Aqueous OH Radical Reaction Rate Constants for Organophosphorus

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Aqueous OH Radical Reaction Rate Constants for Organophosphorus Flame Retardants and Plasticizers: Experimental and Modeling Studies Chao Li, Gaoliang Wei, Jingwen Chen, Yuanhui Zhao, Ya-Nan Zhang, Limin Su, and Weichao Qin Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05429 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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Environmental Science & Technology

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Aqueous OH Radical Reaction Rate Constants for Organophosphorus Flame

2

Retardants and Plasticizers: Experimental and Modeling Studies

3

Chao Li,†, Gaoliang Wei, Jingwen Chen,*, Yuanhui Zhao,**,† Ya-Nan Zhang,† Limin Su,†

4

Weichao Qin†

5

†

6

Restoration, School of Environment, Northeast Normal University, Changchun 130117, China.

7

‡

‡

‡

‡

State Environmental Protection Key Laboratory of Wetland Ecology and Vegetation

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of

8

Environmental Science and Technology, Dalian University of Technology, Dalian 116024,

9

China.

10 11

*

12

**

Corresponding author phone/fax: +86-411-84706269; e-mail: [email protected]. Corresponding author phone/fax: +86-413-89165610, e-mail: [email protected]

13 14

Table of Contents (TOC)

15 16 17

Abstract. Aqueous ⋅OH reaction rate constants (kOH) for organophosphate esters (OPEs) are

18

essential for assessing their environmental fate and removal potential in advanced oxidation

19

processes (AOPs). Herein experimental and in silico approaches were adopted to obtain kOH

20

values for a variety of OPEs. The determined kOH for 18 OPEs varies from 4.0 × 108 M-1 s-1 to 1

ACS Paragon Plus Environment


21

1.6 × 1010 M-1 s-1. Based on the experimental kOH values, a quantitative structure-activity

22

relationship model that involves molecular structural information on the number of heavy

23

atoms, content index and the most negative charge of C atoms, was developed for predicting

24

kOH of other OPEs. Furthermore, appropriate density functional theory (DFT) and solvation

25

models were selected, which together with transition state theory were employed to predict

26

kOH of three representative OPEs. The deviation between the DFT calculated and the

27

experimental kOH values (kcal/kexp) is within 2. Half-lives of the OPEs were estimated to be 0.5

28

- 22791.3 days in natural waters and 0.044 - 19.7 s in AOPs, indicating the OPEs are

29

potentially persistent in natural waters and can be quickly eliminated by AOPs. The

30

determined kOH values and the in silico methods offer a scientific base for assessing OPEs fate

31

in aquatic environments.

32 33

INTRODUCTION

34

Organophosphate esters (OPEs) are an important class of industrial chemicals. OPEs can

35

have diverse molecular structures and have been extensively used as flame retardants and

36

plasticizers in a variety of household and industrial products.1-3 As OPEs are not chemically

37

bonded to the materials in products, they can be released into the environment via

38

volatilization, abrasion and leaching processes.2-9 OPEs have been reported to be present in

39

aquatic environments including river waters, seawaters, ground waters, and even drinking

40

water and effluents/influents of wastewater treatment plants, with concentrations ranging from

41

ng/L to a few hundred µg/L.1-3,5 It has also been reported that some OPEs have adverse effects

42

on humans and on ecosystems, including thyroid disruption, neurotoxicity, carcinogenicity,

43

mutagenicity and hormone disturbance.3,10-15 Therefore, it is necessary to investigate the

44

transformation behavior of OPEs in aqueous phase so as to assess their environmental

45

persistence and removal in engineering facilities. 2


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Hydroxyl radical (·OH) with a high redox potential (2.8 V) is a powerful and important

47

oxidant for degradation of organic pollutants in natural aquatic environments and artificial

48

facilities based on advanced oxidation processes (AOPs).16-23 In natural waters (such as

49

atmospheric water droplets and sunlit surface waters), ⋅OH can be generated via

50

photochemical reactions of dissolved organic matter (DOM), nitrate ions and transition metal

51

complexes, as well as by sunlight-independent pathways during oxidation of reduced aquatic

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DOM and iron.21-23 In AOPs, ·OH can be generated at high concentrations by various

53

electrochemical, photochemical and sonochemical methods.19,20 Therefore, the reaction

54

with ·OH is important in determining the fate of OPEs in aquatic environments since many

55

OPEs (especially chlorinated OPEs) are resistant to microbial degradation, hydrolysis and

56

direct photolysis.1,24-27 The ·OH reaction rate constants (kOH) are important for assessing fate

57

and persistence of OPEs in the aquatic environment, and for designing AOPs in engineering

58

facilities. However, to our knowledge, kOH values are not available for most OPEs.

59

Generally, most kOH values for organic compounds are obtained by experimental

60

measurements.

However,

experiments

are

subject

to

some

disadvantages

like

61

equipment-dependence, time-consuming and huge expenditure. With the advancement of

62

computational capacities and software, “it is now possible to imagine a day when many of the

63

constants that we carefully measure in the lab will be accessible with the click of a mouse”.28

64

Therefore, it was the purpose of this study to develop in silico models that can be employed to

65

predict kOH of OPEs.

66

Quantitative structure-activity relationship (QSAR) models on which the Organization for

67

Economic Co-operation and Development has issued guidelines with regard to model

68

development and validation, have been proven to be useful for kOH prediction.23,29-34

69

Nevertheless, the utility of QSAR models is constrained by their applicability domains (ADs),

70

and most OPEs (e.g., aryl OPEs and phosphoric acid diesters) are not covered in the ADs of 3



71

previously developed QSAR models for prediction of kOH (detailed in the Supporting

72

Information, SI). Thus, it is of importance to develop new QSAR models for predicting kOH of

73

OPEs within their ADs.

74

Quantum chemical calculation is another alternative to experiments for kOH determination,

75

which can allow for accurately predicting chemical reactivity of organics, and provide

76

kinetics data without the restriction of ADs of QSARs.35,36 Furthermore, it can provide

77

important information about reaction sites and favorable reaction pathways, and this

78

information is a valuable complement to both QSARs and experimental results. As far as we

79

know, there are no previously reported results of quantum chemical calculations on aqueous

80

kOH of OPEs. Thus, the second purpose of this study is to predict kOH of OPEs by quantum

81

chemical calculations.

82

In this study, we applied both experimental and in silico approaches to obtain the aqueous

83

kOH values of OPEs. The kOH values of 18 OPEs were determined with the conventional

84

competition kinetics methods in the UV/H2O2 process. Based on the measured kOH values, a

85

reliable QSAR model was developed for kOH prediction. Additionally, with density functional

86

theory (DFT) calculations, we developed a directly predictive method for aqueous kOH of 3

87

classes of OPEs independent of experimental data.

88 89

MATERIALS AND METHODS

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Chemicals and reagents. The 18 OPEs under study can be classified into 4 subgroups,

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alkyl-substituted organophosphate triesters (Class I), aryl-substituted organophosphate

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triesters (Class II), halogenated organophosphate triesters (Class III), and remaining OPEs

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(Class IV) that do not fit structurally into any of the previous 3 classes (Table 1). Information

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on the sources of the authentic standards of these OPEs and organic solvents are presented in

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the SI. 4


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Table 1. CAS, chemical names, abbreviations and molecular structures of the OPEs under

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study. Class

Class I

CAS

Chemical name

Abbr.

Structure

78-40-0

Triethyl phosphate

TEP

O O P O O

513-08-6

Tripropyl phosphate

TPP

126-73-8

Tributyl phosphate

TnBP

115-86-6

Triphenylphosphate

TPhP

26444-49-5

Cresyl diphenyl phosphate

CDPP

78-30-8

Tri-2-cresylphosphate

ToCP

78-32-0

Tri-4-cresylphosphate

TpCP

115-96-8

Tris(2-chloroethyl) phosphate

TCEP

O

O O

O

P

P

O

O O

O

Class II

O

Cl Cl

O

P

O

Cl

O

Cl Cl

13674-84-5

Tris(2-chloroisopropyl) phosphate

O O P O O

TCIPP

Class III

Cl

Cl

13674-87-8

Tris(2-chloro-1-(chloromethyl) ethyl) phosphate

TDCPP

Cl O O P O Cl O

Cl Cl

Cl

5



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Cl

38051-10-4

Tetrakis(2-chloroethyl) dichloroisopentyl diphosphate (V6)

V6

Cl

O

Cl

O

P

O

O

O

P

O Cl

O

Cl

O

Cl

Br

126-72-7

Tris(2,3-dibromopropyl) phosphate

T23Br

O

Br Br

O

P

Br

O

Br

O

Br

78-38-6

Diethyl ethyl phosphonate

DEEP

107-66-4

Dibutyl hydrogen phosphate

DnBP

O O P O

OH O P O O OH

2781-11-5

Diethyl bis(2-hydroxyethyl) aminomethyl phosphonate

O

DBAP

O

P

O

N

OH

Class IV 298-07-7

Di(2-ethylhexyl) phosphate

O OH P O O

DEHP

O

78-51-3

Tris(2-butoxyethyl) phosphate

TBEP O

838-85-7

Diphenyl phosphate

O

O

P

O

O

O

DPhP

98 99

UV/H2O2 competitive kinetics. Details on the UV/H2O2 competitive kinetics method can be

100

found in previous studies37-40 and are elaborated in the SI. In this study, ⋅OH was generated in

101

a UV/H2O2 system, and atrazine (kOH = 2.6 × 109 M-1 s-1) was selected as a reference

102

compound.41 Initial concentrations of H2O2, atrazine and OPEs were 5 or 10 mM, 1−4 µM

103

and 0.34−5 µM, respectively. The measured pH values of the reaction systems were ~5.5

104

without adjustment. The pKa values of the OPEs containing ionization groups are 14.3

105

(DBAP) and 1~2 (DnBP, DEHP and DPhP), and the pKa of atrazine is 1.68.42-44 Therefore, 6


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the molecular forms (molecular/anionic/cationic) of these OPEs and atrazine at pH = 5.5 are

107

similar to those in the pH range of 5−8 in water treatment applications and 6−9 in natural

108

waters.

109

The competition kinetics experiment was performed with an XPA-7 merry-go-round

110

photochemical reactor (Xujiang Technology Co., Nanjing) with a water-refrigerated 500 W

111

Hg lamp equipped with 290 nm cut-off filters. Controls were included for each batch of the

112

experiments. All the experiments were carried out in triplicate. The temperature in the reactor

113

was kept around 25 ±1 oC with a water cooling system.

114

Analytical methods. Details on the sample pre-treatment are provided in the SI. Quantitative

115

analyses of TBEP, ToCP, TPhP and TpCP were performed using an Agilent 6890 GC equipped

116

with FID and an Agilent DB-17ms column (0.25 µm × 30 m × 0.25 mm). As the pretreatment

117

process for the samples quantified by GC was found to be very tedious and time-consuming,

118

all the other OPEs including TPhP were quantified with an Agilent 1100 HPLC-tandem

119

6410B quadrupole mass spectrometer (HPLC-MS/MS) equipped with an electrospray

120

ionization (ESI) source and an XTerra MS C18 column (2.1 mm × 100 mm × 3.5 µm; Waters,

121

Milford, MA, USA) using multiple reaction monitoring positive mode. All the analytical

122

conditions are detailed in Tables S1 and S2. TPhP was quantitatively analyzed by both GC

123

and LC-MS/MS to verify the reliability of the different sample treatment methods. The

124

corresponding GC chromatograms are shown in Figure S1.

125

QSAR modeling. Two classes of molecular structural descriptors were considered for the

126

modeling, including quantum chemical descriptors that have clear physicochemical

127

definitions and DRAGON descriptors that can describe the structural diversity of the

128

compounds. Before calculating the descriptors, the molecular structures of the 18 OPEs were

129

optimized by the M06-2X/6-311+G(d,p) method using the Gaussian 09 program suite.45

130

Frequency analysis was conducted to verify that the optimized geometries were the 7



131

corresponding local minima. To consider solvent effects of water, the integral equation

132

formalism polarized continuum model (IEFPCM) based on the self-consistent-reaction-field

133

with the default radii UFF (IEFPCM-UFF) was employed.46,47 The quantum chemical

134

descriptors were obtained via calculations by both Gaussian 09 and MOPAC 2016 as detailed

135

in Table S3, since the two programs generate different descriptors. The DRAGON descriptors

136

were calculated by the DRAGON software (version 6.0) (TALETE srl, Italy).48 The

137

data/compounds were randomly split into a training set (14 data) used for QSAR model

138

development and a validation set (4 data) for model validation. Stepwise multiple linear

139

regression (MLR) analysis was employed to construct the QSAR model. The AD of the model

140

was characterized by the Euclidean distance-based approach with AMBIT Discovery (version

141

0.04), as detailed in the SI.

142

Quantum chemical calculation. Considering computational costs, 3 model compounds

143

(TPP, TCIPP and TpCP) from 3 classes of OPEs were selected for the computation. Since

144

there are many sp3-hybridized C-X (X = H, C, Cl, O) bonds in the 3 OPE molecules, these

145

OPEs have a range of conformations. A global minimum search (GMS) is needed to identify

146

the optimal structures of the reactants, followed by kinetics calculations. The GMS was

147

performed as described in our previous studies.35,36 The conformations from the GMS were

148

selected as the starting points for geometry optimization at the M06-2X/6-311+G(d,p) level

149

using the Gaussian 09 program package, and the conformation with the lowest energy was

150

identified as the global minimum. The selection of the DFT methods were detailed in the SI.

151

A gradual scheme was used to screen reliable theoretical methods for kOH prediction. As

152

TPP has the most simple structure in these 3 compounds and thus requires the lowest

153

computational time, the reaction of TPP with ⋅OH was taken as an initial test to select the

154

different DFT methods [X/6-311+G(3df,2p)//X/6-311+G(d,p), where “X” stands for the

155

functional B97-1,49 B3LYP,50,51 M06-2X52 and ab initio MP253]. The IEFPCM-UFF approach 8


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156

was initially employed to mimic the water solvent effects. In this study, the IEFPCM-UFF

157

approach was used for all geometry optimizations and vibrational frequency calculations. As

158

the rate constant calculations are sensitive to activation free energies (∆G‡), the high accuracy

159

MP2/6-311+G(3df,2p) and B2PLYP/6-311+G(3df,2p) methods with the IEFPCM-UFF were

160

used for single point energy calculations (SPECs) for further evaluation.

161

Since the method used for solvation calculation is important in determining the accuracy

162

of ∆G‡, we further evaluated another two solvation methods (IEFPCM-Bondi54 and SMD55) in

163

SPECs combined with the B2PLYP/6-311+G(3df,2p) method. Since the B2PLYP method

164

showed significant spin contamination, has a high demand in storage hardware, and is

165

time-consuming for such a large system TpCP+⋅OH, the M06-2X/6-311+G(3df,2p) method

166

was employed for its SPECs with consideration of the IEFPCM-UFF, IEFPCM-Bondi and

167

SMD. The aqueous kOH values of TPP, TCIPP and TpCP were calculated based on the

168

transition state theory as detailed in the SI, and the Wigner correction was used to account for

169

the quantum mechanical tunneling effect. Additionally, natural bond orbital (NBO) population

170

analysis was performed to obtain the natural charges that may correlate to the reactivity of

171

OPEs.

172 173

RESULTS AND DISCUSSION

174

The UV-vis absorption spectra of the 18 OPEs and atrazine with consideration of solvent

175

subtraction are shown in Figure S2. As can be seen from Figure S2, direct photolysis of the

176

OPEs and atrazine at λ > 290 nm was insignificant because of their negligible photon

177

absorption. Moreover, the control experiments without addition of H2O2 under light irradiation

178

with λ > 290 nm further verified that the OPEs did not undergo direct photolysis. Other

179

controls showed that direct oxidation of the OPEs by H2O2 and hydrolysis of the OPEs are

180

negligible (< 1% of disappearance was observed in the period of the competition kinetics 9



181

experiment).

182

Experimentally determined kOH values. Figure 1 shows the measured kOH values for the 18

183

OPEs. Among these OPEs, the ones in Class IV including DBAP, DEHP and DPhP degrade

184

rapidly by ⋅OH. Their reaction rate constants (kOH > 1010 M-1 s-1) are significantly higher than

185

those of the other OPEs, followed by TPhP, TBEP, CDPP and TnBP with kOH values around 8

186

× 109 M-1 s-1. All the halogenated OPEs exhibit a high stability to ⋅OH oxidation, and their kOH

187

values are < 6 × 108 M-1 s-1.

188

As revealed by the NBO population analysis, the high reactivity of the OPEs in Class IV

189

can be due to the atomic formal charge (q) distribution in these molecules, which can be

190

concluded from DPhP and DnBP as examples. The q value for one phenyl in DPhP is

191

0.32253e, and the q value of a chain in TnBP is 0.37332e (Table S4). These charges are lower

192

than those of TPhP (0.34382e) and DnBP (0.37777e). Since ⋅OH is a strong electrophile, the

193

addition and H-abstraction reactions occurring at sites with lower charges are more favorable.

194

Thus, DPhP and TnBP have higher reactivity towards ⋅OH than TPhP and DnBP. Additionally,

195

it is found that logkOH of OPEs positively correlates with their chemical potential [χ = (ELUMO

196

+ EHOMO)/2, p < 0.05] that measures the tendency of electrons to escape from a system (Figure

197

S3). Molecules with high χ values are more inclined to provide electrons for ⋅OH and

198

eventually be oxidized, leading to their higher ⋅OH reactivity. This can also well explain the

199

high reactivity of DBAP and TBEP in Class IV.

200

For the alkyl-substituted OPEs, the degradation rate constants increase with the length of

201

the alkyl carbon chain in the range of C2 to C4. However, upon further increase of the alkyl

202

chain length and ramification of the alkyl chains, the reactivity of OPEs toward ⋅OH decreases.

203

No significant degradation was observed for tris(2-ethylhexyl) phosphate (TEHP), which is

204

consistent with the results of a previous study for this compound.56 Therefore, its kOH value

205

cannot be obtained, and TEHP was excluded from the modeling study. The measured kOH 10


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206

values of aryl-substituted OPEs have the order TPhP > CDPP >ToCP > TpCP. This ranking

207

suggests that methyl substitution in the phenyl moiety of the aryl OPEs reduces the reactivity.

208

It can also be observed from Figure 1 that ortho-methyl substituted tricresyl phosphate

209

possesses a higher reactivity than para-methyl substituted tricresyl phosphate.

210

It is worth mentioning that the kOH values of TPhP measured by means of GC and

211

HPLC-MS/MS are very close (8.89 × 109 and 8.80 × 109 M-1 s-1), suggesting reliability of the

212

different sample pretreatment and analytical methods. Moreover, the measured kOH values of

213

several OPEs (TnBP, TCEP, TCIPP and TBEP) are generally consistent with the ones reported

214

in previous studies,26,57-59 indicating reliability of the methods/techniques used in this study

215

(Table S5).

216

1.6 × 1010

Class I

Class II

Class III

Class IV

1.2 × 1010 kOH (M-1s-1) 8.0 × 109 4.0 × 109

217

TEP TP TnBP TPhP CD P P ToCP TpCP TC P TD EP CP TC P IPP V T23 6 B DE r EP Dn B DB P AP DE H TB P E DP P hP

0

218

Figure 1. Experimentally determined kOH values for the 18 OPEs under study (the

219

corresponding experimental kOH values are listed in Table S5)

220 221

QSAR modeling. The optimum QSAR model for logkOH prediction was obtained as follows:

222

logkOH = 8.785 – 0.182 × nHM + 0.573 × IC5 + 1.485 × QC-

223

ntr = 14, R2adj = 0.877, RMSEtr = 0.175, Q2LOO = 0.842, p < 1× 10-5, 11



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224

next= 4, Q2ext = 0.862, RMSEext = 0.235

225

This model contains 3 molecular structural descriptors: the number of heavy atoms (nHM), the

226

information content index (5th order neighborhood symmetry, IC5) and the most negative

227

charge of C atoms in an organic molecule (QC-). Among them, IC5 and nHM are DRAGON

228

descriptors from the block of information indices and constitutional indices, respectively.

229

Values of these molecular structural descriptors are listed in Table S6 together with the

230

predicted logkOH values of the 18 OPEs. The statistical parameters, including the adjusted

231

square of determination coefficient (R2adj), the root mean squared error (RMSEtr) and the

232

leave-one-out cross-validated square of determination coefficient (Q2LOO) for the training set,

233

suggest high goodness-of-fit and robustness of the established model. The difference between

234

R2adj and Q2LOO does not exceed 0.3, indicating lack of over-fitting of the model.60 All variance

235

inflation factors (VIF) of these 3 descriptors are < 3 (Table S7), indicating that this model is

236

free of multi-collinearities. As shown in Figure 2, the predicted logkOH values are close to the

237

observed values for both the training and validation set compounds (Table S6). The high Q2ext

238

value and the low RMSEext value of the validation set demonstrate the good predictive

239

performance of the model. 10.5 10.0

logkOH (Predicted)

9.5 9.0 8.5 8.0 8.0

Training set Validation set 8.5

9.0

9.5

10.0

10.5

logkOH (Experimental)

240 241

Figure 2. Plot of predicted versus experimental logkOH values for OPEs in the training and

242

validation sets. 12


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243 244

The descriptor space of the QSAR model is described in Figure S4. It may be deduced

245

from Figure S4 that all the OPEs in the training and in the validation sets are in the domain,

246

and none of them is particularly influential in the model space. This implies that the training

247

set compounds have good representativeness. As shown in Figure 3, the values of the

248

Euclidean distance of each OPE (di) are lower than their warning value d* (the largest

249

Euclidean distance of the OPEs in the training set) and all the standardized residuals are < |2|,

250

indicating there are no outliers in either the training set or the validation set of the QSAR

251

model. Thus, it can be inferred that the developed model can be employed to predict kOH

252

values of other OPEs with molecular structures similar to those in the present study and

253

within the AD of the model. As far as we know, this QSAR model with the AD description is

254

the first for predicting kOH of OPEs.

255

As indicated by the t test statistics and the corresponding significance level (p values) for

256

the 3 descriptors (Table S7), the descriptor nHM is the most important factor governing kOH.

257

The negative coefficient of nHM indicates that the kOH values of OPEs decrease with the

258

increase of heavy atom numbers that count all the atoms with principal quantum number > 2

259

(such as phosphorus, sulfur and halogen atoms).48 The comparatively smaller kOH values of

260

halogenated OPEs than those of non-halogenated OPEs confirm this prediction. The

261

descriptor IC5 is calculated on the basis of the pairwise equivalent atoms in an H-filled

262

molecule, which measures the structural complexity per vertex.61-63 OPE molecules with high

263

IC5 values tend to have high reactivity with ⋅OH. The descriptor QC- imparts a positive

264

influence on logkOH. In an OPE molecule, the H atoms connected to C atoms with a more

265

negative charge are more inclined to have a more positive charge, leading to their low

266

reactivity toward ⋅OH due to the electrophilicity of ⋅OH.

13



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4 3 2 1 Standardized 0 Residuals

-1 -2 Training set Validation set

-3 -4 267

0.2

0.4

0.6

0.8

*

d = 0.952

1.0

1.2

1.4

Euclidean Distance

268

Figure 3. Plot of standardized residuals versus Euclidean distance [The transverse dash-dotted

269

lines represent ±2 standardized residuals, and the vertical dash-dotted lines represent the

270

waring leverage (d*, the largest Euclidean distance of the OPEs in the training set)]

271 272

DFT calculation results-TPP reaction with ⋅OH. The reaction of TPP with ⋅OH proceeds via

273

H-atom abstraction to form H2O and TPP-radicals (PH-n, where n denotes different H atoms in

274

TPP, Figure S5) since the addition of ⋅OH to the P=O bond is unfavorable.35 All the

275

H-abstraction reaction channels were considered in the calculation (Figure S5), and the results

276

show that each pathway proceeds through a pre-reactive complex (RC), a transition state (TS)

277

and a product-complex (PC), and finally leads to formation of a TPP-radical and H2O.

278

The calculation method was evaluated firstly by comparing the calculated rate constants of

279

3 selected pathways from the different methods with IEFPCM-UFF. We found that the rate

280

constants (2.7 × 1010 and 1.1 × 1010 M-1s-1) for ⋅OH+TPP are greatly overestimated based on

281

the calculated ∆G‡ values and the imaginary frequency of the TSs from the B97-1 and B3LYP

282

functional, whereas the rate constant (2.1 × 106 M-1s-1) is greatly underestimated by the MP2

283

method when compared with the experimental kOH value [(2.82 ± 0.09) × 109 M-1s-1] (Table

284

S8). The M06-2X functional (3.2 × 109 M-1s-1) yielded a relatively small deviation between 14


Page 15 of 31


285

the calculated and measured rate constants compared with the other methods. Thus, the

286

M06-2X/6-311+G(d,p) method with the IEFPCM-UFF was selected for geometry

287

optimization and frequency analysis.

288

The

high

level

SPECs

results

show

that

the

∆G‡

values

from

the

289

MP2/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) method are greatly overestimated like the ∆G‡

290

values obtained from the MP2/6-311+G(3df,2p)//MP2/6-311+G(d,p) method, whereas the

291

∆G‡ values obtained from the B2PLYP/6-311+G(3df,2p) method are slightly underestimated

292

(Table S9). Further investigation on the solvation methods at the B2PLYP/6-311+G(3df,2p)/

293

/M06-2X/6-311+G(d,p) level revealed that the calculated ∆G‡ values from the IEFPCM-Bondi

294

and SMD are close to each other (Table 2). According to TST [eq. (9) in the SI], the kOH value

295

for TPP at T = 298 K was calculated (from the IEFPCM-Bondi and SMD) to be about 5.3 ×

296

109 M-1s-1.

297

The accuracy of the quantum chemically calculated free energy is within 1 kcal mol-1, and

298

generally an error of 1 kcal mol-1 in ∆G‡ corresponds to an error of a factor of 10 in rate

299

constants, which is acceptable.64-66 Therefore, the DFT calculated rate constant is considered

300

to be well consistent with the experimental value [(2.82 ± 0.09) × 109 M-1s-1]. Thus, as to

301

alkyl-substituted OPEs, their aqueous kOH can be predicted by the B2PLYP/6-311+G(3df,2p)

302

method with the IEFPCM-Bondi or SMD based on the geometries optimized at the

303

M06-2X/6-311+G(d,p) level with the IEFPCM-UFF. The corresponding schematic potential

304

energy

305

B2PLYP/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) method with the IEFPCM-Bondi is

306

displayed in Figure S6.

surface

for

TPP+⋅OH

calculated

with

the

307

We further calculated kOH values of TPP in the temperature range 273−313 K at the

308

B2PLYP/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) level with the IEFPCM-Bondi method

309

(Figure 4). The kOH of TPP increases with temperatures, suggesting that increased temperature 15



Page 16 of 31

310

promotes the ⋅OH-initiated degradation of TPP.

311

TCIPP reaction with ⋅OH. The selected methods were subsequently employed for calculating

312

kOH of TCIPP. As concluded from our previous study, the favorable reaction pathways of

313

TCIPP with ⋅OH are associated to H-abstraction.35 Therefore, we just considered all

314

H-abstraction pathways. The reaction pathways proceed as those of TPP+⋅OH. The calculated

315

∆G‡ values at the B2PLYP/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) level with two different

316

solvation methods are listed in Table 2. A general trend for the favorable H-abstraction

317

channels is observed as: (-CH< group) > (-CH2Cl group) > (-CH3 group). In addition, it can

318

be seen that the ∆G‡ values calculated by the SMD method are generally lower than those

319

obtained by the IEFPCM-Bondi method. In contrast, the calculated kOH at T = 298 K from the

320

IEFPCM-Bondi method (6.9 × 108 M-1 s-1) is lower than that from the SMD (9.2 × 108 M-1 s-1),

321

but agrees with the experimental values measured in this study [(4.32 ± 0.74) × 108 M-1s-1]

322

and in previous studies [1.98 × 108 M-1s-1,26 (7 ± 2) × 108 M-1s-1 57] (Table 2).

323

In our previous study, the kOH value of TCIPP was overestimated by the M06-2X method

324

when IEFPCM-UFF was used.35 Thus, we calculated the kOH value of TCIPP with the

325

IEFPCM-Bondi method to further check whether the M06-2X method works well for this

326

system. When the IEFPCM-Bondi was considered, the calculated kOH is 4.6 × 108 M-1 s-1,

327

which

328

B2PLYP/6-311+G(3df,2p)/

329

M06-2X/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) method with the IEFPCM-Bondi can be

330

employed for estimating kOH of chlorinated OPEs. The schematic potential energy surface for

331

TCIPP+⋅OH calculated at the M06-2X/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) level with

332

the IEFPCM-Bondi is shown in Figure S7. Additionally, the calculated kOH of TCIPP in the

333

range of 273−313 K also displays a positive temperature dependence (Table 2).

is

consistent

with

the

experimental

values.

/M06-2X/6-311+G(d,p)

334 16


Therefore,

method

both and

the the

Page 17 of 31


335

Table 2. Activation free energy (∆G‡, kcal·mol-1) of H-abstraction pathways of TPP and

336

TCIPP with ⋅OH calculated by the B2PLYP/6-311+G(3df,2p) method with the

337

IEFPCM-Bondi and SMD based on the structures optimized at the M06-2X/6-311+G(d,p)

338

level with IEFPCM-UFF. TPP Species a

TCIPP Species b

∆G‡

∆G‡

∆G‡

∆G‡

(IEFPCM-Bondi)

(SMD)

(IEFPCM-Bondi)

(SMD)

TSH-15

9.4

9.0

TS10h

10.4

10.2

TSH-16

7.2

7.3

TS11h

9.5

9.5

TSH-17

9.0

8.6

TS12h

8.1

8.1

TSH-18

7.5

7.6

TS13h

10.6

10.5

TSH-19

7.2

7.4

TS14h

10.3

10.1

TSH-20

9.4

9.0

TS15h

11.0

10.2

TSH-21

9.1

8.8

TS17h

8.6

8.5

TSH-22

8.6

8.4

TS19h

10.5

10.4

TSH-23

9.2

8.9

TS20h

11.5

11.8

TSH-24

8.6

8.4

TS21h

10.9

10.6

TSH-25

8.7

8.0

TS23h

10.1

10.3

TSH-26

9.1

8.8

TS24h

9.6

8.8

TSH-27

9.0

8.8

TS27h

11.2

11.1

TSH-28

8.7

8.4

TS29h

10.5

10.6

TSH-29

9.0

8.7

TS30h

12.5

11.5

TSH-30

9.4

9.2

TS31h

11.2

10.9

TSH-31

9.0

8.8

TS33h

9.7

9.7

TSH-32

9.1

8.8

TS34h

9.4

8.7

17



TSH-33

9.1

8.9

TSH-34

9.0

8.7

TSH-35

8.7

8.4

Page 18 of 31

339

a

340

atoms in TPP (corresponding to the reaction pathways shown Figure S6). b TSnh stands for the

341

transition states in the reaction of TCIPP+⋅OH, and n denotes different H atoms in TCIPP

342

(corresponding to the reaction pathways shown in Figure S7).

TSH-n stands for the transition states in the reaction of TPP+⋅OH, and n denotes different H

343 344

TpCP reaction with ⋅OH. The reaction of TpCP with ⋅OH proceeds mainly via either

345

⋅OH-addition to the phenyl groups to form TpCP-adducts (PCn, where n denotes different C

346

atoms in TpCP) or H-abstraction from the –CH3 groups to form TpCP-radicals (PHn, where n

347

denotes different H atoms in TpCP.) and H2O.67 Totally, there are 3 cresyl groups (denoted as

348

R1, R2 and R3) and there are therefore 18 different addition positions and 9 different

349

H-abstraction positions for ⋅OH (Figure S5). All these reaction pathways were considered, and

350

their corresponding ∆G‡ values are listed in Table 3. The M06-2X/6-311+G(3df,2p) method

351

was utilized in SPECs for this system, since the B2PLYP method showed significant spin

352

contamination for this system, is also time-consuming, and has a high demand for storage

353

hardware as is common to such large molecules.

354

Just like in the case of TCIPP+⋅OH, the ∆G‡ values of TpCP+⋅OH calculated from the

355

IEFPCM-UFF and SMD are generally lower than those from the IEFPCM-Bondi.

356

Nevertheless, the calculated kOH of TpCP at T = 298 K from the IEFPCM-Bondi method (1.3

357

× 109 M-1s-1) is lower than the value obtained from SMD (3.2 × 109 M-1s-1) and IEFPCM-UFF

358

(1.2 × 1010 M-1s-1), but agrees well with the experimental values [(1.81 ± 0.77) × 109 M-1s-1]

359

determined

360

M06-2X/6-311+G(3df,2p)//M06-2X/6-311+G(d,p) with the IEFPCM-Bondi can be applied to

in

this

study

(Figure

4).

Therefore,

the

18


combination

method

of

Page 19 of 31


361

predict the aqueous kOH of aryl-substituted OPEs. The corresponding schematic potential

362

energy surface is shown in Figure S8.

363

Using the selected method, the kOH values of TpCP in the temperature range of 273−313 K

364

were calculated, as shown in Figure 4. The kOH values of TpCP display a negative temperature

365

dependence. Based on the calculated ∆G‡ values from IEFPCM-Bondi, it can be concluded

366

that the H-abstraction pathways occurring at the cresyl group R1 are generally more favorable

367

than the other H-abstraction pathways. As indicated by the ∆G‡ values, R2 has the highest

368

reactivity toward ⋅OH, whereas R1 and R3 have an almost similar reactivity towards ⋅OH.

369 370

Table 3. Activation free energy (∆G‡, kcal·mol-1) of reaction pathways of TpCP with ⋅OH

371

calculated by M06-2X/6-311+G(3df,2p) with the IEFPCM-UFF, IEFPCM-Bondi and SMD

372

based on the molecular structures optimized at the M06-2X/6-311+G(d,p) level with the

373

IEFPCM-UFF. ∆G‡ Species a

IEFPCM

∆G‡ Species a

IEFPCM-

IEFPCM-

IEFPCM-

UFF

Bondi

SMD

SMD

-UFF

Bondi

TSC10

8.9

10.4

9.8

TSC6

9.3

10.2

9.1

TSC11

8.0

9.6

8.8

TSC7

9.7

10.9

10.0

TSC12

9.5

11.0

10.0

TSC8

9.3

10.3

9.1

TSC13

7.4

8.6

7.8

TSC9

7.7

8.8

8.1

TSC14

10.2

11.2

10.3

TSH37

9.1

10.4

9.8

TSC15

8.9

10.2

9.6

TSH38

7.4

8.8

8.7

TSC16

8.3

9.5

8.9

TSH39

9.1

10.4

9.8

TSC17

9.3

10.7

9.8

TSH40

7.7

9.0

8.6

TSC18

7.8

8.9

8.1

TSH41

7.6

9.1

8.9

19



Page 20 of 31

TSC19

8.1

9.3

8.4

TSH42

6.5

7.9

7.8

TSC20

7.6

8.8

7.9

TSH43

8.6

10.0

9.5

TSC21

8.8

9.7

8.8

TSH44

7.1

8.4

8.1

TSC22

9.2

10.2

9.3

TSH45

8.4

10.1

9.2

TSC23

7.6

8.5

7.6

374

a

375

denotes abstracting different H atoms from -CH3 groups; and Cn denotes ⋅OH-addition to

376

different C atoms of phenyl groups in TpCP.

The symbols “TSHn and TSCn” stand for transition states in the reaction of TpCP+⋅OH; Hn

377 10

1.0 × 10

TPP (DFT) TpCP (DFT) TCIPP (DFT) TPP (Experimental value, current study) TpCP (Experimental value, current study) TCIPP (Experimental value, current study) TCIPP (Ref. 57 ) TCIPP (Ref. 26 )

9

8.0 × 10

9

kOH (M-1 s-1)

6.0 × 10

9

4.0 × 10

9

2.0 × 10

270

280

290 T (K)

378

300

310

379

Figure 4. DFT calculated kOH values for TPP, TCIPP and TpCP over the temperature range of

380

273 to 313 K together with the experimental values at T = 298 K.

381 382

It deserves mentioning that the formed product radicals and TpCP-adducts in the primary

383

reactions of 3 OPEs with ·OH are open-shell species that are not stable. As can be inferred

384

from the degradation of TCEP in UV/H2O2 system,68 the final products of OPEs under study

385

in AOPs may include PO43−, Cl−, Br−, chlorinated alcohol/aldehyde, non-chlorinated

386

alcohol/aldehyde with small molecular weight and phenolic compounds. These small organic

387

molecules can further be oxidized to acids, and most of them can be finally mineralized to 20


Page 21 of 31

388


CO2 and H2O in AOPs.

389

As far as we know, this study is the first to systematically investigate the reaction kinetics

390

of ⋅OH with OPEs with diverse molecular structures. Appropriate DFT methods combined

391

with the solvent models for predicting aqueous kOH values of the 3 classes of OPEs were

392

selected. The deviation (kcal/kexp) between the DFT calculated and experimental kOH values for

393

these 3 OPEs at T = 298 K is 0.7–1.9 (for TCIPP with multiple experimental kOH values, the

394

arithmetic mean value was used for the estimation). This good agreement between the

395

experimental and the quantum chemically calculated values validates the reliability of the

396

present calculation methods. Additionally, the NBO population analysis of TPP and TCIPP

397

revealed that the H atoms connected to C atoms with large QC- values tend to have high

398

reactivity towards ⋅OH (Table S10). As to TpCP, the order of H-abstraction pathways from –

399

CH3 groups and ⋅OH addition pathways follow the similar regularity as in case of TPP and

400

TCIPP (Table S11). This well explains the appearance of the descriptor QC- in the QSAR

401

model.

402

In this study, both QSAR modeling and DFT calculation methods were applied for

403

predicting kOH of OPEs. We also calculated the kOH values of the OPEs under study by

404

previously developed models, although most of the OPEs are out of the AD of the respective

405

models listed in Table S12.32,33,69 The ratios of kcal/kexp for these 18 OPEs estimated by the

406

group contribution method of Minakata et al.69 and the QSAR models of Jin et al.,32 Luo et

407

al.,33 and this study (QSAR model) are in the range of 0.1–5.2, 0.01–239.9, 0.4–12.1 and 0.4–

408

2.0, respectively. Therefore, the newly developed QSAR model shows the most satisfying

409

performance. Furthermore, the quantum chemical calculations combined with TST can predict

410

temperature-dependent rate constants, which is superior to the above models.

411

Table S13 lists the half-life (t1/2) values calculated by the formula t1/2= ln2/(kOH × cOH),

412

where cOH represents ⋅OH concentration in different water systems. cOH in natural waters 21



413

ranges from 10-17 to 10-15 M,33,70,71 and the corresponding t1/2 is calculated to be 0.5–22791.3

414

days for the 18 OPEs. Organic pollutants with t1/2 > 60 days in aquatic environments are

415

classified as persistent organic pollutants, according to the screening criteria of the Stockholm

416

Convention. cOH in AOPs is 4–8 orders of magnitude (10-11–10-9 M) higher than those in

417

natural aquatic environments.72 This implies that t1/2 for the OPEs in AOPs is in the range of

418

0.044–19.7 s, indicating that the OPEs are eliminated by AOPs quickly.

419 420

ASSOCIATED CONTENT

421

Supporting Information. Pseudo-first-order reaction and the competitive kinetics method;

422

Details on samples preparation and analytical conditions, characterization of the AD, kinetics

423

calculation, gas chromatograms, UV-vis absorbance spectra, atomic formal charges, quantum

424

chemical descriptors and schematic potential energy surfaces. This material is available free

425

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

426 427

AUTHOR INFORMATION

428

Corresponding Authors

429

*Phone/fax: +86-411-84706269; e-mail: [email protected].

430

** Phone/fax: +86-413-89165610; e-mail: [email protected].

431 432

Acknowledgements

433

We thank Prof. Donald G. Truhlar (University of Minnesota) for providing the POLYRATE

434

2010-A program and Prof. W. Peijnenburg (Leiden University) for valuable suggestions. This

435

study was supported by the National Natural Science Foundation of China (21607022,

436

21661142001), the Fundamental Research Funds for the Central Universities (2412016KJ031),

437

the Jilin Province Science and Technology Development Projects (20180520078JH) and the 22


Page 22 of 31

Page 23 of 31


438

Key

Laboratory

439

(KLIEEE-16-02).

of

Industrial

Ecology

and

Environmental

Engineering

(MOE)

440 441

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Aqueous OH Radical Reaction Rate Constants for Organophosphorus

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