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Article
A New Reaction Pathway for Bromite to Bromate in the Ozonation of Bromide Alexandra Fischbacher, Katja Löppenberg, Clemens von Sonntag, and Torsten C Schmidt Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02634 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015
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Environmental Science & Technology
A New Reaction Pathway for Bromite to Bromate in the Ozonation of Bromide
3 4
Alexandra Fischbacher,a Katja Löppenberg,a Clemens von Sonntag†,a,b and Torsten C.
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Schmidta,c*
6
(a) Instrumental Analytical Chemistry, University of Duisburg-Essen, Universitätsstraße 5,
7
45141 Essen, Germany
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(b) Max-Planck-Institut für Bioanorganische Chemie, Stiftstr. 34-36, 45413 Mülheim an der
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Ruhr, Germany
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(c) Centre for Water and Environmental Research (ZWU), University of Duisburg-Essen,
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Universitätsstraße 2, 45141 Essen, Germany
12
*
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E-Mail: torsten.schmidt@uni-due.de
Corresponding author: Torsten C. Schmidt Phone: +49 201-183-6772
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Fax: +49 201-183-6773 †
Clemens von Sonntag passed away April 5, 2013
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Graphical abstract
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Abstract
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Ozone is often used in drinking water treatment. This may cause problems if the water to be
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treated contains bromide, as its reaction with ozone leads to bromate formation, which is
24
considered to be carcinogenic. Bromate formation is a multistep process, resulting from the
25
reaction of ozone with bromite. Although, this process seemed to be established, it has been
26
shown that ozone reacts with bromite not by the previously assumed mechanism via O-transfer
27
but via electron transfer. Besides bromate, the electron transfer reaction also yields O3• ͞ , the
28
precursor of OH radicals. The experiments were set-up in a way, that OH radicals are not
29
produced from ozone self-decomposition, but solely by the electron transfer reaction. This study
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shows that hydroxyl radicals are indeed generated using tBuOH as OH radical scavenger and
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measuring its product formaldehyde. HOBr and bromate yields were measured in systems with
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and without tBuOH. As OH radicals contribute to bromate formation, higher bromate and HOBr
33
yields were observed in absence of tBuOH than in its presence, where all OH radicals are
34
scavenged. Based on the results presented here a pathway from bromide to bromate revised in
35
the last step was suggested.
36
Introduction
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Ozone is often used for disinfection and oxidation purposes and has a wide application in
38
drinking water treatment. However, its use in water treatment may cause problems when the
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water to be treated contain bromide at concentrations higher than 50 µg/L 1, as the reaction of
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ozone with bromide leads to bromate formation. Bromide concentrations in natural waters
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usually range between 4 and 1000 µg/L (in some cases, e.g., oceans even more)2 and thus the
42
critical value for bromide of 50 µg/L is environmentally relevant. Therefore, the bromide
43
concentration must be controlled before applying an ozone-based treatment step. Bromate is
44
considered a potential carcinogen, with a threshold limit value in drinking water of 10 µg/L in
45
America and Europe3, 4. Therefore, it is of great interest to elucidate the mechanism of bromate
46
formation in drinking water treatment. The low drinking water standard may not be justified
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anymore, as preliminary experiments conducted in real and synthetic gastric juices predict a
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short lifetime for bromate in human stomach.5-7 It was shown that at H+, Cl
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concentrations typically found in gastric juices, 99 % of bromate is reduced within its retention
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time of 20 - 30 minutes in the stomach. However, as long as the low drinking water standard is
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and H2S
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still present, bromate is one of the most relevant by-products formed in ozone-based processes.
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Furthermore, once formed, it is neither biodegradable nor removable by conventional treatment.
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Bromate formation is a multi-step process as has been so far described in reactions (1) – (4) 8.
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Br ͞ + O3 → BrO ͞ + O2
k = 160 M-1 s-1
(1)
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BrO ͞ + O3 → BrO2 ͞ + O2
k = 100 M-1 s-1
(2)
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BrO ͞ + O3 → Br ͞ + 2 O2
k = 330 M-1 s-1
(3)
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BrO2 ͞ + O3 → BrO3 ͞ + O2
k > 1 × 105 M-1 s-1
(4)
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This mechanistic concept has been studied in detail and seems to be well established.
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However, it has been suggested that bromite reacts with ozone by electron transfer (reaction 5) 9
60
and not by O-transfer (reaction 4), as previously assumed. The authors investigated the reaction
61
of bromite with ozone and postulated that electron transfer provides a lower kinetic barrier than
62
breakage of the O–O bond, and, therefore, the reaction (4) is unlikely to proceed despite
63
comparable reaction rate constants. The authors provided experimental evidence for the electron
64
transfer reaction by measuring the intermediate BrO2• (reaction 5). However, a further
65
experimental or theoretical corroboration is missing.
66 67 68 69 70
BrO2 ͞ + O3 → BrO2• + O3• ͞
k = 8.9 × 104 M-1 s-1
(5)
The electron transfer (5) yields BrO2• and O3• ͞ , the precursor of •OH (overall reaction 6; for details see10). O3• ͞ + H2O → •OH + O2 + OH ͞
(6)
BrO2• disproportionates into bromite and bromate (reaction 7) 11, 12.
71
2 BrO2• ⇌ Br2O4
k = 1.4 × 109 M-1 s-1
(7a)
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Br2O4 + OH ͞ → BrO2 ͞ + BrO3 ͞ + H+
k = 7 × 108 M-1 s-1
(7b)
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In the ozone-based reaction pathway from bromide to bromate, •OH can also act as an
74
oxidation agent and contribute to bromate formation.13 In a cascade of reactions (8 – 12),
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bromide reacts with •OH yielding relevant intermediates (e.g. HOBr) that can further react with
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ozone or •OH giving bromate (reactions 13 – 16 and 7). 3 ACS Paragon Plus Environment
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Br ͞ + •OH ⇌ BrOH• ͞
k = 1.06 × 1010 M-1 s-1
(8) 14
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BrOH• ͞ + H+ ⇌ Br• + H2O
k = 4.4 × 1010 M-1 s-1
(9) 14
79
Br• + Br ͞ → Br2• ͞
k ≃ 1010 M-1 s-1
(10) 14
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Br2• ͞ + Br2• ͞ → Br ͞ + Br3 ͞
k = 1.8 × 109 M-1 s-1
(11) 15
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Br3 ͞ + H2O → HOBr + 2 Br ͞ + H +
82
HOBr + •OH → BrO• + H2O
k = 2 × 109 M-1 s-1
(13) 16
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OBr ͞ + •OH → BrO• + OH ͞
k = 4.5 × 109 M-1 s-1
(14) 11
84
2 BrO• + H2O → OBr ͞ + BrO2 ͞ + 2 H+
k = 4.9 ± 1.0 × 109 M-1 s-1 (15) 11
85
BrO2 ͞ + •OH → BrO2• + OH ͞
k = 1.8 × 109 M-1 s-1
(12)
(16) 11
86
The impact of •OH on the bromate formation is negligible if H2O2 is present.17 As H2O2 is
87
always in large excess over •OH, the formed HOBr is not oxidized further but reduced to
88
bromide (reaction 17) 18, outcompeting the further oxidation. HOBr + HO2 ͞ → Br ͞ + H2O + O2
89
k = 7.6 × 108 M-1 s-1
(17)
90
As •OH contribute to bromate formation, an indication for •OH formed in the system could be
91
the comparison of the bromate formation with and without a suitable •OH scavenger. If •OH are
92
present, the bromate yield should be higher than in systems without •OH. The same would apply
93
to HOBr formation. Due to the fast reaction of bromide with •OH, higher HOBr concentrations
94
should be found in the absence of an •OH scavenger. A prerequisite for formation of •OH is the
95
intermediate ozonide radical anion (O3• ͞ ). If the experimental conditions are chosen in a way
96
that no ozone self-decomposition occurs, O3• ͞ is only generated in pure water if reaction (5)
97
occurs.
98
Ozone self-decomposition in water is initiated by the reaction of ozone with hydroxide ions
99
(reaction 18). Therefore, it is strongly pH dependent. At pH 6.5, the second-order rate constant
100
for ozone decomposition in water is 2.42 M-1 s-1, while the second order rate constant for the
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reaction of bromide with ozone is 160 M-1 s-1. When ozone decomposes, •OH are generated
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(reactions 18 – 21). The ozone concentration was chosen not to be higher than the bromide
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concentration, so no ozone is present in excess. The fraction of ozone self-decomposition was
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calculated for the worst-case scenario in which the ozone concentration was equal to the
105
bromide concentration and was only around 1.5 %. If reactions of ozone with other bromine
106
species are considered (alongside bromide), the fraction would be even smaller. Therefore, self-
107
decomposition of ozone can be excluded as an •OH source in the present study.19
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O3 + OH ͞ → HO2 ͞ + O2
(18)
109
O3 + HO2 ͞ → O2•͞ + •OH + O2
(19)
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O3 + O2•͞ → O3•͞ + •O2
(20)
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O3• ͞ + H2O → •OH + O2 + OH ͞
(21)
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Even though the electron transfer reaction was suggested about a decade ago based on UV-Vis
113
spectroscopic observation of BrO2•, in the context of water treatment the previously established
114
concept of the O-transfer reaction in the last step of the pathway is still prevalent20, 21. It is the
115
intention of the present paper to determine the •OH yield and based on these data to provide
116
further evidence of electron transfer occurring in the oxidation of bromite on route to bromate
117
formation. This seems essential to establish a correct mechanistic concept for bromate formation
118
and has consequently already been picked up in the recent textbook by von Sonntag and von
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Gunten 22, that, however, does not provide any experimental details.
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Material and Methods
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Ozone stock solutions (0.9 mM) were prepared by generating ozone with an oxygen-fed
122
ozonator (BMT 802X, BMT Messtechnik, Berlin) and passing it through ice-cooled (4 °C) ultra
123
pure water (Purelab Ultra, Elga LabWater, Celle). The ozone concentration of these stock
124
solutions was determined spectrophotometrically using ε(260 nm) = 3300 M-1 cm-1.23
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•
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Hydroxyl radicals (•OH) are highly reactive and therefore short-lived, so they cannot be
127
quantified by direct measurement. As direct measurement of •OH is difficult an appropriate
128
indirect method must be selected. The indirect quantification is performed by scavenging •OH
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by a suitable probe compound and determinatingthe formed products. Such a probe compound
130
must react rapidly with •OH and barely with other substances present in the sample solution.
OH quantification
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Furthermore, the yield of the products from this reaction must be known, so that the •OH yield
132
can be derived from these results. In the present system, tBuOH was selected as a scavenger, as
133
it complies with the requirements. It reacts readily with •OH (k = 6 × 108 M-1 s-1)
134
with ozone (k ~ 3 × 10-3 M-1 s-1) 25. In all experiments, the concentrations of tBuOH (180 mM)
135
and bromide (0.6 mM) were kept constant, while ozone concentration was varied (0.15 - 0.6
136
mM). The tBuOH concentration was chosen to be 200 times higher than that of bromide for
137
assuring that 99 % of the formed •OH are scavenged by tBuOH and do not react with any
138
intermediates present in the solution (the calculation of the •OH fraction reacting with tBuOH is
139
shown in SI). As nearly all •OH react with tBuOH their quantification is possible. One of the
140
main products of the reaction of •OH with tBuOH is formaldehyde, which can be determined
141
after derivatization as its 2,4-dinitrophenylhydrazone by HPLC-UV at λ = 350 nm
142
derivatization, 0.1 mL perchloric acid (1 M in acetonitrile) and 0.2 mL 2,4-DNPH solution (6
143
mM in acetonitrile) were added to 1.7 mL of sample. After standing for 45 minutes in the dark,
144
the solutions were measured by HPLC-UV (Agilent 1100, Agilent Technologies, Santa Clara,
145
CA; column: NUCLEODUR 5µm C18 ec 250 x 3 mm (Macherey-Nagel), eluent: gradient
146
acetonitrile/water pH 3 within 20 minutes from 45 to 80 % acetonitrile at 0.5 mL/min). The
147
retention times for 2,4-DNPH and derivatized formaldehyde were 5.5 and 9.3 minutes,
148
respectively. The quantification was performed using formaldehyde standards derivatized in the
149
same way as the samples. To control whether DNPH was contaminated with formaldehyde,
150
blank samples were measured containing perchloric acid, DNPH solution and deionized water.
151
The contamination of the DNPH solution was negligible. As the samples were always in the
152
same concentration range as the standards, the impact of an incomplete derivatization can also
153
be neglected.
24
but barely
26
. For the
154
As an alternative probe compound methanol was tested, as it has been applied as •OH
155
scavenger in ozone based systems27. It reacts fast with •OH (k = 9.7 × 108 M-1 s-1) 24 and barely
156
with ozone (k ~ 2 × 10-2 M-1 s-1) 25. To obtain formaldehyde as the main product, HPO42 ͞ and/or
157
OH
158
experiments were conducted in 2 mM H2PO4 ͞ /HPO42
159
concentration of methanol (470 mM) and bromide (0.6 mM) were kept constant while ozone
160
was varied between 0.15 and 0.6 mM. The fraction of •OH reacting with methanol is calculated
161
to be 99 % in the given system (the calculation of the •OH fraction reacting with methanol is
162
shown in SI). Formaldehyde, the product of this reaction, was determined as described above.
͞
are required as the reaction is base-catalyzed (reactions 28 – 31). ͞
28
Therefore, all
buffered water (pH 7 and 8). The
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Quantification of bromate and HOBr
164
Bromate was determined by ion chromatography (883 Basic IC Plus, Metrohm, Filderstadt)
165
using a Metrosep A Supp 4 – 250/4 column and an eluent consisting of 0.17 mM NaHCO3 and
166
0.18 mM Na2CO3 at 1 ml/min. Retention times for bromate and bromide were 12 and 19.3 min,
167
respectively.
168
HOBr stock solution was prepared from a 1 mM pH 4 buffered ozone solution (100 mM
169
phosphate buffer) by addition of 0.8 mM potassium bromide solution29. This solution was stored
170
for 24 h in the dark and residual ozone was removed by purging the solution with nitrogen gas
171
for 15 minutes. The first prepared HOBr solution was standardized iodometrically and by direct
172
photometric determination of hypobromite at 329 nm (ε = 332 L mol-1 cm-1) after adjusting the
173
pH to 11 by sodium hydroxide.30 Both methods gave consistent results and further
174
standardization of HOBr solutions was only done photometrically before every use.
175
A photometric determination of HOBr formed in the samples was not possible due to other
176
possible constituents of the samples, which might absorb light in the same range. Therefore,
177
HOBr formed in the samples was determined by measuring its derivatization products with
178
phenol, which yields 2- and 4-bromophenol.31, 32 The pH for derivatization was varied from 2 to
179
5.
180
Calibration for 2- and 4-bromophenol was prepared from purchased ≥98 % pure 2- and 4-
181
bromophenol, respectively. In order to investigate the reaction of HOBr with phenol and
182
determine the bromophenol yield per HOBr, standards with defined HOBr concentrations were
183
prepared from the produced HOBr stock solution. These solutions were buffered at pH 4 (100
184
mM phosphate buffer). 15 µL phenol (0.5 M) was added to 1 mL of the buffered standards.
185
After 2 h (after the reaction was completed) the solutions were measured by HPLC-UV.
186
2- and 4-bromophenol were determined using HPLC-UV at a wavelength of 271 nm and a
187
flow rate of 0.4 mL min-1 (column: NUCLEODUR 5µm C18 ec 250 x 3 mm (Macherey-Nagel)
188
eluent: gradient acetonitrile/water within 18 minutes from 30 to 70 % acetonitrile). The retention
189
times for phenol, 2- and 4-bromophenol were 9.2, 14.1 and 15.2 minutes, respectively.
190
Results and Discussion
191
•
OH Quantification by tBuOH
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The reaction of tBuOH with •OH initiates a radical chain reaction 33. In the presence of O2, a
193
cascade of reactions leads to stable products. One of these products that can be easily quantified
194
is formaldehyde.
195
to the generation of O3 from O2), the reactions yielding formaldehyde dominate here.
Since in ozonation processes O2 is always present in excess over O3 (due
50
100 80
[CH2O] / µM
40
60 30 40 20
10 100
[•OH] / µM
33, 34
20
200
300
400
500
600
0 700
[O3] / µM
196 197 198
Figure 1 Formation of formaldehyde (⊠, ■ and □ correspond to 3 independent experiments
199
and cannot be shown as mean values as the ozone concentrations differ slightly) in the
200
reaction of •OH and tBuOH (left y-axis). •OH are generated in the reaction of ozone
201
with bromite. The shaded area represents the possible range of •OH yield (right y-
202
axis). The upper limit corresponds to the highest possible yield, if •OH yield is 3.3
203
times the formaldehyde yield. The lower limit corresponds to the lowest possible •OH
204
yield, if •OH yield is twice of the formaldehyde yield. Circles and the linear trend line
205
correspond to the possible calculated •OH yields using a correction factor of
206
2.29±0.06 (details see text).
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The results obtained from the tBuOH/•OH system show that •OH were formed (Figure 1) as
208
indicated by reaction (6). As the formation of bromate from bromide occurs not via one step
209
(reactions 1 – 5), the yields are not linear. The formaldehyde yield always depends on the
210
system generating •OH, as superoxide (O2•͞ ) is formed in the sequence of reactions of tBuOH
211
with •OH.34 It reacts fast with ozone giving rise to additional •OH (reaction 22) that contribute
212
to higher formaldehyde yields. This additional formaldehyde source has to be taken into account
213
when calculating the •OH yield. The formaldehyde yield is about 30±4 % of •OH (•OH yield is
214
3.3 times the formaldehyde yield), if no ozone is present. In the presence of ozone it is estimated
215
to be around 50 % (•OH yield is 2 times the formaldehyde yield). 34 If the yield is not corrected,
216
it leads to overestimation of •OH yields. In this system, the •OH yield is just an approximate
217
value, as O2•͞ reacts not only with ozone, but also with other species present in the sample
218
(reactions 22 – 27). HOBr must be the most relevant bromine species reacting with O2•͞ due to
219
its high concentration in the sample and its fast reaction rate with O2• ͞ . As the pH of the solution
220
is 6.5 and the pKa of HOBr is 8.69 35, the slower reaction with OBr ͞ can be neglected, as under
221
these conditions HOBr (99%) is present in excess over OBr ͞ (1%). O2• ͞ reacts with HOBr (k =
222
3.5 × 109 M-1 s-1) even faster than with ozone, hence HOBr acts here as competitor for ozone
223
and makes it difficult to define the formaldehyde yield per •OH, as it is not possible to use the
224
known correction factor for the formaldehyde yield in ozone-based processes.
225
O3 + O2• ͞ → O3• ͞ + O2
226
HOBr + O2• ͞
→ O2 + OH ͞
227
OBr ͞ + O2• ͞
→ O2 + 2 OH ͞ HO
Br ͞
Br ͞
+ Br2• ͞ + Br2• ͞
k = 1.6 × 109 M-1 s-1
(22)
k = 3.5 × 109 M-1 s-1
(23)
k > 2 × 108 M-1 s-1
(24)
2
228
Br2• ͞ + O2• ͞ → O2 + 2 Br ͞
k = 1.7 × 108 M-1 s-1
(25)
229
Br3• ͞ + O2• ͞ → O2 + Br ͞ + Br2• ͞
k = 1.5 × 109 M-1 s-1
(26)
230
Br2• ͞ + Br2• ͞ → Br ͞ + Br3 ͞
k = 1.8 × 109 M-1 s-1
(27)
231
The •OH yield is shown in Figure 1. Due to the reaction of O2• ͞ with ozone and HOBr, it is
232
not possible to give an exact •OH yield per ozone. The shaded area indicates the possible range
233
for the yield. The lower line gives the lowest limit considering solely the reaction of O2• ͞ with
234
ozone and the upper line indicates the highest yield excluding the reaction of O2• ͞ with ozone.
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As the system is very complex it cannot be surely predicted to what extent O2• ͞ reacts with
236
ozone and to what extent with bromine species. If one assumes that the most important bromine
237
species reacting with superoxide is HOBr, a possible •OH yield can be calculated. Therefore, the
238
fractions of superoxide reacting with ozone and HOBr were calculated, from which the
239
correction factors were adapted for every ozone concentration. The calculated correction factor
240
is 2.29 ±0.06 (•OH yield is 2.29 times the formaldehyde yield), if the only bromine species
241
reacting with O2• ͞ is HOBr. The calculated possible •OH yields are shown in Figure 1 (white
242
circles and corresponding linear trend line).
243
The exact quantification of •OH would be possible using the DMSO/•OH system, which gives
244
methanesulfinic acid, that is formed in 92 % yield.36 However, this system could not be used
245
here, as high DMSO concentrations would have to be present to scavenge all •OH, but at the
246
same time it would compete with bromide for O3. To comply with the prerequisites of a •OH
247
scavenger the DMSO concentration must be at least 100 times higher than the bromide
248
concentration. Under these conditions, the fraction of •OH reacting with DMSO would be 99 %,
249
but the fraction of ozone reacting with bromide only 45 %.
250
•
251
The reaction of methanol with •OH yields formaldehyde (ractions 28 – 31).28 The
252
formaldehyde yield per •OH is unity. Once a •OOCH2OH is formed it reacts with HPO42 ͞ and
253
OH ͞ to formaldehyde (reaction 30 and 31).
OH Quantification by MeOH
254
•
OH + CH3OH → •CH2OH + H2O
k = 9.7 × 108 M-1 s-1
(28)
255
•
CH2OH + O2 → •OOCH2OH
k = 4.5 × 109 M-1 s-1
(29)
256
•
OOCH2OH + HPO42 ͞ → HCHO + O2• ͞ + H2PO4 ͞
k ≈ 2 × 106 M-1 s-1
(30)
257
•
OOCH2OH + OH ͞ → HCHO + O2• ͞ + H2O
k = 1.8 × 1010 M-1 s-1
(31)
258
In the present system the formaldehyde yield and thus the •OH yield is 83 % with respect to
259
the applied ozone (Figure 2). Such a high •OH yield could not exclusively emerge from the
260
reaction of ozone with bromide. Thus, there must be another relevant source of •OH in the
261
system. In the sequence of the reactions between methanol and •OH, O2• ͞ is formed. Once it is
262
formed, it reacts readily with ozone (reaction 22) and outcompetes the reaction of bromide with
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͞
giving •OH, that react with methanol
263
ozone. Hence, nearly all ozone is consumed by O2•
264
yielding formaldehyde. This undesirable reaction leads to an overestimated •OH yield.
265
Therefore, methanol cannot be used for the quantification of •OH in ozone-based reactions.
0.5 y = 0.828x + 0.04 [CH2O] / mM
0.4
0.3
0.2
0.1 0.1
0.2
0.3
268 269
0.5
0.6
[O3] / mM
266 267
0.4
Figure 2 Formation of formaldehyde in the reaction of •OH with MeOH buffered at pH 7 (■) and 8 (□) with 2 mM H2PO4 ͞ /HPO42 ͞ . Formation of Bromate and HOBr
270
Since •OH contribute to bromate formation (reactions 8 – 16) scavenging of •OH must lead to
271
a reduced bromate yield.13 This could be observed in experiments with and without the addition
272
of tBuOH as an •OH scavenger (Figure 3). Such a significant difference between the bromate
273
yield in presence and absence of tBuOH is a further evidence for the •OH formation in the
274
multistep reaction of ozone with bromide to bromate. The bromate yield without tBuOH is
275
around twice its yield in the presence of tBuOH.
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[BrO−3 ] / µM
20
10
0 100
200
300
276
400 500 [O3] / µM
600
700
277
Figure 3 Formation of bromate in presence of tBuOH as •OH scavenger (squares) and in
278
absence of tBuOH (circles) in the reaction of ozone with bromide plotted against the
279
ozone concentration (n = 3).
280
HOBr is the most important and stable intermediate in the multi-step process en route to
281
bromate. Based on published results presenting the mechanism of the formation of 2- and 4-
282
bromophenol as an example for the formation of brominated phenolic compounds in natural
283
waters in ozone treatment a well working method for HOBr quantification was developed
284
HOBr was quantified indirectly by measuring 2- and 4-bromophenol, the products from its
285
reaction with phenol. The pH was varied from 2 to 5 and the highest yields were obtained at pH
286
4. The slope of the graph in Figure 4 gives the yield as 0.99 with an error of 0.026. Hence, the
287
bromophenol yield with respect to the applied HOBr is ~100 %.
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[Bromophenols] / µM
160
y = 0.99x + 2.04
140 120 100 80 60 40 20 0 0
20
40
60
80
100
120
140
160
180
[HOBr] / µM 288 289 290
Figure 4 2-bromophenol (open circles), 4-bromophenol (open triangles) and sum of both
291
bromophenols (solid squares) yielded from the derivatization of HOBr with phenol at
292
pH 4 (n = 3). Relative standard deviations are indicated but often smaller than symbol
293
size. The dashed line indicates a 100 % yield of bromophenols per HOBr.
294
The ratio of 4-bromophenol and 2-bromophenol formed in the reaction of hypobromous acid
295
with phenol is 1.64 at pH 4. This first appears to be contrary to the results of Acero et al. as they
296
reported a ratio of 0.53 for experiments conducted at pH 8-9
297
bromophenols is strongly pH dependent as Tee et al
298
bromophenols is para substituted at pH 0. This means that at low pH 4-bromophenol is
299
prevalent, and at high pH 2-bromophenol is dominant. As the ratio of 2- and 4-bromophenol is
300
pH dependant, it is highly recommended measuring both bromophenols and using their sum
301
concentration to determine the HOBr concentration. HOBr was measured after the reaction was
302
completed and all ozone was consumed. Figure 5 shows the bromophenol yields in the samples.
303
The sum of 2- and 4-bromophenol corresponds to the HOBr formed in the reaction of bromide
304
and ozone. The HOBr concentration in the samples in the absence of tBuOH is higher than in its
31
32
. However, the ratio of the two
published that 95% of formed
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305
presence, as •OH are not scavenged and react rapidly with bromide that is present at high
306
concentration yielding HOBr (reaction 8). Furthermore, if no tBuOH is added, no O2• ͞ is formed
307
in the sequence of the reaction between tBuOH and •OH that could react with HOBr (reactions
308
23 and 24). Thus, the lower HOBr yield observed in the presence of tBuOH provides further
309
evidence for •OH formed from the reaction of bromite with ozone via electron transfer.
310
0.18
[Bromophenols] / mM
0.15 R2 = 0.733 0.12 0.09 R2 = 0.977 0.06 0.03 0.00 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
[O3] / mM 311 312 313
Figure 5 Formation of bromophenols at pH 4 from the reaction of phenol and HOBr formed as
314
intermediate from bromide and ozone. The sum of 2- and 4-bromophenol formed in
315
the presence of tBuOH (solid squares) and in the absence of tBuOH (open squares) as
316
a function of the applied ozone concentration (n = 3).
317
From the results discussed above, it can be concluded that in the sequence of the ozone
318
reactions from bromide to bromate, •OH must be generated. Thus, the original concept of
319
bromate formation has to be revised. Based on the obtained results the known pathway from
320
bromide to bromate was modified (Figure 6). It was updated at the last ozone reaction step. The
321
O-transfer reaction was replaced by the electron transfer reaction.
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Figure 6 (a) Old pathway for the bromate formation in the reaction of ozone with bromide
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established by U. von Gunten1,
325
of bromide with ozone based on this study adapted from U. von Gunten’s reaction
326
scheme1,
327
reaction of ozone on the right hand side.
328
13
13
and (b) new recommended pathway for the reaction
. Note the difference between the old and new mechanism in the last
Practical Implications
329
The present paper shows that in the multi-step reaction of ozone with bromide to bromate, •OH
330
are generated. This is due to the fact that the last of these steps is not as previously assumed an
331
O-transfer but an electron transfer reaction. The electron transfer reaction also yields bromate,
332
but this new fact could be essential for reaction modeling. Until now, the modeling of the
333
reaction of bromide with ozone does not give good results, as there are still too many not well
334
known reactions included (U. von Gunten 2015, personal communication). In systems with low
335
•
OH scavenger capacity (e.g., waters with low dissolved organic matter content) OH radical
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336
formation in the electron transfer step might enhance the bromate yield, as the •OH formed on
337
site may contribute to bromate formation. Under which conditions this is indeed relevant in
338
natural waters requires further investigation.
339
Supporting Information
340
Additional information on the calculation of the •OH fraction reacting with a probe compound as
341
noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.
342
Acknowledgement
343
We dedicate this paper to the memory of our co-author and dear friend Clemens von Sonntag
344
who was an inspiration to us all. We thank Dr. Holger Lutze and Prof. Urs von Gunten for
345
fruitful discussion and Alua Suleimenova for reviewing the manuscript.
346 347 348
References
349
Formation in Presence of Bromide, Iodide or Chlorine. Water Res. 2003, 37, (7), 1469-1487.
350
2.
351
Ozonation of Bromide-Containing Waters. Aqua (Oxford) 1992, 41, (5), 299-304.
352
3.
353
Disinfection Byproducts. In Federal Register, Environmental Protection Agency: 1994; Vol. 59,
354
(145).
355
4.
356
European Communities, Council Directive 98/83/EC: 1998.
357
5.
358
Bromate Ion in Simulated Gastric Juices. Ozone Sci. Eng. 2006, 28, (3), 165-170.
359
6.
360
Reaction of Bromate Ion with Synthetic and Real Gastric Juices. Toxicology 2006, 221, (2-3),
361
225-228.
362
7.
363
Simulated Gastric Juice. J. Am. Water Works Ass. 2010, 102, (11), 77-86.
364
8.
365
Formation of Hypobromous Acid and Bromate. Environ. Sci. Technol. 1983, 17, (5), 261-267.
1.
von Gunten, U., Ozonation of Drinking Water: Part II. Disinfection and By-Product
von Gunten, U.; Hoigne, J., Factors Controlling the Formation of Bromate During
Drinking Water; National Primary Drinking Water Regulations: Disinfectants and
On the Quality of Water Intended for Human Consumption. In Official Journal of the
Keith, J. D.; Pacey, G. E.; Cotruvo, J. A.; Gordon, G., Preliminary Data on the Fate of
Keith, J. D.; Pacey, G. E.; Cotruvo, J. A.; Gordon, G., Experimental Results from the
Cotruvo, J. A.; Keith, J. D.; Bull, R. J.; Pacey, G. E.; Gordon, G., Bromate Reduction in
Haag, W. R.; Hoigne, J., Ozonation of Bromide-Containing Waters: Kinetics of
16 ACS Paragon Plus Environment
Page 17 of 19
Environmental Science & Technology
366
9.
Nicoson, J. S.; Wang, L.; Becker, R. H.; Hartz, K. E. H.; Muller, C. E.; Margerum, D.
367
W., Kinetics and Mechanisms of the Ozone/Bromite and Ozone/Chlorite Reactions. Inorg.
368
Chem. 2002, 41, (11), 2975-2980.
369
10.
370
Peroxide (Peroxone Process): A Revision of Current Mechanistic Concepts Based on
371
Thermokinetic and Quantum-Chemical Considerations. Environ. Sci. Technol. 2010, 44, (9),
372
3505-3507.
373
11.
374
Compounds; the Spectra and Reactions of BrO and BrO2. Proc. R. Soc. London, Ser. A 1968,
375
304, (1479), 427-439.
376
12.
377
Free Radicals in Aqueous Solution: Quantum-Chemical Calculations. Environ. Sci. Technol.
378
2011, 45, (21), 9195-9204.
379
13.
380
Containing Waters: Interaction of Ozone and Hydroxyl Radical Reactions. Environ. Sci.
381
Technol. 1994, 28, (7), 1234-1242.
382
14.
383
Pulse Radiolytic Investigation. J. Phys. Chem. 1972, 76, (3), 312-319.
384
15.
385
Deaerated Aqueous Bromide Solutions. J. Phys. Chem. 1966, 70, (7), 2092-2099.
386
16.
387
Aqueous Solution - Reactions of Cl- and Br-Atoms. Ber. Bunsen-Ges. Phys. Chem 1985, 89,
388
(3), 243-245.
389
17.
390
Bromate Formation Mechanisms. Environ. Sci. Technol. 1998, 32, (1), 63-70.
391
18.
392
Hypobromous Acid: Implication on Water Treatment and Natural Systems. Water Res. 1997,
393
31, (4), 900-906.
394
19.
395
Influence of pH and Temperature. Russ. J. Phys. Chem. A 2009, 83, (8), 1295-1299.
396
20.
397
Formation During Ozonation. Environ. Technol. 2012, 33, (15), 1747-1753.
Merenyi, G.; Lind, J.; Naumov, S.; von Sonntag, C., Reaction of Ozone with Hydrogen
Buxton, G. V.; Dainton, F. S., The Radiolysis of Aqueous Solutions of Oxybromine
Naumov, S.; von Sonntag, C., Standard Gibbs Free Energies of Reactions of Ozone with
von Gunten, U.; Hoigne, J., Bromate Formation During Ozonization of Bromide-
Zehavi, D.; Rabani, J., Oxidation of Aqueous Bromide Ions by Hydroxyl Radicals -
Matheson, M. S.; Mulac, W. A.; Weeks, J. L.; Rabani, J., The Pulse Radiolysis of Klaning, U. K.; Wolff, T., Laser Flash-Photolysis of HClO, ClO-, HBrO, and BrO- in
von Gunten, U.; Oliveras, Y., Advanced Oxidation of Bromide-Containing Waters:
von Gunten, U.; Oliveras, Y., Kinetics of the Reaction Between Hydrogen Peroxide and
Ershov, B. G.; Morozov, P. A., The Kinetics of Ozone Decomposition in Water, the
Antoniou, M. G.; Andersen, H. R., Evaluation of Pretreatments for Inhibiting Bromate
17 ACS Paragon Plus Environment
Environmental Science & Technology
Page 18 of 19
398
21.
Han, Q.; Wang, H. J.; Dong, W. Y.; Liu, T. Z.; Yin, Y. L., Suppression of Bromate
399
Formation in Ozonation Process by Using Ferrate(VI): Batch Study. Chem. Eng. J. 2014, 236,
400
110-120.
401
22.
402
Treatment - From Basic Principles to Applications. IWA Publishing: London, 2012.
403
23.
404
Bands of Ozone in Aqueous Solutions. Anal. Chem. 1983, 55, (1), 46-49.
405
24.
406
Constants for Reactions of Hydrated Electrons, Hydrogen-Atoms and Hydroxyl Radicals
407
(.OH/.O-) in Aqueous-Solution. J. Phys. Chem. Ref. Data 1988, 17, (2), 513-886.
408
25.
409
Compounds in Water-I Non: Dissociating Organic Compounds. Water Res. 1983, 17, (2), 173-
410
183.
411
26.
412
Automobile Exhaust with an Improved 2,4-Dinitrophenylhydrazine Method. J. Chromatogr.
413
1982, 247, (2), 297-306.
414
27.
415
Propagation Reactions on the Hydroxyl Radical Formation in Ozonation and Peroxone
416
(Ozone/Hydrogen Peroxide) Processes. Water Res. 2015, 68, 750-758.
417
28.
418
Quantum Yield and Applications for Polychromatic UV Actinometry in Photoreactors. Environ.
419
Sci. Technol. 2007, 41, (21), 7486-7490.
420
29.
421
Determination of Reactive Bromine and Chlorine Species with ABTS. Water Res. 2000, 34,
422
(18), 4343-4350.
423
30.
424
Hypobromous Acid Reactions with Iodide and with Sulfite and the Hydrolysis of Bromosulfate.
425
Inorg. Chem. 1991, 30, (18), 3538-3543.
426
31.
427
Phenols and Phenoxide Ions in Aqueous Solution. Diffusion-Controlled Rates. J. Am. Chem.
428
Soc. 1989, 111, (6), 2233-2240.
von Sonntag, C.; von Gunten, U., Chemistry of Ozone in Water and Wastewater
Hart, E. J.; Sehested, K.; Holcman, J., Molar Absorptivities of Ultraviolet and Visible
Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B., Critical-Review of Rate
Hoigne, J.; Bader, H., Rate Constants of Reactions of Ozone with Organic and Inorganic
Lipari, F.; Swarin, S. J., Determination of Formaldehyde and other Aldehydes in
Liu, Y.; Jiang, J.; Ma, J.; Yang, Y.; Luo, C.; Huangfu, X.; Guo, Z., Role of the
Goldstein, S.; Aschengrau, D.; Diamant, Y.; Rabani, J., Photolysis of Aqueous H2O2:
Pinkernell, U.; Nowack, B.; Gallard, H.; Von Gunten, U., Methods for the Photometric
Troy, R. C.; Margerum, D. W., Non-Metal Redox Kinetics: Hypobromite and
Tee, O. S.; Paventi, M.; Bennett, J. M., Kinetics and Mechanism of the Bromination of
18 ACS Paragon Plus Environment
Page 19 of 19
Environmental Science & Technology
429
32.
Acero, J. L.; Piriou, P.; von Gunten, U., Kinetics and Mechanisms of Formation of
430
Bromophenols During Drinking Water Chlorination: Assessment of Taste and Odor
431
Development. Water Res. 2005, 39, (13), 2979-2993.
432
33.
433
Radical-Induced Oxidation of 2-Methyl-2-Propanol in Oxygenated Aqueous Solution - Product
434
and Pulse Radiolysis Study. J. Phys. Chem. 1979, 83, (7), 780-784.
435
34.
436
Determination of .OH, O2.−, and Hydroperoxide Yields in Ozone Reactions in Aqueous Solution.
437
J. Phys. Chem. B 2003, 107, (30), 7242-7253.
438
35.
439
Ohio, 1971-1972.
440
36.
441
Radicals with Sulfoxides. J. Chem. Soc., Perkin Trans. 2 1980, (1), 146-153.
Schuchmann, M. N.; von Sonntag, C., Radiation-Chemistry of Alcohols 22. Hydroxyl
Flyunt, R.; Leitzke, A.; Mark, G.; Mvula, E.; Reisz, E.; Schick, R.; von Sonntag, C.,
Weast, R. C., Handbook of Chemistry and Physics. 52nd ed.; The Chemical Rubber Co.:
Veltwisch, D.; Janata, E.; Asmus, K. D., Primary Processes in the Reaction of OH-
442 443
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