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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
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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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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
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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
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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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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
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thus represent the
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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.
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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
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(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)
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-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
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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
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!C
(‰)
6 4
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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
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15 N
is
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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
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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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−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
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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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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
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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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material is available free of charge via the Internet at http://pubs.acs.org/.
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Environmental Science & Technology
ACS Paragon Plus Environment
