Transformation of an amine moiety of atenolol during water treatment

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Transformation of an amine moiety of atenolol during water treatment with chlorine/UV: Reaction kinetics, products, and mechanisms Ji Woon Ra, Hoonsik Yoom, Heejong Son, Tae-Mun Hwang, and Yunho Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b01412 • Publication Date (Web): 05 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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Environmental Science & Technology

1

Transformation of an amine moiety of atenolol during water

2

treatment with chlorine/UV: Reaction kinetics, products, and

3

mechanisms

4

Jiwoon Ra1, Hoonsik Yoom2, Heejong Son2, Tae-Mun Hwang3, Yunho Lee1*

5 6 7

1School

8

Technology (GIST), Gwangju, 61005, Republic of Korea

9

2Busan

10

3Water

11

Technology, 2311, Goyang, Gyeonggi 411-712, Republic of Korea

of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and

Water Quality Institute, Gimhaesi, Kyungnam 621-813, Republic of Korea Resources and Environmental Research Division, Korea Institute of Construction

12 13

*Corresponding author. Mailing address: School of Earth Sciences and Environmental Engineering,

14

Gwangju Institute of Science and Technology (GIST), Gwangju, 61005, Korea.

15

715-2468, fax: 82-62-715-2434, email: [email protected]

Phone: 82-62-

16 17

Word equivalent count: Text: 5538, Figures: 2 regular (600), Scheme: 2 large (1200), Table: 1

18

large (600), Sum: 7938

19 20 21 22 23 24 25 26

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Abstract

28

Transformation of atenolol (ATN), a micropollutant containing a secondary (2o) amine moiety,

29

can be significantly enhanced in water treatment with sequential and combined use of chlorine and

30

UV (chlorine/UV) through photolysis of the N-Cl bond. This study investigated the transformation

31

kinetics, products, and mechanisms of the amine moiety of ATN in chlorine/UV (254nm). The

32

fluence-based, photolysis rate constant for N-Cl ATN was 2.010-3 cm2/mJ. Transformation

33

products (TPs) with primary (1) amines were mainly produced, but TPs with 2 and 3 amines

34

were also formed, based on liquid chromatography (LC)/quadrupole-time-of-flight/mass

35

spectrometry and LC/UV analyses. The amine-containing TPs could be further transformed in

36

chlorine/UV (with residual chlorine in post UV) via formation and photolysis of new N-Cl bonds.

37

Photolysis of N-Cl 1 amine TPs produced ammonia as a major product. These data could be

38

explained by a reaction mechanism in which the N-Cl bond was cleaved by UV, forming aminyl

39

radicals that were transformed via 1,2-hydrogen shift, -scission, intramolecular addition, and 1,2-

40

alkyl shift. Among these, the 1,2-alkyl shift is newly discovered in this study. Despite enhanced

41

transformation, only partial mineralization of the ATN’s amine moiety was expected, even under

42

UV/chlorine advanced oxidation process conditions. Overall, the kinetic and mechanistic

43

information from this study can be useful for predicting the transformation of amine moieties by

44

chlorine/UV water treatment.

45 46 47 48 49 50 51


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Table of Contents (TOC)

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72

Introduction

73

Chlorine has been widely used as a disinfectant for drinking water and swimming pool water.1

74

Chlorination, however, has drawbacks, such as formation of toxic disinfection byproducts (DBPs)

75

and low inactivation efficacy against protozoan microorganisms.2,3 Ultraviolet (UV) light has been

76

increasingly used as a secondary disinfectant in drinking water, due to its high germicidal

77

effectiveness to chlorine-resistant microorganisms, and the general absence of toxic byproduct

78

formation.4,5 UV disinfection is typically followed by chlorination, as it is mandatory to maintain

79

a residual disinfectant in drinking water distribution systems in many countries.6 Application of

80

UV has also been increasing for swimming pools, as it can improve the water and air quality of

81

pools by removing volatile chlorinated byproducts.7,8

82

Chlorination followed by UV is relevant for drinking water treatment as chlorine (referring

83

here to free available chlorine, HOCl/OCl) is used for a pre-oxidation process before coagulation

84

and filtration processes.9,10 In this sequential use of chlorine and UV, chlorinated waters are

85

subjected to UV photolysis in the presence or absence of residual chlorine. Combined use of

86

chlorine and UV has been proposed as an advanced oxidation process (commonly denoted as

87

UV/chlorine AOP)11,12 and intensively tested in bench-scale studies13-22 and also occasionally in

88

pilot- and full-scale studies.23,24 The performance of UV/chlorine as an AOP is superior or

89

comparable to that of UV/H2O2 at acidic and neutral pH, but lower at basic pH, due to the pH-

90

dependent hydroxyl radical (OH) oxidation efficiency.13,25 Elucidating the major reactive species

91

responsible for organic contaminant degradation in UV/chlorine has been the subject of

92

investigations, since UV photolysis of chlorine produces a range of radical species, such as OH,

93

chlorine radicals (Cl), dichlorine radicals (Cl2), and oxychlorine radicals (ClO) as described in

94

reactions 15 (R1–R5).25 It has been found that OH was the main oxidant for recalcitrant

95

contaminants with electron-deficient substituents, while Cl, Cl2, and ClO (termed as reactive


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chlorine species, RCS) become important for the degradation of contaminants with electron-rich

97

moieties.15,18

98

HOCl (OCl) + hv  OH (O) + Cl

99

O

+ H+  OH

100

Cl

+ OH

101

Cl

+ Cl

102

OH

(1) (2)

ClOH-

OH

+ Cl

Cl2

(3) (4)

+ OCl  ClO + OH

(5)

103

Synthetic amine compounds are often present in water sources impaired by the discharge of

104

municipal or industrial wastewaters.26-29 This is related to the fact that primary (1), secondary (2),

105

tertiary (3), and quaternary (4) amine moieties are abundant in the structures of wastewater-

106

relevant contaminants, such as pharmaceuticals and personal care products.30,31 The presence of

107

amines in drinking water sources is a concern, as they are the precursors of toxic nitrogenous DBPs

108

(N-DBPs) such as halonitriles, halonitroalkanes, or nitrosamines.32 The chlorination chemistry of

109

amines shows that, apart from 4 amines, they all react rapidly with chlorine, to produce organic

110

chloramines with mono- (R2N-Cl) or di-chloro (RN-Cl2) bonds.33-35 Organic chloramines are

111

relatively stable, and can revert to the parent amines by chlorine transfer to reductants, such as

112

sulfite or thiosulfate.35-37 Decomposition of organic chloramines sometimes yields toxic products,

113

including halonitriles, halonitroalkanes, and aldehydes.38,39

114

Transformation of amine compounds can be significantly enhanced in sequential and

115

combined chlorine and UV treatment (denoted as chlorine/UV hereafter), compared to chlorine or

116

UV alone, due to their reactions with radicals (OH and RCS) and the UV photolysis of the organic

117

chloramines. UV photolysis of N-Cl bonds generates aminyl radicals (R2-N) and Cl.33,40 The latter

118

pathway is more specific to the transformation of an amine moiety, compared to the less selective

119

OH-induced

120

water treatment with chlorine/UV has been shown to be relevant for the enhanced elimination of

transformation of compounds. The role of organic chloramine photolysis during



121

microcystin-LR41, and for the formation of N-DBPs such as cyanogen chlorine42-44, chloropicrin45,

122

and nitrosamines.46 Nevertheless, organic chloramine photolysis has sometimes been neglected as

123

an additional transformation pathway of amine-containing contaminants.15,47,48 Furthermore,

124

information on the reaction pathways and mechanisms is currently too limited to be able to assess

125

the fate of amine moieties in the chlorine/UV process.

126

To fill this information gap, the transformation of a 2 amine moiety of atenolol (ATN) during

127

water treatment with sequential and combined use of chlorine and UV (254nm) was investigated

128

in this study. ATN is a -blocker pharmaceutical and wastewater-relevant contaminant.37

129

Systematic investigations were carried out on the following: (i) the kinetics and transformation

130

products (TPs) of UV photolysis of organic chloramines of ATN, plus several 1 and 2 amine

131

compounds, including the TPs from ATN or structural model compound of ATN, (ii) the

132

transformation pathways of a 2 amine moiety of ATN in chlorine/UV, and (iii) the mechanisms

133

of UV-induced decomposition of organic chloramines.

134 135

Materials and Methods

136

Standards and Reagents. All chemicals and solvents (the highest purity available) were used

137

as received from various commercial suppliers. Further details of chemical sources and stock

138

solutions have been provided in SI-Text-1.

139

Chlorination and UV experiments. ATN and other amine compounds (1100 M) were

140

prepared in phosphate- (210 mM, pH 6–8.5) or borate-buffered solutions (5 mM, pH 910), and

141

in a surface water matrix. These samples were treated with chlorine, UV, and chlorine/UV in

142

laboratory, bench-scale experiments. UV irradiation was carried out in a quasi-collimated beam

143

system, equipped with a low-pressure Hg lamp, emitting UV light at 254 nm with UV intensity of

144

35 mW/cm2. Details of the chlorine and UV experiments have been provided in SI-Text-2.

145

Analytical methods. A liquid chromatography (Infinity 1260, Agilent), coupled to a 6 ACS Paragon Plus Environment

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quadrupole time-of-flight mass spectrometer (LC/Q-TOF/MS) with an electrospray ionization (ESI)

147

source (6520, Agilent) was used for the identification of the TPs. A LC (Ultimate 3000, Dionex)

148

with a UV detector (LC/UV) was used to quantify ATN and its TPs. The LC/UV was also used to

149

quantify low molecular weight (LMW) carbonyl and amine products after pre-column

150

derivatizations. Chlorinated samples were analyzed directly within a few hours to minimize further

151

transformation, otherwise the samples were quenched by thiosulfate and stored at 4C before the

152

analyses. Further details of the analytical methods are provided in SI-Text-3.

153 154

Results and discussion

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Transformation of ATN in dark chlorination. N-Cl ATN was formed from the reaction of

156

ATN with chlorine. The identity of N-Cl ATN could be confirmed by its mass spectrum (Figure

157

S1). The reaction stoichiometry was 1:1 for the decrease of ATN and formation of N-Cl ATN per

158

chlorine consumption (Figure S2). Thus, R6 can describe the reaction of ATN with chlorine

159

forming N-Cl ATN.

160

ATN + HOCl  N-Cl ATN

(6)

161

The formation of N-Cl ATN was rapid and completed in less than one min. Using the typical

162

reactivity of HOCl toward deprotonated 2 amines (k = 107108 M-1s-1) and much less reactivity of

163

OCl and protonated amines,34,49 the apparent k for the reaction of chlorine with the 2 amine of

164

ATN (pKa = 9.6) was calculated to be 21032107 M-1s-1 in the pH range of 69. Thus, the N-Cl

165

formation during typical water chlorination conditions (e.g., a few mg/L of chlorine) could actually

166

have been completed within a few seconds.

167

N-Cl ATN was stable in water (Figure S3). The decay of N-Cl ATN (10 M) was less than 5%

168

in 4 hours in the pH range of 6–8. At pH 9, the decay of N-Cl ATN became faster, with a pseudo

169

first-order rate constant of 7.610-5 s-1. N-des-isopropyl ATN and acetone were the major products

170

from the N-Cl ATN decay, indicating hydrolysis of the N-Cl moiety. Similar hydrolysis of the N7 ACS Paragon Plus Environment


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halo moiety has been reported previously.33,37 N-Cl ATN was found to revert readily to ATN upon

172

reaction with thiosulfate (Figure S4), which was consistent with a previous report.37 Overall, N-Cl

173

ATN, a chlorinated product of ATN, was found to be persistent (t1/2 of >38 hrs at pH 6–8) in water

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chlorination.

175

Transformation kinetics of ATN in chlorine/UV. The transformation kinetics of ATN

176

during treatment at pH 8 with UV, chlorine, chlorine/UV (with and without tert-butanol),

177

respectively, are shown in Figure S5. Note that all chlorinated samples were quenched with

178

thiosulfate (1 mM). The transformation of ATN was negligible ( SW > PB-BuOH matrices. The difference in the ATN

202

transformation rate was most significant at pH 6; the kUV values were 8.210-3, 5.210-3, and

203

2.010-3 cm2/mJ for PB, SW, and PB-BuOH, respectively. With increasing pH from 6 to 9, the

204

differences in the ATN transformation rates decreased. At pH 9, the kUV values were 2.610-3,

205

2.510-3, and 2.010-3 cm2/mJ for PB, SW, and PB-BuOH, respectively. Notably, the

206

transformation rate of ATN in the PB-BuOH matrix was almost constant at kUV of 210-3 cm2/mJ,

207

regardless of the pH. This indicated that the transformation of ATN in the PB-BuOH matrix was

208

mainly driven by the photolysis of N-Cl ATN, with little contribution from OH (or RCS) reaction.

209

In the PB and SW matrices, the OH (or RCS) reaction also contributed to the transformation of

210

ATN, with the contribution increasing with decreasing pH.13,25 Thus, the photolysis of N-Cl ATN

211

can become the dominant transformation pathway for ATN at basic pH or in the presence of high

212

concentrations of OH scavengers. The quantum yield (, mol/einstein) for the photolytic

213

decomposition of N-Cl ATN was calculated to be 0.54 (see Table S1 for further details).

214

Transformation products of N-Cl ATN photolysis. N-Cl ATN was prepared at 20 M (or

215

80 M) by reacting 40 M of ATN with 20 M of chlorine (or 100 M of ATN with 80 M of

216

chlorine) at pH 7 for 10 min. The N-Cl ATN samples (containing 20 M of unreacted ATN) were

217

then treated with UV (0–1000 mJ/cm2), and analyzed using LC/Q-TOF/MS and LC/UV. Figure 1

218

shows selected chromatograms of (a) LC/MS, and (b) LC/UV of the N-Cl ATN (20 M) sample

219

treated with UV at 600 mJ/cm2. An overlay of all LC/UV chromatograms of before and after UV

220

treatment of N-Cl ATN is also shown in Figure S7. The elution of N-Cl ATN was at 6.0 min in 9 ACS Paragon Plus Environment


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LC/MS, and 9.5 min in LC/UV, and its peak gradually decreased with increasing UV fluence. With

222

the decreasing N-Cl ATN peak, six other peaks evolved at retention times (RTs) of 1.1, 1.5, 2.1,

223

2.3, 3.1, and 4.6 min in LC/MS (Figure 1a). These LC/MS peaks could be matched to those from

224

LC/UV at RTs of 3.0, 4.1, 5.8, 6.2, 7.5, and 8.1 min, respectively (Figure 1b), based on their elution

225

order in the same column used for the LC/MS and LC/UV analyses. These six peaks were named

226

as TP-225, TP-239, TP-265-I, TP-206, TP-194, and TP-265-II, based on their protonated molecular

227

masses (M+H+) (Table 1). The ATN peak changed little in the tested UV fluence range (Figure

228

S7).

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229

To identify the structure of the TPs, their MS/MS (MS2) spectra (Figures S8S14) and UV

230

absorption spectra (Figure S15) were investigated. The MS2 spectrum of ATN showed twelve

231

fragment ions, which could be explained by the fragmentation patterns proposed in Figure S8.

232

Among the ATN fragment ions, m/z 190.0859, 145.0646, and 133.0645 were formed from 2-(4-

233

(2-hydroxypropoxy)phenyl)acetamide moieties. In addition, m/z 116.1070 and 98.0964 were

234

formed from (2-hydroxy-3-isopropylamino)propoxy moieties. These characteristic ATN MS2

235

spectra were used to interpret the MS2 spectra of the TPs (Table S2). The UV absorption spectrum

236

of ATN showed peaks at 226 nm and 276 nm, which were assigned to phenylacetamide moieties.

237

Changes in these characteristic absorption peaks indicated structural modification of the

238

phenylacetamide moieties (Figure S15). Finally, a chlorine reactivity test was performed to check

239

the presence of chlorine-reactive amine moieties in TPs, in which the rapid disappearance of the

240

TP peak upon chlorine treatment was interpreted as the presence of the amine moiety. Details of

241

the structural identification of the TPs have been provided in SI-Text-4, and the identified structures

242

have been summarized in Table 1. TP-225 was confirmed to be N-des-isopropyl ATN (DIP-ATN),

243

using a commercially available chemical standard. TP-239 was formed by replacement of the ATN

244

isopropylamino by the methylamino moiety. TP-265-I was formed by a dehydrogenation in the

245

isopropyl moiety of ATN. TP-206 was a product from removal of the isopropylamine (deamination) 10 ACS Paragon Plus Environment

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and a water (dehydrogenation), and TP-194 from removal of isopropylamine (deamination) and a

247

carbon from ATN. Finally, TP-265-II was a 3 aromatic amine compound that could be formed via

248

an intramolecular cyclization mechanism (see Scheme 1 and the below section for transformation

249

mechanism).

250

Evolution of the TPs as a function of UV fluence was also investigated. Figure 2 shows the

251

results from UV photolysis of 80 M of N-Cl ATN. The six TPs could be quantified based on their

252

relative peak areas compared with those of ATN in the LC/UV analysis. This semi-quantification

253

method was deemed to be acceptable as ATN and its TPs (except TP-265-II) showed almost the

254

same UV absorption spectra, due to the presence of a common phenylacetamide moiety (Figure

255

S15). The suitability of this approach was also supported by almost the same peak area responses

256

from ATN and DIP-ATN under LC/UV analysis. Using the data in Figure 2a, [TPs] vs [N-Cl

257

ATN] were plotted, which showed good linear relationships for all TPs (Figure S16a). From the

258

slopes of these linear plots, the molar yields of the six TPs from the UV photolysis of N-Cl ATN

259

could be determined (Table 1). The sum of the molar yields of the six TPs was 0.85, indicating that

260

most of the major TPs from the UV photolysis of N-Cl ATN were identified.

261

Comparison of the structures of the six TPs with ATN indicated formation of LMW products

262

(i.e., C1 – C4) containing either an amine or carbonyl moiety from the cleavage of C-N or C-C ATN

263

bonds. Figure 2b shows the evolution of isopropylamine, acetone, formaldehyde, and acetaldehyde

264

during the UV photolysis of N-Cl ATN. The molar yields of these LMW products could be

265

determined from the slopes of the linear plots of [LMW product] vs [N-Cl ATN] (Figure S16b).

266

Notably, the molar yield of isopropylamine (0.45) was close to the sum of molar yields of TP-206

267

and TP-194 (0.44=0.20+0.24) in which the latter two compounds lost an isopropylamino moiety

268

from ATN. The molar yields of acetone (0.25), formaldehyde (0.20), and acetaldehyde (0.08) were

269

comparable to those of TP-225 (0.20), TP-194 (0.24), and TP-239 (0.10), respectively. These data

270

indicated an association of the formation of isopropylamine with both TP-206 and TP-194, 11 ACS Paragon Plus Environment


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formaldehyde with TP-194, and acetaldehyde with TP-239, respectively.

272

Photo-decomposition of N-Cl DIP-ATN. From the UV photolysis of N-Cl ATN, TPs

273

containing a 1 amine moiety were formed as the major products (Table 1). These amine-containing

274

TPs can be further transformed during UV/chlorine AOP treatment via formation of a 1 N-Cl bond

275

and its photolysis. To elucidate the full transformation pathways of the ATN amine moiety, DIP-

276

ATN was selected as a representative 1 amine TP, and the UV photolysis of N-Cl DIP-ATN was

277

investigated.

278

N-Cl DIP-ATN was prepared by reacting 20 M of DIP-ATN with 10 M of chlorine at pH

279

7. From this reaction, N-Cl DIP-ATN was formed as the major product, but di-chloro (N-Cl2) DIP-

280

ATN was also formed – at a relatively low yield (Figure S17). The photolysis of N-Cl2 DIP-ATN

281

will be discussed in the next section. With increasing UV fluence, the N-Cl DIP-ATN peak

282

decreased and three additional peaks evolved at RTs of 4.9, 6.2, and 7.5 min, respectively (Figure

283

S17). The kUV value of 2.010-3 cm2/mJ (=0.49, Table S1) was determined for the UV photolysis

284

of N-Cl DIP-ATN. The TP structures were identified by investigating their MS and MS2 spectra

285

(Table S3, Figures S18S20), and UV absorption spectra (Figure S15). The peaks at RTs of 6.2

286

and 7.5 min were identified as TP-206 (m/z 206.0813) and TP-194 (m/z 194.0817), respectively.

287

Note that these two are the identical TPs to those produced from the UV photolysis of N-Cl ATN

288

(Table 1). The peak at 4.9 min showed m/z of 223.1088 and was denoted as TP-223. This product

289

was derived through loss of H2 from DIP-ATN, and determined to be the product of an

290

intramolecular cyclization of DIP-ATN, which was comparable to the formation of TP-265-II from

291

ATN (Table 1).

292

In order to determine product yields, 80 M of N-Cl DIP-ATN (prepared by reacting 100 M

293

of DIP-ATN with 80 M of chlorine) was irradiated with UV, and the products were quantified by

294

LC/UV analysis. Note that the samples were analyzed after thiosulfate quenching, and thus N-Cl

295

DIP-ATN was determined in the form of DIP-ATN. Figure S21 shows the decrease of DIP-ATN 12 ACS Paragon Plus Environment

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and evolution of (a) TP-223, TP-206, and TP-194, and (b) LMW products, such as formaldehyde

297

and ammonia, with increasing UV fluence. The molar yields were 0.11, 0.25, and 0.37, for TP-223,

298

TP-206, and TP-194 (Figure S22a), and 0.61 and 0.27, for ammonia and formaldehyde (Figure

299

S22b), respectively. The sum of the molar yields of the three TPs was 0.73. The molar yield of

300

ammonia (0.61) was close to the sum of molar yields of TP-206 and TP-194 (0.62=0.25+0.37), in

301

which the latter two compounds lost an amino moiety from DIP-ATN. These data indicated an

302

association of the formation of ammonia with both TP-206 and TP-194 (Table 1).

303

Photo-decomposition of N-Cl2 DIP-ATN. N-Cl2 DIP-ATN was prepared by reacting 20 M

304

of DIP-ATN with 50 M of chlorine at pH 7, and then the mixture was treated with UV in the

305

presence of tert-butanol (10 mM). The latter was used to minimize degradation of N-Cl2 DIP-ATN

306

by OH (or RCS) and to allow its transformation exclusively by the direct photolysis. With

307

increasing UV fluence, the N-Cl2 DIP-ATN peak decreased, and two new peaks evolved at RTs of

308

6.2 and 7.5 min, respectively (Figure S23). The kUV value of 3.910-3 cm2/mJ (=0.73, Table S1)

309

was determined for the UV photolysis of N-Cl2 DIP-ATN, which was higher than kUV of N-Cl DIP-

310

ATN by a factor of 2. The two product peaks were identified as TP-206 (RT 6.2 min) and TP-194

311

(RT 7.5 min), which were the same as the products formed from N-Cl DIP-ATN. TP-223 was not

312

formed. Figure S24a shows the decrease of N-Cl2 DIP-ATN, and evolution of the two products

313

with increasing UV fluence. The molar yields were 0.51 for TP-194 and 0.10 for TP-206 (Figure

314

S24b).

315

Overall, the UV photolysis of N-Cl2 DIP-ATN produced products similar to N-Cl DIP-ATN,

316

but with different yields. Notably, the yield of TP-194 was higher, and the yield of TP-206 was

317

lower, for N-Cl2 DIP-ATN than for N-Cl DIP-ATN. Some level of N-Cl DIP-ATN was initially

318

present as a minor product, despite the fact that molar excess chlorine over DIP-ATN was used.

319

Interestingly, the peak area of N-Cl DIP-ATN changed little in the tested UV fluence range (0500

320

mJ/cm2) (Figure S24). This indicated that N-Cl DIP-ATN was produced from the UV photolysis 13 ACS Paragon Plus Environment


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of N-Cl2 DIP-ATN, which counterbalanced the decrease of N-Cl DIP-ATN by UV photolysis.

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Transformation pathways and mechanisms of N-Cl ATN photolysis. Scheme 1 shows the

323

transformation pathways and mechanisms of the photolysis of N-Cl ATN, forming six TPs

324

containing the phenylacetamide moiety and LMW products. As the first step, homolytic cleavage

325

of the N-Cl bond of ATN by UV generated an aminyl radical and Cl (R7a).51,52 Aminyl radicals

326

can be protonated, forming aminyl radical cations. The pKa of the conjugated acids of the dimethyl-

327

and diethyl-aminyl radicals have been reported to be 6.8 and 5.3, respectively.53 The aminyl radical

328

of ATN (and its conjugated acid) is transformed following the five reaction pathways (R8, R11,

329

R15, R18, and R20) that are based on four reaction mechanisms (1,2-hydrogen (H) shift, -scission,

330

intramolecular addition, and 1,2-alkyl (R) shift) described further in the following sub-sections.

331

The Cl (or OH from Cl) was expected to react with ATN and N-Cl ATN, or be scavenged by

332

tert-butanol, however, it was found that the rate and product formation pattern of N-Cl ATN

333

photolysis were similar, with or without tert-butanol (data not shown). This indicated that the Cl

334

(or OH) from the UV photolysis of N-Cl ATN had an insignificant effect on the transformation of

335

N-Cl ATN, in the experimental conditions applied.

336

1,2-H shift (R8 and R11). Aminyl radicals are known to undergo 1,2-hydrogen(H) shifts, in

337

which the corresponding C-centered radicals are formed.54 For the aminyl radical of ATN, the 3

338

and 2 C-centered radicals could be formed via 1,2-H shifts following R8 and R11, respectively. It

339

has been well established that C-centered radicals with a neighboring amino group are converted

340

into imines, via formation of peroxyl radicals and following liberation of a hydroperoxyl radical

341

(HO2).55 Hydrolysis of the imines generates the corresponding carbonyls and 1 amines.55

342

Following this mechanism, the 3 C-centered radical of ATN was transformed into acetone and

343

DIP-ATN following R9a and R10. The 2 C-centered radical of ATN was transformed following

344

R12 and R13 into isopropylamine and 2-(4-(2-hydroxy-3-oxopropoxy)phenyl)acetamide, from

345

which the latter was further transformed into TP-206, via dehydration of an -hydroxy carbonyl 14 ACS Paragon Plus Environment

Page 15 of 31


346

moiety (R14). Formation of TP-265-I could be explained by C-C double bond formation instead of

347

the imine from the peroxyl radical of 3 C-centered radical of ATN (R9b). Nevertheless, this was

348

a minor pathway (molar yield of 0.01).

349

-scission (R15). Cleavage of C-C bonds at the -position (-scission) has been observed for

350

aminyl radicals, especially for amino acids.56,57 For the aminyl radical of ATN, the scission was

351

expected to occur mainly at the -carbon with the hydroxyl (OH) group, considering the radical

352

stabilization effect by the OH. As a result of the -scission, N-isopropylmethanimine and the C-

353

centered radical of 2-(4-((2-hydroxyethoxy)phenyl)acetamide were formed (R15). Hydrolysis of

354

N-isopropylmethanimine generated isopropylamine and formaldehyde (R16), and the C-centered

355

radical of 2-(4-((2-hydroxyethoxy)phenyl)acetamide was converted into TP-194 via the

356

corresponding peroxyl radical chemistry (R17).

357

Intramolecular addition (R18). Aminyl radical cations are known to react readily with

358

aromatic or olefinic moiety, via an addition mechanism generating amine compounds.52 These

359

reactions of aminyl radical cations have been proposed as a novel synthetic route for various

360

structured amine compounds.40,58 For ATN, the aminyl radical cation was expected to undergo

361

intramolecular attack on the phenyl moiety, forming TP-265-II following R18 and R19. TP-265-II

362

was seen as a 3 aromatic amine compound, whose light absorption, mass fragmentation, and

363

chlorine reactivity were distinct in comparison with those of the other TPs containing 2 or 1

364

amines. Similar intramolecular cyclization has been observed for the aminyl radical cations with

365

neighboring olefinic or aromatic moiety.59

366

1,2-R shift (R20). The formation of TP-239 and acetaldehyde from ATN could not be explained

367

by the known reaction mechanisms of aminyl radicals as described above. To explain the

368

unexpected transformation of N-isopropyl of ATN to N-methyl of TP-239 and formation of

369

acetaldehyde, a mechanism based on a 1,2-alkyl (R) shift is proposed. As the result of a 1,2-methyl

370

shift from the isopropyl to aminyl, which is analogous to the 1,2-H shift, a 3 amine intermediate 15 ACS Paragon Plus Environment


Page 16 of 31

371

with a C-centered radical at the N-ethyl moiety was formed (R20). Subsequent transformation of

372

the C-centered radical via the peroxyl radical (R21) and imine hydrolysis (R22) generated TP-239

373

and acetaldehyde. As this type of rearrangement based on a ‘1,2-R shift’ is rare, additional

374

confirmation experiments were conducted, by treating N-isopropylmethylamine as a 2 amine

375

model compound with chlorine/UV. As the products of UV photolysis of N-Cl

376

isopropylmethylamine, dimethylamine and acetaldehyde were indeed formed, with molar yields of

377

0.17 and 0.11, respectively (Figure S25a). This demonstrated that the same type of carbon

378

rearrangement occurring in both N-Cl ATN and N-Cl isopropylmethylamine via the 1,2-methyl

379

shift (Scheme S1). It has also been found that UV photolysis of N-Cl isopropylamine, a 1 amine

380

model compound, generated methylamine and acetaldehyde (Figure S25b). This indicated that the

381

aminyl radicals derived from the 1 amine could also undergo transformations via the 1,2-R shift

382

mechanism (Scheme S2).

383

Effect of dissolved oxygen on the transformation pathway was tested by photolyzing N-Cl

384

ATN in N2(g)-, Air(g)-, and O2(g)-purged solutions. The product formation pattern was almost the

385

same for the Air- and O2-purged solutions while it was different for the N2-purged solution (Table

386

S4). The results support that the aminyl radicals of ATN do not directly react with O2 due to its

387

slow reaction,60,61 and are rapidly converted into the C-centered radicals that subsequently react

388

with O2 forming peroxyl radicals as summarized in Scheme 1.

389

Overall, the following order was determined for the relative importance of each transformation

390

pathway for the aminyl radical of ATN: 1,2-H shift (0.41) > -scission (0.24) > intramolecular

391

addition (0.1)  1,2-R shift (0.1), with the parentheses containing the molar TP yields generated by

392

each mechanism.

393

Transformation pathways and mechanisms of N-Cl DIP-ATN photolysis. Scheme 2 shows

394

the transformation pathways and mechanisms for the photolysis of N-Cl DIP-ATN. The aminyl

395

radical of DIP-ATN is transformed following three mechanisms, 1,2-H shift, -scission, and 16 ACS Paragon Plus Environment

Page 17 of 31


396

intramolecular addition. Overall, the transformation pathway of N-Cl DIP-ATN was similar to that

397

of N-Cl ATN, except that the 1,2-alkyl shift mechanism was not observed for N-Cl DIP-ATN. The

398

1,2-H shift of the aminyl radical of DIP-ATN generated the 2 C-centered radical (R23), which

399

was transformed into TP-206 and ammonia, via imine formation (R24), imine hydrolysis (R25),

400

and dehydration of an -hydroxy carbonyl moiety (R26). The -scission of the aminyl radical

401

generated N-methanimine and the C-centered radical of 2-(4-((2-hydroxyethoxy)phenyl)acetamide

402

(R27). Hydrolysis of N-methanimine generated ammonia and formaldehyde (R28) and the C-

403

centered radical was converted into TP-194 (R29), which was identical to the reaction described

404

previously for ATN (R17 in Scheme 1). The intramolecular addition of the aminyl radical cation

405

to the phenyl moiety of DIP-ATN produced TP-223 (R30 and R31), which was a 2 aromatic amine

406

compound. For N-Cl DIP-ATN, the relative importance of each transformation mechanism of the

407

aminyl radical was: -scission (0.37) > 1,2-H shift (0.25) > intramolecular addition (0.11), with the

408

parentheses containing the molar TP yields.

409

Transformation pathways and mechanisms of N-Cl2 DIP-ATN photolysis. The

410

transformation pathways of N-Cl2 DIP-ATN are shown in Scheme S3. The Cl-aminyl radical was

411

generated from the UV photolysis of N-Cl2 DIP-ATN in the first step. Due to the presence of an

412

electronegative Cl atom, the Cl-aminyl radical cation could be more acidic than the aminyl radical

413

cation. This explained the observation that TP-223 was not formed from N-Cl2 DIP-ATN, because

414

only the aminyl radical cation, not the neutral aminyl radical, could undergo an addition reaction

415

to the aromatic moiety (intramolecular addition in this case).52 The transformation pathways of the

416

Cl-aminyl radical included the 1,2-H shift and -scission mechanisms, which were comparable to

417

those of the aminyl radical, except that the -scission (0.51) became more dominant compared to

418

the 1,2-H shift (0.10). This indicated that the electronegative Cl atom could facilitate C-C cleavage

419

at the  position.



Page 18 of 31

420

Fate of the amine moiety of ATN during chlorine/UV. ATN (10 M) was treated with molar

421

excess chlorine (100 M) followed by UV photolysis, in order to investigate the full transformation

422

pathway of the amine moiety of ATN in chlorine/UV (with residual chlorine in post UV). The

423

experiment was conducted in the presence of tert-butanol (10 mM), to exclude N-Cl ATN

424

degradation by OH (or RCS). Figure S26 shows the evolution of ATN, its N-containing TPs,

425

ammonia, and nitrate, as a function of UV fluence (01960 mJ/cm2). ATN almost fully disappeared

426

at 1960 mJ/cm2. As the primary TPs, DIP-ATN, TP-239, and isopropylamine could be quantified.

427

The concentration of these primary TPs reached the maximum at 500 mJ/cm2, and then decreased

428

with increased UV fluence. The concentrations of TP-265-I and TP-265-II, as the other N-

429

containing primary TPs, were below the method quantification limit. Methylamine and ammonia

430

were formed as the secondary or (more than) tertiary degradation products. For instance, these

431

compounds could be formed from the UV photolysis of N-Cl isopropylamine as the primary

432

products (Figure S25b). The concentration of methylamine reached its maximum at 1500 mJ/cm2,

433

while the concentration of ammonia continued to increase up to 1960 mJ/cm2. It should be noted

434

that some ammonia could be produced as (more than) tertiary products from the UV photolysis of

435

1 N-Cl TPs, in addition to DIP-ATN and isopropylamine, which were not identified in this study.

436

Nitrate was formed as the final amine oxidation product, and its concentration continued to increase

437

up to 16% of the initial ATN concentration at 1960 mJ/cm2. Nitrate can be formed from the UV

438

photolysis of chloramines that are produced from the reaction of ammonia with chlorine.62,63

439

Transformation of the amine moiety of ATN could be kinetically modeled based on the results of

440

this study (Scheme S4) and information from the literature (e.g., UV photolysis of chloramines62,

441

63, breakpoint chlorination64) (see SI-Text-5 and Table S5 for further details). The developed kinetic

442

model was able to simulate the experimental results reasonably well (Figures S26S28).

443

Implications for the fate and control of organic amines in water treatment. A chlorine/UV

444

process could significantly enhance the transformation of 2 amine to 1 amine moieties via the 18 ACS Paragon Plus Environment

Page 19 of 31


445

UV photolysis of N-Cl bonds. The transformation of 1 amine moieties to ammonia and then to

446

nitrate is also accelerated. The UV photolysis rates of 2 N-Cl and 1 N-Cl & N-Cl2 varied within

447

a factor of 5, and the photolysis rates of N-Cl2 was usually larger than those of N-Cl (Table S1).

448

UV photolysis of 3 N-Cl is expected unimportant as 3 N-Cl are quickly transformed to 2 amines

449

before its photolysis.39 The kinetic and mechanistic information obtained from this study for the

450

UV photolysis of some selected 2 and 1 organic chloramines can be useful for a generalized

451

prediction of the transformation of amine moieties in chlorine/UV process, which is relevant for

452

advanced treatments of impaired source water by wastewater effluent or pool water. Only partial

453

mineralization of the amine moieties is expected, with formation of lower grade amines and

454

ammonia, under typical UV/chlorine AOP conditions (see Figure S27). Transformation of amine

455

moieties can be enhanced by OH or RCS in addition to the pathway via N-Cl photolysis, but the

456

radical pathway is usually less efficient due to its low selectivity. It remains unclear how such

457

partial mineralization of the amine moieties affects their N-DBP formation potential in post

458

chlorination, and this warrants further investigation.

459 460 461 462

Supporting Information Five texts, 5 tables, 28 figures, and 4 schemes for addressing materials, experimental procedures, additional data and discussions, and kinetic modeling.

463 464

Acknowledgements

465

This study was supported by the National Research Foundation of Korea funded by the

466

Ministry of Science, ICT and Future Planning (NRF-2017R1A2B2002593 and NRF-

467

2017M3A7B4042273).

468 469

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International Edition in English 1983, 22, (5), 337-350.

626

60.

627

superoxide radical anion by the OH radical-induced oxidation of trimethylamine in oxygenated

628

aqueous solution. The kinetics of the hydrolysis of (hydroxymethyl)dimethylamine. Chemische

629

Berichte 1987, 120, (3), 319-323.

630

61.

631

of •NH2 with O2 in Aqueous Solutions. The Journal of Physical Chemistry A 1998, 102, (44),

632

8498-8504.

633

62.

634

Science & Technology 2009, 43, (1), 60-65.

635

63.

636

photodecomposition of monochloramine and dichloramine in aqueous solution by UV irradiation

637

at 253.7 nm. Water Res 2010, 44, (10), 3261-9.

638

64.

639

Environmental Science & Technology 1992, 26, (3), 577-586.

Chow, Y. L.; Danen, W. C.; Nelsen, S. F.; Rosenblatt, D. H., Nonaromatic aminium Jonsson, M.; Wayner, D. D. M.; Lusztyk, J., Redox and Acidity Properties of Alkyl- and

Vrček, V.; Zipse, H., Rearrangemements in Piperidine-Derived Nitrogen-Centered

Wisniowski, P.; Carmichael, I.; Fessenden, R. W.; Hug, G. L., Evidence for β Scission in Svejstrup, T. D.; Ruffoni, A.; Juliá, F.; Aubert, V. M.; Leonori, D., Synthesis of

Stella, L., Homolytic Cyclizations of N-Chloroalkenylamines. Angewandte Chemie Das, S.; Schuchmann, M. N.; Schuchmann, H.-P.; Sonntag, C. V., The production of the

Laszlo, B.; Alfassi, Z. B.; Neta, P.; Huie, R. E., Kinetics and Mechanism of the Reaction

Li, J.; Blatchley Iii, E. R., UV Photodegradation of Inorganic Chloramines. Environmental De Laat, J.; Boudiaf, N.; Dossier-Berne, F., Effect of dissolved oxygen on the

Jafvert, C. T.; Valentine, R. L., Reaction scheme for the chlorination of ammoniacal water. 24 ACS Paragon Plus Environment

Page 25 of 31

641


Table 1. Atenolol (ATN) and its transformation products (TPs) from UV photolysis of N-Cl ATNa and N-Cl DIP-ATNb identified by LC/Q-TOF/MS. Compounds

Measured m/z

Theoretical m/z

Mass error (ppm)

Chemical formula

Molar yieldc

H2 N

ATN

267.1708

267.1703

1.8

C14H23N2O3

-

TP-225 (DIP-ATN) a

225.1240

225.1234

2.8

C11H17N2O3

0.20

TP-239 a

239.1394

239.1390

1.6

C12H19N2O3

0.10

H2 N

H2 N

TP-265-I

a

265.1555

265.1547

3.1

C14H21N2O3

0.01

TP-206 a

206.0819

206.0812

3.6

C11H12N1O3

0.20

TP-194 a

194.0816

194.0812

2.2

C10H12N1O3

0.24

Coupled LMW product d

Structure OH O

PhAc



OH

O

H 3N

O OH

O

O

PhAc

OH O

O

O

PhAc

O

O

PhAc H+

PhAc

 NH3

H+

NH3

Ac



PhAc

O

TP-265-II a

265.1551

265.1547

1.6

C14H21N2O3


0.10

HO

H N

+ O


Page 26 of 31

O

TP-223 b

223.1088

223.1077

4.9

C11H15N2O3

0.11

HO

TP-206 b

206.0813

206.0812

1.6

C11H12N1O3

0.25

TP-194 b

194.0817

194.0812

2.7

C10H12N1O3

0.37

O

O

O

O

Cl

N-Cl ATN

301.1322

301.1313

2.8

C14H22ClN2O3



NH

N-Cl DIP-ATN

259.0848

259.0844

1.6

C11H16ClN2O3



H2 N

C11H15Cl2N2O3



N-Cl2 DIP-ATN

293.0456

293.0454

0.6

Ac

N H2

Cl

Cl Cl

NH



H+ NH4

PhAc H+

PhAc

O NH4 +

OH O

PhAc

OH O

PhAc

OH O

PhAc



 

642

aTPs from N-Cl ATN, bTPs from N-Cl des-isopropyl ATN (DIP-ATN), c[TP]/[N-Cl ATN] (Figure S16) and [TP]/[N-Cl DIP-ATN] (Figure S22),

643

dLow

molecular weight (LMW) product(s) that is coupled with each TP containing the phenylacetamide moiety.


Page 27 of 31


OH

H N

O2 / - HO2

O

PhAc

(9b)

TP-265-I (0.01)

(8)

Cl

OH

N

O

PhAc (11)

1,2-H shift (21%)

PhAc

N

O

(9a)

PhAc

O

(20%)

PhAc

OH

O

H2O

O2 / - HO2

N

O

(12)

PhAc

O +

H

H2O (16)

PhAc (15) -scission

+

(24%)

OH O

OH

NH2

H

+

OH

NH2

H2O

O

+

(13)

O

- H2O

O

Scheme S2

PhAc

(14) O

PhAc

TP-206 (0.20)

H Isopropylamine Formaldehyde (0.20) (0.45) O2 / - HO2

PhAc

(17)

O O

PhAc

TP-194 (0.24)

PhAc O

Intramolecular addition HO (18) (10%)

PhAc

TP-225 (0.20)

O

+

O

O

(10)

Isopropylamine (0.45)

N

N

H2N

+

Acetone (0.25) OH

OH

H N

1,2-H shift

OH

H N

O

O2 / - HO2

(7a)

hv / -Cl

-H

OH

OH

H N

O O2 / - HO2 Ac

N

HO Ac

N

(19)

TP-265-II (0.10)

(20)

644

1,2-R shift (10%)

OH N

O

O2 / - HO2

PhAc

OH N

(21)

O

PhAc


H2O (22)

O

OH +

Acetaldehyde (0.08)

HN

O

PhAc

TP-239 (0.10)

Scheme 2 Scheme S3


Page 28 of 31

645

Scheme 1. UV-induced transformation pathways and mechanisms for N-Cl ATN (compound marked with rectangle). Five pathways are proposed for

646

the transformation of the ATN aminyl radical, based on four different mechanisms: 1,2-H-shift (R8 and R11), -scission (R15), intramolecular addition

647

(R18), and 1,2-R-shift (R20). The parentheses indicate the molar yields of TPs and LMW products.

648 649 650 651 652


Page 29 of 31


Cl

(23)

OH

HN

O

1,2-H-shift (25%)

OH H2N

O

O

(24)

OH

(27)

PhAc

-scission (37%)

(28)

+ OH O

-H+ H+

H

H2O

HN

O

PhAc

OH HN

PhAc

H2O (25)

PhAc

hv / -Cl

HN

O2 / - HO2

+

NH3

H Ammonia (0.61) Formaldehyde (0.27) O2 / - HO2

PhAc

O

(29)

OH NH3

+

O

Ammonia (0.61)

O

- H2O O

PhAc

(26) O

PhAc

TP-206 (0.25)

O O

PhAc

TP-194 (0.37)

OH H2N

O

PhAc (30)

Intramolecular addition (11%)

O

O2 / - HO2

HO N H

Ac

(31)

653

O HO Ac N H TP-223 (0.11)

654

Scheme 2. UV-induced transformation pathways and mechanisms for N-Cl DIP-ATN (compound marked with rectangle). Three pathways are

655

proposed for the transformation of the DIP-ATN aminyl radical, based on three different mechanisms: 1,2-H-shift (R23), -scission (R27), and

656

intramolecular addition (R30). The parentheses indicate the molar yields of TPs and LMW products.



657

658 659

Figure 1. (a) LC/MS, and (b) LC/UV (225nm) chromatograms, for N-Cl ATN treated by UV

660

irradiation at 600 mJ/cm2. The initial concentration of N-Cl ATN was 20 M for LC/MS ([ATN]0 =

661

40 M and [Chlorine]0 = 20 M), and 80 M for LC/UV ([ATN]0 = 100 M and [Chlorine]0 = 80 M),

662

which were prepared by chlorinating ATN at pH 7 for 10 min.

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Page 30 of 31

Page 31 of 31


80

35

(a)

40

20

Concentration, M

Concentration, M

N-Cl ATN TP-225 TP-239 TP-265-I TP-206 TP-194 TP-265-II Total

60

(b)

Isopropylamine TP-206 + TP-194 Acetone TP-225 Formaldehyde TP-194 Acetaldehyde TP-239

30 25 20

1,2-H-shift  -scission O NH2 + TP-194 O PhAc O Isopropylamine O PhAc TP-206

1,2-H-shift OH O O + H2N Acetone

H 10

H

Formaldehyde

0

0

100

200

300

400

500

600

+

O

TP-194

1,2-R-shift OH O + HN O

5

0

TP-225

-scission O O

15

0

100

200

300

400

500

600

Acetaldehyde

TP-239

PhAc

PhAc

PhAc

UV fluence, mJ/cm2

UV fluence, mJ/cm2

663 664

Figure 2. (a) Decrease of N-Cl ATN and evolution of transformation products (TPs) containing a

665

phenylacetamide moiety, and (b) evolution of low molecular weight (LMW) products and their

666

associated TPs as a function of UV fluence during UV photolysis of N-Cl ATN. For (a), the ‘total’

667

indicates the summed molar concentrations of N-Cl ATN and its six TPs. For (b), the associated

668

formation of LMW products and TPs via different reaction mechanisms is depicted on the right. The

669

experimental conditions were as for Figure 1b.

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Transformation of an amine moiety of atenolol during water treatment

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