Enhanced Permanganate Oxidation of Sulfamethoxazole and

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Remediation and Control Technologies

Enhanced Permanganate Oxidation of Sulfamethoxazole and Removal of Dissolved Organics with Biochar: Formation of Highly Oxidative Manganese Intermediate Species and in-situ Activation of Biochar Shi-Qi Tian, Lu Wang, Yu-Lei Liu, Tao Yang, Zhuangsong Huang, Xian-Shi Wang, Hai-Yang He, Jin Jiang, and Jun Ma Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00180 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 21, 2019

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Environmental Science & Technology

1

Enhanced

Permanganate

Oxidation

of

2

Sulfamethoxazole and Removal of Dissolved

3

Organics with Biochar: Formation of Highly

4

Oxidative Manganese Intermediate Species and

5

in-situ Activation of Biochar

6 7 8

Shi-Qi Tian1, Lu Wang1*, Yu-Lei Liu2, Tao Yang1, Zhuang-Song Huang1, Xian-Shi

9

Wang1, Hai-Yang He1, Jin Jiang1, Jun Ma1*

10 11

1

12

Environment, Harbin Institute of Technology, Harbin 150090, China

13

2

14

Technology, Dongguan 523808, China

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

Technology R & D Center for Environmental Engineering, Dongguan University of

15 16 17

*

18

*

19

*

Corresponding authors: Lu Wang, Phone/ Fax: 86 451 86283010; e-mail: [email protected];

Jun Ma, Phone/ Fax: 86 451 86283010; e-mail: [email protected];

20 21

ACS Paragon Plus Environment


22

Abstract

23

Sulfamethoxazole (SMX) is a broad-spectrum antibiotic and was largely used in

24

breeding industry. The reaction rate of SMX with KMnO4 is slow, and the adsorption

25

efficiency of biochar for SMX was inferior (less than 11% in 30 min). By adding

26

biochar powder into SMX solution with addition of permanganate, the oxidation ratio

27

of SMX surged to 97% in 30 min, and over 58% of the total organic carbon (TOC) was

28

simultaneously removed. KMnO4 interacted with biochar and resulted in the formation

29

of highly oxidative intermediate manganese species, which transformed SMX into

30

hydrolysis products, oxygen-transfer products, and self-coupling products. Brunauer-

31

Emmett-Teller (BET) analysis showed that surface area, total pore volume, and

32

micropore volume of biochar increased by 32.1%, 36.4%, and 80.6%, respectively, after

33

reaction process. This in situ activation of biochar with KMnO4 enhanced its adsorption

34

capacity and led to great improvement of TOC removal. Besides KMnO4 oxidation,

35

biochar also enhanced TOC removal in Mn(III) oxidation (KMnO4 + bisulfite) and

36

ozonization of SMX. Considering that KMnO4 could react with biochar and result in

37

the formation of intermediate manganese species, while biochar can be simultaneously

38

activated and exhibit high capacity for organic adsorption, the combination of biochar

39

with the chemical/advanced oxidation could be a promising process for the removal of

40

environmental pollutants.

41


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42

1. Introduction

43

Permanganate (KMnO4) is a stable, low cost, and easy-to-use oxidant. KMnO4 has

44

been applied for decades on the control of dissolved manganese, taste/odor compounds,

45

algae and emerging micropollutants in water treatment and environmental remediation

46

1, 2

47

rich moieties. This property limits the application of KMnO4 on organic pollutants

48

control, and many investigations focus on exploring relevant methods for enhancing

49

the oxidation capacity of KMnO4.

. KMnO4 is a selective oxidant and tends to react with organics containing electron-

50

Previous studies showed that in the reaction of high valent metal-oxo oxidants

51

(such as permanganate and ferrate) with organics, highly reactive intermediate species

52

would be formed

53

forms, but unstable. They would swiftly interact with organics, ligands or self-decay

54

and then transform into stable products. Jiang et al. found that ligands (such as

55

phosphate and humic acid) could stabilize intermediate Mn species and accelerate the

56

removal of endocrine disrupting chemicals in surface water 5, 6. Sun et al. reported that

57

Mn(III) may be the dominant species in the system where KMnO4 reacting with

58

bisulfite, which are responsible for the oxidation of organic pollutants at extraordinarily

59

high rates 7. Guo et al. revealed that Mn(V) peroxide and hydroxyl radicals may be

60

formed in ultraviolet (UV) irradiated KMnO4 solution and enhanced the oxidation of

61

micropollutants 4. Finding proper agents for improving the stability of intermediate

62

species could provide useful guidance for environmental remediation.

63

3-5

. These intermediate species are more reactive than their parent

For the elimination of organic pollutants from source water, oxidants could



64

effectively transform the chemical structure of target pollutants and decrease their

65

toxicity 8. However, the oxidation products still exist in water and contribute to the total

66

content of dissolved organics in the system. Some oxidation products are small

67

molecular weight organics and easy to be bio-consumed 9. This may influence the

68

microbial stability of the treated water. Some products may act as disinfection-precursor

69

in water treatment and lead to the formation of disinfection by-products 10. The residual

70

organics may also impact the aesthetic properties of treated water such as odor and color

71

9

. Enhanced removal of dissolved organics could improve the quality of treated water.

72

Biochar is prepared by the pyrolysis of organic materials (straw, bark, wood,

73

sludge, peat etc.) at 400–500 ℃ to drive off volatile components and leave carbon

74

behind 11. Due to low pyrolysis temperature and no further treatment, the pore volume

75

and the surface area of biochar are not high, and the adsorption capacity of biochar is

76

inferior. Biochar is normally used as soil conditioner in agriculture and as a carbon

77

sequestration method against carbon dioxide release 12. Another similar carbon material

78

is activated carbon, which is made by carbonization of carbonaceous source materials

79

at 600–900 ℃ and subsequent activation (including chemical activation and physical

80

activation) of the carbonized material. Activation is the dominant step for the formation

81

of micropores in activated carbon, and the amount/volume of micropore is directly

82

correlated with the surface area and the adsorption capacity of carbon material.

83

Activated carbon is extensively used in water/air purification while biochar is mainly

84

used as soil-conditioner. However, compared with activated carbon, biochar is easy to

85

prepare, low cost and environment friendly. Previous studies also found that biochar


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86

can be used as the reductant for catalyzing the degradation of organic contaminants. Xu

87

et al. found that in black carbon/sulfide system, reductive transformation of

88

nitroglycerin was involved with electron transfer from sulfides to target contaminant

89

through conductive carbon regions 13. Fang et al. revealed that biochar could activate

90

persulfate to produce more sulfate radicals (SO4•–) to degrade polychlorinated biphenyls

91

14

92

method for improving environmental remediation technologies.

. Exploring the potential of biochar for eliminating pollutants could be a practical

93

Sulfamethoxazole (SMX) was used in breeding industry as a broad-spectrum

94

antibiotic due to its cheapness. Previous studies estimated that SMX is one of the mostly

95

consumed veterinary drugs in China 15. SMX is biodegradable, while it takes long time

96

(weeks to months) to accumulate the reactor microbe

97

normally ranged from 70 to 150 ng/L in natural waters, and from 200 to 2000 ng/L in

98

wastewater treatment plant (WWTP) effluents 15, 17-19. SMX could be bio-accumulated

99

through food chains

20

16

. The concentration of SMX

, and it may influence the balance of microbial flora in

100

environment. Relevant remediation methods should be developed to eliminate SMX in

101

WWTP effluent and source water. Herein, the effect of biochar on KMnO4 oxidation of

102

SMX was explored. The variation of SMX and KMnO4 content was determined in the

103

reaction process, and the dominant species for SMX oxidation were recognized. After

104

that, removal of TOC in the system was examined, and transformation pathway of SMX

105

was studied. The properties of biochar before and after reaction process were analyzed.

106

Combining these results, the mechanism of biochar on enhancing KMnO4 oxidation of

107

SMX and the removal of TOC in the system was elucidated.



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108 109

2. Material and methods

110

2.1 Chemicals

111

SMX

(99%)

and

2,2-azino-bis(3-ethylbenzothiazoline)-6-sulfonic

acid

112

diammonium (ABTS, > 98% purity) were obtained from Sigma Aldrich (Germany).

113

KMnO4 (99.5%) was purchased from Sinopharm Chemical Reagent (China). A

114

commercial biochar powder was purchased from Shanghai Furui Chemical Industry

115

Co.Ltd (Shanghai, China) and the detail parameters will be shown in the next section.

116

All other chemicals were at least of analytical grade and used without further

117

purification. All solutions were prepared with deionized (DI) water (18.2 MΩ/cm)

118

produced by a Milli-Q purification system (Millipore, Billerica, MA). Stock solutions

119

of KMnO4 were freshly prepared by dissolving weighed amounts of KMnO4 in DI water,

120

and standardized by ABTS method

121

solution was prepared following the procedure described as the modified Murray’s

122

method by mixing the appropriate amounts of MnSO4 and KMnO4 stock solution 22.

123

2.2 Oxidation experiment

21

. A stable particulate manganese dioxide stock

124

Oxidation experiment was carried out in 100 mL glass conical bottles in water bath

125

at 25 ± 1 °C under magnetic stirring (500 r/min). Reactions were initiated by adding

126

SMX and permanganate simultaneously to pH buffered (10 mM borate buffer) solutions

127

containing biochar. At given time intervals, 1.0 mL of the solution was sampled and

128

filtered through a glass fiber membrane of 0.22 μm pore size, and then added into a 2.0

129

mL vial containing 10 μL of 1 M hydroxylamine hydrochloride.


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130

Besides KMnO4, Mn (III) [prepared by reduction of KMnO4 with bisulfite] and O3

131

were also used for the oxidation of SMX in the absence/presence of biochar. For

132

oxidation of SMX by Mn(III) (KMnO4 + bisulfite), the solutions were adjusted to pH

133

5.0 by adding HCl or NaOH, and SMX (10 μM) was first mixed with 500 μM NaHSO3

134

solution with/without biochar (50 mg/L). Reactions were initiated by adding Mn(VII)

135

(100 μM) into the above mixture. O3 stock solution was prepared by sparging

136

oxygen/ozone mixture gas in 4 °C deionized water, and quantified at 260 nm (ε = 3200

137

M-1 cm-1). Then 5 mg/L of O3 stock solution and SMX were added into 10 mM borate

138

buffer solution of pH 7.0 in the absence/presence of biochar. After 30 min reaction,

139

samples were withdrawn and filtered for TOC measurement. Hydroxylamine

140

hydrochloride was used for quenching the residual intermediate manganese species.

141

Pure N2 was purged into relevant solution samples to drive off residual O3.

142

KMnO4 oxidation of SMX in the presence of biochar under actual raw water

143

background condition was carried out in ground water and surface water. The ground

144

water sample was taken from a well of Mopanshan reservoir in Harbin, China (TOC =

145

2.3 mg C/L, alkalinity =103 mg/L as CaCO3, and pH 7.6). Another surface water sample

146

was taken from Songhua River of Harbin, China (TOC = 7.8 mg C/L, alkalinity =230

147

mg/L as CaCO3, and pH 7.3). The water samples were filtered through the glass fiber

148

filters and stored at 4 °C prior to use.

149

2.3 Characterization

150

Biochar samples before and after reaction were characterized to explore the reaction

151

mechanism. After 30 min reaction of SMX degradation in KMnO4/biochar system, the



152

suspending biochar was centrifugal-separated, washed 3-6 times with deionized water

153

until the pH stabilized, and then it was dried at 60 ℃ for 12 h. The average surface

154

areas of biochar samples were measured by the Brunauer-Emmett-Teller method (BET)

155

using nitrogen adsorption-desorption isotherm measurements at 77 K on a surface area

156

and porosity analyzer (ASAP 2020, Micromeritics, USA). Elemental C, H, O, N, S

157

abundances of biochar samples were determined using a vario EL II elemental analyzer

158

(Elementar, Germany). To examine the variation of surface functional groups and the

159

chemical state of Mn on biochar surface before and after reaction process, aliquots of

160

samples were characterized by Fourier transform infrared spectrometer (FTIR,

161

Spectrum 100, PerkinElmer, USA) and X-ray photoelectron spectrometer (XPS,

162

ESCALAB 250Xi, Thermo Scientific, USA). The XPS spectra was measured on a PHI

163

5700 ESCA System using Al Kα radiation (1486.6 eV). Fourier transform infrared

164

spectroscopy (FTIR) analysis of biochar samples was conducted on a PerkinElmer

165

Spectrum One FTIR. Biochar samples were diluted to a concentration of 2% with IR-

166

grade KBr. FTIR spectra was collected at 4 cm-1 resolution in the IR region of 4000–

167

400 cm-1 for pure KBr and the samples.

168

2.4 Analytical methods

169

Concentration of SMX was determined by Waters 2695 series high-performance

170

liquid chromatography (HPLC) (2695, Waters, USA) at wavelength of 300 nm with a

171

flow rate 1.0 mL/min. The mobile phase was composed by 0.1% aqueous acetic acid

172

and methanol (60:40, v/v). The concentration of ABTS used to reflect the content of

173

permanganate was measured by a UV–vis spectrometer at 415 nm (2550, Shimadzu,


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174

Japan). The TOC content of solution samples was determined by TOC-VCHS (Kyoto,

175

Shimadzu, Japan).

176

Degradation products of SMX were analyzed by an ABSciex QTrap 5500 MS

177

coupled with an Agilent 1260 HPLC under ESI positive ionization mode with the

178

HPLC/ESI-QQQ mass spectrum analysis. A Waters XBridge C18 column (2.5 μm

179

particle size, 3.0 × 100 mm) was used for separation. The gradient mobile phase was

180

consisted of acetonitrile/water (v/v, 80/20) at a flow rate of 0.2 mL/min, and the

181

injection volume of each sample solution was 10 μL. MS instrumental parameters were

182

optimized and set as follows: ion spray voltage: +5500 V; source temperature, 450 °C;

183

collision cell exit potential (CXP), 18 V; declustering potential (DP), 90 V; entrance

184

potential (EP), 10 V; curtain gas 35 arbitrary units; gas I: 50 arbitrary units, gas II: 50

185

arbitrary units.

186 187

3 Results and discussion

188

3.1 Effect of biochar on KMnO4 oxidation of SMX

189

When 100 μM of KMnO4 reacted with 10 μM of SMX for 30 min, less than 10%

190

of SMX was oxidized (Figure 1A). This result is in accordance with previous study,

191

that KMnO4 alone could not effectively oxidize SMX

192

selectively attacking unsaturated bonds of emerging pollutants such as steroid estrogens

193

and phenols 6. However, SMX is composed by benzene sulfinic part and oxazole part,

194

and both of them are recalcitrant.

195

23

. KMnO4 has advantages on

Similar to the case in KMnO4 oxidation group, less than 5% of SMX was removed



196

by biochar powder (Figure 1A). Theoretically, biochar was prepared by the pyrolysis

197

(400–500 ℃) of biomass, while activated carbon was prepared by the carbonization

198

(600–900 ℃) of carbonaceous source materials (bamboo, coconut husk, wood, coal,

199

pitch etc.) and subsequent activation (including chemical activation and physical

200

activation) of the carbonized material. Activation is the main process for the formation

201

of micropores in activated carbon, and high surface area is the main reason for the

202

adsorption capacity of activated carbon. Compared with activated carbon, biochar is

203

easy to be prepared but its adsorption capacity is much limited. In the experiment,

204

biochar powder removed less than 10% of SMX.

205

Interestingly, when KMnO4 and biochar powder were simultaneously added into

206

the SMX solution, almost complete removal of SMX (> 97%) was achieved within 30

207

min. Reaction dynamic could be fitted well (R2 > 0.996) with pseudo-first-order

208

kinetics law, and the determined apparent rate constant (kobs) is 0.1626 min-1

209

(Supporting Information, Figure S1). Considering that less than 10% of SMX was

210

removed in 30 min in the control groups (KMnO4 alone and biochar alone) respectively,

211

the effect of KMnO4/biochar system for the elimination of SMX was obvious.

212

Biochar contains organic functional groups and KMnO4 is a strong oxidant.

213

Biochar may interact with KMnO4 and consume its oxidation capacity. Stability of

214

KMnO4 in different systems was examined (Figure 1B). No obvious depletion of

215

KMnO4 was observed within 30 min in the solution of KMnO4 spiked with SMX. This

216

result further suggested that the reaction of KMnO4 with SMX was sluggish. In

217

KMnO4/biochar system, less than 15% of KMnO4 was decreased in 30 min. This


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218

indicated that biochar would interact with KMnO4 but the reaction rate was slow, and

219

oxidation capacity of KMnO4 would not be rapidly consumed. In another experiment,

220

it was found that after blending of KMnO4 with biochar for 12h, 24h, 36h, and 48h, the

221

oxidation percentage of SMX decreased from 97% to 89%, 83%, 69%, and 53%,

222

respectively (Figure 1C). Long time of interaction of KMnO4 with biochar negatively

223

impacted the removal of SMX. Detailed mechanism would be revealed in section 3.6

224

(Reaction mechanism).

225

KMnO4 was stable in KMnO4/biochar system, while after the addition of SMX,

226

purple color of KMnO4 swiftly shifted into pink color of MnO2 in the system (Figure

227

1B). Around 58% of KMnO4 was consumed in the process. It seemed that after addition

228

of organic pollutants into KMnO4/biochar system, the oxidation capacity of KMnO4

229

was gradually released. Detailed mechanism would be revealed in section 3.6 (Reaction

230

mechanism).

231

3.2 Possible effects of MnO2 and cations on the oxidation of SMX by KMnO4

232

Previous studies reported that MnO2 could catalyze the KMnO4 oxidation of

233

organics 24. MnO2 itself is also an oxidant (standard reduction potential = 0.464 V) and

234

can react with organics

235

may participate in the elimination of SMX. Effects of different dosage of MnO2 (50 μM

236

and 100 μM) on the KMnO4 oxidation of SMX were investigated (Figure 2A).

237

Generally, MnO2 showed no obvious effect on the oxidation of SMX (< 4%), and SMX

238

elimination percentage in KMnO4/MnO2 system was less than 5%. Around 10% of

239

SMX was eliminated in biochar/MnO2 group. Compared with that in KMnO4/biochar

25, 26

. MnO2 may be formed in KMnO4/biochar system and it



Page 12 of 35

240

group, the effects of MnO2 on enhancing KMnO4 oxidation and biochar adsorption were

241

not obvious. MnO2 was not the main reason for the enhanced removal of SMX in

242

KMnO4/biochar system.

243

Besides MnO2, metal ions may catalyze the formation of persistent free radicals in 14

244

biochar, and these radicals may participate in the oxidation of SMX

. Removal of

245

SMX in KMnO4 alone and KMnO4/biochar systems with the presence of Al, Fe, Cd,

246

and Cu (initial concentration: 10 μM) was separately investigated (Figure 2B and S2).

247

Generally, adding cations showed no obvious effect on improving the oxidation of SMX,

248

and SMX removal tendency in KMnO4/biochar system was not greatly influenced.

249

Metal ions in biochar were not the main reason for the enhanced oxidation of SMX in

250


251

Organics could act as electron shuttles in chemical and biological systems and

252

enhance the oxidation of pollutants 27. Previous studies showed that HA could enhance

253

the oxidation capacity of KMnO4 by forming intermediate Mn species, and the in-situ

254

formed intermediate Mn species showed strong oxidation capacity

255

also act as reductant and contribute for the formation of oxidative intermediate Mn

256

species. To confirm this speculation, some biochar powder was ozonized for an hour

257

(O3 content = 20 mg/L) to fully oxidize function groups of biochar. When the ozonized

258

biochar powder was added into the solution containing SMX and KMnO4, around 16%

259

of SMX was eliminated (Figure 2C), much lower than that in biochar group (Figure

260

1A). This indicated that reduction property of biochar (reduction groups on biochar)

261

may be the main reason for the enhanced removal of SMX in KMnO4/biochar system.


5, 6

. Biochar may

Page 13 of 35

262


3.3 Formation of intermediate manganese species in KMnO4/biochar system

263

Previous studies reported that intermediate Mn species could be formed in the

264

reduction of KMnO4, and they have strong capacity for the oxidation of organic

265

contaminants 5, 7. Pyrophosphate could complex with intermediate Mn and Fe species

266

and negatively influence their oxidation capacity 3. Hence, pyrophosphate was used as

267

an indicator to study the role of intermediate Mn species in KMnO4 oxidation process.

268

When the content of pyrophosphate increased from 0 mM to 1 mM, percentage of SMX

269

oxidation decreased from 97% to 12% (Figure 3A). Even 200 μM of pyrophosphate

270

could make the percentage removal of SMX decrease to 70% around. Pyrophosphate

271

inhibited the removal of SMX in KMnO4/biochar system, and the inhibiting effect

272

increased with the elevation of pyrophosphate concentration. The intermediate Mn

273

species formed in the reaction process may be complexed by pyrophosphate and thus

274

the oxidation strength was inhibited 28. Intermediate Mn species may be main reason

275

for the enhanced removal of SMX in KMnO4/biochar system.

276

Mn(III) is one of the intermediate Mn species, and previous investigations

277

speculated that Mn(III) may have strong oxidation capacity for organic pollutants 7, 29.

278

Under UV irradiation, Mn(III)-complex would absorb energy and show a characteristic

279

absorbance peak 7. UV-visible spectrum could be an effective way for studying the

280

existence

281

spectrophotometer, it was found that (1), no absorbance peak appeared in SMX solution

282

with and without the presence of biochar (Figure S3); (2), no absorbance peak appeared

283

in KMnO4 solution without the presence of biochar (Figure 3B); (3) an absorbance peak

of

Mn(III). After

analyzing

solution


samples

with

UV-visible


284

appeared at 231 nm in KMnO4 solution with the presence of biochar (Figure 3B).

285

Previous studies revealed that the characteristic absorbance peak of Mn(III)-complex

286

would be influenced by the properties of ligand. For example, the absorbance peaks of

287

Mn(III)-pyrophosphate and Mn(III)-quinone complexes were at 258 nm and 330 nm,

288

respectively

289

spectra of KMnO4/biochar solution with the presence of 5 mM of pyrophosphate

290

(Figure S4). We speculate that the absorbance peak appearing at 231 nm represents

291

intermediate Mn species formed on the surface of suspended biochar powder in

292

KMnO4/biochar system, while the valence of Mn was uncertain.

28, 30, 31

. However, no obvious peak at 258 nm was observed in full scan

293

FTIR, XPS, and elemental composition analyses were further carried out to study

294

the chemical properties of biochar powder before and after reaction process

295

(biochar/used biochar) in KMnO4/biochar system. Generally, both raw and used biochar

296

showed FTIR peaks at 3409 cm-1, 1610 cm-1, 1087 cm-1 and 1045 cm-1 position (Figure

297

4A). These peaks could be assigned to -OH (of H2O), C=O, phenolic-OH, and C-O-C

298

groups. Compared with the raw biochar, the intensity of C=O bond of the used biochar

299

in KMnO4/biochar system was more intense. This indicated that biochar was oxidized

300

in the reaction process and oxygen atoms may be transferred from KMnO4 to biochar.

301

For the used biochar, a new peak was observed at 520 cm-1 in comparison with the

302

spectra of raw biochar, which is possibly due to the formation of Mn-O bond 32. This

303

suggested that Mn species were loaded onto the surface of biochar after reaction process.

304

XPS was used to analyze biochar powder before and after reaction process. C1s

305

photoelectron spectrum of biochar could be deconvoluted into five signals attributed


Page 14 of 35

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306

to C=C, C-C, C-H (284.3 eV and 285 eV), C-OH (286 eV), C=O (287.6 eV) and

307

COOH (288.6 eV) (Figure 4C and 4D). Compared with raw biochar, spectrum of used

308

biochar showed a decrease of C=C proportion from 60.33% to 45.11%, whereas the

309

relative content of oxygen-containing groups (C-OH, C=O and COOH) increased

310

markedly from 7.67% to 13.25%, 2.61% to 3.98%, and 3.63% to 9.25% respectively.

311

This phenomenon was in agreement with FTIR analysis results, in which the amount

312

of hydroxyl, carbonyl and carboxylic groups increased in the interaction of KMnO4,

313

biochar and SMX. Moreover, an obvious peak at 642 eV for Mn(2p 3/2) was observed

314

in the wide-scan spectra of KMnO4/biochar (Figure 4B), and spectrum of Mn 2p 3/2

315

could be fitted with three peaks, corresponding to Mn(III) (642.03 eV) and Mn(IV)

316

(640.53 and 644.03 eV)

317

were formed on the surface of oxidized biochar in the reaction process.

318

33

. This result further revealed that intermediate Mn species

After analyzing the elemental composition of raw and used biochar (Figure 4E), it

319

was found that relative intensity of C element of biochar decreased from 97.9% to 81.8%

320

after reaction, while intensity of O element increased from 0.4% to 14.1%. This result

321

was in accordance with FTIR and XPS data, which biochar was partly oxidized by

322

KMnO4, leading to the formation of oxygen-containing groups (such as C=O, C-OH,

323

and COOH). Meanwhile, C, H, N, O, and S elements make up 99.9% of raw biochar,

324

while the total content of C, H, N, O, and S elements in used biochar decreased to 97%.

325

Considering that only biochar, SMX, electrolytes (buffer), and KMnO4 were added into

326

the system, and the used biochar was rinsed 3 to 6 times to remove unbonded substances

327

(such as electrolytes and dissolved organics). The difference of biochar elemental



328

composition indicated that the used biochar was partly composed by Mn, which may

329

come from the complexation of intermediate Mn species with biochar.

330

3.4 Improved removal of TOC

331

For the purification of polluted water, both elimination of dissolved organics and

332

degradation of hazardous organics should be critical steps for improving chemical and

333

microbial stability of treated water 34. TOC removal efficiencies in the reaction of SMX

334

with biochar, KMnO4, and KMnO4/biochar were separately determined (Figure 5A).

335

Surprisingly, the percentage of TOC removal in KMnO4/biochar system surpassed 58%,

336

while the percentage removal in biochar adsorption group and KMnO4 oxidation group

337

were 11% and 6%, respectively. Biochar not only enhanced KMnO4 oxidation of SMX,

338

but also largely improved removal of total organics in the whole system. Chemically,

339

increasing TOC removal ratio with oxidant is difficult: even radicals could not fully

340

mineralize organics in water, and the oxidation products still contribute to the solution

341

TOC. Adsorbents could remove organics from water, but they would interact with

342

background constituents (such as microbes, natural organic matters, cations, anions,

343

and suspended particles), and adsorption sites would be gradually filled. Enhanced

344

removal of TOC in KMnO4/biochar system can be a promising method for eliminating

345

organic pollutants and improving stability (chemical and biological) and quality of

346

water.

347

Besides KMnO4 oxidation, other oxidation processes also have difficulty for

348

eliminating dissolved organics. Similar with that in KMnO4 oxidation process, Mn(III)

349

and O3 showed limited effect (< 8%) on the removal of TOC in the reaction with SMX


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350

(Figure 5A). In comparison, the addition of biochar improved the removal efficiency of

351

TOC, and the percentage removal of TOC increased to 52.4% in [KMnO4 +

352

bisulfite]/biochar group and to 42.2% in ozonization/biochar group, respectively. It

353

seems that the combination of chemical oxidation with biochar makes the dissolved

354

organics easy to be captured.

355

The adsorption capacity of carbon material is correlated with its relative surface

356

area. BET analysis was conducted to compare the relative surface area and pore volume

357

of raw and used biochar (Figure 5B). Interestingly, BET surface area and Langmuir

358

surface area of used biochar increased from 86.3 m2/g to 114.1 m2/g, and from 119.8

359

m2/g to 158.3 m2/g, respectively. Compared with raw biochar, the surface area of used

360

biochar increased by 32.1%. On the other hand, the total pore volume of raw biochar

361

and used biochar was 0.260884 cm3/g and 0.355954 cm3/g, respectively; the micropore

362

volume of raw biochar and used biochar was 0.001679 cm3/g and 0.003032 cm3/g,

363

respectively (Figure S5 and S6). The total pore volume and micro pore volume of

364

biochar increased by 36.4% and 80.6%, respectively. The average pore width of biochar

365

increased from 120.8913 Å to 124.7876 Å. These data suggested that in the reaction

366

with KMnO4, large amount of micropore was formed in biochar. Since micropore is

367

critical for the adsorption of target pollutants with carbon material, great increasement

368

(80.6%) of micropore formation could be interpreted as a core reason for the enhanced

369

removal of TOC in the system (Figure 5A). This result also suggested that in the

370

reaction of biochar with KMnO4, biochar was in situ activated with the formation of

371

large amount of micropore. This process is similar to chemical activation of carbonized



372

material in the production of activated carbon, which result in the formation of

373

micropores.

374

3.5 Transformation pathway of SMX

375

By analyzing solution samples at different reaction time with HPLC/ESI-QQQ

376

mass spectrum, 7 possible transformation products were identified (Figure 6A and

377

Figures S7-S14). The HPLC peak at 22.04 min could be attributed to SMX, whose

378

concentration decreased as a function of time in the reaction process. Previous studies

379

reported that SMX would be hydrolyzed in chemical and biological degradation

380

processes with the formation of 4-amino benzene sulphinic acid and 3-amino-5-

381

methylisoxazole 16, 35, 36. The identified TP 174 could be attributed to 4-amino benzene

382

sulphinic acid, and TP 99 could be attributed to 3-amino-5-methylisoxazole (pathway

383

1). On the other hand, C-S bond may also be hydrolyzed in the reaction process, with

384

the formation of p-aminophenol (TP 110) and TP 163 (pathway 2).

385

Besides hydrolysis pathways, 3 products with m/z value higher than SMX (254.1

386

Da) were identified (268.0 Da, 284.3 Da, and 503.0 Da). After analyzing the ionization

387

pattern of these products, we speculate that TP 268 was an oxidation product formed in

388

the oxygen-transfer from intermediate Mn species to the amino group of SMX, and TP

389

284 was a product formed in the oxidation of TP 268. Amino group of SMX was

390

stepwise oxidized into nitro group in the reaction process (pathway 3).

391

Besides hydrolysis products and oxygen-transfer products, a product with m/z =

392

503.0 Da was identified at 28.97 min (Figure 6A). Molecular weight of SMX is 253 Da.

393

After analyzing the ionization pattern of TP 503, we speculate that it is a self-coupling


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394

product formed in the coupling of amino groups of two SMX molecules. Chemical

395

structure of TP 503 is proposed in Figure 6B (pathway 4). According to the intensity of

396

HPLC peak (Figure 6A), TP 503 may be a main product formed in the transformation

397

of SMX.

398

Previous study reported that in the permanganate oxidation of triclosan,

399

intermediate Mn(III) species would result in the formation of triclosan-dimer, while

400

other manganese intermediates would transform triclosan into 2,4-dichlorophenol and

401

other products 37. By analyzing the transformation products formed in KMnO4/biochar

402

system, not only self-coupling products (dimer) were identified, but also hydrolysis

403

products and oxygen-transfer products were identified. Hence, we speculate that

404

different intermediate Mn species may participate in the transformation of SMX in

405


406

3.6 Reaction mechanism

407

Based on above information, reaction mechanism of biochar assisted (enhanced)

408

KMnO4 oxidation of SMX is illustrated in Scheme 1. Generally, reactivity of KMnO4

409

with SMX is much lower. KMnO4 slowly reacted with reductive groups of biochar

410

(such as C-H, C=C, C=O) and was transferred into biochar-complexed intermediate Mn

411

species [Figure 3B, no free Mn(III) species were detected in solution sample]. The

412

intermediate Mn species complexed on biochar surface is more reactive than KMnO4,

413

and can readily oxidize SMX into hydrolysis products, oxygen-transfer products, and

414

self-coupling products (Figure 6). These transformation products were oxygen-

415

containing organics or larger molecular weight products that may be easy to be captured



416

by the Mn-loaded biochar through physical adsorption and hydrogen bond. On the other

417

hand, intermediate Mn species could react with reductive groups of biochar, and lead

418

to the formation of oxygen-containing functional groups (Figure 4).

419

The newly formed intermediate Mn species may also react with biochar and

420

corrode its physical structure (in situ activation). This process would lead to the

421

formation of micropores in biochar and increase its surface area and total pore volume,

422

which in turn largely improve adsorption capacity of biochar (Figure 5). If target

423

pollutants (SMX) were not added into the system in the initial stage, and KMnO4

424

reacted with biochar for a long time, the intermediate Mn species would be consumed

425

in the reaction with biochar, and the oxidation efficiency of SMX would decrease as a

426

function of blend time (Figure 1C). Besides KMnO4, O3 could also oxidize biochar and

427

change its physical structure. The TOC removal efficiency in ozonization of SMX

428

would be enhanced by in situ activated biochar either (Figure 5A).

429

3.7 Influencing factors

430

Influence of biochar dosage, KMnO4 concentration, re-used biochar, and authentic

431

waters on the removal of SMX in KMnO4/biochar system was systematically examined.

432

As the dosage of biochar varied from 25 mg/L to 100 mg/L, percentage oxidation of

433

10 μM of SMX increased from 87% to 100% with 100 μM of KMnO4 (Figure 7A). As

434

mentioned in above sections, merely 9% of SMX could be oxidized by KMnO4. Even

435

low dosage of biochar (25 mg/L) showed an obvious effect for the enhanced KMnO4

436

oxidation of SMX. Similarly, as 50 mg/L of biochar existed in the solution, 25 μM and

437

50 μM of KMnO4 could oxidize 77% and 93% of SMX (10 μM) in 30min, respectively


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438

(Figure 7B). Increasing the dosage of biochar or KMnO4 in the system could facilitate

439

the removal of SMX, and the optimum dosage of biochar and KMnO4 could be

440

determined in relevant operation processes based on the percentage removal of target

441

pollutants and operation cost.

442

When used biochar was re-added into the solution to enhance KMnO4 oxidation of

443

SMX, the percentage oxidation of SMX decreased with the increasement of biochar

444

used cycles (Figure 7C). This result is in accordance with the above proposed reaction

445

mechanism. KMnO4 would consume reductive groups of biochar, and leads to the

446

formation of biochar-complexed Mn(III) species. Reductive groups of reused biochar

447

were less than the raw biochar and this would impact the formation of intermediate Mn

448

species, thus attenuate the oxidation of SMX.

449

Determined oxidation rate of SMX in KMnO4/biochar system was 0.1626 min-1 in

450

deionized water. In comparison, the oxidation rate of SMX in a ground water from a

451

well and a surface water (from Songhua River) was 0.1948 min-1 and 0.0831 min-1,

452

respectively. Compared with that conducted in deionized water, the oxidation of SMX

453

was faster in the ground water from a well and was slower in surface water. Almost

454

100% of SMX was oxidized in the ground water from a well in 30 min, while only 80%

455

of SMX was oxidized in surface water. Excessive background constituents such as

456

natural organic matters and inorganic species may competitively react with KMnO4 or

457

interact with the reduction sites of biochar, and thus impact the oxidation of SMX. For

458

the treatment of authentic water, optimizing the dosage of added KMnO4 and biochar is

459

a critical step for improving the elimination of pollutants.



460

4 Environmental Implications

461

KMnO4 received enduring interests from both research and application aspects as

462

an environmental remediation agent. KMnO4 is stable compared with other oxidants

463

such as HClO, H2O2, ozone and ferrate. This property makes KMnO4 easy to be

464

produced, stored and delivered. However, the stability of KMnO4 in turn hindered its

465

reactivity with target pollutants in water treatment, and the methods such as adding

466

bisulfite

467

pollutants. Herein, we demonstrated that by adding biochar powder, the percentage

468

oxidation of SMX with KMnO4 could be elevated from 9% to 97%.

7

and UV irradiation

4

were explored for enhancing the removal of organic

469

Conventional oxidation processes mainly focused on the transformation of target

470

pollutants into lower molecular weight products, while the reaction process would be

471

influenced by various environmental factors such as reaction time, oxidant dosage,

472

solution temperature and co-existing background constituents. Accurate transformation

473

pathway of target pollutants in actual treatment procedures was difficult to fully

474

revealed. Toxic degradation products may be formed under certain conditions.

475

Meanwhile, the lower molecular weight products formed in degradation process would

476

still contribute to the total organic content in water, and negatively influence the

477

chemical and microbial stability of the treated water. Ultimately removing dissolved

478

organics from water could effectively improve the quality and the stability of treated

479

water, but it is always costly.

480

In this study, we showed that adding biochar not only improved the oxidation

481

capacity of KMnO4, but also facilitated the removal of dissolved organics in water.


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482

KMnO4 oxidation and biochar adsorption removed 6% and 11% of solution TOC

483

respectively, while the combination of KMnO4 and biochar removed over 58% of TOC

484

in the system. More importantly, biochar could be in situ activated in the oxidation

485

process, with the formation of micropore and increasement of surface area and pore

486

volume. Biochar not only enhanced the TOC removal in KMnO4 oxidation process, but

487

also improved TOC removal in intermediate Mn oxidation (KMnO4 + bisulfite) and

488

ozonization processes. Oxidation of SMX leads to the formation of larger molecular

489

weight products (self-coupling products) and OH-, COOH- containing products. These

490

products may be easily captured by Mn-loaded biochar (formed in KMnO4/biochar

491

system) through chemical and physical adsorption. Considering that biochar is lower

492

cost and easier to be prepared than activated carbon, the combination of KMnO4 or O3

493

with biochar powder is promising for the elimination of organic pollutants from

494

polluted water.

495 496

497 498

Supporting Information

The supporting information including 11 figures is available free of charge on the ACS Publications website.

499 500

Acknowledgement

501

This work was financially supported by the National Key R&D Program of China

502

(2017YFA0207203), the National Natural Science Foundation of China (NSFC,



503

51808163), and the Major Science and Technology Program for Water Pollution

504

Control and Treatment (2017ZX07201003-03).

505 506

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507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542

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Abstract picture



Figure 1. Variation of SMX content in the reaction with biochar, KMnO4, and KMnO4/biochar (A), and the depletion of KMnO4 in different systems (B). Inlet photo is about the color of solution samples of KMnO4/biochar and KMnO4/biochar + SMX with and without the presence of biochar. Reaction of KMnO4 with biochar is slow and KMnO4 is stable in the system (B3). After the addition of SMX, KMnO4 was readily reduced into MnO2 in 30 min (B4). Effect of blend time on the elimination of SMX in the system (C). Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar]0 = 50 mg/L, solution pH = 7.0, T = 25 ℃.


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Figure 2. Effects of MnO2 on the KMnO4 oxidation and biochar adsorption of SMX (A); effects of cations on the KMnO4 oxidation of SMX (B); and effects of biochar and ozonized biochar on the KMnO4 oxidation of SMX (C). Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar] = 50 mg/L, solution pH = 7.0, T = 25 ℃.



Figure 3. Effects of different concentration of pyrophosphate on the removal of SMX in KMnO4/biochar system (A), and UV-visible spectrum of different solution samples in KMnO4/biochar system (B). Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar] = 50 mg/L, solution pH = 7.0, T = 25 ℃.


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Figure 4. FTIR result about the biochar before and after reaction with KMnO4 (biochar/used biochar) (A); high resolution XPS spectra of Mn2p of biochar after reaction with KMnO4 (B); high resolution XPS spectra of C1s of biochar before and after reaction with KMnO4 (C and D); and elemental composition of biochar before and after reaction with KMnO4. Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar] = 50 mg/L, solution pH = 7.0, T = 25 ℃.



Figure 5. Removal ratio of TOC in biochar adsorption, KMnO4 oxidation, KMnO4 + biochar, KMnO4 + bisulfite oxidation [Mn(III)], KMnO4 + bisulfite + biochar, ozonization, and O3 + biochar systems (A), and BET analysis result about the raw biochar and biochar reacted with KMnO4 (B). Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar] = 50 mg/L, [bisulfite]0 = 500 μM, [O3] = 5 mg/L, T = 25 ℃.


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Figure 6. HPLC/ESI-QQQ XIC chromatograms of solution samples at different reaction time (A), and proposed oxidation pathway of SMX in the reaction process (B). Experimental condition: [SMX]0 = 10 μM, [KMnO4]0 = 100 μM, [biochar] = 50 mg/L, solution pH = 7.0, T = 25 ℃.



Scheme 1. Proposed reaction mechanism of biochar with KMnO4 and SMX. KMnO4 reacted with reductive groups on biochar and resulted in the formation of highly reactive intermediate Mn species [Mn(int)]. Biochar was oxidized in the process, leading to the increasement of surface area, total pore volume, and micropore volume, which can be recognized as “in situ activation”. SMX was oxidized by Mn(III), and the transformation products were subsequently adsorbed by the activated biochar.


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Figure 7. Effect of biochar dosage on SMX oxidation with KMnO4/biochar ([KMnO4]0 = 100 μM, [SMX]0 = 10 μM, pH = 7.0) (A); effect of permanganate concentration on SMX oxidation with KMnO4/biochar ([biochar]0 =50 mg/L, [SMX]0 = 10 μM, pH = 7.0) (B); performance of reused biochar on SMX oxidation with KMnO4/biochar ([KMnO4]0 = 100 μM, [SMX]0 = 10 μM, [biochar]0 =50 mg/L, pH = 7.0) (C); and oxidation of SMX with KMnO4/biochar in authentic water samples ([KMnO4]0 = 100 μM, [SMX]0 = 10 μM, [biochar]0 =50 mg/L).


Enhanced Permanganate Oxidation of Sulfamethoxazole and

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