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Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution Marco Ratti, Silvio Canonica, Kristopher McNeill, Jakov Bolotin, and Thomas B. Hofstetter Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03119 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on September 30, 2015

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Environmental Science & Technology

Isotope Fractionation Associated with the Indirect Photolysis of Substituted Anilines in Aqueous Solution Marco Ratti,†,‡ Silvio Canonica,† Kristopher McNeill,‡ Jakov Bolotin,† and Thomas B. Hofstetter∗,†,‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, CH-8600 Dübendorf, Switzerland, and Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zürich, CH-8092 Zürich, Switzerland E-mail: [email protected]

Fax: +41 58 765 50 28, Phone: +41 58 765 50 76

∗ To

whom correspondence should be addressed

† Eawag ‡ ETH

Zürich

1 ACS Paragon Plus Environment


1

Abstract

2

Organic micropollutants containing aniline substructures are susceptible to different light-

3

induced transformation processes in aquatic environments and water treatment operations.

4

Here, we investigated the magnitude and variability of C and N isotope fractionation during

5

the indirect phototransformation of four para-substituted anilines in aerated aqueous solutions.

6

The model photosensitizers, namely 9,10-anthraquinone-1,5-disulfonate and methylene blue

7

were used as surrogates for dissolved organic matter chromophores generating excited triplet

8

states in sunlit surface waters. Transformation of aniline, 4-CH3 -, 4-OCH3 - and 4-Cl-aniline by

9

excited triplet states of the photosensitizers was associated with inverse and normal N isotope

10

fractionation whereas C isotope fractionation was negligible. The apparent 15 N-kinetic isotope

11

effects (AKIE) were almost identical for both photosensitizers, increased from 0.9958 ± 0.0013

12

for 4-OCH3 -aniline to 1.0035 ± 0.0006 for 4-Cl-aniline, and correlated well with the electron

13

donating properties of the substituent. N isotope fractionation is pH-dependent in that H+

14

exchange reactions dominate below and N atom oxidation processes above the pK a -value of the

15

substituted aniline’s conjugate acid. Correlations of C and N isotope fractionation for indirect

16

phototransformation were different from those determined previously for direct photolysis of

17

chloroanilines and offer new opportunities to distinguish between abiotic degradation pathways.


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Page 3 of 27


18

Introduction

19

Numerous organic micropollutants contain aniline substructures, which are often responsible for

20

the initial steps of degradation in aquatic environments as well as in water treatment processes. 1

21

Such reactions include oxidations by dissolved or particle-bound oxidants and enzymes, additions to

22

electrophilic sites of organic matter, as well as direct photolysis. 2–15 In addition to adverse effects

23

caused by the parent compound, an identification of the predominant transformation pathways

24

is essential for the assessment of risks for human health and the environment because these

25

processes can give rise to products of equal or even greater (eco)toxicity. 16 However, products of

26

such reactions (e.g., high molecular weight radical coupling products) are numerous and usually

27

difficult to analyse quantitatively, for example, when bound to organic matter. We have therefore

28

proposed to use compound-specific isotope analysis (CSIA 17–20 ) to help quantify the share of

29

different degradation reactions from the change in stable C, H, and N isotope ratios in the residual

30

contaminants containing aromatic amine moieties. 21–25

31

Stable isotope-based approaches for organic micropollutants offer interesting possibilities to

32

assess the fate of N-containing contaminants, even if competing reaction pathways occur and

33

reaction products are partially unknown. 22,26–35 Depending on the chemical bond(s) broken or

34

formed, stable isotope compositions measured in the remaining fraction of a contaminant molecule

35

change over time and distance from the pollution source due to kinetic and equilibrium isotope

36

effects. 36 Examples of redox reactions of triazine and phenylurea herbicides as well as nitroaromatic

37

explosives show that especially the combinations of N isotope fractionation with those for C and

38

H are indicative for the ongoing transformation processes and, in many instances, also enable the

39

quantification of the extent of degradation. 29,34,35,37,38 An application of CSIA-based procedures

40

for micropollutants with aromatic amine functional groups is currently hampered by the fact that

41

the isotope effects associated with the important degradation routes are not fully known.

42

In aerobic surface waters, enzymatic, mineral-catalyzed and photochemical oxidations are

43

considered major elimination routes for organic compounds. Some of these processes have been

44

shown to cause substantial C and N isotope fractionation, which originates from isotope effects of 3 ACS Paragon Plus Environment


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45

different reaction mechanisms. For example, the direct photolysis of chlorinated anilines is subject

46

to C and N isotope fractionation from photophysical processes during dechlorination of excited state

47

species. 22,39 Enzymatic oxidations in metabolic reactions, on the other hand, involve dioxygenation

48

of the aromatic ring 21 while cometabolic oxidations have been proposed to be initiated by single

49

electron transfer at the N atom. 25,40 The latter is also the predominant pathway of MnO2 -catalyzed

50

oxidations of aromatic amines. 24,25 The apparent kinetic isotope effects (AKIEs) of oxidative

51

degradation routes originate predominantly from the change of bonding to the N atom of the amine

52

group. We have proposed that an increase in C–N bond strength in the resulting imine bonds was

53

responsible for the often observed inverse N isotope fractionation (i.e.,

54

Even though the understanding of KIEs requires further investigation, current evidence suggest that

55

thermal reactions lead to only minor C isotope fractionation as opposed to direct photolysis, where

56

both significant C and N isotope fractionation occur. However, it is currently unclear whether

57

oxidations caused by dissolved oxidants, for example those generated in the presence of light (e.g.,

58

excited triplet states of dissolved organic matter) also follow the trends found for mineral-catalyzed

59

oxidations.

15 N-AKIE

< 1). 21,24,25

60

The goal of the present work was therefore to investigate the C and N isotope effects associated

61

with the sensitised photolysis of aromatic amines and to evaluate whether the resulting C and N

62

isotope fractionation trends enable the elucidation of aromatic amine transformation pathways in

63

sunlit surface waters. To this end, we studied

64

substituted anilines by triplet states of transient photooxidants generated through light absorption of

65

dissolved organic matter (3 DOM∗ ). Laboratory experiments were carried out with an anthraquinone

66

disulfonate and methylene blue as model compounds for the chromophores of the DOM generating

67

3 DOM∗ .

68

photolysis of substituted anilines and its pH dependence with 4-methylaniline as an example. For the

69

discussion of isotope effects from different photooxidants as well as those from mineral-catalyzed

70

oxidations, we compare data from experiments with aniline and three para-substituted anilines,

71

namely, 4-methyl-, 4-methoxy-, and 4-chloroaniline with regard to their substituent effects on the

13 C-

and

15 N-AKIEs

pertinent to the oxidation of

We illustrate the typical C and N isotope fractionation trends associated with the indirect


Page 5 of 27


72

observable C and N isotope fractionation.

73

Experimental Section

74

A list of chemicals including their purities and suppliers can be found in the Supporting Information

75

(SI).

76

Chemical Analysis

77

We measured the concentrations of aniline, 4-methylaniline (4-CH3 -aniline), 4-methoxyaniline (4-

78

OCH3 -aniline) and 4-chloroaniline (4-Cl-aniline) with reversed-phase HPLC and UV-vis detection

79

(Dionex UltiMate 3000). Two different eluent compositions (40/60 vol % and 30/70 % vol) of

80

methanol and 1 mM KH2 PO4 buffered at pH 7.0 were used with a flow rate of 1 mL min−1 and

81

a sample injection volume of 20 µL on a Supelcosil LC-18 (25 cm ×4.6 mm, 5µm). 22,24 Trans-

82

formation products were analysed by LC-MS/MS in positive ion mode using an LTQ (Linear Trap

83

Quadrupole) Orbitrap mass spectrometer (Thermo) with electrospray ionization as documented

84

previously. 24,25 Gradient elutions were run from 90/10 to 5/95 vol % ratios of H2 O/MeOH (con-

85

taining 0.1 vol % formic acid) on Atlantis C-18 (15 cm × 3.0 mm, 3 µm) and XBridge C-18 (5

86

cm × 2.1 mm, 3.5 µm) columns from Waters. Identification of the products was based on exact

87

mass and fragmentation patterns due to the lack of standard reference material. The postulated

88

molecular structures are shown in the SI.

89

Stable Isotope Analysis

90

C and N isotope ratios of substituted anilines were measured by gas chromatography coupled

91

to isotope ratio mass spectrometry (GC/IRMS) after solid-phase microextraction (SPME) with

92

polydimethylsiloxane/divinylbenzene (PDMS/DVB, Supelco) fibers as described previously. 22,23

93

The combustion interface of the GC/IRMS was equipped with a Ni/Pt oxidation reactor. 41 Due to



94

pK a -values of the substituted aniline’s conjugate acid between 4.0 and 5.3, 1 we adjusted pH-values

95

of aqueous samples to pH 7.0 and 7.5 to avoid isotope fractionation from H+ -exchange reactions.

96

C and N isotope signatures, δ13 C and δ15 N, are given as arithmetic mean of triplicate mea-

97

surements (±σ) in per mil (h). Isotopic standard materials purchased from Indiana University

98

were used to report δ13 C- and δ15 N-values relative to Vienna PeeDee Belemnite and air, respec-

99

tively. C and N isotope analysis was performed within the method quantification limits (MQLs)

100

of the SPME-GC/IRMS procedure. 42 MQLs for δ13 C and δ15 N amounted to 1.0 and 2.5 µM for

101

4-Cl-aniline, 2.0 µM and 3.5 µM for aniline and 4-CH3 -aniline, as well as 4.0 µM to 25.0 µM for

102

4-OCH3 -aniline. Samples were diluted to concentrations yielding peak amplitudes between 0.5

103

and 3 V. A standard bracketing procedure, referenced to a calibrated in-house standards of aniline,

104

4-Cl-aniline, 4-CH3 -aniline and 4-OCH3 -aniline of known δ13 C and δ15 N-values, was used to

105

ensure accuracy of the isotope ratio measurements.

106

Photochemical experiments

107

Batch experiments were carried out with aniline, 4-CH3 -aniline, 4-OCH3 -aniline, or 4-Cl-aniline

108

and one of the two photosensitizers, that is 9,10-anthraquinone-1,5-disulfonate (AQDS) or methy-

109

lene blue (MB). For experiments with AQDS, 20 mL reaction solutions typically contained 1 mM

110

of the photosensitizer, 10 mM phosphate buffer (pH 7.0), and 100 µM of initial concentration of

111

the substituted aniline. Due to the low extraction efficiency of 4-OCH3 -aniline with solid-phase

112

microextraction (see below), experiments with this substrate were carried out with initial concen-

113

trations of 800 µM at pH 7.5. Experiments with MB were carried out identically except for the

114

lower photosensitizer concentrations (100 µM with 4-OCH3 -aniline, of 30 µM for experiments

115

with all other substituted anilines). The pH dependence of the sensitized photolysis of substituted

116

anilines was studied in reactors containing 1 mM of AQDS and 100 µM of 4-CH3 -aniline in the pH

117

range between 2.0 and 7.0. Control experiments were carried out identically except for the addition

118

of the photo-sensitizers.

119

Sample solutions (20 mL) were irradiated in glass-stoppered quartz tubes (OD 18 mm, ID 15 6 ACS Paragon Plus Environment

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120

mm) equipped with magnetic stirring bars using a DEMA 125 merry-go-round photoreactor (Hans

121

Mangels GmbH, Bornheim-Roidorf, Germany). The photoreactor was equipped with a Heraeus

122

Noblelight model TQ 718 medium-pressure mercury (MP Hg) lamp operating at 500 W. 43 A

123

borosilicate glass cooling jacket and a filter solution containing 0.25 M sodium nitrate and 0.05 M

124

sodium nitrite restricted transmission of wavelengths to ≥ 400 nm. The reaction temperature was

125

kept at 25.0 ± 0.2 ℃ by recirculating the filter solution through a cooling thermostat. Production

126

of excited triplet states of the photosensitizer was initiated by irradiation of polychromatic light

127

at wavelengths above 400 nm, where the investigated substituted anilines did not absorb light.

128

Reactions were stopped by removing the quartz tubes from the photoreactor. Experiments with

129

AQDS as photosensitizer required between 2.5 and 20 hours whereas those with MB could be

130

carried out within 6 to 60 minutes.

131

At predefined time points, which we evaluated in preliminary experiments, reactions were

132

stopped and the entire volume of the quartz tube was used for concentration and isotope ratio

133

analysis. Reactors from experiments with AQDS were also subject to product analysis by high-

134

resolution mass spectrometry as described earlier. 22,24 Typically, 9 to 15 samples were generated

135

with fractional substrate conversion of up to 95% out of which at least 8 reactors were processed

136

further. In experiments with 4-OCH3 -aniline as substrate and MB as photosensitizer, we only

137

achieved fractional conversions of 50%. The pH-values of aqueous samples were adjusted to 7.0

138

and 7.5 to avoid N isotope fractionation by H+ -exchange reactions. 23 UV-vis absorption spectra

139

were acquired on a Cary 100 spectrophotometer, to assess photosensitizer decay. While MB

140

concentration dropped during the experiments, that of AQDS remained constant.

141

Data evaluation

142

A linear regression analysis of δ13 C- and δ15 N- values, vs. fractional amount of remaining reactant

143

ratio (C/C0 ) (eq. 1) was used to derive bulk compound C and N isotope enrichment factors (ǫ C , ǫ N ),

144

respectively. Based on the assumption that oxidation occurs exclusively at the N atom, we derived

145

apparent 13 C- and 15 N-kinetic isotope effects (AKIEs) with eq. 2. 7 ACS Paragon Plus Environment

13 C-AKIEs

thus represent the


146

Page 8 of 27

weighted average of secondary isotope effects of all C atoms.

! ! δh E + 1 C ln h = ǫ E · ln C0 δ E0 + 1 1 AKIEE = 1 + ǫE

(1) (2)

147

where δh E0 and δh E stand for the measured isotope signatures of an element E at time zero and at

148

different amounts of fractional conversion, respectively.

149

We accounted for contributions of primary 15 N and secondary 13 C equilibrium isotope effects

150

from H+ exchange on the observable N and C isotope fractionation assuming a sequential reaction

151

as in eqs. 3 and 4.

k3 k1 BG B + H+ GGGGGGA P BH+ FGGGGGG GGGGG k2 k1 × k3 k obs = k 2 [H+ ] + k 3

(3) (4)

152

where k1 and k2 are reaction rate constants of H+ exchange at the aromatic amino group, k3 stands

153

for the oxidation of the neutral species to radical products (P), and k obs is the overall rate constant

154

of product formation. As shown by Skarpeli-Liati et al. 24 , isotope fractionation on k obs reflects the

155

isotope fractionation of the protonated and neutral species as in eq. 5.

ǫE = 156

157

1 αBH+ ·

+ EIEBH E

·

AKIEBE

−1 + 1 − αBH+ · AKIEBE

(5)

where E stands for either N or C isotopes and αBH+ is the fraction of protonated substituted aniline −1 + corresponding to 1 + 10pH−pKBH+ . EIEBH are the 15 N and 13 C equilibrium isotope effect for the E

158

deprotonation of BH+ , AKIEBE are the apparent 15 N and 13 C kinetic isotope effects of the oxidation

159

reaction of B.


Page 9 of 27


160

Results and Discussion

161

Oxidation of substituted anilines by excited triplet states of 9,10-anthraquinone-

162

1,5-disulfonate (3 AQDS∗ )

163

Isotope fractionation of 4-CH3 -aniline

164

The transformation of 4-methylaniline (4-CH3 -aniline) in irradiated solutions containing 1 mM of

165

AQDS at pH 7.0 is shown in Figure 1a. The disappearance of 4-CH3 -aniline was accompanied

166

by the formation of products of higher molecular weight, typically dimers and trimers of partially

167

reacted substrate (Figure S3). The products are indicative of coupling reactions of radical inter-

168

mediates formed through N atom oxidation. 24,25 Transformation of substituted anilines by such

169

oxidative processes have been observed not only after irradiation of aqueous solutions containing

170

photosensitizers but also in mineral- and enzyme-catalyzed reactions. 4,44,45

171

Oxidation of 4-CH3 -aniline by excited triplet states of AQDS (3 AQDS∗ ) resulted in inverse N

172

isotope fractionation corresponding to a N isotope enrichment factor, ǫ N , of 3.6 ± 0.9h (entry

173

3a in Table 1). C isotope fractionation associated with 4-CH3 -aniline oxidation was also inverse

174

(ǫ C = 0.9 ± 0.4h) but almost negligible, that is δ13 C only decreased by 2h after more than 95% of

175

substrate conversion (Figure 1b). We did not observe changes in C and N isotope ratios of 4-CH3 -

176

aniline in irradiated solutions that did not contain AQDS. The observed combination of inverse

177

N and negligible C isotope fractionation for the oxidation of 4-CH3 -aniline by 3 AQDS∗ matches

178

previous observations where N atom oxidation lead to the formation of radical intermediates. 21,24,25

179

The aromatic amino group in those intermediates was suggested to exhibit partial imine character in

180

the transition state and thus stronger C–N bonds. C isotope fractionation partly reflects this partial

181

imine bonding to a minor extent because none of the C atom is directly involved in the reaction.

182

The apparent primary 15 N- and secondary 13 C-kinetic isotope effects calculated with eq. 2 amount

183

to 0.9960 ± 0.0009 and 0.9991 ± 0.0004, respectively (Table 1).

184

Note that the disappearance kinetics of 4-CH3 -aniline were faster initially than at latter stages of



(a)

100

4-CH3-aniline AQDS 1 mM pH = 7.0

80 60

40

40

20

20 0.0

1.0

2.0 t (h)

4-Cl-aniline MB 30 uM pH = 7.0

0

3.0 (d)

-5 -10

0.0

2.0 4.0 t (min)

6.0

4 0

15

15

C (!M)

60

0

! N (‰)

120 100

! N (‰)

C (!M)

80

(b)

(c)

Page 10 of 27

-15

-4

! C (‰)

-28 -30 -32

13

13

! C (‰)

-20

1.0

0.8 0.6 0.4 0.2 C/C0

0.0

-26 -28 1.0

0.8 0.6 0.4 0.2 C/C0

0.0

Figure 1 Transformation of 4-CH3 -aniline (panels a, b) by 3 AQDS∗ and of 4-Cl-aniline (panels c, d) by 3 MB∗ at pH 7.0. Panels (a) and (c): substituted aniline concentrations. Solid lines show pseudofirst-order decay kinetics. Panels (b) and (d): C and N isotope signatures (δ 13 C, circles, δ 15 N, squares) vs. fraction of remaining substrate (C/C0 ). Uncertainties represent standard deviations of triplicate measurements. Solid lines were calculated with eq. 1; gray lines are 95% confidence interval. Control experiments were treated identically but reactors did not contain photosensitizers (empty symbols, dotted lines).

185

the reaction. The latter could have been due to quenching of 3 AQDS∗ , light screening by reaction

186

products, and a reduction of the reaction intermediates to the parent compound. 10,11,43,46,47 Because

187

the C and N isotope fractionation trends remained constant throughout the reaction, none of the

188

processes seemed to impact the isotope fractionation behaviour from the oxidation reaction.

189


ACS Paragon Plus Environment

11

Aniline 4-Cl-aniline 4-CH3 -aniline 4-OCH3 -aniline

Aniline 4-Cl-aniline 4-CH3 -aniline 4-CH3 -aniline 4-CH3 -aniline 4-CH3 -aniline 4-CH3 -aniline 4-OCH3 -aniline

Compound ǫN (h)

−1.4 ± 0.5 1.3 ± 0.2 −1.0 ± 0.1 −3.5 ± 0.6 0.9 ± 0.4 3.6 ± 0.9 −0.1 ± 0.6 4.0 ± 0.9 1.4 ± 0.8 1.3 ± 0.4 0.4 ± 1.3 −3.4 ± 0.6 −0.2 ± 0.2 −5.2 ± 0.6 −0.2 ± 0.4 4.2 ± 1.4

ǫC (h)

∆δ15 N/∆δ13 C (-)

b all

uncertainties correspond to 95%

2.0 ± 1.2 4.2 ± 3.5 1.7 ± 0.7 -2.4 ± 1.6

0.9987 ± 0.0002 −0.8 ± 0.3 1.0035 ± 0.0006 3.3 ± 0.5 0.9965 ± 0.0009 3.8 ± 0.4 0.9960 ± 0.0009 −12.3 ± 3.5 0.9988 ± 0.0004 1.0 ± 0.4 1.0034 ± 0.0006 −4.0 ± 1.6 1.0052 ± 0.0006 16.7 ± 1.9 0.9958 ± 0.0013 -11.0 ± 8.4

(-)

(-)

1.0014 ± 0.0005 1.0010 ± 0.0001 0.9991 ± 0.0004 1.0001 ± 0.0006 0.9986 ± 0.0008 0.9996 ± 0.0013 1.0002 ± 0.0002 1.0002 ± 0.0004

15 N-AKIE c

13 C-AKIE c

7.0 0.7 ± 1.1 2.1 ± 0.7 0.9993 ± 0.0011 0.9979 ± 0.0007 7.0 −0.2 ± 0.4 −1.7 ± 0.3 1.0002 ± 0.0004 1.0017 ± 0.0003 7.0 0.5 ± 1.0 2.3 ± 0.6 0.9995 ± 0.0010 0.9977 ± 0.0006 7.5 −1.4 ± 2.3 4.2 ± 1.3 1.0014 ± 0.0023 0.9958 ± 0.0012

7.0 7.0 7.0 6.0 5.3 4.0 2.0 7.5

pH

experiments were carried out in 10 mM phosphate buffer adjusted to the given pH c calculated with eq. 2; confidence intervals;

a All

5 6 7 8

3 MB+∗

1 2 3a 3b 3c 3d 3e 4

3 AQDS∗

Entry

Table 1 C and N isotope enrichment factors (ǫ C , ǫ N ), apparent 13 C-, 15 N-kinetic isotope effects (13 C and 15 N-AKIE), as well as isotope fractionation slopes ∆δ15 N/∆δ 13 C of experiments with irradiated solution containing 9,10-anthraquinone-1,5-disulfonate (AQDS) and methylene blue (MB), respectively. a,b

Page 11 of 27 Environmental Science & Technology


!C

(‰)

6 4

Page 12 of 27

4-CH3-aniline AQDS 1mM

2 0

!N

-2 -4 -6 2

3

4

5

6

7

pH Figure 2 N anc C isotope enrichment factors, ǫ N and ǫ C , for 4-CH3 -aniline oxidation by 3 AQDS∗ in the pH-range 2.0 to 7.0. The solid line corresponds to ǫ -values calculated from eq. 5 with p KBH+ = + 5.17 1 and the following isotope effects: 15 N-EIEBH = 1.0095 ± 0.0008, 15 N-AKIEB = 0.9950 ± + 0.0005, 13 C-EIEBH = 1.0009 ± 0.0004, 13 C-AKIEB = 0.9992 ± 0.0003.

190

pH dependence

191

4-CH3 -aniline was also used as a model compound to probe the pH dependence of C and N

192

isotope fractionation during the oxidation of by 3 AQDS∗ . ǫ N -values derived in the pH range 2.0

193

to 7.0 (Table 1, entries 3a-e) were fit well with eq. 5 as shown in Figure 2 assuming that only the

194

neutral 4-CH3 -species reacts with 3 AQDS∗ and that deprotonation precedes this oxidation step. The

195

resulting

196

cation amounted to 1.0095 ± 0.0008, which is slightly smaller than previous

197

from the pH-dependent oxidation of 4-CH3 -aniline by MnO2 (1.015 ± 0.004, 24 ). Regardless of

198

this differences, the

199

found preferentially in the protonated organic N species. 23,33,48 The calculated 15 N-AKIE for the

200

oxidation of the neutral species of 0.9950 ± 0.0005 matches the values from experiments performed

201

at solution pH 6.0 and 7.0, where this species predominates. The agreement with the 15 N-AKIE for

202

the oxidation of 4-CH3 -aniline by MnO2 of 0.9941±0.0003 suggests that the N isotope fractionation

203

may not be modulated by the type of oxidant (see further discussion below). We also derived the

15 N-equilibrium

isotope effect,

15 N-EIE

15 N-EIE,

for the deprotonation of the 4-CH3 -anilinium 15 N-EIEs

derived

larger than unity adequately describe the phenomenon that


15 N

is

Page 13 of 27


204

corresponding data for C isotope fractionation associated with deprotonation and substituted aniline

205

oxidation, which had so far not been determined. The

206

AKIEB of 0.9992 ± 0.0003 confirm the small secondary isotope effects on C in agreement with

207

earlier findings. 23,24

208

Substituent effects

209

Following the procedures discussed above for 4-CH3 -aniline, we investigated the transformation of

210

aniline, 4-OCH3 -, and 4-Cl-aniline by 3 AQDS∗ . Note that experiments with 4-OCH3 -aniline were

211

carried out at pH 7.5 and at higher initial concentrations (a) to avoid contributions of H+ -exchange

212

reactions on the measured isotope fractionation and (b) because of a smaller extraction efficiency

213

by SPME. Pseudo-first-order substrate disappearance kinetics, C and N isotope fractionation, as

214

well as evidence for formation of radical coupling products are shown in the SI (Figures S1-S5)

215

and in Table 1. N isotope fractionation was larger than that for C and we observed both normal

216

and inverse fractionation for the two elements. ǫ N - and the corresponding

217

the four substituted anilines correlated with the electronic properties of the aromatic substituents

218

as quantified in σ −p -substituent constants 49 (Figure 3). The most electron-donating substituent

219

(4-OCH3 ) led to the most inverse N isotope fractionation whereas that of 4-Cl-aniline was normal.

220

This type of correlation was observed previously 24 for the oxidation of substituted anilines

221

in MnO2 -containing suspensions and the data are also shown in Figure 3. The similarity of the

222

correlations implies the same N-atom oxidation mechanism regardless of the oxidant. However,

223

we hypothesized earlier that N-atom oxidations are associated with the formation of partial iminie

224

bonds after the first electron transfer and that N isotope fractionation was always inverse due

225

to stronger C–N bonds. 24,25 This assumption regarding the origin of N isotope fractionation no

226

longer holds. The

227

(1.0035 ± 0.0006, Table 1, entry 2) but we have no evidence that 4-Cl-aniline was transformed by

228

an alternative mechanism.

15 N-AKIEs

13 C-EIEBH+

of 1.0009 ± 0.0004 and

13 C-

15 N-AKIE-values

of

measured for the oxidation of 4-Cl-aniline by 3 AQDS∗ is normal



3

MB*

MnO2

4-Cl

AQDS*

4-H

3

0.0

0.2

1.005

0.995

4-CH3

0.990

4-OCH3

15

N-AKIE (-)

1.000

-0.2

!p– Figure 3 Correlation of 15 N-AKIE-values for the oxidation of aniline (4-H), 4-CH3 -, 4-OCH3 -, and 4-Cl-aniline by excited triplet states of two photosensitziers (3 AQDS∗ , 3 MB∗ ) and suspensions of 49 MnO2 24 with σ − Note that the 15 N-AKIEs for oxidation of 4-OCH3 -aniline p substituent constants. by 3 AQDS∗ and 3 MB∗ are identical.

229

Oxidation of substituted anilines by excited triplet states of methylene blue

230

(3 MB∗ )

231

The transformation of four substituted anilines from irradiated solutions containing methylene blue

232

(3 MB∗ ) at pH 7.0 is shown for the example of 4-Cl-aniline in Figure 1c/d and the data for the

233

other compounds can be found in the SI (section S4.3). Note that the kinetics of 4-CH3 - and

234

4-OCH3 -aniline oxidation by 3 MB∗ was biphasic in contrast to its reaction with aniline and 4-Cl-

235

aniline as well as oxidation of the four substituted anilines by 3 AQDS∗ . Biphasic kinetics (and

236

incomplete oxidation) can be attributed to the fast oxidation of 4-CH3 - and 4-OCH3 -aniline by

237

3 MB∗

238

AQDS, the total concentration of MB was > 30 times smaller. The oxidation of substituted anilines

239

by 3 MB∗ was accompanied with measurable N isotope fractionation with ǫ N -values ranging from

and quenching of 3 MB∗ by radical coupling products. Compared to reaction solutions with


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240

−1.7 ± 0.3h to +4.2 ± 1.3h (Table 1, entries 5-8). δ13 C-values changed only to a minor extent and

241

none of the ǫ C -values is significantly different from zero. The 15 N-AKIEs for oxidation by 3 MB∗

242

are largely identical to those found with 3 AQDS∗ as oxidant except for 4-Cl-aniline, which differ by

243

0.0015 AKIE units. The oxidation of substituted anilines by 3 MB∗ also shows the same substituent

244

effects on 15 N-AKIE-values (Figure 3) suggesting that the mechanisms of initial electron transfer

245

from the substrates were identical for both triplet photosensitizers.

246

The factors controlling the reactivity by 3 MB∗ were investigated recently in detail by Erickson

247

et al. 15 , who reported bimolecular rate constants for quenching of 3 MB∗ by substituted anilines,

248

k q , as proxy for their oxidation rate constants. The k q -values of substituted anilines increase

249

with the electron donating properties of the substituent of the aromatic amines. Bimolecular

250

rate constants for quenching of 3 MB∗ exceeding 4.0 · 109 M−1 s−1 indicated that oxidation of

251

4-CH3 - and 4-OCH3 -aniline is diffusion controlled whereas aniline and 4-Cl-aniline reacted one

252

order of magnitude slower. However, we do not have any evidence that diffusion control would

253

limit the extent of isotope fractionation.

254

reacting substituted anilines. This finding implies that the electron transfer reaction alone was not

255

responsible for N isotope fractionation but that the latter is also determined by the bonding changes

256

associated with the formation of the radical intermediates. 24,25 This interpretation is consistent

257

with the results from experiments with 3 AQDS∗ . The bimolecular rate constants for oxidation of

258

substituted anilines by 3 AQDS∗ are not known, but the reduction potential of 3 AQDS∗ exceeds that

259

of 3 MB∗ (EH0 (3 AQDS2−∗ /AQDS3− ) of 1.86 V 50 vs. EH0 (3 MB+∗ /MB• ) of 1.43 V 15 ). Because of

260

the much larger driving force of electron transfers from substituted anilines to 3 AQDS∗ , we assume

261

that the latter happens at diffusion controlled rates for all studied substituted anilines. Nevertheless,

262

we observe N isotope fractionation and the

263

follow those where 3 MB∗ was the oxidant.

15 N-AKIEs

were even more inverse for the two fast

′

′

15 N-AKIE

values for oxidation by 3 AQDS∗ closely

264

Note that the very similar results for the two photo-oxidants also provide indirect evidence

265

that singlet oxygen, 1O2 , did not contribute to the transformation and thus isotope fractionation

266

of substituted anilines in our experiments. 1O2 can, in principle, be formed in aqueous solution



267

268

from reactions of dissolved O2 with excited triplet states sensitisers. Current data suggest that 1

O2 could be formed from 3 MB∗ due to its longer triplet lifetime 15,51,52 whereas production of is unlikely based on data for anthraquinone-2-sulfonate. 53 Assuming that 1O2 reacted by a

269

3 AQDS∗

270

different mechanisms with substituted anilines than the two photosensitizers, we would expect that a

271

reaction of substituted anilines with 1O2 leads to a distinctly different C and N isotope fractionation

272

in experiments with MB. As shown in Figure 3 and Table 1, however, we cannot observe such

273

trends.

274

Origin of N isotope fractionation

275

In Figure 3, we compiled the currently available 15 N-AKIEs for the transformation of substituted

276

anilines through N atom oxidation by excited triplet states of two photosensitizers and MnO2 miner-

277

als. While N atoms oxidation exhibits rather small 15 N-AKIEs compared to aniline dioxygenation 21

278

or N atom reduction, 34,54,55 we observe consistent substituent effects. Changes of the electron donor

279

(or acceptor) properties of aromatic substituent lead to a very similar change in 15 N-AKIE-values

280

regardless of the oxidant. Despite this correlation, however, 15 N-AKIE values for indirect photolysis

281

are offset relative to those for MnO2 suspensions by approximately 0.003 to 0.005 AKIE units. We

282

cannot explain this offset with different rate-limiting steps of photochemical vs. mineral-catalyzed

283

reactions, which, in the present case, would lead to a partial masking of isotope effects in reactions

284

with 3 AQDS∗ and 3 MB∗ . This scenario would require that 15 N-AKIEs for 4-Cl-aniline approach

285

unity. Instead, 15 N-AKIEs switch from inverse to normal, implying that one can no longer invoke

286

a tightening of N bonding as exclusive origin of N isotope fractionation. One could speculate that

287

N isotope fractionation of the one-electron oxidation by transient photooxidants were normal, for

288

example due to increasing driving force 56 and would thus contribute to more positive 15 N-AKIEs.

289

Theoretical studies on the oxidation of substituted anilines by different oxidants in part support the

290

hypothesis but 15 N-AKIEs calculated so far only approach 1.001. 24 Normal 15 N-AKIEs were found

291

for the oxidation of 4-Cl-N-methyl- and 4-Cl-N,N-dimethylanilines by horseradish peroxidase in

292

presence of H2 O2 . 25 Substituted N-alkyl-anilines, however, are subject to oxidative N-dealkylation 16 ACS Paragon Plus Environment

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Page 17 of 27


3

10

AQDS*

3

MB* DP

6

15

!" N (‰)

8

4

2

4-Cl-aniline pH = 7.0

0 0

2

4

13

!" C (‰) Figure 4 Correlation of C and N isotope fractionation (∆δ 13 C, ∆δ 15 N) of 4-Cl-aniline for direct photolysis (DP) and indirect photolysis experiments, by using AQDS and MB as photosensitizers, at pH = 7.0. Initial 4-Cl-aniline isotope composition: δ 13 C = −27.1h, δ 15 N = −3.1h, dashed grey lines are 95% confidence intervals of the linear regression.

293

and the data of Skarpeli-Liati et al. 25 suggest that this reaction pathways was responsible for 15 N-

294

AKIEs larger than unity rather than N atom oxidation. Even though the results presented in this

295

study enable one to delineate typical

296

rationalize the origin of N isotope fractionation for all compounds.

15 N-AKIEs

for oxidation of substituted anilines, we cannot



297

Environmental Significance

298

With the data presented in this and earlier work 22,39 it is, for the first time, possible to assess the

299

diagnostic power of CSIA for photochemical transformation of aromatic amines in contaminated

300

surface waters. In Figure 4, we have compiled the C and N isotope signature trends of 4-Cl-aniline

301

associated with its direct photolysis at 254 nm as well as the oxidation by the two photosensitzers

302

discussed above at pH 7.0. Slopes of the correlation of ∆δ15 N vs. ∆δ13 C are equivalent to the ratio

303

ǫ N /ǫ C . The ratios reflect the mechanisms of 4-Cl-aniline transformation regardless of whether

304

isotope fractionation is masked or not. 17 ∆δ15 N/∆δ13 C-values for direct and indirect photolysis are

305

0.9 ± 0.3 22 and 3.3 ± 0.5 (Table 1, entry 2), respectively, and thus sufficiently different to distinguish

306

the two degradation pathways if they occurred simultaneously. Notice that ∆δ15 N vs. ∆δ13 C data

307

for 3 MB∗ follows the same trend as for 3 AQDS∗ but the linear correlation is statistically uncertain

308

(see entry 6 in Table 1) here due to the small number of measurements. The combined C and N

309

isotope fractionation analysis reveals isotopic fingerprints with which transformation of 4-Cl-aniline

310

through N atom oxidation or photolytic dechlorination can be tracked. In contrast to this kind of data

311

treatment for non-photochemical degradation processes of other organic contaminants, 28,30–34,57–63

312

however, the correlations of δ15 N vs. δ13 C of photolytic processes are quite variable due to the spin

313

sensitivity of kinetic isotope effects for changes in solution pH and ionic composition and require

314

a thorough evaluation of reaction conditions.

315

Acknowledgement

316

This work was supported by the EU Initial Training Network CSI:Environment (Grant agreement

317

no. 264’329).

318

Supporting Information Available

319

Concentration dynamics as well as C and N isotope fractionation of aniline, 4-Cl-aniline, 4-OCH3 -

320

aniline during transformation by 3 MB∗ and 3 AQDS∗ , respectively. Data on reaction products. This


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321

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

322

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