Thermochemical Formation of Polybrominated Dibenzo-p-Dioxins and

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Thermochemical formation of polybrominated dibenzo-pdioxins and dibenzofurans mediated by secondary copper smelter fly ash, and implications for emission reduction Mei Wang, Guorui Liu, Xiaoxu Jiang, Minghui Zheng, Li Li Yang, Yuyang Zhao, and Rong Jin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02119 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 30, 2016

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Environmental Science & Technology

1

Thermochemical formation of polybrominated

2

dibenzo-p-dioxins and dibenzofurans mediated by

3

secondary copper smelter fly ash, and implications

4

for emission reduction

5 6

Mei Wang1,2, Guorui Liu1,2,*, Xiaoxu Jiang1,2, Minghui Zheng1,2, Lili Yang1,2, Yuyang

7

Zhao1,2, Rong Jin1,2

8 9

1

State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research

10

Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871,

11

Beijing 100085, China

12

2

University of Chinese Academy of Sciences, Beijing 100049, China

13 14

*Corresponding author. Tel: +86 10 6284 9356; Fax: +86 10 6284 9355;

15

E-mail: [email protected]

1

ACS Paragon Plus Environment


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16

Abstract

17

Heterogeneous reactions mediated by fly ash are important to polychlorinated

18

dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) formation. However, the formation of

19

polybrominated

20

heterogeneous reactions is not yet well understood. Experiments were performed to

21

investigate the thermochemical formation of PBDD/Fs at 150–450 °C through

22

heterogeneous reactions on fly ash from a secondary copper smelter. The maximum

23

PBDD/F concentration was 325 times higher than the initial PBDD/F concentration in

24

the fly ash. The PBDD/F concentration after the experiment at 150 °C was five times

25

higher than the initial concentration. PBDD/Fs have not previously been found to

26

form at such a low temperature. Secondary-copper-smelter fly ash clearly promoted

27

PBDD/F formation, and this conclusion was supported by the low activation energies

28

that were found in Arrhenius’s law calculations. Thermochemical formation of

29

PBDD/Fs mediated by fly ash deposited in industrial facilities could explain “memory

30

effects” that have been found for PCDD/Fs and similar compounds released from

31

industrial facilities. Abundant polybrominated diphenyl ethers (PBDEs) that were

32

formed through fly-ash-mediated reactions could be important precursors for

33

PBDD/Fs also formed through fly-ash-mediated reactions. The amounts of PBDEs

34

that formed through fly-ash-mediated reactions suggested that secondary copper

35

smelters could be important sources of reformed PBDEs.

dibenzo-p-dioxins

and

dibenzofurans

36 2


(PBDD/Fs)

through

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37

INTRODUCTION

38

Heterogeneous reactions on fly ash are widely recognized as being the dominant

39

mechanisms through which polychlorinated dibenzo-p-dioxins and dibenzofurans

40

(PCDD/Fs) form during industrial thermal processes. These reactions can occur

41

because carbon-containing residues and catalytic metal particles are abundant in ﬂy

42

ash.1-4 Copper compounds are some of the most efficient catalysts of the formation of

43

PCDD/Fs.1, 5-8

44

The catalysis of the formation of PCDD/Fs and other chlorinated organic

45

contaminants by copper compounds (copper oxides and copper chlorides) under

46

different combustion conditions has been investigated in numerous studies.4, 6, 9, 10 Olie

47

et al. reviewed the catalysis of PCDD/F formation by several metals during the

48

incineration of municipal waste, and they found that copper has the strongest catalytic

49

effect.11 Stieglitz et al. found that copper (II) ions catalyze reactions on the surfaces of

50

carbon particles in the presence of oxygen, promoting the oxidation of the carbon to

51

CO2 and the formation of chlorinated aromatic compounds.12 Nganai et al. investigated

52

the formation of PCDD/Fs through pyrolysis mediated by copper oxide, and they found

53

that divalent copper oxides increased the PCDD/F yield to a remarkable extent.7 The

54

studies mentioned above showed that copper compounds are effective catalysts for the

55

formation of PCDD/Fs during waste incineration processes.

56

Polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) are the

57

brominated analogs of PCDD/Fs, and are also toxic.13 PBDD/Fs have recently attracted 3



58

much attention because higher PBDD/F toxic equivalents (TEQs) than PCDD/F TEQs

59

have been found in some environmental matrices.14, 15 Controlling PBDD/F emissions

60

should be prioritized so that the amounts of PBDD/Fs present in the environment and

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to which humans are exposed will decrease. PBDD/Fs and PCDD/Fs have similar

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sources, including waste incinerators, secondary nonferrous smelting processes, and

63

iron and steel making processes. Secondary copper smelting (SCu) processes have

64

been found to be important sources of PBDD/Fs, and PBDD/Fs have been found at

65

higher concentrations in stack gases produced during SCu processes (2.5 ng Nm−3)

66

than in stack gases produced during the incineration of waste (0.025 ng Nm−3),

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steelmaking using electric arc furnaces (1.5 ng Nm−3), and other processes.16, 17

68

The underlying reason for PBDD/F emissions during SCu processes being higher

69

than emissions during other processes has not yet been determined. Copper

70

compounds are found at much higher contents in SCu systems than in waste

71

incinerators and other industrial systems.18 The raw materials (recycled metal wire)

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used in secondary copper smelters normally contain plastic and cable residues that

73

contain polybrominated diphenyl ethers (PBDEs) or other brominated flame retardants

74

(BFRs). These BFR residues are widely recognized as being important sources of

75

organic bromine for the formation of PBDD/Fs. Metal bromides in fly ash could also

76

act as sources of bromine for the formation of PBDD/F, as has been reported

77

previously.19, 20 The large amounts of copper compounds present during SCu processes

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may catalyze thermochemical reactions between bromine (inorganic or organically 4


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bound) and carbon in organic material in scrap metal, allowing PBDD/Fs to form.

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Heterogeneous PBDD/F formation reactions mediated by fly ash with a high copper

81

compound content could contribute to larger amounts of PBDD/Fs being emitted

82

during SCu processes than during other processes. However, no attempt has yet been

83

made to quantitatively evaluate the potential for PBDD/Fs forming through

84

heterogeneous reactions mediated by real fly ash from SCu processes.

85

In this study, thermochemical experiments were performed to allow the potential

86

for the formation of PBDD/Fs mediated by real fly ash from a SCu process to be

87

quantified. The primary objective of the study was to gain, for the first time, insights

88

into the extent to which real fly ash promotes the formation of PBDD/Fs during SCu

89

processes. The results of this study will help characterize the role fly ash plays in the

90

formation of PBDD/Fs, and this will help in the development of techniques for

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controlling PBDD/F emissions.

92 93

94

Thermochemical experiment design

EXPERIMENTAL SECTION

95

The fly ash that was used as a matrix for the formation of PBDD/Fs through

96

heterogeneous reactions was collected from a bag filter in a typical SCu plant. This fly

97

ash was used to simulate, as closely as possible, the active surfaces of fly ash produced

98

during thermochemical processes.3 The SCu fly ash had a particularly high copper

99

concentration, 260 ± 2.0 mg/g, which was determined using an inductively coupled 5



100

plasma-optical emission spectrometer.21, 22 The fly ash had a bromine content of 0.3%,

101

determined using a scanning electron microscope with energy dispersive X-ray

102

analysis.22 The porosity and specific surface area of the fly ash were determined using a

103

Brunauer–Emmett–Teller analytical system.22 The specific surface area of the SCu fly

104

ash was 3.5 m2 g−1 (Table S3). The high surface area of the SCu fly ash could have

105

allowed the surface and external reactants involved in reactions to easily come into

106

contact with each other.

107

The thermochemical experiments were performed in a temperature-controlled

108

quartz horizontal laboratory furnace that had an internal diameter of 45 mm and was

109

600 mm long. Synthetic air was passed through the furnace at a constant flow rate of

110

50 mL/min to simulate the conditions in an open SCu smelter system. A 200-mg

111

sample of SCu fly ash in a uniform layer in a porcelain boat was placed in the furnace,

112

then the furnace was heated to the desired temperature. Experiments were performed

113

at 150, 200, 250, 350, and 450 °C. The set temperature in each experiment was held

114

for between 10 and 240 min. The experiments were performed to allow the potential

115

for PBDD/Fs to be formed at different temperatures to be quantified. The gas phase

116

reaction products that were evolved in an experiment were trapped by passing the gas

117

leaving the furnace through two ice-cooled impingers containing toluene. After each

118

experiment, the quartz tube and the fittings and connecting tubes were rinsed three

119

times with toluene to recover any products that had been deposited on the surfaces.

120

The gaseous products of the thermochemical experiments were therefore in two 6


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solutions, the cleaning solution (the toluene used to rinse the apparatus) and the

122

absorption solution (the toluene in the impingers). The toluene solutions and the solid

123

residue that remained after each thermochemical experiment had been completed were

124

stored at around 4 °C until they were analyzed.

125 126

Analytical procedure

127

The PBDD/F congeners of interest were identified and quantified using

128

isotope-dilution high-resolution gas chromatography (HRGC) combined with

129

high-resolution mass spectrometry (HRMS). PBDEs, which could act as PBDD/F

130

precursors, were also determined using isotope-dilution HRGC/HRMS. The analytical

131

technique and the methods used have previously been described in detail.17, 23 Briefly,

132

each solid residue sample was spiked with known amounts of 13C12-labeled PBDD/F

133

and PBDE internal standards (EDF-5408 and EO-5403; Cambridge Isotope

134

Laboratories, Andover, MA, USA). The sample was then digested in hydrochloric

135

acid, filtered, rinsed with distilled water, dried, and Soxhlet extracted with 250 mL of

136

toluene for about 24 h. Each gas phase sample (i.e., each cleaning solution and each

137

absorption solution) was spiked with known amounts of the

138

and PBDE internal standards. Each extract was evaporated to a small volume using a

139

rotary evaporator, then subjected to a series of cleanup steps, including passing the

140

extract through a multilayer silica gel column and a basic alumina column. After the

141

clean-up steps had been completed, the PBDD/Fs and PBDEs were separated by 7


13

C12-labeled PBDD/F


142

passing the extract through an activated carbon column, which was eluted with 80 mL

143

of dichloromethane/hexane (5/95, v/v) to elute the PBDEs and then 250 mL of toluene

144

to elute the PBDD/Fs. Each fraction was evaporated to about 20 µL using a rotary

145

evaporator and then under a gentle stream of N2. A 13C12-labeled PBDD/F injection

146

standard (EDF-5409; Cambridge Isotope Laboratories) was added to the PBDD/F

147

fraction and a

148

Laboratories) was added to the PBDE fraction before the fractions were analyzed by

149

HRGC/HRMS. The injection standards were used to allow the recoveries of the

150

internal standards to be calculated.

13

C12-labeled PBDE injection standard (EO-5404; Cambridge Isotope

151

The HRGC/HRMS instrument was a Trace GC Ultra coupled to a double

152

focusing mass spectrometer with an electron impact ion source (Thermo Fisher

153

Scientific, Waltham, MA, USA). A DB-5MS fused-silica column that was 15 m long

154

(with a 0.25-mm i.d. and a 0.10-µm film; Agilent Technologies, Santa Clara, CA,

155

USA) was used to separate the PBDD/Fs and decabromodiphenyl ether, using the

156

splitless/surge injection technique (using an injection port temperature of 280 °C and

157

a glass transfer line temperature of 280 °C). A DB-5MS fused-silica column that was

158

30 m long (with a 0.25-mm i.d. and a 0.10-µm film; Agilent Technologies) was used

159

to separate the mono- to nona-brominated PBDE congeners (using an injection port

160

temperature of 300 °C and a glass transfer line temperature of 280 °C). The HRMS

161

instrument was operated at a resolution of approximately 10,000, in selected ion

162

monitoring mode, and with an electron energy of 45 eV. 8


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163 164

Quality assurance and quality control

165

The reproducibility of the experimental runs was evaluated. The relative standard

166

deviations of the PBDD/F concentrations in the samples produced in duplicate runs

167

were 1%–19%, indicating that the experimental system and analytical procedure gave

168

an acceptable level of reproducibility. Experimental blanks were produced in

169

thermochemical experiments using a porcelain boat containing no SCu fly ash. A few

170

of the target compounds, including octabromodibenzo-p-dioxin (OBDD) and

171

octabromodibenzofuran (OBDF) were detected in the experimental blanks, but the

172

concentrations were less than 0.1% of the concentrations in the samples produced in

173

the experiments using SCu fly ash.

174

Three quality-control criteria were used to identify a target compound: (a) the

175

HRGC retention time had to match the retention time of the corresponding

176

13

177

the quantification to qualification ion ratio had to be within ±15% of the theoretical

178

value. The recoveries of the 13C12-labeled tetra- to hepta-brominated PBDD/F internal

179

standards were 23%–128%, and the recoveries of the 13C12-labeled OBDD and OBDF

180

internal standards were 20%–65% (Table S1). The recoveries of the

181

PBDE internal standards were 43%–124% (Table S2). The isotope dilution

182

HRGC/HRMS method therefore gave satisfactory recoveries of the PBDD/Fs and

183

PBDEs.

C12-labeled standard compound; (b) the signal-to-noise ratio had to be >3; and (c)

9


13

C12-labeled


184 185

186

Potential for PBDD/Fs forming through thermochemical reactions mediated by

187

SCu fly ash

RESULTS AND DISCUSSION

188

The products of the thermochemical reactions that took place in each experiment

189

were present in the solid residue and the gas phase (captured by the impingers or in

190

the cleaning solution). The relative amounts of the PBDD/Fs that were produced at

191

different temperatures in the solid residue and gas phase samples are shown in Figure

192

S1. The PBDD/Fs formed at 150 °C were mainly (64% of the total amount) found in

193

the solid residue. Larger amounts of the PBDD/Fs formed during the experiments

194

would have been transferred from the solid matrix to the gas phase at higher

195

temperatures. It can be seen from Figure S1 that the PBDD/Fs formed were

196

predominantly found in the gas phase at 200–450 °C (87% of the total at 200 °C, 95%

197

at 250 °C, and >99% at 350–450 °C). This indicates that increasing the temperature

198

increased the proportion of the PBDD/Fs formed that entered the gas phase. The sum

199

of the PBDD/F concentrations found in the two phases (solid residue and gas phase) in

200

each test were used in the subsequent assessments of the formation of PBDD/Fs.

201

The promotion of the formation of PBDD/Fs by SCu fly ash during the

202

thermochemical experiments at 150–450 °C for between 10 and 240 min was

203

evaluated. The PBDD/F concentrations in the SCu fly ash after the experiments at

204

different temperatures and for different periods of time had been performed are shown 10


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205

in Figure 1. The PBDD to PBDF ratios were in the range 0.4–0.8, indicating that

206

larger amounts of PBDFs than PBDDs were formed in the experiments. The influence

207

of the temperature on the formation of PBDD/Fs was assessed by performing 30 min

208

experiments at 150, 200, 250, 350, and 450 °C. The results of these experiments are

209

shown in Figure 1A.

210

It is widely accepted that unintentionally produced persistent organic pollutants 24-26

211

(POPs) can be formed at between 250 and 450 °C

but that temperatures below

212

250 °C do not favor the unintentional formation of POPs. However, it is not yet clear

213

whether PBDD/Fs can form below 250 °C. It can be seen from Figure 1 that the

214

PBDD/F concentration after the experiment at 200 °C was 8.8 ng/g, which was about

215

34 times higher than the initial PBDD/F concentration in the SCu fly ash (0.26 ng/g).

216

The PBDD/F concentration after the experiment at 150 °C was 1.3 ng/g, about five

217

times higher than the initial concentration. PBDD/Fs have not previously been found

218

to form at temperatures as low as 150 °C. These results indicate that SCu fly ash

219

clearly promoted the formation of PBDD/Fs even at low temperatures.

220

Extremely high PBDD/F concentrations (58–325 times higher than the initial

221

PBDD/F concentrations in the SCu fly ash) were found after the experiments at 250–

222

450 °C. The highest concentration (325 times higher than the initial PBDD/F

223

concentration) was found after a 30-min experiment at 450 °C. These results indicate

224

that the SCu fly ash strongly promoted the formation of PBDD/Fs at 250–450 °C.

11



Page 12 of 33

225

The PBDD/F concentrations found after experiments lasting different lengths of

226

time are shown in Figure 1B. Thermochemical experiments were performed at 350 °C

227

for 10, 30, 60, 120 and 240 min. The PBDD/F concentration had increased

228

dramatically, to 72 times higher than the concentration in the original SCu fly ash,

229

after a reaction time of 10 min. It can be seen from Figure 1B that the PBDD/F

230

concentration increased slightly as the reaction time increased. The PBDD/F

231

concentration increased from 18.7 ng/g after 10 min to 30.3 ng/g after 240 min.

232

Toxic equivalent factors (TEFs) for PBDD/Fs and mixed brominated/chlorinated

233

dibenzo-p-dioxins and dibenzofurans relative to 2,3,7,8-tetrachlorodibenzo-p-dioxin

234

(the TEF of which was assigned a value of 1.0) have recently been determined. These

235

TEFs

236

dibenzo-p-dioxins and dibenzofurans in environmental matrices to be estimated.13, 27-29

237

The PBDD/F TEQ concentrations were calculated using TEF values (shown in Table

238

S4) determined by Behnisch et al.13 and the concentrations of the individual PBDD/F

239

congeners. The TEQ concentrations of PBDD/Fs formed during the thermochemical

240

reactions were 0.18–4.33 ng TEQ/g (as shown in Table S5), and the maximum TEQ

241

value was about 144 times higher than the initial PBDD/F TEQ concentration in the

242

SCu fly ash (0.03 ng TEQ/g). Fly ash is normally recycled by adding it to the raw

243

materials during the feeding–fusion stages, or fly ash is simply thermally treated during

244

the smelting process. Taking into consideration the increases in the PBDD/F TEQ

245

concentrations during the thermochemical reactions, fly ash needs to be disposed of

allow

the

TEQs

of

PBDD/Fs

and

mixed

12


brominated/chlorinated

Page 13 of 33


246

safely rather than simply being recycled by adding it to the raw materials or being

247

thermally treated in SCu and other metallurgical plants.

248

Emissions of PCDD/Fs from many industrial thermal sources have been found to

249

increase significantly as the bag filters or other facilities increase in age, and this

250

phenomenon is normally called the “memory effect”. It has been suggested that the

251

memory effect in waste incinerators is caused by POPs being adsorbed and later

252

desorbed.30 The memory effect includes the “sorption and desorption memory effect”

253

(which is caused by POPs being absorbed by amorphous materials or adsorbed onto the

254

installation walls or onto deposits of materials in the installation) and the “de novo

255

memory effect” (in which soot deposited in some parts of a system act as a raw material

256

in the de novo synthesis of POPs).31 Sorption and desorption may not fully explain the

257

significant increases (by one to two orders of magnitude) in the amounts of PBDD/Fs

258

that have been found to form in secondary metal smelting plants over time. Fly ash

259

can be deposited in a facility or in a bag filter over time. Our results indicate that the

260

“de novo memory effect” (the thermochemical formation of pollutants mediated by fly

261

ash deposited within a facility) might be the underlying cause (in addition to sorption

262

and desorption) of significant increases in the amounts of PBDD/Fs that are formed in

263

secondary metal smelting plants over time.

264

13



265

Congener

266

thermochemical reactions

patterns

of

the

PBDD/Fs

formed

during

Page 14 of 33

fly-ash-mediated

267

The concentrations of the different PBDD/F homologs after experiments at

268

different temperatures had been performed are shown in Figure 2. Very different tetra-

269

to octa-brominated PBDD/F homolog patterns were found after experiments at

270

different temperatures. It can be seen from Figure 2 that the tetrabromodibenzofuran

271

(TeBDF)

272

tetrabromodibenzo-p-dioxin (TeBDD) concentration reached a maximum at 250 °C,

273

and the TeBDF and TeBDD concentrations decreased as the temperature increased

274

further. The more-brominated PBDD/F homologs were more dominant than the

275

less-brominated PBDD/Fs at 350–450 °C. The hepta- and octa-brominated PBDD/Fs

276

were the dominant homologs, together contributing 56–83% of the total PBDD/Fs, at

277

350–450 °C. The change from the less-brominated homologs dominating at lower

278

temperatures to the more-brominated homologs dominating at higher temperatures

279

indicated that bromination reactions may have occurred at higher temperatures. The

280

curves of the TeBDD and TeBDF profiles in Figure 2 were different from the curves of

281

the other congeners, and an explanation for this is given below. The TeBDD and

282

TeBDF activation energies were much lower than the highly brominated congener

283

activation energies, as shown in Table 1. TeBDD and TeBDF will therefore be much

284

more easily formed than highly brominated congeners at relatively low temperatures.

285

Increasing the temperature could increase the formation rates of highly brominated

concentration

reached

a

maximum

14


at

200

°C,

the

Page 15 of 33


286

congeners and cause TeBDD and TeBDF to be transformed into highly brominated

287

congeners through bromination reactions. The TeBDD and TeBDF concentrations will

288

therefore slightly decrease when the temperature increases, and the highly brominated

289

congener concentrations will increase at relatively high temperatures.

290

The PBDD/F congener profiles in the SCu fly ash and in the products of the

291

experiments at different temperatures are shown in Figure 3. Three tetra-brominated

292

congeners (2,4,6,8-TeBDF, 2,3,7,8-TeBDF, and 2,3,7,8-TeBDD) made small

293

contributions (6.9%, 5.8%, and 6.7%, respectively) to the total PBDD/F concentration

294

in the original SCu fly ash. The PBDD/F congener profiles in the products and the SCu

295

fly ash were different. The 2,3,7,8-TeBDD contributions increased until the

296

temperature reached 250 °C, then decreased as the temperature increased further. The

297

2,4,6,8-TeBDF and 2,3,7,8-TeBDF contributions clearly decreased once the

298

temperature became higher than 200 °C. The more-brominated congeners (including

299

1,2,3,4,6,7,8-heptabromodibenzo-p-dioxin

300

1,2,3,4,6,7,8-heptabromodibenzofuran (HpBDF), and OBDF) clearly increased as the

301

temperature increased. OBDF was the most abundant congener after the experiment at

302

450 °C, contributing 31% of the total PBDD/F concentration. The next most abundant

303

congeners (in decreasing order of contribution) were 1,2,3,4,6,7,8-HpBDF

304

(contributing 29% of the total), OBDD (12%), and 1,2,3,4,6,7,8-HpBDD (11%).

305

These four more-brominated congeners together contributed 83% of the total PBDD/F

306

concentration after the experiment at 450 °C. These results indicated that the

(HpBDD),

15


OBDD,


Page 16 of 33

307

formation of highly brominated congeners was favored at relatively high

308

temperatures.

309

The Arrhenius equation, shown below, describes the dependence of the rate

310

constant (k) of a chemical reaction on the absolute temperature T (in Kelvin). In the

311

equation, k is the rate constant for the formation of PBDD/F congeners involved in the

312

thermochemical reactions, A is a pre-exponential factor, Ea is the activation energy, and

313

R is the universal gas constant.

݇ = Aeିா௔/(ோ்) 314

The Arrhenius equation was used to calculate, from the PBDD/F congener

315

formation rates, the activation energy and pre-exponential factor for each PBDD/F

316

congener during the thermochemical reactions. These calculations were performed

317

with the aim of improving our understanding of the thermochemical behaviors of the

318

PBDD/F congeners. The activation energies and pre-exponential factors that were

319

calculated are shown in Table 1. As can be seen in Figure 3, the formation of the

320

more-brominated

321

thermodynamically favored at higher temperatures. This would have been why we

322

found that larger amounts of the more-brominated PBDD/F congeners were formed in

323

our experiments at higher temperatures. It can be seen from Table 1 that the activation

324

energies of the hexa- and octa-brominated PBDD/F congeners (38.7–54.7 kJ/mol) were

325

higher than the activation energies of the tetra- to penta-brominated PBDD/F congeners

326

(8.6–35.2 kJ/mol). This supported our conclusion that larger amounts of the

PBDD/F

congeners

was

found

16


to

be

very

strongly

Page 17 of 33


327

more-brominated PBDD/Fs would be produced at higher temperatures. Ortuno et al.

328

studied the de novo synthesis of PBDD/Fs using model mixtures of active carbon and

329

CuBr2.32 They found activation energies for the de novo synthesis of PBDD/Fs of 78.7–

330

158.4 kJ/mol, which are higher than the activation energies of 8.6–54.7 kJ/mol that we

331

found for the formation of PBDD/Fs when SCu fly ash was used as the matrix for

332

thermochemical reactions. This further supports our conclusion that SCu fly ash is a

333

strong catalyst for the formation of PBDD/Fs.

334

The different congener patterns found after different reaction times are shown in

335

Figure 4. The contributions of 2,3,7,8-TeBDD, 2,4,6,8-TeBDF, and 2,3,7,8-TeBDF to

336

the total PBDD/F concentration decreased as the reaction time increased. The

337

contributions of the more-brominated congeners, including 1,2,3,4,6,7,8-HpBDD,

338

OBDD, 1,2,3,4,6,7,8-HpBDF, and OBDF, clearly increased as the reaction time

339

increased. These trends indicated that the 2,3,7,8-TeBDD, 2,4,6,8-TeBDF, and

340

2,3,7,8-TeBDF formation reactions tended to reach equilibrium more quickly than the

341

1,2,3,4,6,7,8-HpBDD, OBDD, 1,2,3,4,6,7,8-HpBDF, and OBDF formation reactions.

342

A longer reaction time would therefore favor the formation of the more-brominated

343

congeners.

344

17



345

Formation of PBDEs during fly-ash-mediated thermochemical reactions and the

346

relationship between the formation of PBDEs and the formation of PBDD/Fs

347

Extensive use of brominated ﬂame retardants, especially PBDEs, in the past

348

means that more brominated precursors of PBDD/Fs are now present in the raw

349

materials used in secondary metal smelting than would have previously been the

350

case.33 The likelihood of PBDD/Fs forming during industrial thermal processes is

351

therefore higher than it would previously have been.34-36 High PBDD/F yields have

352

been found when PBDEs have been thermally treated.33, 37, 38 We therefore measured

353

the PBDE concentrations in the products of the experiments and in the SCu fly ash to

354

allow the relationship between the formation of PBDEs and PBDD/Fs to be

355

investigated.

356

It is widely accepted that PBDEs are important precursors of PBDD/Fs formed

357

during industrial thermal processes.19, 35, 39 The PBDE concentrations in the original

358

fly ash and in the products of the experiments are presented in Figure 5. The PBDE

359

concentration in the original fly ash was 0.08 µg/g. It can be seen from Figure 5 that

360

the PBDE concentration increased significantly after the SCu fly ash had been

361

thermally treated. The PBDE concentrations were 3.4–5.3 µg/g after 30 min

362

experiments at 250–450 °C had been performed. The highest PBDE concentration

363

(5.3 µg/g, more than 67 times higher than the PBDE concentration in the original fly

364

ash) was found after the SCu fly ash had been heated to 350 °C for 30 min. The

365

considerably higher PBDE concentrations after the fly ash had been heated than in the 18


Page 18 of 33

Page 19 of 33


366

original fly ash indicated that large quantities of PBDEs were formed on the SCu fly

367

ash through thermochemical reactions. It has been found that de novo synthesis of

368

unintentionally produced POPs, including PCDD/Fs, polychlorinated naphthalenes,

369

and other halogenated organic compounds, from carbon sources can be catalyzed by

370

compounds of copper or other metals.20, 40, 41 It has also been suggested that de novo

371

synthesis of PBDEs from organic residues (acting as carbon sources) produced by

372

plastics and cables in the raw materials used in SCu processes can occur,42 and that this

373

is catalyzed by copper compounds.

374

The PBDE to PBDD/F concentration ratios for the experiments performed under

375

different conditions are shown in Figure 5. It can be seen that the PBDE

376

concentrations were 41–328 times higher than the PBDD/F concentrations. Vetter et al.

377

found that about 0.5‰ of the decabromodiphenyl ether present could be transformed

378

into PBDD/Fs when complex matrices containing decabromodiphenyl ether were

379

heated.38 The PBDE to PBDD/F ratios found by Vetter et al.38 were similar to the

380

PBDE to PBDD/F ratios for our experiments. Altarawneh et al. performed a detailed

381

mechanistic and kinetic study of the formation of PBDD/Fs from PBDEs, and their

382

theoretical calculations indicated that the loss of a Br or H atom from the ortho

383

position of a PBDE then a ring-closure reaction is the most accessible

384

PBDF-production pathway with modest reaction barriers.39 Weber and Kuch found

385

that PBDD/Fs can be formed from PBDEs through the simple intra-molecular

386

elimination of HBr.19 The large amounts of PBDEs formed during fly-ash-mediated 19



387

thermochemical reactions could therefore provide abundant brominated PBDD/F

388

precursors, allowing PBDD/Fs to be formed on SCu fly ash through thermochemical

389

reactions. Weber and Kuch summarized the possible pathways for PBDD/Fs to be

390

formed from brominated flame retardants.19 The results of our study on the formation

391

of PBDD/Fs during SCu processes supported the mechanisms Weber and Kuch

392

suggested for the formation of PBDD/Fs from PBDEs.19 The possible pathways

393

through which PBDD/Fs can be formed from PBDEs during thermochemical processes

394

were determined from our results and the results of the study performed by Weber and

395

Kuch,19 and the pathways are shown schematically in Figure S3.

396 397

Environmental implications and strategies for decreasing PBDD/Fs emissions

398

Fly ash is normally recycled by adding it to the raw materials used in a smelter or

399

simply by thermally treating it during the smelting process.43 The significant increases

400

in PBDD/F concentrations caused by fly-ash-mediated thermochemical reactions

401

mean that fly ash must be disposed of safely rather than simply recycled by adding it to

402

the raw materials or by thermally treating it in SCu plants and other metallurgical plants.

403

Our results also indicate that fly ash in air pollution control devices (APCDs) and

404

other facilities in a plant should be cleaned out periodically to avoid fly ash

405

accumulating and causing PBDD/Fs to be produced.

406

Air pollution control devices that are used to decrease particle emissions from

407

industrial facilities efficiently decrease emissions of PCDD/Fs and similar compounds 20


Page 20 of 33

Page 21 of 33


408

to the atmosphere because the devices remove particles containing the pollutants.44, 45

409

APCDs are normally operated below 200 °C because PCDD/Fs and similar

410

compounds are not generally formed below 200 °C. However, PBDD/Fs clearly

411

formed on fly ash at below 200 °C in our study, suggesting that PBDD/Fs could form

412

in APCDs in industrial plants, particularly in secondary nonferrous smelting plants,

413

even when the APCDs are operated at temperatures below 200 °C.

414

The promotion of PBDD/F formation by fly ash allows us to suggest a novel way

415

of controlling the formation and emission of unintentionally produced POPs. Fly ash

416

deposited in a smelting facility and APCDs could contribute to significant increases in

417

the amounts of PBDD/Fs formed during SCu processes. Therefore, periodically

418

removing fly ash deposited in a smelting facility and APCDs can directly and

419

effectively decrease the amounts of PBDD/Fs formed during industrial processes.

420

The intentional production of PBDEs for use as brominated flame retardants has

421

been banned because PBDEs have been added to Annex A of the Stockholm

422

Convention.46 POPs that can be unintentionally produced during industrial thermal

423

processes (e.g., PCDD/Fs, polychlorinated biphenyls, polychlorinated naphthalenes,

424

hexachlorobenzene, and pentachlorobenzene) are included in Annex C of the

425

Stockholm Convention, which requires measures aimed at decreasing unintentional

426

emissions of these compounds to be taken. PBDE emissions from some industrial

427

thermal processes have been explained by the incomplete destruction of PBDEs

428

(present as impurities) in the raw materials.47, 48 No direct experimental evidence is 21



429

available to support the reformation of PBDEs during industrial thermal processes,

430

especially during SCu processes. Our results indicate that significant amounts of

431

PBDEs can be formed through fly-ash-mediated thermochemical reactions, suggesting

432

that SCu processes could be important sources of PBDE emissions. This indicates that

433

unintentional emissions of PBDEs must be considered (in addition to the production

434

and release of PBDEs used as brominated flame retardants) when compiling a PBDE

435

inventory.

436 437

438

We gratefully acknowledge support from the National 973 program (2015CB453100),

439

the National Natural Science Foundation of China (21477147), the Strategic Priority

440

Research Program of the Chinese Academy of Sciences (XDB14020102), and the

441

Youth Innovation Promotion Association Chinese Academy of Sciences (2016038).

Acknowledgments

442 443

444

Tables S1–S5 and Figures S1–S3. This information is available free of charge via the

445

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

Supporting Information Available

22


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Page 23 of 33


446

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27. Birnbaum, L. S.; Staskal, D. F.; Diliberto, J. J., Health effects of polybrominated dibenzo-p-dioxins (PBDDs) and dibenzofurans (PBDFs). Environ. Int. 2003, 29, (6), 855-860. 28. Loganathan, B. G.; Kannan, K.; Watanabe, I.; Kawano, M.; Irvine, K.; Kumar, S.; Sikka, H. C., Isomer-specific determination and toxic evaluation of polychlorinated biphenyls, polychlorinated/brominated dibenzo-p-dioxins and dibenzofurans, polybrominated biphenyl ethers, and extractable organic halogen in carp from the Buffalo River, New York. Environ. Sci. Technol. 1995, 29, (7), 1832-1838. 29. Ma, J.; Addink, R.; Yun, S.; Cheng, J.; Wang, W.; Kannan, K., Polybrominated dibenzo-p-dioxins/dibenzofurans and polybrominated diphenyl ethers in soil, vegetation, workshop-floor dust, and electronic shredder residue from an electronic waste recycling facility and in soils from a chemical industrial complex in eastern China. Environ. Sci. Technol. 2009, 43, (19), 7350-7356. 30. Li, H.-W.; Wang, L.-C.; Chen, C.-C.; Yang, X.-Y.; Chang-Chien, G.-P.; Wu, E. M.-Y., Influence of memory effect caused by aged bag filters on the stack PCDD/F emissions. J. Hzard. Mater. 2011, 185, (2–3), 1148-1155. 31. Weber, R.; Sakurai, T.; Ueno, S.; Nishino, J., Correlation of PCDD/PCDF and CO values in a MSW incinerator - indication of memory effects in the high temperature/cooling section. Chemosphere 2002, 49, (2), 127-134. 32. Ortuño, N.; Conesa, J. A.; Moltó, J.; Font, R., De novo synthesis of brominated dioxins and furans. Environ. Sci. Technol. 2014, 48, (14), 7959-7965. 33. Zhang, M.; Buekens, A.; Li, X., Brominated flame retardants and the formation of dioxins and furans in fires and combustion. J. Hazard. Mater. 2016, 304, 26-39. 34. D'Silva, K.; Fernandes, A.; Rose, M., Brominated organic micropollutants—igniting the flame retardant issue. Crit. Rev. Env. Sci. Tec. 2004, 34, (2), 141-207. 35. Sindiku, O.; Babayemi, J.; Osibanjo, O.; Schlummer, M.; Schluep, M.; Watson, A.; Weber, R., Polybrominated diphenyl ethers listed as Stockholm Convention POPs, other brominated flame retardants and heavy metals in e-waste polymers in Nigeria. Environ. Sci. Pollut. R. 2014, 1-13. 36. Ortuno, N.; Lundstedt, S.; Lundin, L., Emissions of PBDD/Fs, PCDD/Fs and PBDEs from flame-retarded high-impact polystyrene under thermal stress. Chemosphere 2015, 123, 64-70. 37. Ren, M.; Peng, P. a.; Cai, Y.; Chen, D.; Zhou, L.; Chen, P.; Hu, J., PBDD/F impurities in some commercial deca-BDE. Environ. Pollut. 2011, 159, (5), 1375-1380. 38. Vetter, W.; Bendig, P.; Blumenstein, M.; Hägele, F.; Behnisch, P. A.; Brouwer, A., Formation of polybrominated dibenzofurans (PBDFs) after heating of a salmon sample spiked with decabromodiphenyl ether (BDE-209). Environ. Sci. Pollut. R. 2015, 22, (19), 14530-14536.

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39. Altarawneh, M.; Dlugogorski, B. Z., A mechanistic and kinetic study on the formation of PBDD/Fs from PBDEs. Environ. Sci. Technol. 2013, 47, (10), 5118-5127. 40. Weber, R.; Iino, F.; Imagawa, T.; Takeuchi, M.; Sakurai, T.; Sadakata, M., Formation of PCDF, PCDD, PCB, and PCN in de novo synthesis from PAH: Mechanistic aspects and correlation to fluidized bed incinerators. Chemosphere 2001, 44, (6), 1429-1438. 41. Iino, F.; Imagawa, T.; Takeuchi, M.; Sadakata, M., De novo synthesis mechanism of polychlorinated dibenzofurans from polycyclic aromatic hydrocarbons and the characteristic isomers of polychlorinated naphthalenes. Environ. Sci. Technol. 1999, 33, (7), 1038-1043. 42. Hu, J. C.; Zheng, M. H.; Liu, W. B.; Li, C. L.; Nie, Z. Q.; Liu, G. R.; Xiao, K.; Dong, S. J., Occupational exposure to polychlorinated dibenzo-p-dioxins and dibenzofurans, dioxin-like polychlorinated biphenyls, and polychlorinated naphthalenes in workplaces of secondary nonferrous metallurgical facilities in China. Environ. Sci. Technol. 2013, 47, (14), 7773-7779. 43. Hu, J.; Zheng, M.; Nie, Z.; Liu, W.; Liu, G.; Zhang, B.; Xiao, K., Polychlorinated dibenzo-p-dioxin and dibenzofuran and polychlorinated biphenyl emissions from different smelting stages in secondary copper metallurgy. Chemosphere 2013, 90, (1), 89-94. 44. McKay, G., Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chem. Eng. J. 2002, 86, (3), 343-368. 45. Kulkarni, P. S.; Crespo, J. G.; Afonso, C. A., Dioxins sources and current remediation technologies—a review. Environ. Int. 2008, 34, (1), 139-153. 46. UNEP, Stockholm Convention on Persistent Organic Pollutants. 2015. http://chm.pops.int/TheConvention/ThePOPs/TheNewPOPs/tabid/2511/Default.a spx. 47. Wang, L.-C.; Hsi, H.-C.; Wang, Y.-F.; Lin, S.-L.; Chang-Chien, G.-P., Distribution of polybrominated diphenyl ethers (PBDEs) and polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) in municipal solid waste incinerators. Environ. Pollut. 2010, 158, (5), 1595-1602. 48. Sakai, S.-i.; Watanabe, J.; Honda, Y.; Takatsuki, H.; Aoki, I.; Futamatsu, M.; Shiozaki, K., Combustion of brominated flame retardants and behavior of its byproducts. Chemosphere 2001, 42, (5–7), 519-531.

26


Page 26 of 33

Page 27 of 33


606

Table 1. Arrhenius formulae for the thermochemical reactions involved in the

607

formation of polybrominated dibenzo-p-dioxins and dibenzofurans on secondary

608

copper smelter fly ash at between 423 and 723 K Congener

Arrhenius formula

OBDD

k = 2.82×10−10exp(−39.2/RT)

OBDF

k = 5.99×10−10exp(−38.7/RT)

1234678-HpBDD

k = 6.84×10−10exp(−44.9/RT)

1234678-HpBDF

k = 6.62×10−10exp(−44.8/RT)

123478/123678-HxBDD

k = 3.21×10−10exp(−46.9/RT)

123789-HxBDD

k = 6.71×10−10exp(−50.5/RT)

123478-HxBDF

k = 1.53×10−9exp(−54.7/RT)

12378-PeBDD

k = 2.41×10−11exp(−35.2/RT)

23478-PeBDF

k = 2.18×10−11exp(−34.3/RT)

12378-PeBDF

k = 2.98×10−13exp(−13.8/RT)

2378-TeBDD

k = 1.51×10−11exp(−23.1/RT)

2378-TeBDF

k = 4.75×10−13exp(−8.6/RT)

609

k is the rate constant for the formation of the PBDD/F congener of interest during a

610

thermochemical reaction, the first term in parentheses after exp is the activation

611

energy (Ea in kJ/mol), R is the universal gas constant, and T is the temperature (in K).

612

BDD = bromodibenzo-p-dioxin, BDF = bromodibenzofuran, Te = tetra, Pe = penta, Hx

613

= hexa, Hp = hepta, O = octa.

27



100.0

Page 28 of 33

A PBDDs

PBDFs

Factor

300.0

60.0

40.0

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

6.0

200.0

4.0 2.0

Factor

Concentration ng/g

80.0

0.0 Flyash

150°C

100.0

20.0

0.0

0.0

Flyash

150°C

200°C

250°C

350°C

450°C

614 20.0

PBDFs

150

Factor

120 15.0

90

Factor

Concentration ng/g

B

PBDDs

10.0 60

5.0 30

0.0

0

Flyash

615

10 min

60 min

dibenzo-p-dioxin

120 min

(PBDD)

Figure

617

dibenzofuran (PBDF) concentrations (bars and left y-axis) and the number of times

618

higher than the concentration in the original fly ash the concentration was (factor;

619

points and right y-axis) after the experiments (A) at different temperatures and (B) for

620

different periods of time

28


and

240 min

616

1.

Polybrominated

30 min

polybrominated

Page 29 of 33


30000

10000 PeBDD

HxBDD

Concentration pg/g

Concentration pg/g

TeBDD 8000 HpBDD

OBDD

6000

4000

TeBDF

PeBDF

HpBDF

OBDF

HxBDF

25000 20000 15000 10000

2000 5000 0 0

50

100

150

200

250

300

350

400

450

0

500

0

50

100

Temperature / ºC

150

200

250

300

350

400

450

500

Temperature / ºC

621 622

Figure 2. Polybrominated dibenzo-p-dioxin (on the left) and dibenzofuran (on the right) homolog profiles in the original fly ash and after the

623

thermochemical reaction experiments at different temperatures had been performed (BDD = bromodibenzo-p-dioxin, BDF = bromodibenzofuran,

624

Te = tetra, Pe = penta, Hx = hexa, Hp = hepta, O = octa)

29



Page 30 of 33

2378-TeBDD 12378-PeBDD

Fraction

20.0% 123478/123678HxBDD 123789-HxBDD 10.0% 1234678-HpBDD OBDD

0.0% Flyash 150 °C 200 °C 250 °C 350 °C 450 °C

2468-TeBDF 2378-TeBDF 40.0%

Fraction

12378-PeBDF 23478-PeBDF 20.0%

123478-HxBDF 1234678HpBDF

0.0% Flyash 150 °C 200 °C 250 °C 350 °C 450 °C

OBDF

625 626

Figure 3. (Top) Polybrominated dibenzo-p-dioxin and (bottom) polybrominated

627

dibenzofuran congener patterns in the original fly ash and after the thermochemical

628

reaction experiments at different temperatures had been performed (BDD =

629

bromodibenzo-p-dioxin, BDF = bromodibenzofuran, Te = tetra, Pe = penta, Hx = hexa,

630

Hp = hepta, O = octa)

30


Page 31 of 33


2378-TeBDD

Fraction

20.0%

12378-PeBDD 123478/123678HxBDD

10.0%

123789-HxBDD 1234678-HpBDD

0.0%

OBDD

2468-TeBDF 2378-TeBDF

40.0%

Fraction

12378-PeBDF 23478-PeBDF

20.0%

123478-HxBDF 1234678-HpBDF

0.0%

OBDF

631

Figure 4. (Top) Polybrominated dibenzo-p-dioxin and (bottom) polybrominated

632

dibenzofuran congener patterns in the original fly ash and after the thermochemical

633

reaction experiments with different reaction times had been performed (BDD =

634

bromodibenzo-p-dioxin, BDF = bromodibenzofuran, Te = tetra, Pe = penta, Hx = hexa,

635

Hp = hepta, O = octa).

31



PBDEs

Factor

6

350 300

5

250 4 200 3

Factor

Concentration µg/g

Page 32 of 33

150 2 100 1

50 0

0

Flyash

250 ºC

350ºC

450ºC

636 637

Figure 5. Polybrominated diphenyl ether concentrations (bars and left y-axis) after

638

thermochemical reaction experiments at different temperatures had been performed,

639

and the polybrominated diphenyl ether to polybrominated dibenzo-p-dioxin and

640

dibenzofuran concentration ratios (factor; points and right y-axis)

32


Page 33 of 33


641

TOC/Abstract Art

642

33


Thermochemical Formation of Polybrominated Dibenzo-p-Dioxins and

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