Article pubs.acs.org/est
Bromine and Carbon Isotope Effects during Photolysis of Brominated Phenols Yevgeni Zakon, Ludwik Halicz, and Faina Gelman* The Geological Survey of Israel, 30 Malkhei Israel St., Jerusalem 95501, Israel S Supporting Information *
ABSTRACT: In the present study, carbon and bromine isotope effects during UVphotodegradation of bromophenols in aqueous and ethanolic solutions were determined. An anomalous relatively high inverse bromine isotope fractionation (εreactive position up to +5.1‰) along with normal carbon isotope effect (εreactive position of −12.6‰ to −23.4‰) observed in our study may be attributed to coexistence of both mass-dependent and mass-independent isotope fractionation of C−Br bond cleavage. Isotope effects of a similar scale were observed for all the studied reactions in ethanol, and for 4-bromophenol in aqueous solution. This may point out related radical mechanism for these processes. The lack of any carbon and bromine isotope effects during photodegradation of 2-bromophenol in aqueous solution possibly indicates that C−Br bond cleavage is not a rate-limiting step in the reaction. The bromine isotope fractionation, without any detectable carbon isotope effect, that was observed for 3bromophenol photolysis in aqueous solution probably originates from mass-independent fractionation.
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INTRODUCTION Brominated organic compounds are widely used as biocides, disinfectants, solvents, and flame retardants and, therefore, are often released into the environment in large quantities as industrial, agricultural, and urban pollutants.1 As brominated phenols are frequently found as environmental contaminants and, due to their high toxicity and possible harmful effects on human health, investigation of their natural attenuation and enhanced degradation is of utmost importance. Several previous studies have proven that bromophenols can be degraded under different biological and chemical conditions,2−4 as well as by ultraviolet (UV) and sunlight irradiation.5,6 Despite the fact that the photodegradation of halophenols has been intensively investigated previously7−13 and possible degradation pathways based on identification of metabolites and degradation products were proposed, many mechanistic aspects of this process still remain unresolved. In the present study we apply a novel approach using dual carbon−bromine isotope effects to investigate the UVphotolysis of brominated phenols. During the last decades, compound-specific isotope analysis (CSIA) has become an effective technique to assess the fate of different environmental contaminants.14−16 Moreover, twodimensional CSIA has been recently introduced as a powerful tool for the study of mechanistic pathways of degradation. Thus, for example, δ13C-δ2H analysis has been employed for the determination of methyl tert-butyl ether (MTBE) biodegradation in contaminated aquifers;17 δ13C-δ37Cl analysis was employed for the evaluation of the reaction mechanisms of dichloroethylene (DCE) biodegradation;18 Elsner et al. reported application of two-dimensional δ13C-δ15N analysis for mechanistic studies of atrazine degradation.19 These and many other studies employing CSIA have proven one of the main advantages of this techniquean ability to predict a © 2013 American Chemical Society
decomposition pathway of organic contaminants based on the changes in their isotope composition without a need to identify the products of the reaction. In addition to the well-established analytical methods for carbon, hydrogen and nitrogen isotope analysis, compoundspecific bromine isotope analysis has been recently proposed as a tool for investigation of transformations of brominated organics.20−22 Recent improvements in the method’s precision enabled its application for the determination of even small bromine kinetic isotope effects (Br-KIE). Our group showed that microbial debromination of bromophenols is accompanied by bromine apparent kinetic isotope effect (AKIE) in the range of 1.0006−1.0009.23 Although the obtained Br−KIEs were rather small, their values are still high enough to be applied for the study of contaminated sites. Since photodegradation is a potential pathway of bromophenols degradation in the environment,24 data on isotope effects (IEs) associated with this process can be very helpful for the assessing the degree of degradation. Moreover, this approach may be very informative for the elucidation of the degradation pathway, and may enable us to predict the reaction mechanism without a need to identify photodegradation products. Noteworthy, pronounced carbon isotope effects during UV-photolysis of some organic compounds were reported by several research groups.25−27 Considering the isotope enrichment factors provided in those reports, AKIEs in the range of 1.008−1.05 can be deduced. However, studies regarding bromine isotope effects during UV-photolysis of bromoaromatic compounds have not been reported in the Received: Revised: Accepted: Published: 14147
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analysis of the samples. δ13C values of the analytes were measured against internal laboratory standard CO2 gas that was introduced at the beginning and at the end of each run. Calibration of carbon isotope composition of CO2 gas was performed against international standards USGS-40, urea #1, #2, #3 (Biogeochemical Laboratories Indiana University) and reported in per mil (‰) relative to Vienna-Peedee Belemnite (VPDB). Bromine isotope analysis of the brominated phenols was performed by GC-MC-ICPMS as reported earlier.23 The analyte solution in ethyl acetate or ethanol was injected into a GC (HP 6890) interfaced to the MC-ICPMS (Nu Instruments). Strontium external spike solution (SRM 987 U.S. National Institute of Standards and Technology), was continuously introduced into the system by an Aridus desolvation nebulizer for correction of instrumental mass bias. Fine-tuning of the MC-ICPMS instrument was performed according the maximum signal intensity of 86Sr. Signals of 86Sr, 84 Sr, 81Br, 79Br, and 83Kr (83Kr was measured for correcting an isobaric interferences of 86Kr and 84Kr on 86Sr and 84Sr, respectively) were simultaneously measured by Faraday cup detectors. Operating parameters for the GC-MC-ICPMS system are listed in the Table 1.
literature so far. In the present study, we investigated both the carbon and bromine isotope effects during UV-photolysis of bromophenols in aqueous and ethanolic solutions, and tried to find a correlation between the observed isotope fractionation and possible reaction mechanism.
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EXPERIMENTAL SECTION Chemicals. 2-Bromophenol (2-BP), 3-Bromophenol (3BP) and 4-Bromophenol (4-BP) (97−99% purity) were purchased from Sigma-Aldrich and used without further purification. All used solvents were of analytical grade, MilliQ water of conductivity 95%. Carbon and bromine isotope analysis of the extracted nonirradiated substrates did not result in any detectable isotope fractionation. Stable carbon isotope ratios were determined by GC-CIRMS (Trace GC Ultra, Delta V plus; Thermo Scientific). The oxidation oven (CuO/Ni/Pt) was held at 980 °C and the reduction oven at 650 °C. The same temperature program that was used for GC-MS analysis was also applied for the isotope
Table 1. Instrumental Parameters for the GC-MC/ICPMS Measurement GC parameters injector conditions 250 °C, splitless 1 min, liner i.d. 2 mm column HP-5 (30m, 0.25 mm, 0.25 μm) oven temperature 60 °C, 10 °C min−1 to 250 °C He carrier flow 3 mL min−1 transfer line temperature 240 °C MC/ICPMS parameters RF power coolant flow auxiliary flow nebulizer gas flow interface cones instrument resolution nebulizer spray chamber temp. desolvator temp. sweep gas (argon)
1200 W 12 L min−1 1.25 L min−1 0.69 L min−1 nickel 300 m/Δm PFA 50 μL min−1 75 °C 160 °C 3.45 L min−1
Each of the samples was analyzed at least three times and the external precision for all samples did not exceed 0.2‰ (2σ) for bromine and 0.5‰ for carbon isotope analysis. Calculation of Isotope Effects. The measurement of the isotopic fractionation during the photodegradation process was performed on the basis of carbon and bromine isotopic composition of the reacting compounds at different intervals of the degradation. The Rayleigh equation (eq 1) was used to find a relation between the change in the isotopic composition and degree of degradation.
⎛ Rx , t ⎞ ln⎜ ⎟ = εbulk ·ln f ⎝ Rx , 0 ⎠
(1)
Where εbulk is the isotopic enrichment factor observed for the compound, f is the fraction of degradation (often described as C/C0, where C0 and C are the concentrations of the compound at time zero and t, respectively), Rx is the isotope composition 14148
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of elements (carbon or bromine) in the substrate at time zero and t, respectively. The isotope enrichment factor, εbulk was obtained as a slope of the linear regression line of natural logarithm of the isotopic enrichment, Rx,t/Rx,0, against the natural logarithm of the extent of degradation, f . Kinetic isotope effect can be calculated as: KIE = 1/(1 + εbulk )
Scheme 1. Bromophenols Photodegradation in Ethanol and Aqueous Solutiona
(2)
Whereas the εBrbulk represents also the εBrreactive position, the actual isotope effect on the reacting carbon is diluted by the nonreacting carbons. The εCreactive position was calculated according the following equation:28 εCreact. pos. = n·εbulk
(3)
where n is a total number of C atoms in the molecule. Apparent kinetic isotope effects were calculated according to eq 4:28
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AKIE = 1/(1 + εCreact. pos.)
(4)
RESULTS AND DISCUSSION Photodegradation Pathways and Kinetics. Considering the studies on bromophenols photodegradation published so far in the literature, it can be concluded that the reaction pathway and kinetics depend on the reaction conditions, such as substrate concentration, nature of solvent, pH, oxygen availability, UV intensity, etc. In the present study good linearity was obtained by fitting the studied reactions with first order kinetics (Table 2). Based on the assumption that the photodegradation of bromophenols follows a pseudo-first order model, the rate constants (k) for the studied processes were determined (Table 2).
Table 2. Photodegradation Rate (k) of Bromophenols in Aqueous and Ethanol Solutions substrate
ethanol −1
k, min 2-BP 3-BP 4-BP
0.0072 0.0032 0.0072
a
(A) Main products observed during photodegradation of bromophenols in ethanol and aqueous solutions; (B) Suggested ionic and radical mechanisms of photodegradation.
water r
2
0.97 0.99 0.98
k, min
−1
0.041 0.011 0.0049
r2 0.98 0.99 0.98
of different halophenols in alcohol solutions and proposed a radical mechanism induced by carbon−bromine bond homolysis. Formation of resorcinol and pyrocatechol as the main products of halophenols’ photolysis has been also reported8,30 and a dependence of the reaction mechanism upon the position of the bromine atom in the aromatic ring was suggested.30 Based on experimental results, LipczynskaKochany proposed9,30 that 3-bromophenol reacts by ionic mechanism that includes heterolysis of C−Br bond, whereas 4BP undergoes photodegradation by radical pathway accompanied by homolytic C−Br bond cleavage (Scheme 1B). Although photodecomposition of 4-BP has been further investigated by several research groups, the pathway details of 4-BP photodecomposition remained questionable and heterolytic cleavage of C−Br was proposed as well.31 Isotope Fractionation. The Rayleigh plots used for the determination εbulk for carbon and bromine are represented in Figure 1. Based on the εbulk obtained from the Rayleigh plots, εreact. pos. and AKIEs values for carbon and bromine were calculated (Table 3). As shown above, for all the studied reactions, with an exception of 2-BP in aqueous solution, a significant inverse bromine isotope fractionation (meaning depletion of the
As can be seen, in the case of ethanol solution, 2-BP and 4BP degraded at comparable rates, while debromination of 3-BP was significantly slower. On the other hand, the reaction pathways in the buffered aqueous solution were quite different: the fastest photolysis was observed for 2-BP, followed by 3-BP, whereas 4-BP degraded much slower. In addition to the different kinetic patterns, photodegradation of bromophenols in ethanol and aqueous solutions in our study resulted in the formation of different primary products, but no quantitative data was collected for them in the present study. Accumulation of phenol (I) as the primary product was observed for all the studied bromophenols in ethanol (Scheme 1A, Ethanol). However, photodegradation of 2-BP and 3-BP in aqueous solution resulted in the formation of hydroxyphenols (pyrocatechol (II) and resorcinol (III), respectively), whereas benzoquinone (IV) was observed upon 4-BP degradation (Scheme 1A, Water). These results are in agreement with the findings of Pinhey and Rigby29 that reported the formation of phenol as a main photodegradation product in UV-photolysis 14149
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Figure 1. Rayleigh plots along the photodegradation of 2-BP, 3-BP, and 4-BP in ethanol and aqueous solutions.
Table 3. Carbon and Bromine Fractionation Factors and Apparent Kinetic Isotope Effects Observed during Bromophenols Photodegradationa substrate 2-BP 3-BP 4-BP a
ethanol
water
εCreact.pos.,‰
C-AKIE
εBrreact.pos., ‰
Br-AKIE
εCreact.pos., ‰
C-AKIE
εBrreact.pos.,‰
Br-AKIE
−19.2 ± 1.8 −23.4 ± 3.0 −16.2 ± 2.4
1.0192 ± 0.0018 1.0234 ± 0.0030 1.0162 ± 0.0024
2.1 ± 0.3 3.3 ± 0.4 3.2 ± 0.5
0.9979 ± 0.0003 0.9967 ± 0.0004 0.9968 ± 0.0005
0 0 −12.6 ± 1.2
1.0000 1.0000 1.0126 ± 0.0012
0 5.1 ± 0.2 2.2 ± 0.3
1.0000 0.9949 ± 0.0002 0.9978 ± 0.0003
The ± represents the 95% confidence interval for the regression line of the Rayleigh plots.
accompanied by the usual mass-dependent carbon and bromine isotope fractionation, resulting in a normal KIE (enrichment of the remaining substrate by the heavier isotope, KIE >1) for both of the atoms. Applying semiclassical Streitweiser limits,32 we estimate Br-KIE of 1.002 and C-KIE of 1.040 (see Supporting Information). The magnitudes of the observed CAKIEs upon photolysis in the present study are between 1.0126 and 1.0234. The obtained values are lower than the C-AKIE = 1.05 reported by Rosenfelder et al.25 for UV-debromination of tetrabromodiphenyl ether BDE-47, however, they are still of the same order of magnitude.
remaining substrate fraction by the lighter isotope, KIE < 1) was observed. In contrast, carbon isotopes behaved in a different manner: the remaining fraction of the substrates became enriched with the heavier isotope for all the studied bromophenols in ethanol solution, as well as for 4-BP in aqueous solution, but no change in the carbon isotope composition was observed for 2-BP and 3-BP in aqueous solution. In principle, it is known that for aryl halides, photoreaction usually involves an excitation of the ground state followed by a homolytic or heterolytic C-X bond cleavage. If C-X bond cleavage is a rate-limiting step, the reaction is supposed to be 14150
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Scheme 2. Possible Mechanistic Scenarios for MIE and KIE
The effect can be also observed when both of the nuclei are magnetic, since nuclei of different isotopes generally possess different magnetic moments and therefore exert different hyperfine interactions on electron spins. In contrast to the classical mass-dependent KIE occurring in energy-demanding reaction steps, MIE usually appears in chemical reaction steps which do not limit the rate of the overall process.41 Whereas the sign of the KIE is dictated by the ratio of the masses of the isotopes, the sign of the MIE depends on the direction of the spin conversion.41 In the case of bromine atoms, both 79Br and 81Br are stable isotopes that have an identical nuclear spin but slightly different nuclear magnetic moments (2.7089 and 2.9210, respectively 42 ). Thus, in principle, both of the isotopes are able to undergo spin conversion; however, the rate of this process may be slightly different due to the differences in hyperfine constant for two isotopes. As for now, we speculate that although the difference in the magnetic moments of the bromine isotopes is small it is significant enough to affect the rates of spin conversion and lead to the observed Br isotope effect (Br-IE). We propose that Br isotope effect observed in the present study during photodegradation of bromophenols is a superposition of mass-dependent kinetic isotope effect and massindependent magnetic isotope effects (eq 5):
Interpretation of the observed bromine isotope fractionation (up to +5.1‰) is more confusing. The cleavage of C−Br bond as a rate limiting step of the reaction is expected to be accompanied by the conventional bromine KIE, leading to enrichment of the remaining substrate by the heavy isotope. Thus, normal Br-KIE was reported for the solvolysis of butyl bromide via a nucleophilic substitution,33 for Grignard reagent formation34 and enzymatic reductive dehalogenation of bromophenols23 as well. Recently, the evidence of inverse nitrogen isotope effect during the oxidation of anilines was reported by Hofstetter group.35 The authors rationalized it by the formation of a radical intermediate after one-electron oxidation and corresponding increase in C−N bond strength. However, this scenario is unlikely in the present case due to electron withdrawing properties of bromine. Thus, a different explanation for the observed inverse Br isotope effect is required. We assume that the anomalous inverse bromine isotope effect found in the present study is associated with the molecule excitation. It has been demonstrated that in some cases photochemical reactions are accompanied by magnetic field isotope effect (MIE).36−40 MIE is a mass-independent isotope effect, resulting from the interaction of the magnetic field associated with electron spin with the magnetic field associated with nuclei spin. The effect is based on the principle that chemical reactions are spin -selective and, therefore, allowed for those spin states of reagents whose total electron spin is identical to that of the products. Usually, a significant MIE is observed for the nuclei consisting of zero and nonzero spin isotopes. So, the radical pairs containing magnetic nuclei undergo spin conversion and recombination to the starting molecule much faster than nonmagnetic spin nuclei. One of the most popular reactions for the MIE studies is photolysis of dibenzyl ketone (DBK) which is known to occur via the fragmentation of the excited DBK molecule in triplet state and generation of triplet radical pair.40 The radical pair consists of 13 C in a carbonyl group that undergoes fast triplet-singlet conversion and recombination to the starting DBK molecule, whereas radical pairs that consist of 12C predominantly dissociate into the products. However, when DBK contains a natural abundance of 13C, the effect is barely outside of the experimental error.40 We suppose that C-MIE is insignificant in the studied reactions of bromophenols photodegradation due to a low 13C content in the substrate molecules.
Br‐IE = Br‐KIE + Br‐MIE
(5)
After a closer examination of the obtained results, some interpretations of the mechanistic aspects of the studied reactions concerning observed isotope effects can be suggested (Scheme 2): - Similar values of C-IE, as well as Br-IE, observed for photolysis of bromophenols in ethanol and for 4-BP in aqueous solution, are possibly associated with a similar radical pathway based on homolytic C−Br bond cleavage in the rate-limiting step. Both, classical KIE resulting from a more favored cleavage of a bond between the lighter isotopes, and MIE originating from different rates of intersystem-crossing between singlet−triplet states contribute to the observed isotope fractionation. - The absence of isotope effects for 2-BP in aqueous solution along with much higher rate of photolysis than for two other bromophenols studied, leads us to conclude that C−Br cleavage is not the rate-determining step in this case, and therefore is not accompanied by 14151
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(2) Häggblom, M. M.; Knight, V. K.; Kerkhof, L. J. Anaerobic decomposition of halogenated aromatic compounds. Environ. Pollut. 2000, 107 (2), 199−207. (3) Ahn, Y.-B.; Rhee, S.-K.; Fennell, D.-E.; Kerkhof, L. J.; Hentschel, U.; Häggblom, M. M. Reductive dehalogenation of brominated phenolic compounds by microorganisms associated with the marine sponge Aplysina aerophoba. Appl. Environ. Microbiol. 2003, 69 (7), 4159−4166. (4) Keum, Y.-S.; Qing, X.; Li, Q. X. Reductive debromination of polybrominated diphenyl ethers by zerovalent iron. Environ. Sci. Technol. 2005, 39 (7), 2280−2286. (5) Jayaraman, A.; Mas, S.; Tauler, R.; de Juan, A. Study of the photodegradation of 2-bromophenol under UV and sunlight by spectroscopic, chromatographic and chemometric techniques. J. Chromatogr., B: Anal. Technol. Biomed. Life Sci. 2012, 910, 138−48. (6) Mas, S.; Carbó, A.; Lacorte, S.; Juan, A.; Tauler, R. Comprehensive description of the photodegradation of bromophenols using chromatographic monitoring and chemometric tools. Talanta 2011, 83 (4), 1134−1146. (7) Omura, K.; Matsuura, T. Photoinduced reactionsL: Photolysis of halogenophenols in aqueous alkali and in aqueous cyanide. Tetrahedron 1971, 27, 3101−3109. (8) Boule, P.; Guyon, C.; Lemaire, J. Photochemical behavior of monochlorophenols in dilute aqueous solution. Chemosphere 1982, 11 (12), 1179−1188. (9) Lipczynska-Kochany, E.; Kochany, J. Electron paramagnetic spin trapping detection of free radicals generated in direct photolysis of 4bromophenol in aqueous solution. J. Photochem. Photobiol., A 1993, 73, 23−33. (10) Grabner, G.; Richard, C.; Köhler, G. (1994) Formation and reactivity of 4-oxocyclohexa-2,5-dienylidene in the photolysis of 4chlorophenol in aqueous solution at ambient temperature. J. Am. Chem. Soc. 1994, 116 (25), 11470−11480. (11) Bonnichon, F.; Richard, C.; Grabner, G. Formation of an aketocarbene by photolysis of aqueous 2-bromophenol. Chem. Commun. 2001, 73−74. (12) Ouardaoui, A.; Steren, C. A.; van Willigen, H.; Yang, C. FT-EPR Study of the Photolysis of 4-Chlorophenol. J. Am. Chem. Soc. 1995, 117 (25), 6803−6804. (13) Sage, A. G.; Oliver, T. A.; King, G. A.; Murdock, D.; Harvey, J. N.; Ashfold, M. N. UV photolysis of 4-iodo-, 4-bromo-, and 4chlorophenol: Competition between C-Y (Y = halogen) and O-H bond fission. J. Chem. Phys. 2013, 138 (16), 164318. (14) Elsner, M.; Jochmann, M.; Hofstetter, T.; Hunkeler, D.; Bernstein, A.; Schmidt, T.; Schimmelmann, A. Current challenges incompound-specific stable isotope analysis of environmental organic contaminants. Anal. Bioanal. Chem. 2012, 403 (9), 2471−2491. (15) Thullner, M.; Centler, F.; Richnow, H. H.; Fischer, A. Quantification of organic pollutant degradation in contaminated aquifersusing compound specific stable isotope analysisReview of recent developments. Org. Geochem. 2012, 42 (12), 1440−1460. (16) Hofstetter, T. B.; Berg, M. Assessing transformation processes of organic contaminants by compound-specific stable isotope analysis. Trends Anal. Chem. 2011, 30 (4), 618−627. (17) Kuder, T.; Wilson, J. T.; Kaiser, P.; Kolhatkar, R.; Philp, P.; Allen, J. Enrichment of stable carbon and hydrogen isotopes during anaerobic biodegradation of MTBE: Microcosm and field evidence. Environ. Sci. Technol. 2005, 39 (1), 213−220. (18) Abe, Y.; Aravena, R.; Zopfi, J.; Shouakar-Stash, O.; Cox, E.; Roberts, J. D.; Hunkeler, D. Carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride and cis-1,2-dichloroethene. Environ. Sci. Technol. 2009, 43 (1), 101− 107. (19) Meyer, A. H.; Elsner, M. 13C/12C and 15N/14N isotope analysis to characterize degradation of atrazine: Evidence from parent and daughter compound values. Environ. Sci. Technol. 2013, 47 (13), 6884−6891. (20) Sylva, S. P.; Ball, L.; Nelson, R. K.; Reddy, C. M. Compound specific 81Br/79Br analysis by capillary gas chromatography/multi-
KIE. The lack of MIE may imply that the reaction from the singlet state proceeds through an ionic mechanism. - The reaction of 3-BP in aqueous solution is pretty fast and accompanied by an increased inverse Br isotope effect without any carbon isotope fractionation. We hypothesize that in this case the bromine isotope fractionation is caused entirely by magnetic isotope effect associated with triplet-singlet conversion, whereas C−Br bond cleavage is not a rate-limiting step and is not accompanied by a significant mass-dependent kinetic isotope effect. This scenario may agree with an ionic mechanism for this reaction. However, it is obvious, that further experiments and theoretical calculations are necessary to prove the above-mentioned assumptions. Environmental Significance. In the present study, the carbon and bromine isotope fractionation factors during UV photodegradation of bromophenols were determined for the first time. Considering that debromination of brominated phenols by UV irradiation provides a potential method for treatment of contaminated water, carbon and bromine isotope fractionation factors can be used for quantification of the degradation without a need in products identification. The inverse Br isotope effect observed in the present study is opposite, and of a significantly higher value, than the earlier reported for the microbial reductive debromination (εBr = −0.8‰).23 Taking into account that both of these processes may affect bromophenols on the subsurface, the opposite trends in Br isotope fractionation may be used to distinguish between microbial and photochemical degradation in contaminated sites. Moreover, this study demonstrates the power of dual carbon−bromine isotope analysis for elucidation of mechanistic pathways of debromination. Undoubtedly, more work is required to make Br isotope analysis accessible to a wide range of contaminants and to understand the origins of Br isotope effects of debromination pathways.
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ASSOCIATED CONTENT
S Supporting Information *
Spectral distribution of the mercury lamp (Figure 1S) and estimation of maximal carbon and bromine KIEs by Streitwieser Limit Equation are provided. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(F.G.) Phone: +972-2-5314208; fax: +972-2-5314332; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by BMBF-MOST grant for cooperation in Water Technology Research, Ministry of Science and Technology of the State of Israel and FZKForschungszentrum Karlsruhe grant Number WT1101. We thank three anonymous reviewers of this paper for their useful comments.
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