Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced

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Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced Transformation of Triclosan by Mn(VII) in the Presence of Ligands? Yuan Gao, Jin Jiang, Yang Zhou, Su-yan Pang, Chengchun Jiang, Qin Guo, and Jie-Bin Duan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b00120 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Environmental Science & Technology

1

Does Soluble Mn(III) Oxidant Formed in Situ Account

2

for Enhanced Transformation of Triclosan by Mn(VII)

3

in the Presence of Ligands?

4 5

Yuan Gaoa, Jin Jianga,*, Yang Zhoua, Su-Yan Pangb, Chengchun Jiangc, Qin Guod, and

6

Jie-Bin Duand

7

a

8

Environment, Harbin Institute of Technology, Harbin 150090, China

9

b

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

School of Municipal and Environmental Engineering, Jilin Jianzhu University,

10

Changchun 130118, China

11

c

12

518055, China

13

d

14

and Technology, Harbin 150040, China

15

*Corresponding Author: Prof. Jin Jiang, E-mail: [email protected]

School of Civil and Environmental Engineering, Shenzhen Polytechnic, Shenzhen

College of Chemical and Environmental Engineering, Harbin University of Science

16

1

ACS Paragon Plus Environment


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17

Abstract

18

In previous studies, we interestingly found that several ligands (e.g., pyrophosphate,

19

nitrilotriacetate, and humic acid) could significantly accelerate the oxidation rates of

20

triclosan (TCS; the most widely used antimicrobial) by aqueous permanganate

21

(Mn(VII)) especially at acid pH, which was ascribed to the contribution of ligand-

22

stabilized Mn(III) (defined Mn(III)L) formed in situ as a potent oxidant. In this work, it

23

was found that the oxidation of TCS by Mn(III)L resulted in the formation of dimers,

24

as well as hydroxylated and quinone-like products, where TCS phenoxy radical was

25

likely involved. This transformation pathway distinctly differed from that involved in

26

Mn(VII) oxidation of TCS, where 2,4-dichlorophenol (DCP) was the major product

27

with a high yield of ~80%. Surprisingly, we found that the presence of various

28

complexing ligands including pyrophosphate, nitrilotriacetate, and humic acid, as well

29

as bisulfite slightly affected the yields of DCP, although they greatly enhanced the

30

oxidation kinetics of TCS by Mn(VII). This result could not be reasonably explained

31

by taking the contribution of Mn(III)L into account. Comparatively, the degradation of

32

TCS by manganese dioxide (MnO2) was also greatly enhanced in the presence of these

33

ligands with negligible formation of DCP, which could be rationalized by the

34

contribution of Mn(III)L. In addition, it was demonstrated that DCP could not be

35

generated from Mn(VII) oxidation of unstable phenoxy radical intermediates and stable

36

oxidation products formed from TCS by Mn(III)L. These findings indicate that

37

manganese intermediates other than Mn(III) are likely involved

38

Mn(VII)/TCS/ligand systems responsible for the high yields of DCP product.

39

2


in the

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40

Introduction

41

Soluble Mn(III) species can play an important role in a variety of biogeochemical

42

processes in natural environments.1-4 The importance of soluble Mn(III) was previously

43

ignored because it was thermodynamically unstable by rapid disproportionation to

44

Mn(II) and Mn(IV).5 In the presence of ligands, Mn(III) can be stabilized in solution as

45

Mn(III) complexes (Mn(III)L), and these complexes have thermodynamic stability

46

constants similar to or slightly higher than those of Fe(III) analogues.5

47

been several reports on the occurrence of soluble Mn(III) in some aquatic settings,

48

where it is produced via Mn(II) oxidation or MnIVO2 reduction and then stabilized by

49

natural unknown ligands (possibly humic acid).7-10 Since Mn(III)L can serve as either

50

an electron acceptor or an electron donor, it is considered to participate in multiple

51

redox reactions affecting the cycles of carbon, nitrogen, and iron as well as the

52

transformation of anthropogenic contaminants.11-13 For instance, Kostka et al.11

53

reported that Mn(III)-pyrophosphate complex as a potent oxidant exhibited appreciable

54

reactivity towards Fe(II) and sulfide. Wu et al.12 found that Mn(III)-oxalic acid complex

55

served as a reductant resulting in rapid and extensive decomposition of antibiotic

56

carbadox.

6

There have

57

During the past decade, the occurrence of Mn(III)L has also been reported during

58

water treatment with permanganate (Mn(VII)), where it was generated from Mn(VII)

59

reduction in the presence of model ligands or unknown natural ligands.14-19 In previous

60

work, we interestingly found that pyrophosphate (PPP), nitrilotriacetate (NTA), humic

61

acid (HA), and bisulfite could significantly accelerate Mn(VII) oxidation of phenolic

62

contaminants such as antimicrobial triclosan (TCS) especially at acid pH.14, 15 The

63

presence of Mn(III)L species (e.g., Mn(III)-PPP complex) in the Mn(VII)/ligand

64

systems was confirmed E\ RQOLQH 89íYLV VFDQQLQJ DQG FDSLOODU\ HOHFWURSKRUHVLV

65

techniques.15 Therefore, these enhancements could be explained by the contribution of

66

Mn(III)L formed in situ as a potent oxidant. In other words, the presence of complexing

67

ligands could stabilize Mn(III) formed in situ from Mn(VII) to a certain extent to 3



Page 4 of 29

68

prevent its spontaneous disproportionation and thus the relatively long-lived Mn(III)L

69

as a potent oxidant could contribute to the oxidation of TCS (eqs 1 and 2). / OLJDQG

0Q ,,, 1ÛÛÛÛ. 0Q ,,,

70

0Q ,,,

71

/

/

7&6 : 0Q ,, SURGXFWV

(1) (2)

72

However, to date, little information is available on the reaction kinetics of TCS by

73

Mn(III)L. The effect of complexing ligands on the formation of oxidation products from

74

TCS by Mn(VII) also remained unclear so far. In this work, these two issues were

75

explicitly addressed. Firstly, the transformation kinetics and pathway of TCS by

76

Mn(III)L were investigated. Secondly, the effects of complexing ligands including PPP,

77

NTA, HA, and bisulfite on the transformation of TCS by Mn(VII) were investigated

78

with a focus on the formation of oxidation products. Finally, the effects of complexing

79

ligands on TCS transformation by manganese dioxide (MnO2) of environmental

80

relevance were comparatively examined.

81

Materials and Methods

82

Materials. Unless otherwise stated, all chemicals were purchased from Sigma-

83

Aldrich or Chemical Reagent Co. Ltd Sinopharm with a purity of 97% or higher. All

84

solutions were prepared using deionized water from a Milli-pore system. Mn(VII) stock

85

solutions were prepared by dissolving crystal Mn(VII) in deionized water and

86

standardized spectrophotometrically. Stock solutions of colloidal and particulate MnO2

87

were synthesized following the procedure described in our previous work,20-22 and they

88

were standardized by determining total manganese concentration with inductively

89

coupled plasma optical emission spectrometer after dissolution by ascorbic acid. Stock

90

solutions of HA were purified following the procedure as described in our previous

91

work.23, 24

92

Experimental procedure

4


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(i) Oxidation of TCS by Mn(III)L. Reactions ZHUH LQLWLDWHG E\ DGGLQJ 7&6

93

0

0, L= PPP or NTA), which were freshly prepared by

94

into Mn(III)L solutions (10

95

stoichiometric reaction of Mn(VII) with Mn(II) in excess ligands and adjusted to

96

desired pH.5 Aliquots were periodically withdrawn and immediately quenched by

97

ascorbic acid before analysis by high pressure liquid chromatography (HPLC) and UV

98

detection. To identify oxidation products, a series of solutions containing TCS (at a relatively

99

0 ZHUH treated by Mn(III)L at varying doses (10-5

0

100

high concentration of

101

When the reactions reached completion (i.e., Mn(III)L was totally consumed), the

102

resulting solutions were analyzed by HPLC and electrospray ionization-triple

103

quadrupole mass spectrometry (HPLC (6,í4T406 using a powerful precursor ion

104

scan (PIS) approach. (ii) Oxidation of TCS by Mn(VII) in the presence of complexing ligands. Reactions

105

0 into pH-buffered solutions (10 mM acetate

106

were initiated by adding Mn(VII) (6

107

buffer for pH 5) containing TCS (5 M) with/without a ligand of interest at desired

108

concentrations. Samples were collected at specified time intervals and quenched by

109

ascorbic acid before analysis with HPLC and UV detection. All the kinetic experiments were performed in triplicates at room temperature (25±2)

110 111

o

C. The average data and standard deviations were presented.

112

Analytical methods. The HPLC/UV analysis was performed on a Waters 2695

113

HPLC system equipped with a Waters 1525 solvent pump, a Waters 717 autosampler,

114

a Waters Symmetry C18 FROXPQ

115

GXDO

116

INESA Scientific Instrument Co.Ltd). Absorbance was measured by a Varian Cary 300

117

UV±vis spectrometer. The DOC contents of HA stock solutions were determined using

118

Analytikjena Multi N/C 3100.

î

PP

P SDUWLFOH VL]H , and a Waters 2487

GHWHFWRU. Solution pH was measured with Leici PHS±3C pH-meter (Shanghai

119

An Agilent 1260 HPLC combined with an ABSciex QTrap 5500 MS with an ESI

120

source was used for HPLC/ESI-QqQMS analysis. Chromatographic separation was

121

SHUIRUPHG RQ D :DWHUV ;%ULGJH &

FROXPQ

î

PP

5


P SDUWLFOH VL]H 7KH


Page 6 of 29

122

gradient mobile phase consisted of acetonitrile/deionized water at a flow rate of 0.2

123

mL/min. The MS instrumental parameters were provided in SI Text S1.

124

Results and discussion

125

Transformation of TCS by Mn(III)L

126

(i) Kinetics. Previous studies have concluded that the reactions of Mn(III)L with

127

phenolics are first-order with respect to each reactant.15, 16 So, the apparent second-

128

order rate constants can be described by eq 3

129

-

d[TCS] dt

(3)

kMn(III) > Mn(III) L @>TCS@

130

where kMn(III) represents the apparent second-order rate constant. Accordingly, apparent

131

second-order rate constants (kMn(III)) for reactions of Mn(III)L with TCS under various

132

conditions were determined and shown in Table 1. As can be seen, the kMn(III) values

133

were dependent on solution pH, complexing ligands, and [Mn(III)] : [ligands] ratio,

134

which was consistent with previous studies on the reaction kinetics of Mn(III)L with

135

ELVSKHQRO $ DQG

136

increase of [Mn(III)] : [PPP] ratio but almost remained constant with the increase of

137

[Mn(III)L] : [NTA] ratio. At a fixed [Mn(III)] : [ligands] ratio of 1:5, kMn(III) in the case

138

of PPP was smaller than that in the case of NTA. With the increase of solution pH,

139

kMn(III) declined in both cases. Similar trends on the reactivity of Mn(III)L were also

140

reported in the study of Wang el al., where Mn(III)-PPP complex was an effective

141

oxidant for UO2 and its reactivity decreased with the increase of [Mn(III)L] : [PPP] ratio

142

as well as solution pH.13

143

-estradiol.14, 16 For instance, kMn(III) appreciably decreased with the

(ii) Products. Over the past decade, a powerful HPLC/ESI-QqQMS PIS approach 22, 25-27

144

has been developed for selective detection of polar halogenated compounds

145

Given three chlorine atoms in parent TCS, this approach was used to selectively detect

146

the oxidation products of TCS by Mn(III)L. Figure 1 exemplified the HPLC/ESI-

147

4T406 FKURPDWRJUDP RI D VDPSOH FRQWDLQLQJ 7&6

148

complex

149

new peaks compared to the control sample containing TCS and NTA. In addition, each

0 DW S+

.

0 WUHDWHG E\ 0Q ,,, -NTA

ZKHn PIS was set at m/z 35. Apparently, there were seven

6


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150

peak in the chromatogram detected by the PIS at m/z 35 could find its counterpart by

151

the PIS at m/z 37 (data not shown), suggesting that these peaks in pair should

152

correspond to chlorine-containing products. Five products (I-V) at retention time of

153

39.99, 40.90, 42.28, 43.02, 45.36 min had the same molecular ions of m/z

154

573/575/577/579/581 in the PIS of m/z 35, suggesting that they might be isomeric

155

dimers of TCS (with molecular ions of m/z 287/289/291 at 36.45 min). Product VI at

156

30.60 min had molecular ions of m/z 303/305/307 in the PIS of m/z 35, suggesting that

157

it should contain three chlorine atoms. Also, the isotope abundance ratio of 9:6:1 in its

158

peak clusters was accordant with the theoretical prediction.22,25 So, product VI was

159

suggested to be a mono-hydroxylated product of TCS (i.e., m/z 287/289/291+16).

160

Product VII at 34.42 min had even-numbered molecular ions of m/z 332/334/336. This

161

product was assigned to be a quinone-like compound according to several recent

162

studies.22, 23, 28-30. Under negative ESI, quinones were possibly reduced via accepting

163

electron to form even-numbered radicals M‡-, which was dependent on the conditions

164

RI WKH +3/& (6,í4T406 DQDO\VLV H J FROOLVLRQ HQHUJ\ and mobile phase flow rate).

165

Similar products were identified through changes in the reaction conditions (e.g.,

166

varying solution pH, different ligands, or different [Mn(III)L]:[ligands] ratios). In other

167

words, only quantitative but not qualitative differences in TCS products formation were

168

observed under various conditions. So, it seemed likely that similar products could be

169

generated in the cases of Mn(III)-HA and Mn(III)-bisulfite complexes, although these

170

complexes could not be successfully synthesized ex situ due to their relatively low

171

stability.16 31 Several studies reported that dimers as well as hydroxylated and quinone-

172

like products (i.e., products I-VII) were also generated in the reaction of MnO2 with

173

TCS.22, 32 In contrast, Jiang et al.22 and Chen et al.33 found that products I-VII were not

174

formed in the reaction of Mn(VII) with TCS, while 2,4-dichlorophenol (DCP) as well

175

as several ring-opening products was generated. The yield of DCP from the oxidation

176

of TCS by Mn(VII) was quantified to be as high as 80-90%.22 However, in the cases of

177

Mn(III)L and MnO2, negligible DCP was formed (i.e., the yield of DCP was less than

178

1%).32 7



Page 8 of 29

179

(ii) Proposed Reaction Pathway. Based on the identified products, the transformation

180

pathway of TCS by Mn(III)L was proposed as shown in Figure 2. The phenol moiety

181

of TCS initially donates one electron to Mn(III) L, forming TCS phenoxy radicals in

182

different resonance forms (R1, R2, and R3). Subsequently, these phenoxy radicals are

183

coupled to each other in five different ways (C-C and/or C-O coupling) with the

184

generation of products I-V (SI Figure S1). In parallel, phenoxy radicals are oxidized

185

with the generation of product VI, which can be further oxidized leading to the

186

formation of quinone-like product VII.

187

Zhang and Huang

32

reported that the reactions of MnO2 with TCS also proceeded

188

via oxidative coupling pathway leading to the formation of similar products. In

189

comparison, Jiang et al.22 and Chen et al.33 proposed that the ether bond cleavage and

190

benzene ring opening rather than oxidative coupling pathway were involved in the

191

reactions of Mn(VII) with TCS.

192

Formation of DCP from the oxidation of TCS by Mn(VII) in the presence of

193

ligands. According to our previous studies, the presence of complexing ligands can

194

appreciably accelerate oxidation rates of TCS by Mn(VII) due to the contribution of

195

Mn(III)L formed in situ as a strong oxidant.14-16 Given the marked difference in product

196

formation from TCS oxidation by Mn(VII) vs Mn(III)L, it is expected that the presence

197

of ligands would significantly affect DCP formation as well. Experiments were

198

conducted under various conditions to verify this expectation.

199

(i) PPP and NTA. As shown in Figure 3a-3c, the presence of PPP and NTA (60-600

200

M) considerably accelerated the oxidation rates of TCS (5 M) by Mn(VII) (60 M)

201

at acid pH 5 as expected. Compared to ligand free control, the half time of TCS (t1/2) in

202

the presence of 300 M PPP and NTA were shortened from ~30 min to ~ 4 and 1 min,

203

respectively. With the concentration of PPP decreasing from 600 to 60

204

degradation rate of TCS was accelerated. In comparion, the concentration of NTA (60-

205

600 M) had a slight effect.

M, the

206

In parallel, the formation of DCP during TCS oxidation was monitored (Figure 3d-

207

3f). In the absence of ligands, DCP gradually reached to its maxima (~3.3 M) at about 8


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208

60 min and then gradually declined. Similar formation patterns of DCP were observed

209

in the presence of PPP and NTA except that its concentrations changed more rapidly

210

along with the reaction time. For instance, the occurrence of DCP maxima was

211

shortened to ~6 and 1.5 min when 300 M PPP and NTA were present, respectively.

212

The maxima of DCP concentration occurred at ~9 min and ~1.5 min, respectively, with

213

PPP and NTA concentration increasing to 600 M.

214

Meanwhile, the formation of products I-VII was monitored by the HPLC/ESI-QqQ-

215

MS at MRM mode. For instance, in the presence of 300 M NTA, products I-V and

216

product VII were gradually generated as the reactions proceeded while product VI was

217

initially generated and then slowly decayed (SI Figure S2). It was noted that the

218

concentration levels of these products were displayed by the peak areas due to the lack

219

of their authentic standards. Unlike absolute concentrations, these peak areas should

220

not be compared to one another, as diverse compounds have different response values

221

in MS spectra.

222

Experimental yields of DCP. The experimental yields (Rexp) of DCP (i.e., molar ratios

223

of DCP formed to TCS consumed) were calculated, as shown in Figure 3g-3i. The Rexp

224

value in the absence of ligands was in the range of 80-90% at the initial stage ('&3@IRUP‡†á–

50Q 9,, îÞ>7&6?0Q:9,,; W 50Q ,,, îÞ>7&6?0Q ,,, W / /

>7&6@FRQVXPHGá–

Þ>7&6?0Q:9,,; W >Þ>7&6?0Q ,,, W /

(4)

233

where û[DCP]formed,t and û[TCS]consumed,t represented the amounts of DCP formed and

234

TCS consumed over the time period t, respectively; û[TCS]Mn(VII),t and û[TCS]Mn(III)L,t

235

represented the amounts of TCS consumed by individual Mn(VII) and Mn(III)L over 9



Page 10 of 29

236

the time period t, respectively; RMn(VII) and RMn(III)L were the yields of DCP in the cases

237

of individual Mn(VII) and Mn(III)L.

238 239

Due to the negligible generation of DCP from TCS by Mn(III)L (i.e., RMn(III)L~0), eq 4 could be rearranged as: 5SUHá–

240

50Q 9,, îÞ>7&6?0Q:9,,; W

(5)

>7&6@FRQVXPHGá–

241

Considering that the reactions of TCS with both Mn(VII) and Mn(III)L were first-order

242

with respect to each reactant, the reaction kinetics of TCS in the Mn(VII)/ligand

243

systems could be described as G>7&6?

244

GW

N0Q 9,, >0Q:9,,;?W >7&6@W N0Q ,,, >0Q:,,,;/ ?W >7&6@W

(6)

245

where kMn(VII) and kMn(III) were the apparent second-order rate constants for reaction of

246

TCS with Mn(VII) and Mn(III)L, respectively. Under the pseudo-first order condition

247

with Mn(VII) in excess, eq 7 could be derived by making a steady-state assumption for

248

Mn(III)L, G>XGW?

249

GW

kNREV 0Q 9,, NREV 0Q ,,, o>7&6@W = NREV >7&6@W

(7)

250

where kobs,Mn(VII) and kobs,Mn(III) denoted the pseudo first-order rate constants for Mn(VII)

251

and Mn(III)L oxidation, respectively; NREV represented the pseudo first-order rate

252

constant for reactions of Mn(VII) with TCS in the presence of ligands. For any given

253

duration of time, the ratio of the amount of TCS oxidized by Mn(VII) to that by Mn(III)L

254

would be

255 256 257 258 259

¨7&60Q 9,,

NREV 0Q 9,,

¨7&60Q ,,,

NREV 0Q ,,,

(8)

Eq 9 could be obtained from eq 8 ¨7&60Q 9,, á–

NREV 0Q 9,, NREV 0Q 9,, NREV 0Q ,,,

û>7&6@FRQVXPHGá– L

NREV 0Q 9,, NREV

û>7&6@FRQVXPHGá–

(9)

So, eq 5 could be rewritten as 5SUH W

NREV 0Q 9,, NREV

50Q 9,,

(10)

260

As can be seen in Figure 3b-c, the oxidation of TCS by Mn(VII) in the presence of PPP

261

and NTA displayed autocatalysis: an initial lag phase followed by a secondary rapid 10


Page 11 of 29


262

stage (see the following section for discussion). The degradation kinetics of TCS in the

263

secondary rapid stage were fit to first-order rate laws, and the values of kobs as well as

264

the ratio of

265

were calculated by eq 10. Surprisingly, the values of Rpre were approximately 0 in all

266

cases, while the experimental counterparts (i.e., Rexp) were in the range of 20-75%.

NREV 0Q 9,, NREV

were obtained (SI Table S1). Then, the predicted yields (i.e., Rpre)

267

(ii) HA. The degradation of TCS by Mn(VII) was also appreciably accelerated in the

268

presence of HA. With the concentration of HA increasing from 0 to 5 mg C/L, t1/2

269

decreased from ~30 min to ~0.3 min (Figure 4a).Very recently, Xu et al.34 reported

270

that HA also notably accelerated Mn(VII) oxidation of levofloxacin, and they proposed

271

that hydroxyl radical (OH‡) originated from the reaction of Mn(III)-HA complex with

272

oxygen was responsible. To verify this possibility, the influence of radical scavenger

273

tert-butanol (TBA) and nitrogen purge was examined. Neither TBA nor nitrogen purge

274

had discernible effects on the degradation of TCS by Mn(VII) in the presence of HA

275

(SI Figure S3), which definitely confirmed that OH‡ was not involved.

276

Interestingly, the autocatalytic kinetics were not seen in the presence of HA, in

277

contrast to the cases of PPP and NAT. The occurrence of the initial lag phase in the

278

Mn(VII)/TCS/PPP and Mn(VII)/TCS/NTA systems might be attributed to the

279

accumulation of Mn(III)L formed in situ as the reaction of Mn(VII) and TCS slowly

280

progressed. Similar autocatalytic kinetics were also observed in Cr(VI) reduction by

281

Mn(II) in the presence of oxalic acid, where the initial lag phase was proposed to be

282

resulted from the accumulation of Mn(III) stabilized by oxalic acid.35 So, it seemed

283

likely that the reduction of Mn(VII) by HA with a fast generation of Mn(III)L resulted

284

in the absence of autocatalysis in the Mn(VII)/TCS/HA system. To confirm this, the

285

effect of HA by the treatment of ozonation (to attenuate the reducing ability of HA) was

286

comparatively examined. As expected, the degradation of TCS showed autocatalysis in

287

the presence of pre-ozonated HA, similar to the cases of PPP and NTA (SI Figure S4,

288

for example).

11



Page 12 of 29

289

In parallel, the formation of DCP in the presence of HA was also monitored. As

290

shown in Figure 4b, the concentration of DCP changed more rapidly along with the

291

reaction time, and the occurrence of its maxima was shortened from ~30 to ~1 min with

292

the concentration of HA increasing from 0 to 5 mg C/L. Moreover, the experimental

293

yields (Rexp) were calculated and shown in Figure 4c. Similar to the cases of PPP and

294

NTA, the yields of DCP were slightly influenced in the presence of HA, although the

295

kinetics of TCS decay were considerably accelerated. It was found that the Rpre values

296

predicted by eq 10 (~0) also considerably underestimated the experimental counterparts.

297

(iii) bisulfite. Recently, Sun et al.36 reported that bisulfite could greatly enhance the

298

Mn(VII) oxidation of organic contaminates, and these authors proposed that aquo and

299

hydroxo Mn(III) formed in situ from Mn(VII) reduction was responsible. However, this

300

conclusion seems likely to be contrasted to the acknowledged fact that spontaneous

301

disproportionation of aquo and hydroxo Mn(III) results in its unlikely contribution to

302

the oxidation of contaminants unless in the presence of stabilizing agents or under

303

extremely acidic conditions.5, 37 In a very recent work, we showed several lines of

304

experimental evidence to support that bisulfite acted as a complexing ligand for Mn(III)

305

other than a reductant.31 In other words, Mn(III)-sulfito complex rather than aquo and

306

hydroxo Mn(III) formed in situ by the fast reaction of Mn(VII) with bisulfite acted as

307

the oxidant contributing to the fast oxidation of organics. The appreciable complexing

308

ability of bisulfite for metal ions is well documented in literatures.38-40 For instance,

309

Chen et al.40 reported that Fe(III) could be complexed by bisulfite with a

310

thermodynamic stability constant of about 102.4. Harrington et al.6 reported that

311

Mn(III)-ligand complexes always have a similar or slightly greater thermodynamic

312

stability constants as compared to those of Fe(III) analogues.

313

Here, the effect of bisulfite on Mn(VII) oxidation of TCS was investigated with a

314

focus on DCP generation. Similar to our previous finding39, the extent of TCS decay

315

also exhibited a [bisulfite]:[Mn(VII)] ratio dependency, where it maximized at the ratio

316

of 5:1-10:1, appreciably higher than the stoichiometric one of 2:1 (eq 11) (Figure 5).

317

0Q 9,,

+62 :0Q ,,,

62

12


(11)

Page 13 of 29


318

The experimental DCP yields (Rexp) of 20-30% were observed therein. In comparison,

319

Rpre~0 was expected since negligible DCP was generated from TCS by Mn(III) L and

320

the contribution of Mn(VII) was also neglected during the short reaction time (10 s).

321

Comparison with MnO2. The effect of complexing ligands on the transformation of

322

TCS by MnO2 was comparatively investigated with a focus on DCP formation. Similar

323

to the case of Mn(VII), the complexing ligands including PPP, NTA, HA, and bisulfite

324

greatly accelerated the oxidation rates of TCS by MnO2 as compared to ligand free

325

control, regardless of the type of MnO2 (i.e., colloidal vs particulate MnO2) (Figure 6),.

326

Interestingly, these complexing ligands had indiscernible influence on DCP formation.

327

In other word, negligible formation of DCP was observed in the absence and presence

328

of complexing ligands. The acceleration of TCS degradation as well as negligible DCP

329

formation in the presence of ligands could be reasonably explained by the contribution

330

of Mn(III)L formed in situ from MnO2 reduction.

331

Mechanistic

insights.

The

unexpected

high

yields

of

DCP

in

the

332

Mn(VII)/TCS/ligand systems as well as the marked discrepancy in DCP formation

333

between Mn(VII)/TCS/ligand and MnO2/TCS/ligand systems suggest that manganese

334

intermediates other than Mn(III)L may participate in the Mn(VII)/TCS/ligand systems.

335

Due to multiple oxidation states of manganese, manganese intermediates (e.g., Mn(VI),

336

Mn(V), Mn(IV) and Mn(III)) are always involved in Mn(VII) oxidation processes.41-51

337

For instance, Simandi et al.43,

338

involvement of Mn(VI) during bisulfite and (chloro)phenols oxidation by alkaline

339

Mn(VII). Ogino et al.46, 48 and Simandi et al.46, 48 reported that Mn(V) intermediate was

340

generated from Mn(VII) oxidation of As(III) and olefins. The occurrence of soluble

341

Mn(IV) as an intermediate during Mn(VII) oxidation of cinnamic acid or Mn(II) was

342

also documented by Simandi et al. and Reisz et al.41, 51 Similarly, many studies have

343

reported that high-valent metal-oxo intermediates such as Cr(IV) and Cr(V) also occur

344

in redox reactions involving Cr(VI).52-57 The stability of these high-valent oxo

345

intermediates (e.g., Cr(IV) and Cr(V)) is greatly enhanced by complexing ligands (e.g.,

50

as well as Lee and Sebastian43,

13


50

proposed the


346

oxalate, citrate, and HA).52-56 For instance, Goodgame and Hayman58 reported that

347

short-lived Cr(V) intermediate could stabilized by HA and persisted at least 5 days at

348

pH 5.6. So, it seems likely that PPP, NTA, HA and bisulfite might stabilize manganese

349

intermediates other than Mn(III) and these stabilized species (defined as Mn(X)L)

350

contribute to the enhanced transformation of TCS by Mn(VII). Mn(III)L formed in the

351

initial lag phase likely further participates in the formation of Mn(X)L leading to the

352

subsequent rapid degradation of TCS in the Mn(VII)/ligand systems. Comparatively,

353

the autocatalytic kinetics of TCS decay are not observed in the Mn(VII)/ligand systems,

354

where Mn(III)L formed in situ plays an important role and Mn(X)L is not involved. In

355

addition, similar to Mn(III)L, the reactivity of Mn(X)L might be related to the nature of

356

ligands and the [manganese] : [ligands] ratio, resulting in the difference in the oxidation

357

rates under various conditions.

358

Another possible explanation might involve Mn(VII) oxidation of unstable phenoxy

359

radical intermediates and/or stable oxidation products, which were formed from TCS

360

by Mn(III)L, leading to the high yield of DCP in Mn(VII)/TCS/ligand systems. To

361

explore this possibility, phenoxy radical scavenging and sequential oxidant addition

362

experiments were conducted. It is documented that 5,5-Dimethylpyrroline-N-oxide

363

(DMPO) and 2,2,6,6-Tetramethyl-4-piperidinol (TMP) can act as scavengers for

364

phenoxy radical.59-61Accordingly, the effect of both DMPO and TMP on the formation

365

of dimers (i.e., coupling products of two phenoxy radicals) from TCS oxidation by

366

Mn(III)L was investigated. It was found that the oxidation rates of TCS (5 M) by

367

Mn(III)L (100 M) was negligibly affected by DMPO (1 mM) and TMP (1 mM), while

368

the formation of dimers was obviously inhibited (SI Figure S5). This result suggested

369

that these two scavengers could appreciably trap TCS phenoxy radical intermediate

370

under the condition investigated. Comparatively, in the Mn(VII)/TCS/ligand systems,

371

we only examined the influence of TMP because the fast reduction of Mn(VII) by

372

DMPO caused significant interference. As shown in SI Figure S6, the addition of TMP

373

slightly affected the degradation of TCS as well as the formation of DCP in the 14


Page 14 of 29

Page 15 of 29


374

Mn(VII)/TCS/ligand systems. This finding indicated that the formation of DCP in the

375

Mn(VII)/TCS/ligand systems was not due to the oxidation of unstable TCS phenoxy

376

radical intermediate by Mn(VII).

377

To test whether DCP could be generated from the oxidation of Mn(III)-involved

378

stable products by Mn(VII), a sample containing dimers as well as hydroxylated and

379

quinone-like products was prepared ([TCS]= 5 M, [Mn(III)L=40 M, reaction time =

380

1 hour) and then treated by Mn(VII) after its pretreatment by solid phase extraction to

381

remove background constituents.28, 62 As can be seen in Figure S7, the peaks of dimers

382

and hydroxylated products disappeared after the treatment by Mn(VII) while the peak

383

height for quinone-like product slightly increased. However, no DCP formation was

384

observed, suggesting that Mn(III)-involved products (including stable dimers,

385

hydroxylated and quinone-like products) could not be further oxidized to DCP by

386

Mn(VII) either.

387 388

The contribution of Mn(X)L to DCP formation. The experimental yields of DCP could be described by eq 16 by taking the contribution of Mn(X)L into consideration 5H[S W

389

50Q 9,, îÞ>7&6?0Q:9,,; W 50Q [ îÞ>7&6?0Q: ;/ W / û>7&6@FRQVXPHG W

(16)

390

where ûTCSMn(X)L was the amount of TCS consumed by individual Mn(X)L, and

391

RMn(X)L was the yield of DCP by Mn(X)L. By comparing eq 5 and eq 16, eq 17 could be

392

obtained,

393

I 5H[S W 5SUH W 5H[S W

50Q [ î¸>7&6@0Q ; W / 50Q 9,, î¸>7&6@0Q 9,, W 50Q ; î¸>7&6@0Q ; W / /

(17)

394

where f represeted the contribution of Mn(X)L to DCP formation. The f values in all

395

case were calculated to be about 1.0, indicating that the formation of DCP was primarily

396

ascribed to the contribution of Mn(X)L.

397

Implications. In this work, the transformation of TCS by soluble Mn(III) species

398

was investigated for the first time, where Mn(III)L exhibited appreciable reactivity

399

towards TCS and its reactivity was dependent on solution pH, complexing ligands, and

400

[Mn(III)] : [ligands] ratio. Oxidation products including dimers, as well as hydroxylated 15



401

and quinone-like products were identified when TCS was treated by Mn(III)L, where

402

the initial formation of phenolic radicals via one-electron transfer and their further

403

coupling and oxidation reactions were likely involved. This transformation pathway

404

was similar to that involved in MnO2 oxidation but markedly differed from that

405

involved in Mn(VII) oxidation, where DCP was the major product of TCS with a high

406

yield of ~80%. This finding may have important implications for assessing the fate of

407

Mn(III) species as well as the transformation of anthropogenic contaminants in natural

408

environments, since soluble Mn(III) is abundant in some aquatic environments.

409

Given the marked difference in product formation from TCS oxidation by Mn(VII)

410

vs Mn(III)L, it is expected that the presence of ligands would significantly affect DCP

411

formation as well. Surprisingly, we found that the presence of various ligands (i.e., PPP,

412

NTA, HA and bisulfite) slightly or negligibly affected the yields of DCP, although they

413

greatly enhanced the oxidation kinetics of TCS by Mn(VII). In other words, the DCP

414

yields predicted by taking the contribution of Mn(III) L into consideration considerably

415

underestimated the experimental yields in all cases. Comparatively, the degradation of

416

TCS by MnO2 was also greatly enhanced in the presence of these ligands but negligible

417

formation of DCP was observed, which could be rationalized by the contribution of

418

Mn(III)L. In addition, it was demonstrated that DCP could not be generated from

419

Mn(VII) oxidation of unstable phenoxy radical intermediates and stable oxidation

420

products formed from TCS by Mn(III)L. These findings indicated that manganese

421

intermediates other than Mn(III) were likely involved in Mn(VII)/TCS/ligand systems

422

responsible for the high yields of DCP. In other words, ligands might stabilize

423

manganese intermediates other than Mn(III) and these stabilized species could oxidize

424

TCS leading to a high yield of DCP. However, the pathway for Mn(X) formation as

425

well as the nature of these manganese intermediates is not fully understood, which

426

warrants further investigations.

427

16


Page 16 of 29

Page 17 of 29


428

Acknowledgments

429

This work was financially supported by the National Key Research and Development

430

Program (2016YFC0401107), the National Natural Science Foundation of China

431

(51578203), and the Funds of the State Key Laboratory of Urban Water Resource and

432

Environment (HIT, 2016DX13).

433

Supporting Information

434

The additional texts, figures, and tables addressing supporting data. This material is

435

available free of charge via the Internet at http://pubs.acs.org.

436

17



437 438

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439

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587

containing a hindered piperidine and a phenolic antioxidantüA review. Polym. Degrad. Stabil. 1990,

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591

2011, 45, 3635-3642.

3RVStãLO -

6HGOi

-

3RVVLELOLWLHV IRU FRRSHUDWLRQ LQ VWDELOL]HU systems

592

21



Page 22 of 29

593 39.99I

100% 90%

R e l. In t. ( % )

80%

TCS 36.45

70%

III

60%

40%

40.90

49.24

VI

30.60

287.0

40%

287.9

0% 284

286

288

290 292 m/z, Da

294

60%

573.8 577.9

296

303.9 301.7

580.0

575

581.0

580

585

331.9

307.8

333.9

60% 40%

0%

310

VII

80%

20%

307.0 305

570

100%

304.9

300

578.8

0%

VI

40%

0%

575.9 40%

m/z, Da

80%

20%

60%

292.1

302.9

100%

572.9

80%

20%

291.0

595

R e l. I n t . ( % )

R e l. I n t . ( % )

289.0

20%

I-V

576.9

80% 60%

575.0

100%

TCS

R e l. I n t . ( % )

R e l. I n t . ( % )

100%

600

45.90

34.42

0% 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 Time, min

594

599

43.02

VII

10%

598

IV

II

20%

597

45.36

50%

30%

596

V

42.28

332.8334.9

335.9 336.8

330

332

m/z, Da

334

336 m/z, Da

338

340

Figure 1. The HPLC/ESI±QqQMS PIS (m/z 35) chromatogram of a sample containing TCS treated by Mn(III)-NTA complex. The underneath showed the corresponding molecular ion mass spectra of chromatographic peaks. Experimental condition: [TCS] 0 >0Q ,,, -17$@ 0 DQG S+ .

601 602 22


Page 23 of 29


Cl

OH O Cl

Cl

O

Cl 603 604

R2

Cl

O

O

Cl

TCS

O Cl

Cl

Cl

+R2 +R3

dimers

dimers

Cl

Cl

OH

O

R1

+R2 +R3

Cl

O

O

R3

Cl Cl

OH

Product VI

Cl

+R3

605 606 607 608

dimers

quinone-like product

Product V Products I and II Product VII Products III and IV Figure 2. Proposed transformation pathways of TCS by Mn(III)L. Structures of dimers (products I-V) were presented in SI Figure S1.

609

23



Page 24 of 29

610 1.2 1.0

0.8

0.8

[TCS]t/[TCS]0

0.6 0.4 0.2

0.6 0.4

NTA = 60 P0 NTA = 300 P0 NTA = 600 P0

1.0 0.8 0.6 0.4 0.2

ligand free control

0.0

0.0 0

20

40 Time (min)

60

80

0

(d) 3

DCP (PM)

3

2

1

3

6 Time (min)

9

12

3

2

1

0

PPP = 60 P0 PPP = 300 P0 PPP = 600 P0

0 0

20

1.2

40 Time (min)

60

80

0

3

1.2

6 Time (min)

9

0.5

1.0 1.5 Time (min)

2.0

2.5

(f)

2

1 NTA = 60 P0 NTA = 300 P0 NTA = 600 P0

0 0.0

12

0.5

1.2

Rexp,PPP = 60 P0

(h)

(g)

0.0

(e)

ligand free control

1.0 1.5 Time (min)

2.0

2.5

Rexp,NTA = 60 P0

(i)

1.0

Rexp,PPP = 300 P0

1.0

Rexp,NTA = 300 P0

0.8

0.8

Rexp,PPP = 600 P0

0.8

Rexp,NTA = 600 P0

0.6

0.6

0.4 0.2

RDCP

1.0

RDCP

RDCP

(c)

PPP = 60 P0 PPP = 300 P0 PPP = 600 P0

0.2

0.0

DCP (PM)

1.2

(b)

DCP (PM)

[TCS]t/[TCS]0

(a) 1.0

[TCS]t/[TCS]0

1.2

0.6

0.4

0.4

0.2

0.2

ligand free control

0

611 612 613 614 615

0.0

0.0

0.0 20

40 Time (min)

60

80

0

3

6 Time (min)

9

12

0.5

1.0

1.5 Time (min)

2.0

Figure 3. The oxidation kinetics of TCS by Mn(VII) in the absence/presence of PPP and NTA (a-c) and formation of DCP (d-f), as well as the experimental yields of DCP (g-i). ([SHULPHQWDO FRQGLWLRQ >7&6@ 0 >0Q 9,, @ 0 >333] = [NTA] = 60, 300, or 0 DQG S+

616 617

24


2.5

Page 25 of 29


618 619 1.2 HA = 1 mg C/L HA = 2 mg C/L HA = 5 mg C/L

(a)

[TCS]t/[TCS]0

1.0 0.8 0.6 0.4 0.2 0.0 0

2.0

3

6 9 Time (min)

12

(b)

DCP (PM)

1.5 1.0 0.5

HA = 1 mg C/L HA = 2 mg C/L HA = 5 mg C/L

0.0 0

3

6 Time (min)

9

12

1.2 (c)

Rexp,HA = 1 mg C/L

1.0

Rexp,HA = 2 mg C/L

RDCP

0.8

Rexp,HA = 5 mg C/L

0.6 0.4 0.2 0.0 0

620 621 622 623

2

4

6 8 Time (min)

10

12

Figure 4. The oxidation kinetics of TCS by Mn(VII) in the presence of HA (a) and formation of DCP (b), as well as the experimental yields of DCP (c). Experimental FRQGLWLRQ >7&6@ 0 >0Q 9,, @ 0 >+$@ , 2, or 5 mg C/L, and pH 5.

624

25



Page 26 of 29

625

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0 626 627 628 629 630

1.0

extent of TCS decay experimentaol yields of DCP

2:1

5:1

10:1

20:1

50:1

Expeimental yields of DCP

Extent of TCS decay

1.0

0.0

[bisulfite]:[Mn(VII)] ratio

Figure 5. The extent of TCS decay at the end of experiments by Mn(VII) in the presence of bisulfite and experimental yields of DCP. Experimental conditions: [TCS] 0 >0Q 9,, @ 6 0 [bisulfite] = 120-3000 0, pH 5, and reaction time of 10 s.

631

26


Page 27 of 29


632

ligand free control PPP = 60 PM NTA = 60 PM HA = 2 mg C/L bisulfite = 60 PM

[TCS]t/[TCS]0

1.0 0.8 0.6 0.4 0.2

0

634 635 636

ligand free control PPP = 60 PM NTA = 60 PM HA = 2 mg C/L bisulfite = 60 PM

1.0

(a)

0.0 633

1.2

[TCS]t/[TCS]0

1.2

0.8 0.6 0.4 0.2

(b)

0.0 30

60 90 Time (min)

120

0

30

60 90 Time (min)

120

Figure 6. The effect of complexing ligands on the degradation of TCS by colloidal MnO2 (a) and particulate MnO2 (b) ([SHULPHQWDO FRQGLWLRQ >7&6@ 0 >0Q22] = 0 [PPP] = [NTA] = [bisulfite] = 6 0 [HA]= 2 mg C/L, and pH 5.

637

27



638 639

Table 1. Apparent second-order rate constants for TCS PPP or -NTA complex (100 0 .

Page 28 of 29

0 degradation by Mn(III)-

pH

[Mn(III)]:[ligand] ratio

kTCS,Mn(III)-PPP (M-1s-1)

kTCS,Mn(III)-NTA (M-1s-1)

5

1:5

148.5(±7.2)

538(±11.6)

5 6 6

1:10 1:5 1:10

56.5(±2.1) 14.6(±0.83) 5.0(±0.17)

545(±6.3) 446(±9.7) 460(±4.5)

640

28


Page 29 of 29



Does Soluble Mn(III) Oxidant Formed in Situ Account for Enhanced

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