Transformation of Flame Retardant Tetrabromobisphenol A by

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Transformation of Flame Retardant Tetrabromobisphenol A by Aqueous Chlorine and the Effect of Humic Acid Yuan Gao, Su-yan Pang, Jin Jiang, Jun Ma, Yang Zhou, Juan Li, Li-Hong Wang, Xue-Ting Lu, and Li-Peng Yuan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02844 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 3, 2016

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Environmental Science & Technology

Transformation of Flame Retardant Tetrabromobisphenol A by Aqueous Chlorine and the Effect of Humic Acid Yuan Gao†, Su-Yan Pang‡, Jin Jiang*,†, Jun Ma†, Yang Zhou†, Juan Li†, Li-Hong Wang†, Xue-Ting Lu‡, and Li-Peng Yuan‡ †

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

Environmental Engineering, Harbin Institute of Technology, Harbin 150090, China ‡

Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang

Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, China

*Corresponding Authors: Prof. Jin Jiang (J.J.) Phone: 86−451−86283010; fax: 86 − 451−86283010; E-mail: [email protected].

1

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1

Abstract

2

In this work, it was found that the most widely used brominated flame retardant

3

tetrabromobisphenol A (TBrBPA) could be transformed by free chlorine over a wide pH range of

4

5-10 with apparent second-order rate constants of 138−3210 M−1 s−1. A total of eight products

5

including one quinone-like compound (i.e., 2,6-dibromoquinone), two dimers, and several simple

6

halogenated phenols (e.g., 4-(2-hydroxyisopropyl)-2,6-dibromophenol, 2,6-dibromohydroquinone,

7

and 2,4,6-tribromophenol) were detected by high-performance liquid chromatography tandem

8

mass spectrometry (HPLC-MS/MS) using a novel precursor ion scan (PIS) approach. A tentative

9

reaction pathway was proposed: chlorine initially oxidized TBrBPA leading to the formation of

10

phenoxy radical, and then this primary radical and its secondary intermediates (e.g., 2,6-dibromo-

11

4-isopropylphenol carbocation) formed via beta-scission subsequently underwent substitution,

12

dimerization, and oxidation reactions. Humic acid (HA) considerably inhibited the degradation

13

rates of TBrBPA by chlorine even accounting for oxidant consumption. A similar inhibitory effect

14

of HA was also observed in permanganate and ferrate oxidation. This inhibitory effect was

15

possibly attributed to the fact that HA competitively reacted with the phenoxy radical of TBrBPA

16

and reversed it back to parent TBrBPA. This study confirms that chlorine can transform phenolic

17

compounds (e.g., TBrBPA) via electron transfer rather than the well-documented electrophilic

18

substitution, which also have implications on the formation pathway of halo-benzoquinones during

19

chlorine disinfection. These findings can improve the understanding of chlorine chemistry in water

20

and wastewater treatment.

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21

Introduction

22

Tetrabromobisphenol A (TBrBPA) is the most widely used halogenated flame retardant that is

23

mainly incorporated into epoxy and polycarbonate resins in the manufacture of printed circuit

24

boards for information technology and other electronic equipment.1-3 The global demand for

25

TBrBPA ranges from 120000 to 150000 tons per year. The frequent utilization of TBrBPA has

26

apparently caused its ubiquitous occurrence in the environment.4-6 For instance, Morris et al. and

27

Osako et al. reported the occurrence of TBrBPA at 85 ng L-1 in wastewater treatment effluents of

28

Europe and up to 620 ng L-1 in industrial landfill leachates in Japan.7, 8 Several studies have

29

reported that TBrBPA can exhibit significant thyroid hormonal activities and also act as an

30

endocrine disruptor due to its structural similarity to steroid estrogens.9-11 In view of the potential

31

toxicity of TBrBPA, it is important to investigate its fate in natural environment and engineered

32

processes.

33

Recent studies have demonstrated that selective oxidants/disinfectants such as ferrate [Fe(VI)]

34

and permanganate [Mn(VII)] as well as laccase-catalyzed oxidation can be applied to effectively

35

destruct TBrBPA during drinking water and wastewater treatment as expected by its phenolic

36

structure.12-14 For instance, Yang et al.12 reported that Fe(VI) could rapidly degrade TBrBPA with

37

a second-order rate constant of 7.9 ×103 M-1 s-1 at pH 7.0 and 25 oC.12 Our recent study has shown

38

that Mn(VII) can considerably oxidize TBrBPA with a second-order rate constant of 460 M-1 s-1

39

at pH 7 and 25 oC.13 Feng et al.14 found that TBrBPA could be effectively transformed by laccase-

40

catalyzed reactions with a second-order rate constant of 0.39 U−1·mL·min−1 at pH 7.0 and 25 oC.

41

Moreover, the transformation of TBrBPA by these mild oxidants shared a similar reaction pathway,

42

where an unstable phenoxy radical was initially formed via one-electron transfer and subsequently

43

underwent scission leading to the formation of new radicals.12-14 However, to date little is known



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44

about the transformation of TBrBPA during water and wastewater treatment with free chlorine,

45

which is the globally most used disinfectant.

46

Dissolved organic matters (DOM) ubiquitously exist in surface water, groundwater, and soil

47

pore water, and they might affect the transformation of contaminants in natural environment and

48

engineered processes. Many studies have reported that DOM as a competitor for oxidants can

49

decrease the transformation rates of contaminants in chemical oxidation processes.12,

50

instance, Yang et al. reported that Fe(VI) could be competitively consumed by DOM, thus

51

inhibiting the oxidative removal of TBrBPA.12 Gallare et al.15 found that the depletion rate of

52

bisphenol A (BPA) in the presence of chlorine decreased as DOM concentration increased due to

53

the consumption of chlorine by DOM.

15

For

54

Meanwhile, several studies reported that DOM could also affect the transformation rates of

55

various aqueous contaminants by reducing reactive intermediates back to their parent

56

compounds.17, 18 For instance, Canonica et al.17 found that DOM decreased the reaction rates of

57

naturally occurring contaminants (e.g., phenols, anilines, and phenylurea) when subjected to

58

oxidation by excited triplet states of benzophenone-4-carboxylate (CBBP). These authors

59

interpreted this result as follows: DOM competitively reduced intermediates formed via one-

60

electron oxidation of contaminants by excited triplet states to their parent compounds. Similar

61

inhibitory effects of DOM have been recently reported in laccase-catalyzed oxidation of

62

halogenated phenols.19

63

The primary objective of this work was to evaluate the transformation of TBrBPA by chlorine

64

during water and wastewater treatment. First, reaction kinetics were studied in synthetic buffered

65

waters over pH range of 5-10. Secondly, oxidation products formed during chlorination of

66

TBrBPA were identified by high pressure liquid chromatography and electrospray ionization-triple


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67

quadrupole mass spectrometry (HPLC/ESI−QqQMS) and a tentative reaction pathway was

68

proposed. Thirdly, the effects of DOM on the transformation of TBrBPA by chlorine were

69

examined and compared to the cases of other oxidants (i.e., Mn(VII) and Fe(VI)) under similar

70

conditions. Finally, the oxidation kinetics and products of TBrBPA by chlorine in real water were

71

examined.

72

Experimental Section

73

Materials. TBrBPA, tetrachlorobisphenol A (TClBPA), 2,4,6-tribromophenol (TBrP), and 2,6-

74

dibromo-4-methylphenol (DBrMeP) of 97% purity or higher were purchased from Sigma-Aldrich

75

or J&K Scientific Ltd. Potassium ferrate (K2FeO4) was prepared by the method of Thompson et

76

al.20 Humic acid (HA) as a representative of DOM was also obtained from Sigma-Aldrich. Other

77

reagents used were of high purity and received from Sinopharm Chemical Reagent Co. Ltd.

78

Deionized water (18.2 MΩcm) was obtained from a Milli-pore system. Stock solutions of chlorine

79

were prepared by diluting a commercial solution of sodium hypochlorite (NaClO, 4% active

80

chlorine) and standardized by iodometry. Mn(VII) stock solutions were prepared by dissolving

81

crystal KMnO4 in deionized water, while Fe(VI) stock solutions were prepared by dissolving

82

K2FeO4 in 5 mM Na2HPO4/1 mM borate buffer where aqueous Fe(VI) is known to be most

83

stable.21, 22 Both of them were standardized spectrophotometrically. The stock solutions of HA was

84

purified by repeated pH adjustment, filtration and precipitation following the procedure described

85

by Rebhun et al.23 Stock solutions of TBrBPA and TClBPA were prepared in acetonitrile owing

86

to their high hydrophobicity.24 The contents of acetonitrile introduced in the reaction solutions

87

were k2 >k1 ) can be explained by the much higher activating effect of hydroxyl groups after their

179

deprotonation and thus dissociated forms of TBrBPA were more susceptible to be oxidized12, 13

180

(see the following section).

181

Recently, Lee et al.39 have developed a linear free energy relationship (LFER) for the rate

182

constants of HOCl with 27 substituted phenolate ions (e.g., phenol, halophenols, resorcinol, and

183

triclosan): log kHOCl

 4.46( 0.15)  4.90( 0.44) o ,m, p , where 





was the best descriptor

o ,m , p

184

variable among the Hammett σ (σ, σ+, and σ-). By a structure approximation referring to BPA, the

185





value of TBrBPA was estimated to be 0.37 (see SI Table S1).39 The value of k3 estimated

o ,m , p

186

by LFER was in the range of 215-911 M-1 s-1, one or two orders of magnitude lower than the

187

experimental one (i.e., 4.57 ±0.32 × 104 M-1 s-1).

188

Interestingly, Lee et al. found that the species-specific rate constant (k3) for fully-dissociated

189

BPA structurally similar to TBrBPA could be well predicted by the above LFER.39 The exact

190

reasons for the discrepancy between BPA and TBrBPA are unclear. One possible explanation

191

might be the different mechanisms involved in the chlorination of BPA vs TBrBPA. Gallard et

192

al.15 reported that BPA was transformed by chlorine through electrophilic substitution with the

193

formation of chlorinated BPA. In contrast, we found that chlorine oxidized TBrBPA via electron

194

transfer in this study (see the following section). This pathway during chlorination might be related

195

to the unique chemical structure of TBrBPA, whose ortho- and para-positions on its two aromatic

196

rings are occupied with bromine atoms. In this regard, TBrBPA can be considered as a fully-

197

substituted phenolic compound, although meta-positions are empty but difficult to be substituted.

198

It seems likely that the 27 phenols used to obtain LFER also undergo electrophilic substitution in


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their reactions with chlorine, which is in good agreement with many studies on chlorinated

200

products of these phenolic compounds. This substitution pathway is not difficult to understand by

201

considering their chemical structures of non-fully substituted with available substitutes at ortho-

202

or para- position 15, 31, 40, 41

203

Another possible explanation for the finding that the prediction of k3 by LEFR was much lower

204

than the experimental one may involve the contribution of bromide released from TBrBPA during

205

chlorination. Chlorine can quickly oxidize bromide leading to the formation of active bromine 42-

206

45

207

(see SI Table S1) was examined under similar conditions. The second-order rate constants were

208

comparatively shown in Figure 1. As can be seen, chlorine exhibits comparable reactivity towards

209

TBrBPA vs TClBPA. This suggested a negligible contribution of bromine formed in situ to the

210

chlorination kinetics of TBrBPA (i.e., negligible bromide ions released during the kinetic runs),

211

consistent with the transformation pathway of TBrBPA by chlorine (see the following sections).

212

Moreover, it was noted that the k3 value (944-3126 M-1 s-1) for fully-dissociated TClBPA estimated

213

by LFER was also one or two orders of magnitude lower than the experimental one (2.94 ±0.22×

214

104 M-1 s-1) obtained by using nonlinear least-squares regression of experimental data according

215

to eq 8, similar to the case of TBrBPA.

To explore this possibility, the chlorination kinetics of TClBPA structurally similar to TBrBPA

216

Oxidation products. Recently, a novel and powerful HPLC/ESI-QqQMS PIS approach has

217

been developed for selective detection of polar halogenated compounds.29, 30, 46, 47 Given four

218

bromine atoms in parent TBrBPA, this approach was used for selective detection of transformation

219

products of TBrBPA by chlorine. Figure 2 exemplified the HPLC/ESI-QqQMS chromatograms of

220

a sample containing TBrBPA treated by chlorine at pH 8 when PIS was set at m/z 79 and 81. As

221

can be seen, there were seven new peaks compared to the control sample with TBrBPA only. In



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addition, each peak in the chromatogram detected by the PIS at m/z 79 could find its counterpart

223

at m/z 81 by PIS, in compliance with the fact that the natural isotope abundance of

224

near 1:1. This result also suggested that these peaks in pair should correspond to bromine-

225

containing products.29, 30, 46-49

79

Br:81Br is

226

Product I eluted at 25.20 min had the molecular ions of m/z 307/309 in the PIS of m/z 79,

227

suggesting that it should contain two bromine ions. Also, the isotope abundance ratio of 1:1 in the

228

peak clusters was accordant with the theoretical prediction.29, 30, 46, 47 It was suggested to be 4-(2-

229

hydroxyisopropyl)-2,6-dibromophenol, which was consistent with the major fragments of

230

18(H2O), 59 ((CH3)2−C−OH) and 80/82 (H79Br/ H81Br) observed in the product ion scan spectra.

231

This product was also detected in other mild oxidation cases (e.g., Mn(VII), Fe(VI), MnO2, and

232

laccase-catalyzed oxidation).12-14, 50

233

Product II at the chromatogram retention time of 22.06 min with molecular ions of m/z 265/267

234

in the PIS of m/z 79 was supposed to be 2,6-dibromohydroquinone. It would be further oxidized

235

to 2,6-dibromoquinone (2,6-DBrBQ, product VIII, illustrated in the following section). Product III

236

eluted at 33.33 min with molecular ions of m/z 327/329/331 in the PIS of m/z 79 was assigned to

237

TBrP and further confirmed by the analytical standard (SI Figure S2). Product IV at 24.46 min had

238

molecular ions of m/z 263/265 in the PIS of m/z 79 and the isotope abundance ratio in peak cluster

239

was 3:1. This indicated the presence of one bromine atom and one chlorine atom in product IV.29

240

Product IV was supposed to be (2-hydroxyisopropyl)-2-bromo-6-chlorophenol. Product V eluted

241

at 32.59 min with ion clusters of m/z 283/285/287 in the PIS of m/z 79 should contain two bromine

242

atoms and one chlorine atom according to the isotope abundance ratio of 3:4:1.29 Product VI and

243

VII eluted at 34.5 and 41.4 min with large molecular ions of m/z 555/557/559/561 and


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787/789/791/793/795/797 in the PIS m/z of 79 might be dimeric products, which were also

245

detected in the cases involving Mn(VII), Fe(VI), MnO2, and laccase-catalyzed oxidation.12-14, 50

246

The chromatogram peak of product VIII with retention time at 25.42 min (see SI Figure S3) was

247

co-eluted with the primary product 4-(2-hydroxyisopropyl)-2,6-dibromophenol (product I) which

248

eluted at 25.20 min. So, product VIII could only be observed when product I was further oxidized

249

to a large extent (i.e., in the case of 20 μM chlorine). As shown in SI Figure S3, it had even-number

250

molecular ions of 264/266 and 266/268 in the PIS of m/z 79 and 81 respectively, and thus is

251

assigned to be 2,6-DBrBQ. This product was also confirmed by the analytical standard of 2,6-

252

DBrBQ. The unique MS characteristic of quinone-like compounds has been also reported in

253

several recent studies.51-56 Under negative ESI, quinones were likely reduced via accepting

254

electron to form even-numbered radicals M- • and/or accepting two electrons and losing one proton

255

to

256

HPLC/ESI−QqQMS (e.g., ion source temperature, mobile phase flow rate, and collision energy).

257

In addition, the yield of 2,6-DBBrQ from TBrBPA (i.e., the amounts of 2,6-DBrBQ produced

258

relative to TBrBPA lost) quantified by MRM mode was found to be about 5%.

259

form

odd-numbered

[M+H+2e-]-,

which

was

dependent

on

the

conditions

of

It was noted that PIS approach where product ion was fixed at Cl (m/z=35,37) was also used,

260

while no additional products (i.e., chlorinated TBrBPA) were detected.

261

Oxidation products of DBrMeP (product I) and TBP (product VIII)

262

In order to explicitly explore the pathways of the reaction of chlorine with TBrBPA, chlorination

263

of the major product 4-(2-hydroxyisopropyl)-2,6-dibromophenol (product I) warranted

264

investigation. Unfortunately, its commercial standard was unavailable. So, DBrMeP structurally

265

similar to product I was used. SI Figure S4 showed the PIS chromatogram of a sample containing

266

DBrMeP treated by chlorine at pH 8 in the PIS of m/z 79, where products II (2,6-



Page 14 of 34

267

dibromohydroquinone), III (TBrP), and VIII (2,6-DBrBQ) were generated. This suggested that

268

product I could be further transformed to products II, III, and VIII.

269

Furthermore, transformation products of TBrP (i.e., product III) treated by chlorine at pH 8 were

270

identified and the HPLC/ESI-QqQMS PIS chromatogram at m/z 79 was shown in SI Figure S5.

271

The peaks of products II and VIII were obvious, suggesting that these two products could also be

272

derived from TBrP. In addition, under various experimental conditions (i.e., varying pH and

273

reactant concentrations), 2,4,6-tribromo-3-chlorophenol that was supposed to be the major product

274

of TBrP formed through chlorine electrophilic substitution in literature,57 was not detected. This

275

was consistent with the products results of chlorination of TBrBPA, where no chlorinated TBrBPA

276

was generated.

277

Proposed reaction pathways

278

Based on the identified products, tentative pathways for the reactions of chlorine with TBrBPA

279

were proposed. As shown in Figure 3, the phenol moiety of TBrBPA is initially oxidized by

280

chlorine to lose one electron, forming a phenoxy radical R1. Then, radical R1 undergoes β-scission

281

and releases R2 and R3. The coupling of two R1 radicals also eliminates R2 with the formation of

282

product VII.12-14 Subsequently, the cationic R2 intermediate undergoes substitution reactions with

283

the generation of products I and VI. R3 radical is transformed to product III (TBrP) and product II

284

(2,6-dibromohydroquinone) which can undergo further oxidation leading to the formation

285

corresponding quinone (product VIII).12-14, 50 Products II and III may also be originated from

286

product I. Products IV and V are likely to be generated from products I and III by the exchange

287

of bromine with chlorine, respectively. Zhai and Zhang30 also detected product V during

288

chlorination of TBrP. Similarly, Wendel et al.58 have recently reported the substitution of iodine


Page 15 of 34


289

with chlorine during the transformation of iopamidol by chlorine. This ipso free-radical

290

substitution resulting in halogen exchange have also been reported in many other studies.59-61

291

The time course profiles of products during the kinetic runs were monitored. As shown in Figure

292

4, along with the loss of TBrBPA, product I rapidly reached to its maximum concentration (at

293

about one minute) and then declines quickly. The formation pattern of product IV was similar to

294

that of product I, which might be attributed to the rapid exchange of bromine (of product I) with

295

chlorine. Further, product IV (structurally similar to product I) might undergo the oxidation

296

pathway and halogen exchange pathway. However, the concentration of these secondary products

297

for product IV might below the detection limit and thus were not detected. Comparatively,

298

products II, III, and VIII were gradually formed (within the first two minutes) and then were slowly

299

degraded. Product V was possibly generated form product III through the rapid exchange of

300

bromine by chlorine, and thus its formation pattern was similar to that of product III. The dimer

301

products (i.e., products VI and VII) were of relatively low intensity compared to other products,

302

and thus their formation dynamics were not followed.

303

It was noted that the MS spectral peak areas for compounds with different response values should

304

not be compared to one another. Products III and VIII could be quantified by the authentic

305

standards and their concentrations were shown in Figure 4. Unfortunately, commercial standards

306

of other products were unavailable.

307

It is well known that oxidation, addition, and electrophilic substitution reactions are possible

308

pathways of chlorine with organic compounds. Electrophilic substitution reactions are the main

309

chlorination mechanism for aromatic rings.34 Criquet et al. have reported that phenolic compounds

310

can react with bromine either by oxidation or electrophilic substitution depending on the nature of

311

substituents.62 For instance, hydroquinone and catechol with hydroxyl substituents in para and



Page 16 of 34

312

ortho positions could be oxidized by bromine via electron transfer resulting in the formation of

313

quinones, while resorcinol with hydroxyl substituents in meta positions was attacked by bromine

314

via electrophilic substitution. In this study, it showed that chlorine oxidized fully-substituted

315

phenolic compound (e.g., TBrBPA, DBrMeP, and TBrP) via electron transfer with the formation

316

of halo-benzoquinones (HBQs) during chlorine treatment while chlorine substitution reaction did

317

not occur although meta positions were empty. In contrast, in the case of phenolic compounds with

318

available substitution positions (i.e., non-fully substituted phenolic compounds such as phenol,

319

BPA, and triclosan), chlorine could initially react with them mainly via electrophilic

320

substitutions.15, 31, 40, 41

321

Influence of HA on transformation of TBrBPA by chlorine

322

Influence of HA on kinetics

323

DOM has been shown to affect the transformation rates of various aqueous organic

324

contaminants.15, 17, 18, 63 HA as a major constituent of DOM was selected as a surrogate to explore

325

the effect of DOM on the transformation of TBrBPA by chlorine. Experiments were conducted at

326

an environmentally relevant pH 8 with varying concentrations of HA. The time course profiles of

327

TBrBPA and chlorine during the kinetic runs were presented in Figure 5a and 5b. As can be seen,

328

the degradation of TBrBPA was distinctly suppressed in the presence of HA and such suppression

329

was enhanced with elevated HA concentration. With the concentration of HA increasing from 0.5

330

to 5 mg C/L, TBrBPA degradation greatly decreased from 87% to 25% at one minute, while the

331

consumption of chlorine slightly increased from about 12% to 20%.

332

HA might serve as antioxidant to compete for chlorine with TBrBPA and it was believed to

333

result in the rate decrease in previous studies. When accounting for oxidant consumption, the


Page 17 of 34


334

following equation could be used to describe the chlorination kinetics of a target contaminant (e.g.,

335

TBrBPA) in the presence of HA: Ct

336

C0

t

=exp{-k ∫0 [HOCl]dt}

(9)

337

where Ct was the concentration of TBrBPA at a specific time t, C0 was the initial concentration of

338

TBrBPA, and ∫0 [HOCl]dt was the chlorine exposure (i.e., chlorine concentration integrated over

339

time). The predictions made according to eq 9 substantially overestimated the level of TBrBPA

340

degradation (see Figure 5c for example). In contrast, eq 9 could predict the loss rates of TBrBPA

341

reasonably well (data not shown) in the case of other common reduced species (e.g., NaHSO 3,

342

DMSO, and Fe(II)).

t

343

Recently, Canonica et al.17 and Lu et al.19 have reported similar results that the transformation

344

rates of a variety of aqueous contaminants (e.g., phenols, anilines, and phenylurea and some

345

pharmaceuticals) were inhibited by HA when subjected to oxidation by excited triplet state of

346

CBBP and laccase-catalyzed oxidation. These authors proposed a tentative model (eq. 10-12),

347

where the intermediates of these contaminants formed by one-electron oxidation (e.g., phenoxyl

348

radicals in the case of phenols) were competitively reduced back to their parents by HA.

349 350 351

k4

•

P+Oxidant → P+ • +

k5

P +HA → P • + k6

P → Pt

(10) (11) (12)

352

Contaminants (P) were oxidized by oxidant (e.g., excited triplet state of CBBP) leading to the

353

formation of an intermediate radical •P+ via one electron transfer (eq 10). This radical subsequently

354

went through two parallel pathways: (i) converted to its original form (P) by HA (eq 11), (ii) further

355

transformed to product (Pt) irreversibly (eq 12). The reactions 10 and 11 were both second-order



Page 18 of 34

356

to reactants, where k4 represented the rate constants in the absence of HA and k5 was rate constant

357

for reduction of the intermediate •P+ by HA. k6 was the first-order rate constant for the intermediate

358

• +

359

radical •P+ could be described by eq 13 and 14, respectively.

P leading to the formation of Pt. Thus the degradation rate for P as well as the formation rate for

360

-

dt

-

+

(13) +

=k4 [P][Oxi]-k6 [ •P ]-k5 [ •P ][HA]

(14)

d[P] dt

=k 4 k

k6 [P][Oxi] 6 +k 5 [HA]

(15)

From eq 15, it could be seen

365 366

+

=k4 [P][Oxi]-k5 [ •P ][HA]

By making a steady-state assumption for [•P+], it could be obtained:

363 364

dt

+ d[ •P ]

361 362

d[P]

kapp =k4 k

k6 6 +k5 [HA]

Then, the relationship of kapp and HA concentration could be obtained by eq 16 1

367

kapp

1

k6

4

4 k5

=k +k

[HA]

(16)

368

Canonica et al.17 demonstrated that the effect of HA could be quantitatively described by eq 16.

369

Also, Lu et al.19confirmed the role of HA in laccase catalyzed oxidation of halophenols with the

370

above model as well. Comparatively, a good linear correlation of 1/kapp with HA concentration

371

was obtained in this work (Figure 6), suggesting that HA played a similar role in chlorination of

372

TBrBPA by competitively reducing the oxidation intermediate (possibly phenoxy radical) to

373

parent TBrBPA.

374

Comparatively, it was found that the rate constants of chlorination of BPA remained almost

375

constant in the presence of different HA concentrations when accounting for the chlorine

376

consumption according to eq 9 (see Tabe 1). The contrasting effects of HA in the cases of BPA vs

377

TBrBPA might be related to their different transformation pathways during chlorination. Gallard


Page 19 of 34


378

et al.15 reported that BPA was attacked by chlorine electrophilic substitution with the formation of

379

chlorinated BPA (mono/di/tri/tetra chloro BPA), while TBrBPA of fully substituted with

380

substitutes at ortho- or para- position (i.e., without available substituent positions) underwent

381

oxidation pathways during chlorination in this study.

382

To further confirm the role of HA in chlorination of TBrBPA, the effect of HA on the oxidation

383

of TBrBPA by Mn(VII) and Fe(VI) was also examined, because the generation of phenoxy radical

384

via one electron transfer was well reported in previous studies. The kinetics of the reactions of

385

Mn(VII) and Fe(VI) with TBrBPA in the absence of HA have been well studied in previous work.12,

386

13

387

SI Figure S6 and S7. The loss of TBrBPA was inhibited in the presence of HA and such

388

suppression was increased with elevated HA concentrations in both Mn(VII) and Fe(VI) oxidation.

389

It was found that eq 16 could also fit these experimental data as shown in SI Figure S8, indicating

390

that HA played a similar role in the reactions of Mn (VII), Fe(VI), and chlorine with TBrBPA.

391

This confirmed that TBrBPA underwent a similar reaction pathway by chlorine in comparison to

392

Mn (VII) and Fe(VI), where an unstable phenoxy radical was initially formed via one-electron

393

transfer.12-14

. Time course profiles of TBrBPA and Mn(VII)/Fe(VI) during the kinetic runs were presented in

394

Influence of HA on products

395

Recently, Feng et al.14 reported that HA affected the products formation in laccase-catalyzed

396

oxidation of TBrBPA, where three new products were generated (i.e., molecular ion clusters of

397

m/z 667/669/671, 1039/1041/1043/1045/1047, and 321/323/325 in full scan, respectively).

398

Particularly, the product (IX) with molecular ions of 321/323/325 was also identified when

399

methanol co-solvent was used. Additionally, two recent studies reported the formation of product

400

IX in the reactions of Mn(VII) and Fe(VI) with TBrBPA as well when methanol co-solvent was



Page 20 of 34

401

used.12,13 It was suggested that 2,6-dibromo-4-isopropylphenol carbocation intermediate (R3)

402

underwent substitution with HA or methanol leading to formation of product IX.

403

Interestingly, no additional products were detected in the presence of HA during chlorination of

404

TBrBPA. However, product IX was detected when methanol co-solvent was used by selective ion

405

scan mode in this study (see SI Figure S9). The discrepancy of HA effect on product formation of

406

TBrBPA between this study and Feng’s was not clear so far, and it might be related to the different

407

properties of HA from diverse sources (e.g., Suwannee River NOM in ref 14 and sigma HA in this

408

study) and/or the different experimental conditions (e.g., pH, the initial concentrations of HA),

409

which warrants further investigation.

410

Oxidation in natural waters

411

Kinetic experiments were conducted in natural water samples and time-dependent

412

concentrations of TBrBPA and chlorine were determined, respectively. As can be seen in Figure7a,

413

predictions made according to eq 9 substantially overestimated the level of TBrBPA degradation

414

in natural water samples. This was consistent with above results that HA inhibited the rate constant

415

of TBrBPA by chlorine. Furtherly, it can be inferred that in aquatic environment where TBrBPA

416

concentration was much lower than 100nM, the TBrBPA intermediates might be substantially

417

reduced back to parent by organic matter with relatively high concentration. Thus degradation rate

418

of TBrBPA at environmental relevant concentrations might be considerably inhibited. In addition,

419

six products (i.e., product I, II, III, IV, V, and VIII) were detected by MRM mode. The other two

420

products (i.e., product VI and VII) were not detected which might be due to their low concentration.

421

The time course profiles of these products during the kinetic runs in real water were also

422

monitored (see SI Figure S10), which was similar to that in synthetic buffer.


Page 21 of 34


423

Overall, this study has demonstrated that the flame retardant TBrBPA can be oxidized by the

424

globally most used disinfectant chlorine over a wide pH range of 5-10 with apparent second-order

425

rate constants of 138−3210 M−1 s−1. The transformation pathway involves the formation of

426

TBrBPA phenoxyl radical and subsequent substitution, dimerization, and oxidation reactions of

427

this primary radical and its secondary intermediates (e.g., 2,6-dibromo-4-isopropylphenol

428

carbocation) formed via beta-scission. It seems likely that HA can competitively react with the

429

phenoxy radical by reversing it back to parent TBrBPA and thus affect the transformation rate of

430

TBrBPA by chlorine. These findings were firstly reported and might improve the understanding

431

of chlorine chemistry in water and wastewater treatment.

432

Brominated products formed from TBrBPA by chlorine include (e.g., TBrP, 4-(2-

433

hydroxyisopropyl)-2,6-dibromophenol, and 2,6-DBrBQ, as well as dimers. In addition, these

434

products are also detected during chlorine disinfection in real waters. The adverse effects of these

435

brominated products should receive great attentions. For instance, Suzuki et al. have demonstrated

436

that brominated phenols especially TBrP are potential thyroid-disrupting compounds by strongly

437

inhibiting thyroxine binding to the human thyroid hormone transport protein transthyretin in

438

vitro.64,65 Du et al. have recently reported that HBQs (including 2,6-DBrBQ) are cytotoxic to T24

439

bladder cancer, where the damage to DNA by reactive oxygen species induced by HBQs and

440

protein carbonylation are involved.66 Particularly, the occurrence of HBQs as a group of emerging

441

chlorine disinfection byproducts in drinking water has been also reported in recent years, although

442

their naturally occurring precursors are not clearly known. In this study, free chlorine is shown to

443

be able to oxidize several anthropogenic phenolic compounds (e.g., TBrBPA, DBrMeP, and TBrP)

444

of full substitution at ortho and para positions on aromatic rings leading to the generation of HBQs.

445

This also provides a plausible pathway to account for the occurrence of HBQs during chlorine



Page 22 of 34

446

disinfection process, where halogenated phenolic intermediates generated from non-fully

447

substituted phenolic precursors by stepwise halogen substitution could further undergo oxidation

448

reactions with the formation of HBQS

449 450

Acknowledgments

451

This work was financially supported by the National Natural Science Foundation of China

452

(51178134 & 51378141), the Funds of the State Key Laboratory of Urban Water Resource and

453

Environment (HIT, 2016DX13), the Foundation for the Author of National Excellent Doctoral

454

Dissertation of China (201346), and the Fundamental Research Funds for the Central Universities

455

of China (AUGA5710056314).

456

Supporting Information

457

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

458

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

459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476

References: (1) de Wit, C. A. An overview of brominated flame retardants in the environment. Chemosphere. 2002, 46, (5), 583624. (2) Alaee, M.; Arias, P.; Sjödin, A.; Bergman, Å. An overview of commercially used brominated flame retardants, their applications, their use patterns in different countries/regions and possible modes of release. Environ. Int. 2003, 29, (6), 683-689. (3) Birnbaum, L. S.; Staskal, D. F. Brominated flame retardants: cause for concern? Environ. Health Persp. 2004, 112 (1), 9. (4) Qu, G.; Shi, J.; Wang, T.; Fu, J.; Li, Z.; Wang, P.; Ruan, T.; Jiang, G. Identification of tetrabromobisphenol A diallyl ether as an emerging neurotoxicant in environmental samples by bioassay-directed fractionation and HPLCAPCI-MS/MS. Environ. Sci. Technol. 2011, 45 (11), 5009-5016. (5) Mäkinen, M. S. E.; M Mäkinen, M. R. A.; Koistinen, J. T. B.; Pasanen, A.; Pasanen, P. O.; Kalliokoski, P. J.; Korpi, A. M. Respiratory and dermal exposure to organophosphorus flame retardants and tetrabromobisphenol A at five work environments. Environ. Sci. Technol. 2009, 43 (3), 941-947. (6) Guerra, P.; Eljarrat, E.; Barceló, D. Simultaneous determination of hexabromocyclododecane, tetrabromobisphenol A, and related compounds in sewage sludge and sediment samples from Ebro River basin (Spain).


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Kinetics, equilibrium and thermodynamics of the sorption of tetrabromobisphenol A on multiwalled carbon nanotubes. Appl. Surf. Sci. 2010, 256 (23), 7246-7252. (25) Pinkernell, U.; Nowack, B.; Gallard, H.; von Gunten, U. Methods for the photometric determination of reactive bromine and chlorine species with ABTS. Water Res. 2000, 34 (18), 4343-4350. (26)Jiang, J.; Pang, S.-Y.; Ma, J.; Liu, H. Oxidation of phenolic endocrine disrupting chemicals by potassium permanganate in synthetic and real waters. Environ. Sci. Technol. 2012, 46 (3), 1774−1781. (27) Jiang, J.; Pang, S.-Y.; Ma, J. Oxidation of triclosan by permanganate (Mn(VII)): Importance of ligands and in situ formed manganese oxides. Environ. Sci. Technol. 2009, 43 (21), 8326-8331. (28) Jiang, J.; Pang, S-Y.; Ma, J., Role of ligands in permanganate oxidation of organics. Environ. Sci. Technol. 2010, 44 (11), 4270-4275. (29) Zhang, X.; Talley, J. W.; Boggess, B.; Ding, G.; Birdsell, D. 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Page 26 of 34

614 TBrBPA TClBPA

3200

-1 -1

kapp(M s )

2400

1600

800

0 5

615 616 617 618

6

7

8

9

10

pH

Figure 1. pH-Dependence of measured second-order rate constants (k, M−1 s−1) for the reactions of chlorine with TBrBPA and TClBPA. Symbols represent measured data and the dashed lines show the model fit according to eq 8.

619


Page 27 of 34


620 TIC of -Prec (79.00): Exp 1, from Sample 3 (TBrBPA(10uM)+HOCl(pH=7, 10uM, ascorbic 1:5, t=10min)_ESI(-)_Pre) of 20...

Max. 1.5e7 cps.

25.20 I

100%

(a)

90%

Rel. Int. (%)

80% 70% II 22.06

60%

VI 34.51

50% 40% IV

30%

24.46

V III 32.59 33.33

20%

* 19.81

10% 0%

0

5

10

15

VII

* 26.34

20

TBrBPA

36.20 41.41

25

30 35 40 Time, min TIC of -Prec (81.00): Exp 2, from Sample 3 (TBrBPA(10uM)+HOCl(pH=7, 10uM, ascorbic 1:5, t=10min)_ESI(-)_Pre) of 20...

621

55

Max. 1.6e7 cps.

Rel. Int. (%)

80% 70% 60% 50% 40% 30%

24.47 33.35

10% 0%

19.93 5

10

15

-Prec (79.00): Exp 1, 25.176 min from Sample 2 (TBrBPA(10uM)+HOCl(pH...

20

100%

306.9

25

30 35 Time, min

60% 40% 20%

45

50

55

Max. 4.5e5 cps.

264.8

100%

80%

40

-Prec (79.00): Exp 1, 22.029 min from Sample 4 (246Br(10uM)+HOCl(pH=...

308.9 Rel. Int. (%)

I

Max. 5.7e5 cps.

36.18 41.30

26.34

622

266.8

II 80% 60% 40% 20%

267.9

0%

623

60

(b)

20%

Rel. Int. (%)

50

25.20

100% 90%

45

305

310

315

0%

260

m/z, Da

265 m/z, Da

270

275


60


-Prec (79.00): Exp 1, 33.315 min from Sample 1 (246Br(10uM)(pH=7, bl...

60%

330.9

326.8 40% 20%

624

327.7 320

325

60% 40%

264.9

20%

264.0

330 m/z, Da

335

0%

340

260

Rel. Int. (%)

Rel. Int. (%)

282.9

60% 40%

0%

286.7

625

278.6 280

285 m/z, Da

290

: from 1.txt (Unknown Ion Source)

790.5 792.5

60% 794.5

40% 20% 0% 775

786.5 780

785

796.5 790 795 m/z, Da

800

806.5 805 810

558.7

VI

60% 40%

560.7

554.7

20% 562.7 550

555

560 m/z, Da

564.8 565

80%

570 Max. 1.2e5 cps.

263.8

100%

VII

788.6

275 Max. 4.2e6 cps.

-Prec (79.00): Exp 1, 25.393 min from Sample 3 (246Br(10uM)+HOCl(pH=...

Rel. Int. (%)

Rel. Int. (%)

80%

80%

0%

295 Max. 1.1e7 cps.

100%

556.7

100%

V

20%

270

Max. 1.2e5 cps.

284.7

80%

266.0 266.9

265 m/z, Da

: from 550-570.txt (Unknown Ion Source)

100%

627 628 629 630 631

80%

329.7 331.8


626

IV

Rel. Int. (%)

Rel. Int. (%)

80%

Max. 2.4e5 cps.

263.0

100%

III

0%


Max. 2.4e6 cps.

328.8

100%

Page 28 of 34

265.9

VIII

60% 40% 265.0

20% 0%

260

262

264

266.7

266 m/z, Da

268

270

Figure 2. The HPLC/ESI–QqQMS PIS chromatograms of a sample containing TBrBPA treated by chlorine at m/z 79 (a) and 81(b). Asterisks represent the major impurities contained in commercial TBrBPA standard. The underneath (I-VIII) showed the corresponding molecular ion mass spectra of the chromatographic peaks at m/z 79. Experimental condition: [TBrBPA] = 10 μM, [chlorine] = 10 μM, and pH 8.

632


Page 29 of 34


633

Br

Br CH3 C CH3

HO Br

Br

OH

Br CH 3 C CH 3

O

Br

Br

Br OH

Br

Br CH3 C CH3

O Br

Br

Br HO

O Br

Br

Beta Scission

Br

Br

Br CH3 C CH3

CH3 C+ CH3

HO

OH

Br

Br

OH R3

R2

Ⅶ

+CH3OH CH3 C OCH3 HO CH3

HO Br

Br

+H2O

Br

Br

Br CH3 C OH CH3

Br

IX

+II

Br

R1

Coupling Elimination

R1

OH

Br

OH

III

I

Br HO

Br

OH Br

II

Exchange of bromine for chlorine

Br HO Br

CH 3 O C CH 3

VI

634 635

OH Br

Br

Br

Br

Br

CH 3 C OH CH 3

HO Cl

Cl

OH

O

O

Br

IV

V

Br

VIII

Figure 3. Proposed transformation pathways of TBrBPA by aqueous chlorine.

636



Page 30 of 34

637 638 (a)

0.5

TBrBPA

TBrBPA(M

0.4 0.3 0.2 0.1

(b)

24000

Area

80

Product (I) Product (II) Product (III) Product (IV) Product (V) Product (VIII)

16000

60

40

8000

20

0

0 0

639 640 641

concentration(nM)

0.0 32000

2

4

6

8

Time(min)

Figure 4. Degradation of TBrBPA (0.5 µM) and formation of products during the treatment by chlorine (16.5 µM) at pH 8.

642 643


Page 31 of 34


644

[TBrBPA]t/[TBrBPA]0

0.8 0.6 0.4 0.2

1.0

(b)

0.8

[HOCl]t/[HOCl]0

HA=0 HA=0.5mgC/L HA=1mgC/L HA=2mgC/L HA=5mgC/L

1.0

0.6 0.4


0.2

(a) 0.0

0

30

60

90

30

60

120

(b)

0.8

[HOCl]t/[HOCl]0

[TBrBPA]t/[TBrBPA]0

1.0

0.8 0.6 0.4 0.2

90

T(s)

HA=5mgC/L expermental data HA=5mgC/L model prediction by eq 9

1.0

(c)

0.6 0.4


0.2

0.0

0.0 0

647 648 649 650

0

T(s)

645

646

0.0

120

30

60

90

120

0

T(s)

30

60

90

120

T(s)

Figure 5. Effect of HA concentrations on the degradation of TBrBPA (a) and consumption of chlorine (b). Panel c shows the comparison of measured data and model prediction by eq 9 in the presence of 5 mgC/L HA (c) Experimental condition: [TBrBPA]= 0.5 μM, [chlorine] = 16.5 μM, and pH 8.

651



Page 32 of 34

652

0.0035 0.0030

-1

k (Ms)

0.0025 0.0020 0.0015

2

R =0.98

0.0010 0.0005 0.0000

0

1

2

4

5

HA (mgC/L)

653 654 655

3

Figure 6. Correlation between HA concentrations and the measured second-order constants of the reaction of TBrBPA (0.5µM) with chlorine (16.5 µM) in the presence of HA at pH 8.

656 in real water model prediction by eq 9

(b)

1.0

0.8

0.8

[HOCl]t/[HOCl]0

[TBrBPA]t/[TBrBPA]0

1.0

0.6 0.4 0.2

0.6 0.4 0.2

(a) 0.0

0.0 0

657 658 659

20

40

60

80

100

0

T(s)

20

40

60

80

100

T(s)

Figure 7. The degradation of TBrBPA in real water (a) and consumption of chlorine (b). Experimental condition: [TBrBPA]= 100 nM, [chlorine] = 2mgC/L, and pH 8.

660 661 662 663 664 665


Page 33 of 34

666 667 668


Table1. Apparent second-order rate constants (k, M-1 s-1) for reactions of chlorine with BPA in the presence of varied concentrations of HA at pH 8 HA (mgC/L) 0 0.5 1 2 5

k (M-1s-1) 297 286 301 317 296

669 670



671

Page 34 of 34

TOC Artwork Br

Br CH3 C OH HO CH3

HO Br Br

HA +eBr HO Br

HO Cl

Br CH3 C CH3

CH 3 C OH CH 3

OH Br Br

Br

OH Br

Br

OH

Cl

Br

OH

Br

O

Br

Br

-e-

CH 3 O C CH 3

HO

HOCl

Br

Br

Br HO

O Br

Br

O Br Br

OH Br

Br CH 3 C CH 3

OH Br

672


Transformation of Flame Retardant Tetrabromobisphenol A by

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