Organophosphate Flame Retardants and Plasticizers in Aqueous

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Organophosphate Flame Retardants and Plasticizers in Aqueous Solution: pH-Dependent Hydrolysis, Kinetics and Pathways Guanyong Su, Robert J. Letcher, and Hongxia Yu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02187 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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Environmental Science & Technology

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Organophosphate Flame Retardants and Plasticizers in Aqueous Solution:

2

pH-Dependent Hydrolysis, Kinetics and Pathways

3 4

Guanyong Su †,‡, Robert J. Letcher †,‡*, Hongxia Yuǁ

5 6

† Ecotoxicology and Wildlife Health Division, Environment and Climate Change

7

Canada, National Wildlife Research Centre, Carleton University, Ottawa, ON, K1A

8

0H3, Canada

9

‡ Department of Chemistry, Carleton University, Ottawa, ON, K1S 5B6, Canada

10

ǁ

11

Environment, Nanjing University, Nanjing 210023, China

State Key Laboratory of Pollution Control and Resource Reuse, School of the

12 13 14 15 16 17 18 19 20 21

* Corresponding author: Tel.: 1-613-998-6696, Fax: 1-613-998-0458 E-mail:

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[email protected] (Robert J. Letcher)

23 1

ACS Paragon Plus Environment


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Abstract

25

Despite the growing ubiquity of organophosphate (OP) triesters as environmental

26

contaminants, parameters affecting their aquatic chemical stabilities are currently

27

unknown. The present study examined the pH-dependent (7, 9, 11 or 13) hydrolysis of

28

16 OP triesters in mixtures of 80 ng/mL for each OP triester over a period of 35 days

29

at 20 oC. For the pH=7, 9 and 11 solutions, 10 of the 16 OP triesters were stable and

30

with no significant (p>0.05) degradation. For the remaining 6 OP triesters, significant

31

degradation occurred progressing from the pH=7 to 11 solutions. At pH=13, except

32

for tributyl phosphate and tris(2-ethylhexyl) phosphate, 14 OP triesters were degraded

33

with half-lives ranging from 0.0053 days (triphenyl phosphate) to 47 days (tripropyl

34

phosphate). With increasingly basic pH the order of OP triester stability was group A

35

(with alkyl moieties) > group B (chlorinated alkyl) > group C (aryl). Numerous OP

36

diesters were identified depending on the pH level of the solution, whereas OP

37

monoesters were not detectable. This is consistent with no significant (p>0.05)

38

depletion observed for 5 OP diesters in the same 4 solutions and over same 35 day

39

period, suggesting OP diesters are end products of base-catalyzed hydrolysis of OP

40

triesters. Our results demonstrated that pH-dependent hydrolysis of OP triesters does

41

occur, and such instability would likely affect the fate of OP triesters in aqueous

42

environments where the pH can be variable and basic.

43

Keywords: Organophosphate esters, Flame retardants, Plasticizers, pH, Basic

44

conditions, Hydrolysis, Stability

45 2


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Introduction

47

Organophosphate (OP) triesters are a large group of chemicals that have been

48

used for decades as flame retardants (FRs) and plasticizers in various consumer

49

products, such as plastics, textile, wood and many others materials1-3. Specifically, the

50

halogenated OP triesters, i.e. tris(1,3-dichloro-2-propyl) phosphate (TDCIPP),

51

tris(2-chloroisopropyl) phosphate (TCIPP), tris(2-chloroethyl) phosphate (TCEP),

52

2,2-bis(chloromethyl)propane-1,3-diyltetrakis(2-chloroethyl) bisphosphate (V6), are

53

mainly used as FRs to improve the resistance to fire by chemical or physical

54

mechanisms1. Non-halogenated OP triesters are predominantly used as plasticizers

55

and lubricants to regulate pore sizes4. The exception is triphenyl phosphate (TPHP),

56

which is also used in combination with halogenated FRs, i.e. halogenated

57

bis-(2-ethylhexyl) tetrabromophthalate (TBPH) and tetrabromobenzoate (TBB), and

58

non-halogenated mono-, di- and tri-isopropylated triaryl phosphates (ITPs) in the FR

59

formulation Firemaster® 550 (FM550)4,5. Many OP triesters are additives and not

60

chemically-bonded to polymer products5, and thus are easily released into the

61

environment over the life time of these products. During the last few years, OP

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triesters has received more environmental attention due to an urgent demand for

63

chemical substitutes/replacements for the phased-out, commercial penta- and

64

octa-BDE FR formulations6.

65

OP triesters can be released into the environment through urban wastewater and

66

many of them can further distributed through the water cycle and can subsequently be

67

released into various water bodies1, 7. Thus, there are increasing reports of OP triesters 3



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in various aqueous environments including river water4, marine water8, drinking

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water9-11, precipitation as well as in sewage treatment plant (STP) effluents/influents9.

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The growing environmental ubiquity of OP triesters increases the urgency to

71

continually address the knowledge gaps with respect to their environmental stabilities

72

and fate in aqueous environments1. Given the base molecular structures (i.e. ester

73

bonds) of OP triesters, abiotic hydrolysis of the phosphate ester bond to a phosphoric

74

acid is likely relevant with respect to their stability in aqueous environments.

75

However, to our knowledge the abiotic stability with respect to relevant

76

environmental parameters is limited to several OP triesters, e.g. tris(methyphenyl)

77

phosphate (TMPP), triethyl phosphate (TEP), TCIPP, TNBP, TEHP and TPHP.

78

Specifically, abiotic degradation was reportedly not expected for TCIPP in leachate

79

from a sea-based solid waste disposal site8. High stabilities were also reported for

80

TNBP and TEHP in aqueous solutions under abiotic conditions12. Aqueous stability

81

information is also available for TPHP, TMPP and TEP13-16. For instance, TPHP was

82

shown to be degraded in river water/sediment and pond sediment with half-lives

83

ranging from 3 to 12 d, whereas degradation of TMPP was much slower with a

84

half-life of 96 d15, 16. Regardless, precise experimental data of abiotic stabilities is

85

lacking for most of the OP triesters, and the underlying pathways (i.e. to products of

86

hydrolysis) are essentially unknown.

87

In the present study, the aquatic stability of 16 environmentally relevant OP

88

triesters was investigated as a function of time and under neutral and basic pH

89

conditions. All possible pH-dependent products of hydrolysis (i.e. OP diester and 4


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90

monoester phosphoric acids) were also screened by use of high performance liquid

91

chromatography-quadrupole-time-of-flight mass spectrometry (LC-Q-TOF-MS). To

92

our knowledge, this study provides the first information of OP triester stability as a

93

function of changing pH as well as elucidating the underlying mechanisms/pathways

94

of aquatic degradation.

95 96

2 Material and Methods

97

2.1 Standards and Reagents

98

The chemical structures of all 16 target compounds can be found in Figure 1.

99

Triethyl phosphate (TEP), tripropyl phosphate (TPP), tributyl phosphate (TNBP),

100

TCEP, tris(2-butoxyethyl) phosphate (TBOEP), TPHP, tris(2-ethylhexyl) phosphate

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(TEHP), tris(tribromoneopentyl) phosphate (TTBPP) and 2-ethylhexyl-diphenyl

102

phosphate (EHDPP) were all purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.).

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TCIPP was purchased from AK Scientific (Union City, CA, U.S.A.). TDCIPP and

104

TMPP were purchased from TCI America (Portland, OR, U.S.A.), and the three

105

isomers

106

tris(3-bromo-4-methylphenyl) phosphate (T3B4MP ) and tris(2-bromo-4-methylphenyl)

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phosphate (T2B4MP) were synthesized by GL Chemtech (Oakville, ON, Canada). The

108

V6 was generously provided by Dr. Heather Stapleton (Duke University, NC,

109

U.S.A.).

tris(4-bromo-3-methylphenyl)

phosphate

(T4B3MP),

110

Five isotopically enriched chemical standards were used for quantification of OP

111

triesters. The d27-TNBP and d15-TEP internal standards were purchased from

112

Cambridge Isotope Laboratories (Tewksbury, MA, U.S.A.), and the d15-TPHP internal 5



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113

standard was purchased from Wellington Laboratories (Guelph, ON, Canada). The

114

d12-TCEP and d15-TDCIPP were prepared by and purchased from Dr. Vladimir Belov

115

(Max Planck Institute for Biophysical Chemistry, Germany). The OP diester standards,

116

di-n-butyl phosphate (DNBP) and diphenyl phosphate (DPHP), were purchased from

117

Sigma-Aldrich. The other three OP diester standards, bis(1-chloro-2-propyl)

118

phosphate

119

bis(1,3-dichloro-2-propyl) phosphate (BDCIPP), were purchased from Dr. Vladimir

120

Belov (Max Planck Institute, Germany). Purities of all OP standards were ≥ 97%.

(BCIPP),

bis(2-butoxyethyl)

phosphate

(BBOEP)

and

121

All reagents or solvents, including sodium azide (NaN3), ammonium acetate

122

(NH4COOH) and UPLC-grade methanol were obtained from Sigma-Aldrich, with an

123

exception of sodium chloride (NaCl) which was purchased from Merck KGaA

124

(Darmstadt, Germany).

125 126

2.2 Aqueous Degradation

127

Since the hydroxide anion is a nucleophile for the ester bonds17, base-catalyzed

128

hydrolysis of the suite of OP triesters was investigated in both neutral (nominal pH=7)

129

and increasingly basic (nominal pH=9, 11 and 13) solutions. Degradation experiments

130

were conducted in four different aqueous solutions with volumes of 2 mL each. The

131

compositions of the four aqueous solutions varied as a function of pH, and were

132

detailed in Table S1. For all solutions, NaOH was used for the adjustment of pH level,

133

and NaCl was added as a buffer to maintain the pH level and to be sure that all

134

solutions had comparable ionic strengths. Adding NaN3 (final concentration: 0.02 % 6


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135

(w/v)) was to avoid any possible microbial activities as a confounding factor to any

136

observed degradation. The experiments are conducted and maintained at room

137

temperature (20 ± 1 oC) and with shaking (90 rpm) in a water bath (Thermo Electron

138

Corporation, Waltham, MA, USA). To avoid any potential effects from photolytic

139

degradation, the water bath was covered to exclude light during the experiments. By

140

use of a pH meter (Thermo Scientific, Beverly, MA, USA), accurate pH values (6.96

141

± 0.08, 8.93 ± 0.01, 10.80 ± 0.02 and 12.96 ± 0.01) were determined for the four

142

solutions systems at day 0 and 35 as listed in Table S1.

143 144

2.3 Experimental Design

145

Abiotic degradation experiments of OP triesters was conducted at a single

146

concentration of 80 ng/mL. It has been shown that ester bond hydrolysis is

147

independent of the initial concentration of target compounds18. In the present

148

hydrolysis study, an 80 ng/mL concentration for all individual OP triesters was also

149

used. Although this 80 ng/mL concentration is lower than the aqueous solubility limit

150

of all 16 target OP triesters, we remained concerned that adsorption on glassware

151

surfaces could still be possible for several of the more lipophilic triesters, i.e. EHDPP,

152

TEHP, TMPP, T2B4MP, T3B4MP, T4B3MP, TBOEP, TNBP and TTBPP. See details

153

in the Supporting Information Text-S2.

154

To minimize any possible “false-positive” hydrolysis degradation due to such

155

potential adsorptive effects, two sequential experiments were designed and conducted.

156

In the first experiment, concentrations of the investigated OP triesters were 7



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determined at two time points (0 and 35 d). In brief, 80 µL of the 16 OP triesters in

158

methanol solution (20 µg/mL each) was added to each of 24, 15 mL pre-cleaned glass

159

tubes and blown to dryness under a gentle stream of nitrogen gas. A volume of 2 mL

160

of the pre-warmed reaction solutions were then added into the same tubes (6 replicates

161

for each of four reaction solutions at nominal pH=7, 9, 11 or 13). The bottles were

162

vortexed well for 15 s, and for 3 of the 6 replicate samples from each reaction solution

163

2 mL of 2 M ammonium acetate buffer and 6 mL of fresh methanol were added to

164

neutralize the pH and terminate any further pH-dependent hydrolysis of the OP

165

triesters. It is emphasized that adding methanol is to wash down all the OP triesters

166

that might be adsorbed on the glass surface. Then, a volume of 250 µL of this

167

resulting solution and 20 µL of internal standards (100 ng/mL) were then combined in

168

UPLC injection bottles, after which the collected samples were stored at -20 oC before

169

UPLC-MS/MS analysis. In the next step, the other 3 of the 6 replicates of the

170

pre-warmed reaction solutions for each pH solution, were then placed into a water

171

bath (20 ± 1 oC) for a period of 24 hours, which were subsequently collected for

172

UPLC-MS/MS analysis. In this experiment, 6 mL of methanol was directly added into

173

the hydrolysis system, and then can wash out and nullify any possible OP triester

174

adsorption onto glassware surfaces. This allowed for the determination of the

175

depletion rates of all 16 OP triesters over a period of 35 days at solution pH values of

176

7, 9, 11 and 13 with elimination of possible “false-positive” hydrolysis degradation

177

caused by potential adsorptive effects.

178

The second experiment is a kinetics study, which was performed with respect to 8


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the pH-dependent degradation of several of the OP triesters, which were not

180

susceptible to glass surface adsorption or could be corrected by control comparisons

181

(pH=7), i.e. TEP, TPP, TCEP, TCIPP, TDCIPP, V6, TPHP, TMPP, T2B4MP, T3B4MP,

182

T4B3MP. In brief, 80 µL of the OP triesters in methanol solution (20 µg/mL for each

183

OP triester) was added into 15 mL glass tubes (n=12), and blown to dryness under a

184

gentle stream of nitrogen gas. A volume of 2 mL of the pre-warmed reaction solutions

185

were then added into tubes (3 replicates for each of four reaction systems of pH=7, 9,

186

11 and 13). The tubes were vortexed well for 15 s, and immediately placed into water

187

bath for the degradation experiments. For each of time points (0 min, 15 min, 30 min,

188

1 h, 2 h, 4 h, 8 h, 1 d, 2 d, 4 d, 7 d, 14 d, 21 d, 28 d and 35 d), 50 µL of the reaction

189

solution was collected from each of 12 tubes, pH neutralized with 50 µL of 2 M

190

ammonium acetate buffer, diluted with 150 µL of methanol, spiked with 20 µL of

191

internal standards (100 ng/mL) and stored at -20 oC until UPLC-MS/MS analysis.

192 193

2.4 Quantification of OP Triester/Diesters by UPLC-MS/MS

194

The determination of OP triesters and diesters was performed on a Waters

195

XEVO-TQ-S ultra-high performance liquid chromatography-tandem quadrupole mass

196

spectrometer in the positive atmospheric pressure chemical ionization mode

197

(UPLC-APCI(+)-MS/MS) (Waters Limited, Milford, USA), and the detailed

198

parameters are described in the supporting information. For more method details,

199

please refer to our previous publications19, 20.

200 9



201

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2.5 Screening of Products of Hydrolysis by LC-Q-TOF-MS

202

Screening of products of hydrolysis of OP triesters was carried out using an

203

Agilent 1200 LC system, coupled with an Agilent 6250A quadrupole-time-of-flight

204

mass spectrometer (LC-Q-TOF-MS; Agilent Technologies, Mississauga, ON, Canada).

205

The LC system was equipped with an Luna 3u C18(2) 100A column (2.0 mm×100 mm,

206

3.0 µm particle size) (Phenomenex, Torrance, CA, USA). The mobile phase (A, water;

207

B, methanol; both contain 2mM ammonium acetate) flow rate was 0.5 mL/min and

208

the following gradient was employed: 5% B ramped to 70% B in 3 min (linear), and

209

then ramped to 80% B in 12 min (linear), and ramped to 95% B in 3 min, and held for

210

7 min, followed by a change to 5% B and held for 15 min for the next injection. The

211

MS was operated using ESI source in the negative ion mode (ESI(-)), and its capillary

212

voltage was 3.0 kV. Nitrogen was used as the drying and nebulizing gas and helium

213

was used as the collision gas. Other parameters for MS were optimized as follows: gas

214

temperature, 300 oC; drying gas, 10 L/min; nebulizer, 20 psi; fragmentor voltage, 250

215

V; skimmer voltage, 150 V. The Q-TOF instrument was tuned and calibrated with

216

tuning calibration solution (G1969-85000, Agilent Technologies). The TOF-MS was

217

operated at resolution (R) > 20000 at m/z 601.9790 and within 3 ppm mass error in

218

mass range m/z 50-1700. For each run, TFA anion (m/z 112.9855) and HP-0921 (TFA

219

adduct; m/z 1033.9881) were consistently introduced into the Q-TOF as reference

220

masses.

221 222

2.6 Data Analysis 10


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Data analysis was conducted in three steps. Based on the data generated from the

224

first 3 or 6 replicate samples described in see section 2.3, a t-test analysis was

225

performed on the concentration data of the OP triesters and for the time points of 0 d

226

and 35 d. This was done for each pH-value treatment to check whether there was any

227

significant depletion of the OP triesters. If no significant difference was observed, the

228

half-lives (t1/2) were marked as not available (NA; see in Table 1), meaning that OP

229

triester degradation did not occur for a specific aqueous solution over the period of 35

230

days. For OP triesters that were 100% depleted (i.e. TCEP, TDIPP, V6, TPHP, TMPP,

231

T2B4MP, T3B4MP, T4B3MP and EHDPP at pH=13) in aqueous solutions at day 35,

232

the kinetic rate constants (k, d-1) were calculated from the data generated from the

233

second 3 or 6 replicate samples described in see section 2.3, and from the slope of the

234

line plotting the ln of the fraction of the parent compound vs. time. Here,

235

pseudo-first-order kinetics (p 0.93 for all curves

236

with exceptions of TMPP (0.76) and EHDPP (0.74)) were selected to calculate the

237

half-lives (t1/2, d), and t1/2=ln2/k. It should be noted that concentrations of some OP

238

triesters (i.e. TMPP, T2B4MP, T3B4MP, T4B3MP, EHDPP) at pH=13 have been

239

corrected by controls (pH=7), since glass adsorption was observed for these

240

chemicals.

241

For OP triesters where significant depletion was observed, but not 100 % based

242

on the first 3 or 6 replicate samples described in see section 2.3, and where adsorption

243

occurred during the degradation experiment, their kinetic constants and t1/2 values

244

were estimated based the two time points of 0 and 35 days and according to the 11



245

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following equation: ‫ݐ‬ଵ/ଶ =

35‫ܥ‬଴ 2(‫ܥ‬଴ − ‫ܥ‬ଷହ )

246

where C0 and C35 were concentrations of OP triesters at time points of 0 d and 35 d,

247

respectively. The degradation rate constants are not available for these chemicals

248

(Table 1), and we emphasize that future usage on these t1/2 values should be done with

249

caution.

250 251

3 Results and Discussion

252

3.1 Stability and Degradation Kinetics

253

Sixteen target OP triesters in the present study were classified into four chemical

254

sub-groups, namely Groups A, B, C and D (Figure 1). TEP, TPP, TNBP and TEHP

255

were classified into Group A as all three hydroxyl groups in phosphoric acid were

256

substituted with alkyl groups (a fragment of the molecule with the general formula

257

CnH2(n+1)). OPFRs of Group B were TCEP, TCIPP, TDCIPP and V6 as all have

258

chlorinated alkyl groups (with the general formula CnHxCly, x+y=2(n+1)). TPHP,

259

TMPP, T2B4MP, T4B3MP and T3B4MP comprised Group C as all alkyl moieties are

260

phenyl-based. Among these target OP triesters, TBOEP, EHDPP and TTBPP did not

261

fit structurally in Groups A, B or C, and thus were designated as Group D (Figure 1).

262

No significant (p > 0.05, t-test) degradation was observed for all four OP triesters

263

of Group A (TEP, TPP, TNBP and TEHP) with non-halogenated alkyl moieties and in

264

solutions at pH levels of 7, 9 or 11 over a period of 35 days (Figures 2 and 3). In the

265

solutions at pH=13, significant (p < 0.01, t-test) degradation was observed for the two 12


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266

triesters with shorter alkyl chains (TEP, TPP), but the two OP triesters with longer

267

alkyl chains exhibited great hydrolytic stability in the same solutions. The half-lives

268

of TEP and TPP were estimated as 16 and 33 d, respectively, and the degradation rate

269

constants of TEP and TPP were estimated as 0.0423 and 0.0213 d-1, respectively

270

(Table 1 and Figure 3). These results probably suggest that the degradation rate

271

constants decrease as a function of alkyl carbon chain length of OP triesters. Abiotic

272

stability studies of TEP, TNBP and TEHP have been previously conducted but under

273

different conditions21. Specifically, the stability of TEP was studied in neutral (pH=7)

274

water systems at 101 oC, and its rate constant was reported to be 8.35×10-6 s-1

275

(t1/2=1.39 d)21. This is in contrast to the lack of degradation of TEP we observed in a

276

solution of pH=7 at room temperature and over a period of 35 days. This result

277

difference may suggest that any pH=7 hydrolysis of TEP is likely accelerated by

278

increased solution temperature21. The high stabilities of TNBP and TEHP under the

279

present basic solution conditions are consistent with a study that investigated the

280

feasibility of an aqueous decontamination and an ex situ soil washing technique for

281

the hydrolysis of OP triesters, suggesting that TNBP and TEHP were not hydrolyzed

282

in solutions where sodium perborate (NaBO3, pH=10.3) was added or under basic

283

conditions (Na2CO3, pH=10.7)12.

284

OP triesters (TCEP, TCIPP, TDCIPP and V6) of Group B (Figure 1) showed very

285

similar degradation profiles compared to Group A OP triesters, but seemed to be

286

degraded more rapidly under the same conditions. That is, there was no significant

287

depletion in aqueous solutions of pH levels of 7, 9 or 11 and over a period of 35 days. 13



Page 14 of 34

288

However, in solutions at pH=13, the Group B OP triesters could be degraded

289

extensively over a period of 35 days (Figures 2 and 3). It is worth noting that despite

290

the one atom (chloride (Cl) or hydrogen (H)) difference in each moiety, the

291

degradation rate of TCEP (t1/2=0.083 d) was much more rapid than that of TEP

292

(t1/2=16 d) in solution at a pH=13. Half-lives were estimated to be 0.083, 11, 0.044

293

and 0.11 d, and their degradation constant rates were estimated as 8.36, 0.0621, 15.6

294

and 6.23 d-1 for TCEP, TCIPP, TDCIPP and V6, respectively (Table 1 and Figure 4).

295

Previous hydrolytic stability data on these chloroalkyl OP triesters (i.e. TCEP, TCIPP

296

and TDCIPP) has been predicted mainly from various mathematics models22-24, not as

297

a result of empirical laboratory experiments, and these modeling data are generally

298

inconsistent with our present experimental results. For example, the predictive models

299

and tools for assessing chemicals under the Toxic Substances Control Act (TSCA) in

300

the United States suggested that TCEP may undergo abiotic degradation in the

301

environment based on an estimated degradation half-life of 20 days at pH 5 to 922.

302

However, from our present study, we found that TCEP showed great hydrolytic

303

stability in solutions at pH 7 to 11, but was rapidly degraded at pH=13. Another report

304

suggested that TCIPP hydrolyzes very slowly in both alkaline and acidic aqueous

305

environments23. However, our experimental results suggested that TCIPP can be

306

degraded quite rapidly in solution at pH=13. Limited data for TDCIPP has suggested a

307

resistance to pH-dependent hydrolysis in most environmental waters24. Our data

308

suggested that TDCIPP exhibits very similar degradation profiles compared to TCEP,

309

TCIPP and V6 with rapid hydrolysis in solutions at pH=13. 14


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310

Group C OP triesters Figure 1) exhibited totally different degradation profiles

311

compared to OP triesters of Groups A or B. Specifically, the Group C OP triesters,

312

TPHP, TMPP, T2B4MP, T3B4MP and T4B3MP, contain aryl moieties and could be

313

significantly degraded in all four solutions at pH 7, 9, 11 or 13 over 35 d. In aqueous

314

solution at pH=7, half-lives were estimated to be 112, 94, 22, 18 and 23 d for TPHP,

315

TMPP, T2B4MP, T3B4MP and T4B3MP (Table 1), and these values were very

316

comparable to those obtained in the aqueous solutions at pH 9 and 11. However, when

317

the pH was increased to 13, the degradation of Group C OP triesters increased. At

318

pH=13, the half-lives were estimated to be 0.0053, 0.027, 0.046, 0.066 and 0.067 d

319

for TPHP, TMPP, T2B4MP, T3B4MP and T4B3MP, and the constant rates were

320

estimated at 130, 25.7, 15.2, 10.5 and 10.4 d-1, respectively (Table 1). The hydrolysis

321

of TPHP was also investigated in a previous study, in which saturated solutions of the

322

test substance were prepared by shaking an excess amount of the substance with

323

distilled water or natural lake/river water, followed by filtration (11 µm) to remove

324

most undissolved material and but pass through the microorganisms in natural water25.

325

Thus, this observed hydrolysis (19 d or 3 d at pH 7 or 9 at 21 oC) of TPHP could be

326

contributed to both abiotic and biotic activities, although the reported t1/2 was much

327

lower than in our present study on TPHP. For TMPP, the half-life for TMPP was

328

estimated to be 319 days at pH 7, 31.9 days at pH 8 and 3.19 days at pH 926.

329

For OP triesters of Group D (Figure 1), our data suggested that TBOEP has

330

hydrolytic stability similar to the OP triesters of Groups A and B, and was highly

331

stable in solutions at pH 7, 9 or 11 (Figures 2 and 3). Increasing the pH to 13 resulted 15



Page 16 of 34

332

in a TBOEP half-life of 21 d (Table 1). In aqueous solutions at pH values ranging

333

from 7 to 13, EHDPP hydrolysis was similar to Group C OP triesters, and the

334

half-lives of EHDPP were 110, 130, 130 and 0.11 d at pH of 7, 9, 11 and 13,

335

respectively. TTBPP hydrolysis was observed at all four pH levels with half-lives of

336

estimated to be 23, 22, 23 and 19 d in solutions of pH=7, 9, 11 and 13, respectively

337

(Figure 4 and Table 1). Based on the current literature, limited information is available

338

for degradation of TBOEP, EHDPP or TTBPP. By use of predictive tools developed

339

by USEPA, it is suggested that TBOEP may undergo environmental degradation based

340

on estimated half-lives of 93-95 days at pH 9-522. However, in our present study, we

341

did not observe significant depletion of TBOEP in aqueous solution at pH 7 to 11 over

342

a period of 35 d. Using the same USEPA model, it was predicted that EHDPP would

343

be hydrolyzed in the environment at a high alkaline pH of 9 based on an estimated

344

half-life of 117 days22, which is consistent with the results of our study22.

345

Since the phase-out of penta- and octa-BDE products, reports on the

346

environmental occurrence of OP triesters are increasing, and most of the presently

347

targeted 16 OP triesters have been detected in the aqueous environments. For example,

348

all four Group A non-halogenated OP triesters have been detected in environmental

349

waters, and regardless of the monitored water types, the reported concentrations

350

generally ranked as TNBP > TEP ≈ TPP > TEHP9, 27, 28. Three OP triesters, TCEP,

351

TCIPP and TDCIPP from Group B, have also been reported in various environmental

352

water samples and at high concentrations9. The greatest concentrations of chlorinated

353

OP triesters were reported in raw water samples from a Japan Sea-based solid waste 16


Page 17 of 34


354

disposal site in which concentrations of TCEP, TCIPP and TDCIPP were 4.23-87.4,

355

11.3-48.2 and 0.68-6.18 µg/L, respectively8. Among the five OP triesters of Group C,

356

TPHP has been detected in aqueous environments most frequently8, and the greatest

357

concentrations were reported in snow samples from an airport in northern Sweden,

358

probably suggesting that the hydraulic liquid and oil used in aircraft were its sources

359

to these outdoor environments29. Compared to TPHP, in most cases TMPP has been

360

reported at relatively low concentrations in aqueous samples8, and the highest levels

361

of TMPP were found in the ditch water samples collected around greenhouses in rural

362

area of Higashi-Hiroshima30. To our knowledge, no information is available for the

363

concentrations of T2B4MP, T3B4MP or T4B3MPisomers in environmental waters.

364

Information on the concentrations of three Group D OP triesters in aqueous

365

environments is currently lacking with an exception of TBOEP, where concentrations

366

have been reported frequently and with levels being comparable to that of TNBP9.

367

These reported levels in aqueous environmental compartments are likely due to

368

various factors, i.e. production volume, usage period, biotic transformation, as well as

369

abiotic stability, which may suggest that environmental degradation is minimal for

370

non-halogenated OP triesters over a pH range of 7 to 11 in aqueous media.

371 372

3.2 Screening of Products of Hydrolysis of OP Triesters and the Underlying

373

Mechanisms

374

Based on their molecular base structures, the most probable pathways of

375

hydrolysis of OP triesters was hypothesized to be via the cleavage of ester bonds to 17



Page 18 of 34

376

form OP diester phosphoric acids with possible further degradation to OP monoester

377

phosphoric acids. We investigated the products of hydrolysis of OP triesters, and all

378

possible diester and monoester products were screened in degradation solutions

379

collected at the time points of 0 d (as controls) and 35 d by use of LC-ESI(-)-ToF/MS

380

or UPLC-TQ/MS. Numerous OP diesters, i.e. bis(2-chloroethyl) phosphate (BCEP;

381

from TCEP or V6), bis(1-chloro-2-propyl) phosphate (BCIPP; from TCIPP),

382

bis(1,3-dichloro-2-propyl) phosphate (BDCIPP; from TDCIPP), diphenyl phosphate

383

(DPHP; from TPHP), bis(methylphenyl) phosphate (BMPP; from TMPP);

384

bis(2-butoxyethyl) phosphate (BBOEP; from TBOEP), ethylhexyl monophenyl

385

phosphate (EHMPP; from EHDPP) and three bis(bromo-3-methylphenyl) phosphate

386

isomers (BBMP; from T2B4MP, T4B3MP or T3B4MP), were identified as products of

387

the base-catalyzed hydrolysis of OP triesters. None of these diester degradation

388

products were detectable in the control solutions. Furthermore, OP monoesters were

389

not detectable in the same solutions of the hydrolyzed OP triesters over the period of

390

35 d. These results indicated that OP diesters are more stable than the parent OP

391

triesters in same pH-variable aqueous solutions. This likely due to the phosphoric acid

392

of the OP diester being deprotonated under basic conditions to its corresponding

393

conjugate base. For example, several OP triesters (i.e. TCEP, TDCIPP, V6, TPHP) in

394

solutions at pH=13 were depleted in a very short time (less than 1 d), but their OP

395

diester products were still detectable after 35 d. We subsequently conducted a mass

396

balance study on selected OP triester-diester pairs, i.e. TPHP-DPHP, TBOEP-BBOEP,

397

TDCIPP-BDCIPP and TCIPP-BCIPP, and the results demonstrated that the amounts 18


Page 19 of 34


398

of depleted OP triesters were very comparable to the amounts of OP diesters formed

399

(Table S2). The abiotic stabilities of five OP diesters (DNBP, BCIPP, BDCIPP, DPHP

400

and BBOEP) were also investigated. No significant depletion (t-test, p > 0.01) was

401

observed for all five of these OP diesters in aqueous solutions at all pH levels of 7, 9,

402

11 or 13 and over a period of 35 days (Figure S1).

403

Although OP triesters in aqueous solutions were suspected to break down via

404

cleavage of ester bonds, laboratory experiments have been limited to the study of only

405

TPHP and TMPP31. Specifically, Barnard et al and Howard et al found that alkaline

406

hydrolysis of TPHP can result in a DPHP product25, 31, but further hydrolysis to

407

monophenyl phosphate and (total dealkylated) phosphoric acid (H3PO4) was not

408

observed. A similar hydrolysis pattern was also reported for TMPP, which was shown

409

to be easily hydrolyzed in an alkaline medium to produce dicresyl phosphate and

410

cresol, although TMPP was stable in neutral and acidic media at ambient

411

temperatures31.

412

Strong linear correlative relationships were sometimes observed between log

413

normalized OP triester degradation rate constants (k, d-1) and the leaving alcohol

414

dissociation constants (pKa), i.e. plots between log k versus pKa of the leaving

415

group32-34. We also conducted a correlation analysis between OP triester log k (pH=13

416

solutions) values and the leaving alcohol pKa values, and observed a negative (slope =

417

-0.3010) and significant (p = 0.0087; r2 = 0.5535) linear curve (Figure S2 and Table

418

S3). Our results also indicated that hydrolysis of OP triesters was partly driven by

419

their leaving alcohol pKa values. 19



Page 20 of 34

420 421

3.3 Environmental Implications

422

Given the ongoing reports on adverse effects, i.e. dermatitis35, neurotoxicity4, 36,

423

cardiotoxicity37, genotoxicity38 as well as reproductive toxicity39, 40, of OP triester

424

flame retardants and plasticizers, there is increasing scientific interest in the

425

environmental fate of these chemicals. Based on numerous monitoring studies, most

426

OP triesters currently under study, i.e. TEP, TPP, TNBP, TEHP, TCEP, TCIPP,

427

TDCIPP, V6, TPHP, TMPP, TBOEP and EHDPP, have been reported in river water,

428

marine

429

effluents/influents or precipitation9. The present study demonstrated that the

430

hydrolytic stability to increasingly basic pH is in the order of Group A (with alkyl

431

moieties) > Group B (OP triester with chlorinated alkyl moieties) > Group C (with

432

aryl moieties) ∼ Group D (others). Thus, OP triesters with halogens and/or aryl groups

433

are most unstable, and the most stable are purely alkyl-substituted (Group A) (even

434

after a 35 day degradation period). These results indicate that, in the real environment,

435

OP triesters, especially ones with aryl moieties, may exist more as OP diesters rather

436

than triesters. In fact, the occurrence of OP diesters has been reported in human

437

urine20, 41, 42, which could be another source to the wastewater treatment plants and

438

subsequently to aqueous environments. Current information on the toxicity of OP

439

diesters is relatively rare. A recent study stated that OP diesters may have limited

440

nuclear receptor activity compared to their parent triesters43, but another study of

441

comparison of toxicity between TPHP and DPHP found that DPHP altered more

water,

groundwater,

tap

water,

sewage

treatment

plant

(STP)

20


Page 21 of 34


442

genes than TPHP38. Future research in warranted on the environmental monitoring of

443

OP diesters as well as triesters in aqueous environments, for example in wastewater

444

treatment plant influents and effluents.

445 446

ACKNOWLEDGEMENTS

447

Environment Canada's Chemicals Management Plan (CMP) (to R.J.L.) provided

448

major funding for this project. Supplemental funding was from the Natural Science

449

and Engineering Research Council (NSERC) of Canada (to R.J.L.).

450 451

Supporting Information Available

452

Further details are given on the methods, assessment of adsorptive effects, OP diester

453

hydrolysis studies, mass balance between OP triesters and diesters, and linear

454

correlative relationships between log normalized k and pKa. This material is available

455

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

456

21



Page 22 of 34

457

References:

458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500

1. van der Veen, I.; de Boer, J., Phosphorus flame retardants: Properties, production, environmental occurrence, toxicity and analysis. Chemosphere 2012, 88, (10), 1119-1153. 2. Sjodin, A.; Patterson, D. G., Jr.; Bergman, A., A review on human exposure to brominated flame retardants--particularly polybrominated diphenyl ethers. Environ. Int. 2003, 29, (6), 829-839. 3. Covaci, A.; Harrad, S.; Abdallah, M. A.; Ali, N.; Law, R. J.; Herzke, D.; de Wit, C. A., Novel brominated flame retardants: a review of their analysis, environmental fate and behaviour. Environ. Int. 2011, 37, (2), 532-556. 4. Andresen, J. A.; Grundmann, A.; Bester, K., Organophosphorus flame retardants and plasticisers in surface waters. Sci. Total. Environ. 2004, 332, (1-3), 155-166. 5. Rodriguez, I.; Calvo, F.; Quintana, J. B.; Rubi, E.; Rodil, R.; Cela, R., Suitability of solid-phase microextraction for the determination of organophosphate flame retardants and plasticizers in water samples. J. Chromatogr. A 2006, 1108, (2), 158-165. 6. Stockholm Convention, The new POPs under the Stockholm Convention, 2009; website: http://chm.pops.int/Convention/ThePOPs/TheNewPOPs/tabid/2511/Default.aspx. 7. Rodil, R.; Quintana, J. B.; Concha-Grana, E.; Lopez-Mahia, P.; Muniategui-Lorenzo, S.; Prada-Rodriguez, D., Emerging pollutants in sewage, surface and drinking water in Galicia (NW Spain). Chemosphere 2012, 86, (10), 1040-1049. 8. Kawagoshi, Y.; Fukunaga, I.; Itoh, H., Distribution of organophosphoric acid triesters between water and sediment at a sea-based solid waste disposal site. J. Mater. Cycles Waste Manag. 1999, 1, (1), 53-61. 9. Wei, G. L.; Li, D. Q.; Zhuo, M. N.; Liao, Y. S.; Xie, Z. Y.; Guo, T. L.; Li, J. J.; Zhang, S. Y.; Liang, Z. Q., Organophosphorus flame retardants and plasticizers: sources, occurrence, toxicity and human exposure. Environ. Pollut. 2015, 196, 29-46. 10. Regnery, J.; Puttmann, W., Occurrence and fate of organophosphorus flame retardants and plasticizers in urban and remote surface waters in Germany. Water Res. 2010, 44, (14), 4097-4104. 11. Kim, S. D.; Cho, J.; Kim, I. S.; Vanderford, B. J.; Snyder, S. A., Occurrence and removal of pharmaceuticals and endocrine disruptors in South Korean surface, drinking, and waste waters. Water Res. 2007, 41, (5), 1013-1021. 12. David, M. D.; Seiber, J. N., Accelerated hydrolysis of industrial organophosphates in water and soil using sodium perborate. Environ. Pollut. 1999, 105, (1), 121-128. 13. Faust, S. D.; Gomma, H. M., Chemical hydrolysis of some organic phosphorus and carbamate pesticides in aquatic environments. Environ. Lett. 1972, 3, 171-201. 14. Mabey, W.; Mill, T., Critical review of hydrolysis of organic compounds in water under environmental conditions. J. Phys. Chem. Ref. Data 1978, 7, 383-915. 15. Wagemann, R., Some environmental and toxicological aspects of a tri-aryl phosphate synthetic oil. Verh. Int. Ver. Limnol. 1975, 19, 2178-2184. 16. Pakalin, S.; Cole, T.; Steinkellner, J.; Nicolas, R.; Tissier, C.; Munn, S.; 22


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Eisenreich, S., Review on Production Processes of Decabromodiphenyl Ether (decaBDE) Used in Polymeric Applications in Electrical and Electronic Equipment, and Assessment of the Availability of Potential Alternatives to decaBDE. European Report EUR 22693 EN, Brussel, Belgium. 2007. 17. Tarrat, N., Alkaline hydrolsysi of phosphate triesters in solution: Stepwise or concerted? A theoretical study. Journal of Molecular Structure: THEOCHEM 2010, 941, (1-3), 56-60. 18. Compton, R. G.; Bamford, C. H.; Tipper, C. F. H., Ester Formation and Hydrolysis and Related Reactions (Book). 1972. 19. Chu, S.; Letcher, J. R., Determination of organophosphate flame retardants and plasticizers in lipid-rich matrices using dispersive solid-phase extraction as a sample cleanup step and ultra-high performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry. Anal. Chim. Acta. 2015, 885, 183-190. 20. Su, G.; Letcher, J. R.; Yu, H., Determination of Organophosphate Diesters in Urine Samples by a High-Sensitivity Method Based on Ultra High Pressure Liquid Chromatography-Triple Quadrupole-Mass Spectrometry. J. Chromatogr. A 2015, 1426, 154-160. 21. Lyznicki, E. P.; Oyama, K.; Tidwell, T. T., Reactivity of Organophosphates. IV. Acid-catalyzed Hydrolysis of Triethyl Phosphate: a Comparison with Ethyl Acetate. Can. J. Chem. 1974, 52, (7), 1066-1071. 22. USEPA, Estimation Program Interface (EPI) Suite. Version 4.1. Available from: http://www.epa.gov/oppt/exposure/pubs/episuitedl.htm 2012. 23. WHO, International Programme on Chemical Safety: Flame Retardants: tris(chloropropyl) phosphate and tris(2-chloroethyl) phosphate (1998). Geneva, Switzerland: World Health Organization; Available from: http://www.inchem.org/documents/ehc/ehc/ehc209.htm#SectionNumber:4.1). 2013. 24. Lande, S. S., USEPA: Investigation of Selected Potential Environmental Contaminants: Haloakyl Phosphates USEP-560/2-76-0007. 1976. 25. Howard, P. H.; Deo, P. G., Degradation of aryl phosphates in aquatic environments. Bull. Environ. Contam. Toxicol. 1979, 22, (3), 337-344. 26. Wolfe, N. L., Organophosphate and organophosphorothionate esters: Application of linear free energy relationships to estimate hydrolysis rate constants for use in environmental fate assessment. Chemosphere 2006, 9, 571-579. 27. Marklund, A.; Andersson, B.; Haglund, P., Organophosphorus flame retardants and plasticizers in Swedish sewage treatment plants. Environ. Sci. Technol. 2005, 39, (19), 7423-9. 28. Cristale, J.; Garcia Vazquez, A.; Barata, C.; Lacorte, S., Priority and emerging flame retardants in rivers: occurrence in water and sediment, Daphnia magna toxicity and risk assessment. Environ. Int. 2013, 59, 232-43. 29. Marklund, A.; Andersson, B.; Haglund, P., Traffic as a source of organophosphorus flame retardants and plasticizers in snow. Environ. Sci. Technol. 2005, 39, 3555-3562. 30. Cho, K. J.; Hirakawa, T.; Mukai, T.; Takimoto, K.; Okada, M., Origin and 23



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stormwater runoff of TCP (tricresyl phosphate) isomers. Water Res. 1996, 30, (6), 1431-1438. 31. Barnard, P. W. C.; Bunton, C. A.; Llewellyn, D. R.; A., V. C.; Welch, V. A., 523. The reactions of organic phosphates. Part V. The hydrolysis of triphenyl and trimethyl phosphates. J. Chem. Soc. 1961, 2670-2676. 32. Kirby, A. J.; Medeiros, M.; Mora, J. R.; Oliveira, P. S.; Amer, A.; Williams, N. H.; Nome, F., Intramolecular general base catalysis in the hydrolysis of a phosphate diester. Calculational guidance to a choice of mechanism. J. Org. Chem. 2013, 78, (4), 1343-1353. 33. Grzyska, P. K.; Czyryca, P. G.; Purcell, J.; Hengge, A. C., Transition state differences in hydrolysis reactions of alkyl versus aryl phosphate monoester monoanions. J. Am. Chem. Soc. 2003, 125, (43), 13106-13111. 34. Kulshreshtha, A.; Shinde, C. P., A Review: General Base Catalysis Hydrolysis of Organophosphorus Insectisides by Different Amines. IJSR 2014, 3, (10), 739-746. 35. Camarasa, J. G.; Serra-Baldrich, E., Allergic contact dermatitis from triphenyl phosphate. Contact Derm. 1992, 26, 264–265. 36. Ni, Y.; Kumagai, K.; Yanagisawa, Y., Measuring emissions of organophosphate flame retardants using a passive flux sampler. Indoor Air 2005 - 10th International Conference on Indoor Air Quality and Climate (Part I) 2006, 41, (15), 3235–3240. 37. McGee, S. P.; Konstantinov, A.; Stapleton, H. M.; Volz, D. C., Aryl phosphate esters within a major PentaBDE replacement product induce cardiotoxicity in developing zebrafish embryos: potential role of the aryl hydrocarbon receptor. Toxicol. Sci. 2013, 133, (1), 144-156. 38. Su, G.; Crump, D.; Letcher, R. J.; Kennedy, S. W., Rapid in vitro metabolism of the flame retardant triphenyl phosphate and effects on cytotoxicity and mRNA expression in chicken embryonic hepatocytes. Environ. Sci. Technol. 2014, 48, (22), 13511-13519. 39. Li, H.; Su, G.; Zou, M.; Yu, L.; Letcher, R. J.; Yu, H.; Giesy, J. P.; Zhou, B.; Liu, C., Effects of Tris(1,3-dichloro-2-propyl) Phosphate on Growth, Reproduction, and Gene Transcription of Daphnia magna at Environmentally Relevant Concentrations. Environ. Sci. Technol. 2015, 49, (21), 12975-12983. 40. Zhu, Y.; Ma, X.; Su, G.; Yu, L.; Letcher, R. J.; Hou, J.; Yu, H.; Giesy, J. P.; Liu, C., Environmentally Relevant Concentrations of the Flame Retardant Tris(1,3-dichloro-2-propyl) Phosphate Inhibit Growth of Female Zebrafish and Decrease Fecundity. Environ. Sci. Technol. 2015, 49, (24), 14579-14587. 41. Van den Eede, N.; Neels, H.; Jorens, P. G.; Covaci, A., Analysis of organophosphate flame retardant diester metabolites in human urine by liquid chromatography electrospray ionisation tandem mass spectrometry. J. Chromatogr. A 2013, 1303, 48-53. 42. Van den Eede, N.; Heffernan, A. L.; Aylward, L. L.; Hobson, P.; Neels, H.; Mueller, J. F.; Covaci, A., Age as a determinant of phosphate flame retardant exposure of the Australian population and identification of novel urinary PFR metabolites. Environ. Int. 2015, 74, 1-8. 43. Kojima, H.; Takeuchi, S.; Van den Eede, N.; Covaci, A., Effects of primary 24


Page 25 of 34


589 590

metabolites of organophosphate flame retardants on transcriptional activity via human nuclear receptors. Toxicol. Lett. 2016, 245, 31-39.

591 592

25



593 594 595

Page 26 of 34

Table 1. Half-lives (t1/2, d) and constant rate (k, d-1) of 16 organophosphate (OP) triesters in four aqueous solutions at nominal pH = 7, 9, 11 and 13 (see Table S1 for measured pH values), respectively. This Table was connected with Figure 4 by serial numbers 1-15. OP Triester Group A TEP TPP TNBP TEHP

pH = 7

pH = 9

pH = 11

pH = 13

NAa NA NA NA

NA NA NA NA

NA NA NA NA

t1/2=16 d; k=0.0423 d-1; 1#b t1/2=33 d; k=0.0213 d-1; 2# NA NA

Group B TCEP TCIPP TDCIPP V6

NA NA NA NA

NA NA NA NA

NA NA NA NA

t1/2=0.083 d; k=8.36 d-1; 3# t1/2=11 d; k=0.0621 d-1; 4# t1/2=0.044 d; k=15.6 d-1; 5# t1/2=0.11 d; k=6.23 d-1; 6#

Group C TPHP TMPP T2B4MP T3B4MP T4B3MP

t1/2=112 d; k=0.00618 d-1; 7# t1/2=94 d; Linearc t1/2=22 d; Linear t1/2=18 d; Linear t1/2=23 d; Linear

t1/2=77 d; k=0.00906 d-1; 8# t1/2=110 d; Linear t1/2=21 d; Linear t1/2=18 d; Linear t1/2=21 d; Linear

t1/2=34 d; k=0.0203 d-1; 9# t1/2=70 d; Linear t1/2=26 d; Linear t1/2=19 d; Linear t1/2=26 d; Linear

t1/2=0.0053 d; k=130 d-1; 10# t1/2=0.027 d; k=25.7 d-1; 11# t1/2=0.046 d; k=15.2 d-1; 12# t1/2=0.066 d; k=10.5 d-1; 13# t1/2=0.067 d; k=10.4 d-1; 14#

Group D TBOEP EHDPP TTBPP

NA t1/2=110 d; Linear t1/2=23 d; Linear



t1/2=21 d; Linear t1/2=0.11 d; k=6.58 d-1; 15# t1/2=19 d; Linear

596 597 26


Page 27 of 34

598 599 600 601 602


a

“NA” means not available. No significant degradation is observed for this triester over a period of 35 days; b t1/2 and k was calculated based on an assumption that phosphate ester degradation reaction follow pseudo-first-order kinetics, and see fitted curve in Figure 4 (NO. 1 to 15); c These t1/2 values are estimated based on two time points of 0 and 35 d, because complete curves cannot be obtained due to adsorption on glass surfaces. We emphasize that future usage on these t1/2 values should be done with caution.

27



Page 28 of 34

603

Figure Legends

604

Figure 1. Chemical structures of 16 target organophosphate (OP) triesters. The OP

605

triesters are sub-divided into four groups based on their specific chemical structures. OP

606

triesters of Group A contain non-halogenated alkyl moieties; triesters of Group B contain

607

chlorinated alkyl moieties; triesters of Group C contain aryl moieties; and triesters of

608

Group D do not belong Groups A, B or C.

609 610

Figure 2. Depletion of 16 OP triesters over a 35 day period in aqueous systems at

611

nominal pH = 7, 9, 11 and 13 (see Table S1 for measured pH values), respectively. Using

612

ultra-high performance liquid chromatography-tandem quadrupole mass spectrometry

613

with an atmospheric pressure chemical ionization source (UPLC-APCI(+)-MS/MS), the

614

relative percent of the OP triester concentrations in solutions collected at day 0. The red

615

bars represent the relative percent of the OP triester concentrations in solutions collected

616

at day 35. Where there is a significant difference (p < 0.01) a “*” is marked beside the

617

chemical names.

618 619

Figure 3. Time-dependent hydrolytic degradation of OP triesters at nominal pH = 7, 9,

620

11 and 13 (see Table S1 for measured pH values) over a period of 4 days or 35 days (0

621

min, 15 min, 30 min, 1 h, 2 h, 4 h, 8 h, 1 d, 2 d, 4 d, 7 d, 14 d, 21 d, 28 d and 35 d). For

622

several OP triesters, the degradation curves were shown for a period of 4 days given their

623

rapid degradation (< 1 day) in pH=13 solutions and no significant degradation (t-test,

624

p>0.01; see Figure 2) in pH=7, 9 and 11 solutions over a period of 35 days (i.e. TCEP,

625

TCIPP, TDCIPP and V6); or due to the strong adsorption on glass walls (i.e. TMPP, 28


Page 29 of 34


626

T2B4MP, T3B4MP, T4B3MP and EHDPP). Four OP treisters (TBOEP, TTBPP, TNBP and

627

TEHP) were not shown in this Figure. Because a smooth degradation curve was not

628

observed for TBOEP or TTBPP due to the severely strong adsorption on glass wall, and

629

no significant degradation was observed for TNBP or TEHP in all four pH solutions

630

(t-test,

631

chromatography-tandem quadrupole mass spectrometry with an atmospheric pressure

632

chemical ionization source (UPLC-APCI(+)-MS/MS), and relative to time 0, the percent

633

change in OP triester concentrations over time.

p>0.01;

in

Figure

2).

Using

ultra-high

performance

liquid

634 635

Figure 4. Fitted linear curves for the time-dependent hydrolytic degradation of OP

636

triesters in solutions at nominal pH = 7, 9, 11 and 13 (see Table S1 for measured pH

637

values), and according to pseudo-first-order kinetics. Degradation constants (k, min-1, in

638

Table 1) are calculated from the slope of the line by plotting the log of the fraction of the

639

parent compound concentration versus time, and the half-lives (t1/2, in Table 1) are

640

calculated as follows: t1/2 = ln 2/k. This Figure was connected with Table 1 by serial

641

numbers 1-15. For some OP triesters (i.e. TMPP, T2B4MP, T3B4MP, T4B3MP and

642

EHDPP), adsorption of which might occur on glass surfaces (Text S2 of SI), their

643

measured concentrations were corrected/normalized by those in pH=7 solutions, and

644

these chemicals were assumed to be stable in pH=7 solutions over a short time (all less

645

than 0.2 days).

646

29



647

Page 30 of 34

Figure 1

648 649

30


0

A B

80

40

0 *

*

*

B

*

*

C

*

*

* * *

C

120

pH = 11

80

40

*

*

D

D A

0 * *

A

ACS Paragon Plus Environment * *

B

* *

TP T HP T2 MP B P T 4 4M B P T 3 3M B P 4M P TB O EH E D P T T PP B PP

TE P T TN PP B TE P H P TC T EP TDCIP C P IP P V6

0

TP T HP T2 MP B P T 4 4M B P T 3 3M B P 4M P TB EHOE D P T T PP B PP

pH = 7

Relative Instrumental Response (%)

120

TE P T T N PP B TE P H P TC T EP TDCIP C P IP P V6

TP TMHP T2 P B P T4 4M B P T3 3M B P 4M P TB EHOE D P T T PP B PP

Day 0


A

TP TMHP T2 P B P T4 4M B P T3 3M B P 4M P TB O EH E D P T T PP B PP

TE P T TN P P B TE P H P TC T EP TDCIP C P IP P V6


650

TE P T T N PP B TE P H P TC T EP TDCIP C P IP P V6


Page 31 of 34 Environmental Science & Technology

Figure 2

Day 35

120

pH = 9

80

40

*

*

*

B

*

* *

C

*

*

*

C

*

*

D

120

pH = 13

80

40

* * *

D

651

652

31

ACS Paragon Plus Environment pH=13 pH=11 pH=9 pH=7

40

0

da y

80

4

T4B3MP

4

2

da y

da y

0

da y

40

1

pH=13 pH=11 pH=9 pH=7

0

da y

da y

da y

7

da y D a 14 y D a 21 y D a 28 y D a 35 y D ay

Chemical Relative Response at Different Time Points

TPHP

da y

120

da y

80

0

120


4

2

1

da y


0

2

0

40

da y

40

80

pH=13 pH=11 pH=9 pH=7

1

pH=13 pH=11 pH=9 pH=7

120

da y

80

TCEP

0

T3B4MP

da y

120

4

0

da y

40

2

pH=13 pH=11 pH=9 pH=7

da y

V6


ay

ay

0

1

80

da y Da 14 y D a 21 y D a 28 y D a 35 y D ay

D

ay

ay

40

Chemical Relative Response at Different Time Points 0

35

D

D

D

pH=13 pH=11 pH=9 pH=7

7

120

0

da y

28

21

ay ay

80


4

da y

14

D

D


TPP

da y

0 120

0

40

da y

pH=13 pH=11 pH=9 pH=7

4

80

da y

T2B4MP 2

0

2

40

da y

pH=13 pH=11 pH=9 pH=7

1

TDCIPP 7

ay

ay

ay

ay

0

da y

120

0

D

D

D

ay

ay

40


35

28

21

D

D

D


pH=13 pH=11 pH=9 pH=7

1

80

da y

120

0

da y

da y

14

7

0 80


4

2

da y

da y


TEP

da y

da y

da y

1

0 120

0

4

2

da y

da y


653

1

0

Environmental Science & Technology Page 32 of 34

Figure 3

120

TCIPP

80

pH=13 pH=11 pH=9 pH=7

40 0

120

TMPP

80

pH=13 pH=11 pH=9 pH=7

40 0

120

EHDPP

80

pH=13 pH=11 pH=9 pH=7

40 0

654

655

32

Page 33 of 34

Figure 4

-2

0 -1 -2

-3

-3

-3

-4

-4

-4 0.0

-4

20 30 Time (Days)

40

6: V6, pH=13 Fitted equation: Y = -6.23 X

-1 -2

0

10

20 30 Time (Days)

40

7: TPHP, pH=7 Fitted equation: Y = -0.00618 X

1 0 -1 -2

0.1 0.2 0.3 Time (Days)

0.4


1 0 -1 -2

-3

-4 0.0

-4

-4

-4

11: TMPP, pH=13 Fitted equation: Y = -25.7 X

-1 -2

1 0

10

20 30 Time (Days)

40

12: T2B4MP, pH=13 Fitted equation: Y = -15.2 X

-1 -2

0

1 0

10

20 30 Time (Days)

40


-1 -2

0

0

-3

-3

-4 0.00 0.02 0.04 0.06 0.08 0.10 Time (Days)

-4 0.00

-4 0.00

-4 0.00

0.05 0.10 0.15 Time (Days)

0.20

-4 0.00

40

10

20 30 Time (Days)

40

1 0

0.05 0.10 0.15 Time (Days)

0.20

10: TPHP, pH=13 Fitted equation: Y = -130 X

-1 -2

-4 0.000 0.005 0.010 0.015 0.020 0.025 Time (Days)


-2

-3 0.20

20 30 Time (Days)

-1

-3

0.05 0.10 0.15 Time (Days)

-2

-3

1 Ln (C/C o)

0

0

Ln (C/C o)

Ln (C/C o)

1

0.4

10

-2

-3

5: TDCIPP, pH=13 Fitted equation: Y = -15.6 X

-1


0

-3 0.1 0.2 0.3 Time (Days)

0

-1

-3

0

-3

1 Ln (C/C o)

0

10

Ln (C/C o)

0

1 Ln (C/C o)

-1

4: TCIPP, pH=13 Fitted equation: Y = -0.0621 X

1

Ln (C/C o)

-2

0

-3

1 Ln (C/C o)

-1

3: TCEP, pH=13 Fitted equation: Y = -8.36 X

1

1 Ln (C/C o)

-2

0

Ln (C/C o)

Ln (C/C o)

-1

2: TPP, pH=13 Fitted equation: Y = -0.0213 X

1

Ln (C/C o)

Ln (C/C o)

0

Ln (C/C o)

1: TEP, pH=13 Fitted equation: Y = -0.0423 X

1

Ln (C/C o)

656


0

15: EHDPP, pH=13 Fitted equation: Y = -6.58 X

-1 -2 -3

0.05 0.10 0.15 Time (Days)

0.20

-4 0.00 0.02 0.04 0.06 0.08 0.10 Time (Days)

657 658

33



659

Page 34 of 34

TOC art

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Organophosphate Flame Retardants and Plasticizers in Aqueous

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