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Chlorinated methylsiloxanes generated in papermaking process and their fate in wastewater treatment processes Lin Xu, Xudan He, Liqin Zhi, Chunhui Zhang, Tao Zeng, and Yaqi Cai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03512 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on October 5, 2016

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Environmental Science & Technology

Chlorinated methylsiloxanes generated in papermaking process and their fate in wastewater treatment processes Lin Xu §, Xudan He†, Liqin Zhi §, Chunhui Zhang†, Tao Zengǁ, Yaqi Cai §‡* Affiliations: § State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research

Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China †School of Chemical & Environmental Engineering, China University of Mining & Technology (Beijing), Beijing, 100083, China ǁCollege of Environment, Zhejiang University of Technology, Hangzhou 310032, P. R. China ‡ Institute of Environment and Health, Jianghan University, Wuhan 430056, China * Corresponding author: Tel: +86 (10) 62849182; Fax: 8610-62849182; E-mail: [email protected]

ACS Paragon Plus Environment


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1

ABSTRACT

2

Simulated experiments indicated that chlorinated volatile methylsiloxanes,

3

detected by Q-TOF GC/MS, could be generated in pulp-bleaching process, where

4

poly(dimethylsiloxane)s fluids with volatile methylsiloxanes as impurities, and

5

molecular chlorine were used as de-foamer and bleaching agent, respectively. In the

6

producing processes of one papermaking factory, the mean total concentrations of

7

mono-chlorinated D4, D5, and D6, i.e., D3D(CH2Cl), D4D(CH2Cl), and D5D(CH2Cl),

8

were 0.0430 - 287µg/L in aqueous samples, while 0.0329 -270 µg/g in solid samples.

9

In the coupled papermaking-wastewater treatment processes,

D3D(CH2Cl),

10

D4D(CH2Cl), and D5D(CH2Cl) were detected in all water (0.113 -8.68 µg/L) and

11

solid samples (0.888-26.2µg/g), with solid-water partition values (468-3982 L/Kg)

12

1.08-4.82 times higher than those of their corresponding non-chlorinated

13

The removing efficiencies of D3D(CH2Cl)-D5D(CH2Cl) in the whole wastewater

14

treatment processes were 77.1-81.6%, and sorption to sludge (35.7-74.1%) and

15

removal in primary clarifier (7.19-32.5%) had major contributions to their total

16

removal. Elimination experiments showed that: 1) hydrolysis half-lives of

17

D3D(CH2Cl)-D5D(CH2Cl) (0.9-346 h) in primary clarifier (pH=7.8-9.2) were

18

2.16-3.60 times shorter than those of their non-chlorinated analogs;

19

D3D(CH2Cl)-D5D(CH2Cl) were hardly degraded in oxic sludge treatment process,

20

and their volatilization half-lives (7.38-21.1 h) in oxic sludge were 1.21-1.50 times

21

longer than those of their non-chlorinated analogs.

22

Keywords: chlorinated methylsiloxanes; papermaking; wastewater; hydrolysis


analogs.

2)


23 24

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1. INTRODUCTION

Water chlorination is a commonly used technique in industrial processes (such as

25

pulp bleaching), advanced industrial/municipal wastewater treatment plants (WWTPs)

26

and drinking water treatments. In some circumstances, this process may generate

27

chlorinated organic compounds known as disinfection byproducts.1, 2 In recent years,

28

many studies have been focusing on environmental behavior and health effects of

29

chlorinated products of organic pollutants in such processes.3, 4

30

Due to low surface tension, high thermal stability and lubricating properties,

31

polydimethylsiloxane (PDMS) have vast usage in industrial processes and consumer

32

products.5-7 The impurities of PDMS, cyclic volatile methylsiloxanes (cVMS),

33

been found in air, water, sludge/sediment and biota samples impacted by

34

municipal/industrial wastewater treatment effluent discharge.8-15 cVMS in gas phase

35

could be oxidized by hydroxyl radical, Ozone (O3), Cl atom, and NO3 radicals, and

36

bi-molecular

37

(Hexamethyldisiloxane) by Cl atom is 10 times faster than that for hydroxyl

38

radicals.16 However, until now, there was no study reporting whether methylsiloxanes

39

could be chlorinated by Cl atom or other free available chlorine in aqueous phase

40

under real environmental conditions.

rate

constants

for

oxidization

of

one

have

methylsiloxane

41

More specifically, PDMS have a vast application as de-foamer in many

42

papermaking processes, including pulp-making, bleaching, and dewatering, etc.17, 18

43

Based on the reported dosage levels (0.2-0.8 kg/t) of PDMS de-foamer during

44

pulp-bleaching and percentages (99%) colorimetric

190

method.27 In the chlorination experiment, simulated Stage C samples (pH = 3.0, free

191

chlorine content = 0.42 g/L, 25oC) were prepared by spiking pure water with Cl2,

192

while Stage H samples (pH = 9.5, free chlorine content = 0.25 g/L, 40 oC) were

193

prepared by spiking pure water with NaOH and NaClO. Acetone solution (80 µl, 5

194

mg/L) of cVMS was injected into each simulated sample contained in capped 40

195

mL-glass vial without headspace. After 30 min contact time, the chlorination process

196

was quenched with excessive sodium thiosulfate. Subsequently, water sample was

197

extracted with organic solvent (Section 2.3.2). The exacted compounds were

198

separated with 30 m DB-5MS column (J&W Scientific, Folsom, CA) and



Page 12 of 36

199

qualitatively analyzed with Q-TOF GC/MS. Meanwhile, water samples of cVMS

200

without chlorinating agent were also analyzed as the control for comparison.

201

2.6.2 Hydrolysis of methylsiloxanes and their mono-chlorinated products

202

The experiments were carried out in three pH values - 7.8 [hydrochloric acid–

203

tris(hydroxymethyl)aminomethane

buffer],

204

tris(hydroxymethyl)aminomethane buffer], and 9.2 [glycine-sodium hydroxide buffer].

205

In the batch test, each aqueous sample was added into a 40 ml-glass vial without

206

headspace and sealed with a cap made of aluminum foil disc sandwiched between two

207

Teflon discs. Then, acetone solution (40 µl, 500µg/L) of target compounds [D4-D6,

208

D3D(CH2Cl)-D5D(CH2Cl)] was injected through the side of cap into aqueous sample

209

via appropriate microlitre syringes, respectively. After injection, the cap was rotated

210

back and forth several times to dis-align the three discs so that the original injecting

211

hole will not be aligned through the 3 discs. Subsequently, the vials were incubated in

212

the light-proof shaker (100 rpm) at 22 °C.

213

intervals (0, 1, 3, 10, 24 and 48 h), three vials were taken to determine concentrations

214

of target parent compounds. Meanwhile, in the samples spiked with mono-chlorinated

215

methylsiloxanes

216

[Me2Si(OH)2 and chloromethyl(methyl)silanediol [(CH2Cl)MeSi(OH)2] were also

217

detected. The analysis methods for these silanediols were modified from Xu and

218

Kropscott (2015) as described in Supporting Information.28

219

2.6.3 Removal of methylsiloxanes and their mono-chlorinated products in oxic

220

treatment

[D3D(CH2Cl)-D5D(CH2Cl)]

8.5

[hydrochloric

acid–

At each of the predetermined time

at

pH


8.5,

dimethylsilanediol

Page 13 of 36


Biodegradation.

221

Each glass vial (40 mL) contained 25 mL of oxic sludge (solid

222

content = 3.5 g L-1, pH = 7.2, 22oC) from the studied factory. The headspace of sludge

223

was flushed with pure oxygen, and then seal with a cap made of aluminum foil disc

224

sandwiched between two Teflon discs. And then acetone solution (40 µl, 500µg/L) of

225

target compounds [D4- D6, D3D(CH2Cl)- D5D(CH2Cl)] was spiked in the culture

226

and incubated with the same methods used in hydrolysis experiment. At each of the

227

predetermined time intervals (0, 1, 3, 10, 24, 48h), three vials were taken to determine

228

concentrations of target compounds. Meanwhile, glass vials containing sludge

229

sterilized by 1% NaN3 were prepared and test as the control.

230

Volatilization. 25 mL of oxic sterile sludge, spiked with the target compounds

231

[D4-D6, D3D(CH2Cl)- D5D(CH2Cl)] in acetone solution (25 µl, 500µg/L), was added

232

into glass vial (40 mL). Then, the culture was incubated without capping in the

233

light-proof shaker (100 rpm) at 22 °C. At each of the predetermined time intervals (0,

234

1, 3, 10, 24, 48h), three vials were taken to determine concentrations of target

235

compounds. It should be noted that because water in opened sludge may evaporate

236

during incubating, the target compound concentrations were corrected by water loss.

237 238

3

RESULTS AND DISCUSSION

239 240

3.1 Chlorination of cyclic methylsiloxanes in simulated bleaching process

241

To the best of our knowledge, there was no previous study on whether

242

methylsiloxanes would be chlorinated by free chlorine in aqueous phase. Therefore,



243

the first section of the present study was to determine the likelihood of the

244

chlorination of methylsiloxanes in aqueous phase of a simulated bleaching process

245

and their major chlorinated products. The simulated bleaching process was used in

246

anticipation of the difficulty in direct identification of their chlorinated products in a

247

non-target analysis of the aqueous samples from the real bleaching process with a

248

complex matrix from paper-making processes.

249

As detailed below, besides the parent D4, D5, and D6, some additional

250

compounds were found in the full scan chromatogram for the chlorinated water

251

samples from the chlorination (C) stage (Figure 2a, 2b and 2c).

252

Chlorination of D4. Figure 2a showed that in the full scan chromatogram for the

253

chlorinated sample of D4, besides the peak of D4, some additional peaks

254

corresponding to mono-chlorinated methyl D4 (RT= 8.65) and di-chlorinated methyl

255

D4 (RT = 11.24, 11.27, and 11.30 min) were also observed.

256

Besides three fragments same with D4 - [C5 H13 O4 (28Si)4]+ (m/z = 248.9892,

257

∆m = -2.8 ppm), [C6 H17 O4 (28Si)4]+ (m/z=265.0206, ∆m = -3.0 ppm), [C7H21O4

258

(28Si)4]+ (m/z = 281.0518, ∆m = -2.5 ppm), the EI spectrum of the peak RT =8.65 min

259

had two major fragment ions both containing Cl atom - [C5H16O4 (28Si)4(35Cl)]+ (m/z

260

= 286.9822, ∆m = -4.5 ppm) and [C7H20O4 (28Si)4(35Cl)]+ (m/z = 315.0133, ∆m = -3.7

261

ppm), suggesting mono-chlorinated D4 as the possible identity. The methane PCI

262

spectrum of the peak RT =8.65 min showed three typical PCI methane adducts (+H,

263

+C2H5 and + C3H5) of the molecular ion of D3D(CH2Cl), i.e. [C8H23O4 (28Si)4(35Cl)]+

264

–[C8H23O4 (28Si)4(35Cl) +H]+ (m/z 331.0441, ∆m = -1.8 ppm ), [C8H23O4(28Si)4(35Cl)


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Page 15 of 36


265

+C2H5]+ (m/z 359.0758, ∆m = - 2.8 ppm), and

[C8H23O4 (28Si)4 (35Cl) +C3H5]+ (m/z

266

371.0760, ∆m = - 3.2 ppm). Therefore, combining EI and PCI spectrum, we

267

concluded that the peak at RT of 8.65 min must arise from the mono-chlorinated

268

product of D4 - D3D(CH2Cl). It should be noted that both Si and Cl have important isotopes - 28Si (abundance

269

29

Si (abundance = 3.1%),35Cl (abundance =

= 92.2%),

271

75.8%) and

272

28

273

that except for those merely containing 28Si and 35Cl, there would be some fragments

274

ions containing Si and Cl isotopes. For example, m/z = 288.9782 in the EI mass

275

spectrum for D3D(CH2Cl) could correspond to both [C5H16O4(28Si)3 (

276

(∆m= -1.7ppm) and [C5H16O4 (28Si)4 (37Cl)]+ (∆m = -1.0 ppm). Correcting by the

277

abundance

278

[C7H21O4(28Si)3(30Si)]+ (∆m= -1.7ppm) in EI mass spectrum for D4 and D3D(CH2Cl),

279

we found that the abundance ratio of [C5H16O4 (28Si)4 (37Cl)]+ to [C5H16O4 (28Si)4

280

(35Cl)]+ in the mass spectrum for D3D(CH2Cl) was about 1:3, which further

281

confirming our speculation for the chlorination of D4. Because the possible overlap of

282

mass spectrum for fragments containing

283

fragments in the following discussion about di-chlorinated D4, or mono- / di-

284

chlorinate products of D5/ D6.

Si and

37

Si (abundance = 4.7%),

30

270

37

Cl (abundance = 24.2%), and atomic mass differences between

Cl -

35

30

Si -

Cl were very similar (1.9968 and 1.9970, respectively), suggesting

ratios

of

m/z

281.0518

[C7H21O4(28Si)4]+

30

Si or

37

to

m/z

30

Si)

35

Cl]+

283.0486

Cl, we did not mention these

285

The peaks at RT around 11.24-11.30 min may arise from the chlorinated D4 with

286

two chlorinated methyl group in each molecule, or D3D(CH2Cl)2. In the EI spectrum



Page 16 of 36

287

of peak RT = 11.27 min, two major fragment ions were found same with D3D(CH2Cl)

288

- [C5H16O4 (28Si)4(35Cl)]+ (m/z = 286.9822, ∆m = - 4.5 ppm) and [C7H20O4

289

(28Si)4(35Cl)]+ (m/z = 315.0133, ∆m = -3.7 ppm), while m/z 348.9743 should attribute

290

to an ion with two Cl atoms - [C7H19O4 (28Si)4(35Cl)2]+ (∆m = -3.15 ppm) (or typical

291

“M-CH3” ion for methylsiloxanes). In addition, the PCI spectrum of the peak at RT of

292

11.27 min had typical methane adducts of the molecular ion [C8H22O4 (28Si)4(35Cl)2]+ :

293

[C8H22O4(28Si)4(35Cl)2+H]+

294

[C8H22O4(28Si)4(35Cl)2+C2H5]+ (m/z = 393.0352, ∆m = 1.2 ppm), indicating that the

295

peak at RT of 11.27 min should be assigned to D3D(CH2Cl)2. The peaks RT=11.24

296

and 11.30 min had very similar EI and PCI spectrums with peak RT = 11.27 min,

297

indicating that these peaks RTs may be isomers of D3D(CH2Cl)2.

(m/z

=

365.0039,

∆m

=

1.6

ppm),

and

298

Chlorination of D5. Figure 2b showed that in the full scan chromatogram for the

299

chlorinated sample of D5, some peaks corresponding to mono-chlorinated methyl D5

300

(RT= 10.60) and di-chlorinated methyl D5 (RT = 13.12, 13.14, and 13.19 min) were

301

observed.

302

The EI spectrum for D5 had three major fragment ions- [C3H9

28

Si]+ (m/z =

303

73.0470, ∆m = -2.7 ppm), [C5H15O5(28Si)4]+ (m/z = 267.0004, ∆m = -4.8 ppm), and

304

[C9H27O5(28Si)5]+ (m/z = 355.0709, ∆m = -2.5 ppm).

305

fragment ions same with D5, the EI spectrum of the peak RT=10.60 min, had one

306

major fragment ion containing one Cl atom - [C9H26O5(28Si)5(35Cl)]+ (m/z = 389.0321,

307

∆m = -2.8 ppm). Furthermore, the methane PCI spectrum for peak RT=10.60 min had

308

two methane adducts (+H, +C2H5) of D4D(CH2Cl), i.e. [C10H29O5(28Si)5(35Cl)]+:


Besides the above three

Page 17 of 36


309

[C10H29O5(28Si)5(35Cl)+H]+

310

[C10H29O5(28Si)5(35Cl)+C2H5]+ (m/z = 433.0939, ∆m = -1.2 ppm ). Both EI and PCI

311

spectrum indicated that peak RT=10.60 min corresponded to the mono-chlorinated

312

compound of D5 – D4D(CH2Cl).

(m/z

=

405.0625,

∆m

=

-0.9

ppm

)

and

313

The peaks at RT=13.12, 13.14, and 13.19 min may arise from the chlorinated D5

314

with two chlorinated methyl group in each molecule, or D4D(CH2Cl)2. In the EI

315

spectrums for these three peaks, besides four major fragments same with

316

D4D(CH2Cl), there was one major fragments containing two Cl atom ,

317

[C9H25O5(28Si)5(35Cl)2]+ (m/z = 422.9924, ∆m = -0.9 ppm ). Meanwhile, the PCI

318

spectrum had one methane adduct for molecular ion for [C10H28O5(28Si)5(35Cl)2]+ :

319

[C10H28O5(28Si)5(35Cl)2+H]+ (m/z = 439.0221, ∆m = 2.7 ppm ), indicating these three

320

peaks correspond to isomers of D4D(CH2Cl)2.

321

Chlorination of D6. Figure 2c showed that in the full scan chromatogram for

322

the chlorinated sample of D6, some peaks corresponding to mono-chlorinated methyl

323

D6 (RT= 12.70 min) and di-chlorinated methyl D6 (RT = 15.05 and 15.11 min) were

324

observed.

325

In the EI spectrum for peak RT=12.70 min, [C3H9 28Si]+ (m/z = 73.0471, ∆m = -

326

4.1 ppm), [C6H17O6(28Si)5]+ (m/z = 324.9868, ∆m = - 0.6 ppm), [C7H21O6(28Si)5]+

327

(m/z = 341.0181, ∆m = -0.6 ppm), and [C11H33O6(28Si)6]+ (m/z = 429.0891, ∆m = -

328

0.9 ppm) were same with those of D6, while an additional major fragment ion

329

containing one Cl atom - [C11H32O6(28Si)6(35Cl)]+ (m/z = 463.0517, ∆m = - 4.1 ppm).

330

Furthermore, PCI spectrum for this peak had two methane adducts (+H, +C2H5) of



Page 18 of 36

331

molecular ion [C12H35O6(28Si)6(35Cl)]+: [C12H35O6(28Si)6(35Cl)+H]+ (m/z = 479.0819,

332

∆m = -1.8 ppm ) and [C12H35O6(28Si)6(35Cl)+C2H5]+ (m/z = 507.1122, ∆m = -0.6

333

ppm ). Both EI and PCI spectrum indicated that peak RT=12.70 corresponded to the

334

mono-chlorinated compound of D6 – D5D(CH2Cl).

335

In the EI spectrum for the peaks RT=15.05 and 15.11 min, although there was no

336

same

major

fragment

with

D6

or

D5D(CH2Cl),

the

fragment

ion

337

[C11H31O6(28Si)6(35Cl)2]+ (m/z = 497.0119, ∆m = -2.2 ppm) indicated that the

338

corresponded compounds would have six Si-O bonds at least and there would be two

339

groups of –CH3 replaced with –CH2Cl.

340

peaks had one methane adduct for molecular ion for [C12H34O6(28Si)6(35Cl)2]+ :

341

[C12H34O6(28Si)6(35Cl)2+H]+ (m/z = 513.0425, ∆m = -0.9 ppm ), indicating the peaks

342

RT=15.05 and 15.11 min correspond to isomers of dichlorinated-D6 compounds.

Meanwhile, the PCI spectrum for these two

343

Different from those in chlorination (C) stage, chlorinated products of cVMS in

344

either simulated hypochlorite (H) stage or control samples without chlorinating agent

345

were not detected. Although free chlorine content (0.25 g/L) in H stage was lower

346

than that (0.42 g/L) in C stage, the difference was not large enough to explain the

347

non-detection of chlorinated methylsiloxanes in H stage. The chlorine speciation as

348

affected by pH may be the major factor. In alkalic aqueous environment (pH > 8), the

349

major chlorine species would be OCl- and HOCl.29 The non-detection for chlorinated

350

methylsiloxanes in H stage (pH = 9.5) indicated that either OCl- or HOCl would

351

hardly chlorinate methylsiloxanes. While in chlorination (C) stage – the highly acidic

352

aqueous environment (pH < 3), besides HOCl, there would be residual Cl2, which was


Page 19 of 36


353

the much more reactive chlorinating agent than HOCl.29,

30

354

chlorination of methylsiloxanes in the C stage suggested that Cl2 may be the major

355

chlorinating agent for methylsiloxanes. One possible chlorination pathway may be –

356

because carbon atom has larger electronegativity than Si atom, –CH3 in

357

methylsiloxanes may be electron-rich, and could generate -CH2+ and H-.

358

Subsequently, H- combine with Cl+ generated by heterolytic cleavage of Cl2 in polar

359

solvent (water), while -CH2+ combine with Cl- to form –CH2Cl.

The significant

360 361 362

3.2 Occurrence of mono-chlorinated methylsiloxanes in papermaking and its coupled wastewater treatment processes

363

Section 3.1 indicated that methylsiloxanes mainly had both mono- and di-

364

chlorinated products in bleaching process. However, di-chlorinated products had

365

complex isomers and their standards were not available for this study.

366

only mono-chlorinated methylsiloxanes were measured in actual samples from the

367

studied papermaking factory. In this section, the reported concentrations of target

368

compounds and their calculated solid-water distribution coefficients were mean

369

values for three sampling events.

Therefore,

370

Papermaking processes. In the studied papermaking factory, PDMS were mainly

371

used as de-foamer agent in pulp washing, CEH bleaching and pulp refining processes.

372

However, besides in the above processes, D4, D5, and D6 were also detected in paper

373

formation process (Table S3 and S4), which should be attributed to D4-D6 adsorbed

374

in pulp from pulp refining process. In these four processes, the mean concentrations



375

of total methylsiloxanes (D4-D6) in aqueous samples were 0.936 - 184 µg/L, while

376

0.776 – 122 µg/g in solids. In this factory, free chlorine was only used in CEH

377

bleaching processes. In pulp refining process immediately following bleaching

378

processes, the residual free chlorine was eliminated with excessive sodium thiosulfate.

379

D3D(CH2Cl), D4D(CH2Cl), and D5D(CH2Cl) were only detected in CEH bleaching

380

process (Table S5 and S6). In consistence with the results in Section 3.1, chlorination

381

of methylsiloxanes in CEH bleaching mainly occurred in chlorination (C) stage,

382

where mono-chlorinated methylsiloxanes had the highest total concentrations

383

(287µg/L for water, 270 µg/g for solid), higher than those in E (1.06 µg/L for water,

384

1.15 µg/g for solid) and H stages (0.043 µg/L for water, 0.0329 µg/g for solid) by 2-4

385

orders of magnitude. The residual D3D(CH2Cl)- D5D(CH2Cl) found in the latter two

386

stages may arise from those sorbed in pulp at the chlorination (C) stage and transfer

387

from C to E and H stage with pulp.

388

The mean value of total mass flux in all papermaking processes, calculated with

389

Equation 1, were for 40.6 g/d D4, 174 g/d for D5, 93.0 g/d for D6, 8.76 g/d for

390

D3D(CH2Cl), 44.6 g/d for D4D(CH2Cl), and 38.4 g/d for D5(CH2Cl), respectively.

391

These data suggested that about 16.2% of D4, 19.0% of D5, and 27.7% of D6 in

392

papermaking processes underwent the mono-chlorination - calculated according to Si

393

levels in cVMS and their mono-chlorinated compounds. This was a rough estimation

394

because the removal (including volatilization and degradation, etc) of methylsiloxanes

395

and monochloro-methylsiloxanes in papermaking processes was neglected and the

396

dichlorination products could not be evaluated. Nevertheless, the above results


Page 20 of 36

Page 21 of 36


397

suggested that the chlorination of methylsiloxane will be significant if these siloxanes

398

are used in the conventional pulp-bleaching processes.

399

Wastewater treatment processes. The free chlorine content (about 0.8 mg/L) in

400

the primary influent of WWTP was three orders of magnitude lower than those in

401

bleaching process. Furthermore, the pH values in wastewater treatment units were

402

6.5-9.2, indicating that Cl2 residual in aqueous phase of the studied WWTP could be

403

neglected. Therefore, the chlorination of D4-D6 in the WWTP was expected to be

404

negligible.

405

D4-D6 and D3D(CH2Cl)- D5D(CH2Cl) were detected in all aqueous samples

406

from wastewater treatment processes (Table S3 and Table S5), with concentration

407

ranging from 0.541-32.9 µg/L for total D4-D6 and 0.461-16.7µg/L for total

408

D3D(CH2Cl)- D5D(CH2Cl), respectively. In solid samples from wastewater treatment

409

units, concentrations were 12.0-42.7 µg/g for total D4-D6 (Table S4), while 11.0-49.5

410

µg/g for total D3D(CH2Cl)- D5D(CH2Cl) (Table S6).

411

organic carbon/water partition coefficients (KOC) of methylsiloxanes [log KOC 4.2 for

412

D4, 5.2 for D5, and 5.86 for D6,]. 31, 32 In the studiedWWTP, the mean solid/water

413

distribution coefficients (Kd) of D4, D5 and D6 were 97-1418, 274-2531 and

414

650-3090, respectively (Table S7). After Kd values were normalized by total organic

415

carbon (TOC) of sludge (Table S8) – measured with TOC analyzer (TOC-VCPH,

416

Shimadzu),

417

D4, 5.23-6.29 for D5, and 5.61-6.27 for D6, respectively. Compared with those of

418

D4-D6, Kd values (468-3982, Table S7) of their mono-chlorinated products were

Previous studies reported high

apparent Log(Koc) values (Table S9) in this study were 4.78-6.04 for



419

1.08-4.82 times larger, and their apparent Log(Koc) values were 1.01-1.14 times

420

higher – 5.47-6.27 for D3D(CH2Cl), 5.62-6.43 for D4D(CH2Cl), and 5.65-6.49 for

421

D5D(CH2Cl), respectively (Table S9).

422

The removal efficiencies of mono-chlorinated methylsiloxanes in the whole

423

wastewater treatment processes were calculated as the difference between their total

424

mass flux in influent of primary clarifier (Mpri-inf) and effluent of secondary clarifier

425

(Msec-out), [100% ×(Mpri-inf - Msec-out)/ Mpri-inf]. In general, the mean removal

426

efficiencies at three sampling events were 81.6% for D3D(CH2Cl), 80.8% for

427

D4D(CH2Cl), and 77.1% for D5D(CH2Cl), respectively, which were 1.07-1.16 times

428

larger than those of their paired non-chlorinated compounds (76.2 % for D4, 70.8%

429

for D5, 66.5% for D6). Due to higher Kd values, chlorinated methylsiloxanes would

430

be more likely to be removed by sorption to excess sludge than their non-chlorinated

431

methylsiloxane analogs, which could cause their higher removal efficiencies. In

432

addition, faster hydrolysis rates of chlorinated methylsiloxanes, which would be

433

discussed in the Section 3.3, could also contribute to their higher removal efficiencies.

434 435

3.3 Removal mechanism of mono-chlorinated methylsiloxanes in wastewater treatment processes

436

The fractions (%) of mass loss (RMFLs) for target compounds in each treatment

437 438

unit to the total mass loss in the whole treatment processes were presented in Figure

439

3.

440

Removal by sorption to excess sludge. cVMS and their mono-chlorinated analogs


Page 22 of 36

Page 23 of 36


441

had high apparent Log Koc values (4.78-6.49, Table S8), hence sorption to excess

442

sludge (from primary and secondary clarifiers) had significant contribution to their

443

total removal (Figure 3) – RFMLsludge = 27.5% -70.7% for D4-D6 and 35.7% -74.1%

444

for D3D(CH2Cl) - D5D(CH2Cl). In addition, because both methylsiloxanes and

445

mono-chlorinated methylsiloxanes with larger molecular weights had higher Log Koc

446

values than their analogs with lower molecular weights (Table S8), RFMLsludge of

447

D4-D6 and D3D(CH2Cl) - D5D(CH2Cl) increased with the increasing numbers of

448

Si-O bones, respectively (Figure 3).

449

Removal in primary clarifier. Both methylsiloxanes and their mono-chlorinated

450

products in aqueous phase would be expected to volatilize from aqueous media. The

451

contribution of such removal process to RFML in each water treatment process also

452

depended on many factors including competing removal mechanism. For example,

453

RFMLs of D4 (38.0%) and D5 (28.0%) in oxic tank were larger than those (11.3% for

454

D4, and 3.7% for D5) in the primary clarifier, suggesting that volatilization of

455

chemical compounds in oxic tank would be higher than that in primary clarifier. This

456

can be expected since the hydraulic retention time (3h) was shorter and there was no

457

aeration in the primary clarifier.

458

D4D(CH2Cl) (10.3%) in oxic tank with aeration and longer hydraulic retention time

459

(12 h) were lower than those in primary clarifier [32.5% for D3D(CH2Cl) and 26.3%

460

for D4D(CH2Cl)]. We suspected that besides volatilization, there would be a

461

competing removal mechanism for D3D(CH2Cl) and D4D(CH2Cl) in the primary

462

clarifier, less significant for D4 and D5 under such conditions.

However, RFMLs of D3D(CH2Cl) (17.0%) and



463

In view of fast hydrolysis of D4-D6 in alkaline aquatic environment,33-35 the

464

hydrolysis of D3D(CH2Cl)-D5D(CH2Cl) in primary clarifier (pH=7.8-9.2) was

465

suspected as one important removal pathway. During simulated experiments, the

466

first-order half-lives in alkaline water samples (pH = 7.8, 8.5 and 9.2) were 0.9-4.33 h

467

for D3D(CH2Cl), 8.58-46.8 h for D4D(CH2Cl), 79.6-346 h for D5D(CH2Cl), which

468

were about 2.16-3.60 times shorter than those for their non-chlorinated

469

methylsiloxane analogs (Table 1). In these simulated alkaline water samples for

470

D3D(CH2Cl)-D5D(CH2Cl), both dimethylsilanediol and

471

chloromethyl(methyl)silanediol [(CH2Cl)MeSi(OH)2] were detected as the final

472

hydrolysis products. Based on the concentrations of dimethylsilanediol and

473

chlorodimethyl(methyl)silanediol in the water samples (pH = 9.2) separately spiked

474

with D3D(CH2Cl), D4D(CH2Cl) and D5D(CH2Cl), we found that in all incubation

475

time points, especially at earlier time points (1,3,and 10h ), the ratios of Si mass in

476

(CH2Cl)MeSi(OH)2 to the total Si mass in Me2Si(OH)2 + (CH2Cl)MeSi(OH)2

477

were >1/4(28-34%) in water spiked with D3D(CH2Cl), >1/5(24-31%) in water spiked

478

with D4D(CH2Cl), and >1/6(19-26%) in water spiked with D5D(CH2Cl) (Figure S2).

479

These results indicated that perhaps because -CH2Cl had stronger electrophilicity than

480

–CH3, Si-O bond with one branch of -CH2Cl would be more easily broken during

481

alkali hydrolysis than Si-O bond merely linked with branches of –CH3.

482

Removal in activated treatment processes.

RFMLs of mono-chlorinated

483

methylsiloxanes in anaerobic tank were 2.44-3.22% (Figure 3). Compared with those

484

in anaerobic tank, mono-chlorinated methylsiloxanes had more obvious removal in


Page 24 of 36

Page 25 of 36


485

oxic tank, especially for D3D(CH2Cl) (RFML=17%) and D4D(CH2Cl)

486

(RFML=10.3%). The simulated experiments showed that removal rates of

487

D3D(CH2Cl), D4D(CH2Cl) and D5D(CH2Cl) in capped activated oxic sludge system

488

were approximately equal to those in paired sterile system (Figure S4), indicating that

489

mono-chlorinated methylsiloxanes hardly underwent biodegradation in oxic treatment,

490

and volatilization would be their major removal pathway, a trend similar to that of

491

their non-chlorinated methylsiloxane analogs.10, 36 In oxic treatment, volatilization of

492

D3D(CH2Cl)-D5D(CH2Cl) would be greater due to aeration and long hydraulic

493

retention time (12 h), which could explain their higher RMFLs (4.52-17.0%) in oxic

494

tank than those (2.44-3.22%) in anaerobic tank of the WWTP. Generally, the RMFLs

495

of D3D(CH2Cl) (17.0%), D4D(CH2Cl) (10.3%), and D5D(CH2Cl) (4.51%) in oxic

496

tank was lower than those of their non-chlorinated analogs- 38.0% for D4, 19.4% for

497

D5 and 7.6% for D6, indicating that compared with methylsiloxanes, the chlorinated

498

products had slower volatilization rates. Separate volatilization experiments showed

499

that the half-lives in sterile oxic sludge-liquid mixture were 7.38 h for D3D(CH2Cl),

500

10.7 h for D4D(CH2Cl), and 21.1 h for D5D(CH2Cl), respectively, which were

501

1.21-1.50 times longer than those of their non-chlorinated analogs-5.56 h for D4, 7.13

502

h for D5, and 17.5 h for D6, respectively (Figure S5). We speculated: (1)

503

mono-chlorinated methylslioxanes might have lower vapor pressures than their paired

504

methylsiloxanes - vapor pressures of chemical compounds with similar structures

505

were always negatively related with molecular weights; (2) mono-chlorinated

506

methylslioxanes had 1.01-1.14 times larger apparent Log(Koc) values than those of



507

their paired methylsiloxanes (Table S9), indicating that D3D(CH2Cl)-D5D(CH2Cl)

508

spiked in the sludge would be more likely to be sorbed in solid phase, and then would

509

be more difficult to release from sludge to air. Notably, because removal experiments

510

could not completely simulate all affecting factors in real WWTP, the

511

volatilization/degradation mechanisms of target compounds speculated via

512

experiments may be somewhat different from those in the real WWTP processes.

513

The above data demonstrated the possible production of chlorinated cVMS as

514

by-products in the conventional papermaking processes when both cVMS and

515

element Chlorine are used together in the bleaching process. The detection of

516

chlorinated cVMS in both effluent and excess sludge samples from the studied

517

papermaking wastewater treatment processes indicated that these compounds may

518

enter the environmental compartments. This element chlorine bleaching techniques

519

has been phased out in the developed countries.23 In the developing countries where

520

this technique is still in use, our data suggested that the chlorinated cVMS and related

521

silanols also need to be taken into consideration in the environmental risk assessment.

522 523

ACKNOWLEDGMENT

524

This work was supported by National Natural Science Foundation of China

525

(21537004, 21407159, 21321004) and the Strategic Priority Research Program of the

526

Chinese Academy of Sciences (XDB14010201). We are grateful to Dr. Shihe Xu from

527

Dow Chemical Company, USA, for his consulting in silicone chemistry and review of

528

the draft manuscript, and Dr. Yawei Wang from Research Center for


Page 26 of 36

Page 27 of 36


529

Eco-Environmental Sciences, Chinese Academy of Sciences, for his help in Q-TOF

530

GC/MS analysis.

531

Supporting Information Available. This information is available free of charge via

532

the Internet at http://pubs.acs.org



REFERENCES:

(1) Drinan, J. E.; Spellman, F. R. Water and Wastewater Treatment: A Guide for the Nonengineering Professional, 2nd ed.; CRC Press: Boca Raton, 2012, 108−111. (2) Shah, A. D.; Mitch, W. A. Halonitroalkanes, halonitriles, haloamides, and N-nitrosamines: a critical review of nitrogenous disinfection byproduct formation pathways. Environ. Sci. Technol. 2011, 46 (1), 119−131. (3) Bedner, M.; MacCrehan, W.A. Transformation of acetaminophen by chlorination produces the toxicants 1,4-benzoquinone and N-acetyl-p-benzoquinone imine. Environ. Sci. Technol. 2006, 40 (2), 516-522. (4) Bulloch, D.N.; Nelson, E.D.; Carr, S.A.; Wissman, C.R.; Armstrong, J.L.; Schlenk, D.; Larive, C.K. Occurrence of halogenated transformation products of selected pharmaceuticals and personal care products in secondary and tertiary treated wastewaters from southern California. Environ. Sci. Technol. 2015, 49, 2044-2051. (5) Environment Canada. Screening assessment for the challenge Octamethylcyclotetrasiloxane (D4) 2008. Available at http:// www.ec.gc.ca/substances/ese/eng/challenge/batch2/batch2_556- 67-2_en.pdf. (6) Environment Canada. Screening assessment for the challenge Decamethylcyclopentasiloxane (D5) 2008. Available at http:// www.ec.gc.ca/substances/ese/eng/challenge/batch2/batch2_541- 02-6_en.pdf. (7) Environment Canada. Screening assessment for the challenge Dodecamethylcyclohexasiloxane (D6) 2008. Available at http:// http://www.ec.gc.ca/ese-ees/FC0D11E7-DB34-41AA.../batch2_540-97-6_en.pdf. (8) Cheng, Y.; Shoeib, M.; Ahrens, L.; Harner, T.; Ma, J. Wastewater treatment plants and landfills emit volatile methylsiloxanes (VMSs) to the atmosphere: Investigations using a new passive air sampler. Environ. Pollut. 2011, 159, 2380-2386. (9) Dewil, R.; Appels, L.; Baeyens, J.; Buczynska, A.; Vaeck, L.V. The analysis of volatile siloxanes in waste activated sludge. Talanta. 2007, 74(1), 14-19. (10) Wang, D.; Steer, H.; Tait, T.; Williams, Z.; Pacepavicius, G.; Young, T.; Ng, T.; Smyth, S.A.; Kinsman, L.; Alaee, M. Concentrations of cyclic volatile methylsiloxanes in biosolid amended soil, influent, effluent, receiving water, and sediment of wastewater treatment plants in Canada. Chemosphere. 2013, 93, 5, 766-773. (11) Bletsou, A.A.; Asimakopoulos, A.G.; Stasinakis, A.S.; Thomaidis, N.S.; Kannan, K. Mass loading and fate of linear and cyclic siloxanes in a wastewater treatment plant in Greece. Environ. Sci. Technol. 2013, 47(4), 1824-1832.


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(12) Companioni-Damas, E.Y; Santos, F.J.; Galceran, M.T. Analysis of linear and cyclic methylsiloxanes in water by headspace-solid phase microextraction and gas chromatography-mass spectrometry. Talanta. 2012, 89, 63-69. (13) Sanchís, J.; Martínez, E.; Ginebreda, A.; Farré, M.; Barceló, D. Occurrence of linear and cyclic volatile methylsiloxanes in wastewater, surface water and sediments from Catalonia. Sci. Total Environ. 2013, 443, 530−538. (14) Lee, S.; Moon, H.B.; Song, G.J.; Ra, K.; Kannan, K. A nationwide survey and emission estimates of cyclic and linear siloxanes through sludge from wastewater treatment plants in Korea. Sci. Total. Environ. 2014, 497-498, 106-112. (15) Kierkegaard, A.; Egmond, R. V.; Mclachlan, M.S. Cyclic volatile methylsiloxane bioaccumulation in flounder and ragworm in the Humer Esturary. Environ. Sci. Technol. 2011, 45, 5936–5942. (16) Atkinson, R.; Tuazon, E.C.; Kwok, E.S.C; Arey, J.; Aschmann, S.M.; Bridier, I. Kinetics and products of the gas-phase reactions of (CH3)4Si, (CH3)3SiCH2OH, (CH3)3SiOSi(CH3)3 and (CD3)3SiOSi(CD3)3 with Cl atoms and OH radicals. J. Chem. Soc. Fararday. Trans. 1995, 91(18), 3033-3039. (17) Habermehl, J. Silicone processing benefits pulp brownstock washing operations. China Pulp Paper Technology, http://www.dowcorning.com/content/publishedlit/30-1147-01.pdf, 2005. (18) G. Mudaly, Bubreak siloxane technology: the key to profitable pulping, TAPPSA Journal, http://www.tappsa.co.za/archive/Journal_papers/Bubreak_siloxane/bubr eak_silo xane.html, 2002. (19) Chao, S.H. Silicones in the pulp and paper industry. http://www.dowcorning.com/content/publishedlit/Chapter4.pdf. 2012. (20) Rahmawati, N.; Ohashi, Y.; Honda, Y.; Kuwahara, M.; Fackler, K.; Messner, K.; Watanabe, T. Pulp bleaching by hydrogen peroxide activated with copper 2,2-dipyridylamine and 4-aminopyridine complexes. Chem. Eng. J. 2005, 112, 167-171. (21) Kronberg, L.; Franzen, R. Determination of chlorinated furanones, hydroxyfuranones, and butenedioic acids in chlorine-treated water and in pulp bleaching liquor. Environ. Sci. Technol. 1993, 27, 1811-1818. (22) Available at: http://www.chinabgao.com/freereport/45146.html (In Chinese). 2014. (23) Conservatree. Chlorine free processing.

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(36) Parker, W.J.; Shi, J.; Fendinger, N.J.; Monteith, H.D.; Chandra, G. Pilot plant study to assess the fate of two volatile methyl siloxane compounds during municipal wastewater treatment. Environmental Toxicology and Chemistry. 1999, 18, 172-181.



Figure captions

Figure 1 Flow scheme of the studied papermaking processes and wastewater treatment processes with sampling locations Figure 2 GC/MS total ion chromatograms (the first plot in each panel) and their mass spectrums obtained by different techniques (EI and PCI) for emerging products after chlorination of D4 (a), D5 (b) and D6 (c) in a simulated bleaching process using element Chlorine as the bleaching agent. Figure 3 Relative fractions of mass loss for methylsiloxanes and their mono-chlorinated products due to the sorption to sludge, and other removal mechanisms in each treatment unit (average values from the three sampling events) in a conventional papermaking factory using element chlorine as the bleaching agent.


Page 32 of 36

Page 33 of 36


Figure 1 Flow scheme of the studied papermaking processes and wastewater treatment processes with sampling locations



Page 34 of 36

Figure 2 GC/MS total ion chromatograms (the first plot in each panel) and their mass spectrums obtained by different techniques (EI and PCI) for emerging products after chlorination of D4 (a), D5 (b) and D6 (c) in a simulated bleaching process using element Chlorine as the bleaching agent ACS Paragon Plus Environment

Page 35 of 36


Figure 3 Relative fractions of mass loss for methylsiloxanes and their mono-chlorinated products due to the sorption to sludge, and other removal mechanisms in each treatment unit (average values from the three sampling events) in a conventional papermaking factory using element chlorine as the bleaching agent.



Page 36 of 36

Table 1 The hydrolysis half-lives of methylsiloxanes and their mono-chlorinated products in aqueous environment calculated by first-order elimination kinetics*

pH=7.8

pH=8.5

pH=9.2

T1/2 (h)

R2

p

n*

T1/2 (h)

R2

p

n

T1/2 (h)

R2

p

n

D3D(CH2Cl)

4.33

0.9973

Chlorinated Methylsiloxanes Generated in the ... - ACS Publications

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