation in the Fe3O4/MW/PS system. Reaction conditions: [PNP] = 20 mg L-1, [PS]/[PNP] = 15, [Fe3O4] = 0.1 g L-1.

33

(a)

7

(b) pHini (1) Distilled water (2) Tap water pHfin (3) Songhua River

6

80

5 60

pH

PNP Removal (%)

100

40

3

Distilled water Tap water Songhua River

20

4

2 1

0 5

10

15

20

25

0

30

1

Time (min)

80 60 40 Control [PS]/[PNP]=15/1 [PS]/[PNP]=20/1 [PS]/[PNP]=25/1

20 0

80

5

10

15

20

25

30

100

(d)

80

60

60

40

40

20

20

0

0

3

100

(c)

PNP Removal (%)

PNP Removal (%)

100

2

Systems with different water matrices

Control

15/1

20/1

25/1

TOC Removal (%)

0

0

Songhua River with different [PS/PNP]

Time (min)

Fig. 7. Degradation efficiencies of PNP (a) and the evaluation of pH (b) using different water matrices in the Fe3O4/MW/PS system. The degradation efficiencies of PNP (c) and TOC removal (d) using various [PS]/[PNP] in the Songhua River.

34

Highlights Magnetite (Fe3O4) was successfully prepared with the solvothermal method. Fe3O4 combined with microwave could efficiently activate persulfate. SO4-• was considered the major active radical. The Fe3O4/PS/MW system performed well in actual water (Songhua River).