Isotopic Fractionation of Sn during Methylation and Demethylation

May 15, 2009 - Environmental Science & Technology .... Academy of Sciences, Apatity, Fersman 14, 184209, Russia ... Russian Academy of Sciences...
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Environ. Sci. Technol. 2009, 43, 4399–4404

Isotopic Fractionation of Sn during Methylation and Demethylation Reactions in Aqueous Solution D . M A L I N O V S K I Y , * ,†,‡ L . M O E N S , † A N D F. VANHAECKE† Department of Analytical Chemistry, Ghent University, Krijgslaan 281-S12, B-9000, Ghent, Belgium, and Institute of Ecology, KSC, Russian Academy of Sciences, Apatity, Fersman 14, 184209, Russia

Received January 14, 2009. Revised manuscript received April 21, 2009. Accepted April 28, 2009.

Laboratory experiments, modeling the methylation of inorganic Sn(II) by methylcobalamin and the decomposition of methyltin under irradiation with UV light in aqueous solution, have been performed. Methyltin has been separated from inorganic Sn using anion-exchange chromatography and subjected to Sn isotope ratio measurements via solution nebulization multicollector inductively coupled plasma mass spectrometry (MCICPMS). The methylation of Sn(II) in the dark was accompanied by mass-dependent Sn isotopic fractionation, which resulted in preferential partitioning of the lighter Sn isotopes into the organic phase, with a shift of ∼0.57 ( 0.12‰ in terms of δ124/ 116 Sn between methylated and inorganic Sn. The methylation of Sn(II) by methylcobalamin under UV irradiation resulted in the accelerated formation of methyltin in the beginning of the process, but was followed by the photolytic degradation of methyltin until its complete mineralization. The photolytic degradation of methyltin in the presence of methylcobalamin and inorganic Sn(II) was slower than that of pure solutions of commercially obtained monomethyltin. This is attributed to the methylating action of methyl radicals produced from photolytically decomposing methylcobalamin. Both synthesis and decomposition of methyltin under UV irradiation were accompanied by both mass-dependent and mass-independent Sn isotopic fractionation, with the latter due to the magnetic isotope effect. As a result of this, the lighter Sn isotopes preferentially partition into reaction products, while the odd isotopes, 117Sn and 119Sn, are selectively enriched relative to the other isotopes in the starting molecules. The extent of the observed variations in the isotopic composition of Sn is larger thanthatdocumentedpreviouslyforgeologicalandarcheological samples. These results indicate that Sn isotopic fractionation between various chemical forms of Sn in the natural aquatic systems may be significant and can provide new insights into the biogeochemical cycling of the element.

Introduction Organotin compounds have been used for decades in a variety of industrial applications, including timber preservatives, plastic stabilizers, pesticides, fungicides, and antifouling * Corresponding author e-mail: [email protected]; phone: +32-9-2644830; fax: +32-9-2644960. † Ghent University. ‡ Russian Academy of Sciences. 10.1021/es9000856 CCC: $40.75

Published on Web 05/15/2009

 2009 American Chemical Society

paints. Due to their widespread use, considerable amounts of organotin compounds entered various ecosystems. The documented toxicity of organotin compounds for aquatic organisms and mammals has caused increasing concern about their distribution and fate in the environment. Reviews by Hamasaki et al. (1995), Abalos et al. (1997), Hoch (2001), Suzuki (2003), among others, have discussed developments in the understanding of the biogeochemical cycling of Sn, the toxicity of the different chemical forms in which the element occurs, and analytical methods for their determination (1-4). It has been shown that the man-made butyltin and phenyltin compounds are eventually degraded into inorganic tin, while inorganic tin is methylated in the environment (5-9). Many studies have suggested that environmental methylation is the major source of methyltin compounds (1, 3, 10). In this process, the most important step is the formation of monomethyltin, where the first addition of a methyl (CH3) group takes place (1). Further oxidation-reduction reactions involving methyl donors may produce di-, tri-, and tetramethyltin compounds. Possible reactions that can account for environmental methylation of inorganic Sn, involving such methyl donors as methylcobalamin, methyl iodide, S-adenosylmethionine, and N5methyltetrahydrosylmethionine, have been studied under laboratory conditions (11-20). These studies show that the reaction pathways during methylation of Sn are different, depending on the methyl donor, pH, salinity, and other environmental parameters. Although significant progress was made in developing sensitive analytical methods for the determination of various chemical forms of Sn, uncertainty still exists about the mechanisms of methyltin formation in the environment. For a number of heavy stable elements, including Mg, Ca, Cr, Fe, Mo, Zn, Cu, Se, Cd, Hg, Tl, their participation in some physicochemical processes was shown to be accompanied by isotopic fractionation and thus, isotopic analysis was shown to be a very useful tool for studying the relevant process (21, 22). In the context of environmental studies, it was shown that variations in the isotopic composition of these elements can shed light on their origin in studied systems, migration pathways, and serve as proxies of physicochemical conditions that existed in the past. Tin is the element with the highest number of naturally occurring stable isotopessten. Tin isotopes include 112Sn, 114 Sn, 115Sn, 116Sn, 117Sn, 118Sn, 119Sn, 120Sn, 122Sn, and 124Sn, with relative abundances ranging from 0.34% to 32.58% (Table S1, Supporting Information). Published data for a limited number of archeological and geological samples have demonstrated mass-dependent variations in the isotopic composition of Sn up to 0.15‰ per atomic mass unit (23, 24). Mass-independent isotopic fractionation of Sn was observed during liquid-liquid extraction of the element using a crown ether (25). Pronounced mass-independent Sn isotopic fractionation was reported during photolysis of organotin compounds (26, 27). However, the latter observation was made by using nuclear magnetic resonance (NMR) spectroscopy, which enables qualitative assessment of isotope effects only. In this work, we have made an attempt to monitor and quantify Sn isotopic fractionation due to (i) the methylation of inorganic tin by methylcobalamin and (ii) the photolytically driven decomposition of monomethyltin. Although the concentration of Sn used in the experiments was much higher than that in natural waters, this study under controlled laboratory conditions allows us to determine the extent of variations in the isotopic composition of Sn more precisely VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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and make inferences on the mechanisms of the isotopic fractionation.

Experimental Section Experimental Design. The methylation experiments were performed in 10-ml polypropylene centrifuge vials (with caps) under ambient atmosphere. In general, the sequence of operations was as follows. Five ml of 0.5 M HCl was added into the reaction vial, followed by addition of an amount of methylcobalamin (CH3B12) giving rise to a final concentration in the reaction mixture of ∼0.02 mM. Then, 0.1 mL of 1000 µg mL-1 Sn(II) stock solution was pipetted into the vial. The final volume of the reaction mixture was ∼5.2 mL with Sn(II) concentration of ∼0.16 mM. The Sn(II) stock solution used in the experiments was prepared from SnCl2 · H2O stannous chloride (99%). More details for the reagents used in the experiments can be found in the Supporting Information. The pH of the experimental solutions was adjusted by dropwise addition of NH3. Ammonia was intentionally chosen as the pH-adjusting substance instead of NaOH or other alkaline salts to avoid potential matrix effects during MCICPMS measurements. The reaction vial was capped, covered with aluminum foil, and kept at room temperature (20 ( 1 °C) during time intervals ranging from 3 to 15 days. A separate set of experiments was performed to assess the effect of UVirradiation on the methylation of tin. These experiments included illumination of the solutions prepared as described above with UV light (wavelength ∼260 nm) from a 30-W low-pressure mercury vapor lamp (Philips, Holland). At the end of each experiment, the pH of the solution was determined using a pH meter. The sample was transferred from the reaction vial into a 10 mL polypropylene tube. A 0.4 mL portion of 10.5 M HCl was added to reach HCl concentration of ∼1.05 M. The photolytic demethylation experiments include irradiation of solutions prepared from commercially available monomethyltin with UV light. Monomethyltin trichloride (97%) standard was used in these experiments at a concentration of ∼10 µg mL-1 of Sn. The influence of the duration and the pH of the solution were also assessed in the demethylation experiments. Ion-Exchange Separation of Methylated Tin. Five-mL plastic pipet tips were fitted with plugs of quartz wool at their outlets and filled with AG MP-1M, 100-200 mesh size anion-exchange resin to produce ∼2.5 cm long × 0.5 cm i.d. chromatographic columns. The columns were cleaned with 10 mL of 4 M HNO3 and regenerated to chloride form with 8 mL of 1.05 M HCl. The samples were loaded onto the columns in 1.05 M HCl. Negatively charged inorganic Sn species were efficiently retained on the column in its anionexchange form, whereas positively charged methylated Sn passed through. The solution passed through the chromatographic column is ready for Sn isotope ratio measurements by MC-ICPMS and requires only proper dilution. More details on performance of ion-exchange chromatography for separation of methyltin can be found in the Supporting Information. Mass Spectrometry and Data Processing. Prior to Sn isotope ratio measurements, the concentration of Sn was determined by means of quadrupole-based ICP-MS using an Elan DRCplus instrument (Perkin-Elmer Sciex, Thornhill ON, Canada), operated in vented mode. Then the samples were appropriately diluted with 0.5 M HCl to match Sn concentrations in the standards within 20% and spiked with Sb for instrumental mass discrimination correction at 1:1 of the Sn concentration. Sn isotope ratios were measured by multicollector ICP-MS (MC-ICPMS; Neptune, ThermoScientific, Germany) following an approach modified from Clayton et al. (24). Detailed information on MC-ICPMS measurements is given in the Supporting Information. 4400

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The online data processing included calculation of the raw ratios and removal of outliers by means of a 2σ-test. Further calculations were performed off-line. Results for the isotopic analyses are expressed in the δ-notation, as defined by the relationship δX/116Sn )

[

(XSn/ 116Sn)sample (XSn/ 116Sn)standard

]

- 1 · 1000‰

(1)

where XSn is 117Sn, 118Sn, 119Sn, 120Sn, 122Sn, or 124Sn in the measured ratios for sample and in-house standard, corrected for instrumental mass discrimination using the 123/121Sb ratio. “Capital delta” notation was also used to describe the observed mass-independent fractionation of Sn isotopes as defined in the Supporting Information.

Results and Discussion Mass-Dependent Fractionation during Methylation of Sn in the Dark. Figure 1 provides a summary of the concentration and isotopic data in terms of δ124/116Sn obtained for methyltin formed from inorganic Sn in the dark. All concentration and isotopic data for these experiments are shown in Table S3 in the Supporting Information. It is worth pointing out that the inorganic Sn(II) used in the experiments is isotopically indistinguishable from the in-house Sn isotope standard solution within the limits of instrumental precision. The latter varies from 0.01-0.03‰ for the 117/116Sn ratio to 0.01-0.07‰ for the 124/116Sn ratio at one standard deviation level. As seen from Figure 1, the isotopic composition of methylated Sn is lighter than that of the inorganic source. This means that lighter Sn isotopes preferably partition into the organic phase during the synthesis of methyltin. The shift in the isotopic composition between methylated and inorganic species of Sn is substantially larger than the standard deviation of the measurements. Data obtained from the time series experiments show minor variations in the fraction of methyltin and its isotopic composition only (Figure 1), suggesting that a period of three days is sufficient for a steady state to establish between the methylated and inorganic species of tin. The average value for the fraction of methyltin formed in the time series experiments is 0.022 ( 0.003 with the average isotopic composition in terms of δ124/116Sn of -0.57 ( 0.12‰. There is an inverse relationship between the amount of methyltin produced by methylcobalamin and the pH of the solution (Figure 1D). The fraction of tin methylated at a pH of 3 is ∼2.3 times larger than that at a pH of 6.2. Previous studies suggested that under environmentally relevant conditions, methylation of Sn can occur by addition of a carbonium ion (CH3+), a carboanion (CH3-), or by methyl radical transfer (CH3•) and that the relationship between the pH and the yield of methyltin appeared to be linked with the pH-dependency of the equilibrium states of both inorganic Sn(II) and the methyl donor (1, 11, 17). These studies also provided evidence that monomethyltin was by far the dominant species produced from inorganic Sn(II) by methylcobalamin, whereas di-, tri-, and tetramethyltin were either not detected at all, or present at a near blank level only. An interesting observation is that although variable fractions of Sn were methylated at different pH values, the isotopic composition of Sn in these fractions remains nearly invariant within the limits of instrumental precision (Figure 1C). Thus, the isotopic fractionation between Sn(II) species coexisting in solution appears to be of minor importance. The observed Sn isotopic fractionation during methylation of Sn can be explained by a purely kinetic mechanism. According to this mechanism, lighter and, consequently, more mobile Sn isotopes are favored during synthesis of methyltin.

FIGURE 1. δ124/116Sn values and fraction of methyltin produced due to methylation of inorganic Sn(II) by methylcobalamin in the time series experiments (A and B) and in the pH series experiments (C and D). Uncertainty bars are one standard deviation. See text for more details. Kinetic isotopic fractionation can accompany the reaction of the methyl group with the Sn(II) atom or the oxidation of the CH3Sn(III)• reaction intermediate to CH3Sn(IV) by an oxidant such as oxygen (11). Alternatively, equilibrium isotopic fractionation between inorganic Sn(II) and Sn(IV) species coexisting in solution can also account for the experimental data. This mechanism includes equilibrium isotopic fractionation between Sn(II) and Sn(IV) species in solution, followed by addition of a methyl group to Sn(II) atoms directly or via an CH3Sn(III)• intermediate. In the presence of oxygen, the monomethyltin(II) or CH3Sn(III)• intermediate are quickly oxidized to form monomethyltin(IV). The hypothesis of equilibrium Sn isotopic fractionation during the methylation process is supported by findings from a recent study by Polyakov et al. (28), who calculated equilibrium fractionation between inorganic Sn(II) and Sn(IV) as large as 0.6‰ per atomic mass unit based on spectroscopic data. To distinguish which mechanism of Sn isotopic fractionation operates during the methylation in the dark, the isotope analysis of methyltin formed during first hours of the process is necessary. However, this analysis is difficult due to low concentration of methyltin formed over a short period of time. A significant increase in concentration of parent inorganic tin over that used in the experiment can result in precipitation of hydrolyzed Sn ions. Precipitation of tin may bias the isotope data and limits the use of elevated concentration of inorganic tin in the experiments. Mass-Independent Fractionation during Methylation of Inorganic Sn under Irradiation with UV Light. Figure 2 and Table S4 show concentration and isotopic data on methyltin produced in the experiments under UV light. UV irradiation increased the rate of methyltin formation at the beginning of the process. However, by plotting the concentrations of methyltin against time of UV irradiation, it becomes clear that after reaching its maximum concentration, the methyltin is subject to photolytic degradation, which finally results in complete mineralization of the organic phase (Figure 2). The accelerated formation of methyltin under UV irradiation is

FIGURE 2. (A) Fraction of methyltin produced due to methylation of inorganic Sn(II) by methylcobalamin under irradiation with UV light. (B) δ117/116Sn and δ124/116Sn values of methyltin plotted as a function of time of UV irradiation. The uncertainty bars for the measurement data are smaller than the data points. presumably explained by the presence of highly reactive methyl radicals (CH3•), formed as a result of the photolytic decomposition of methylcobalamin. Previous studies demonstrated that the methyl radical is generated by the homolytic cleavage of the Co-C bonds, photoexcited by UV light (29-31). The concentration data shown in Figure 2 VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Sn isotopic data and fraction of methyltin remaining in solution in the experiments on photolytic demethylation at a pH of 5.6 (A and B) and at a pH of 7.8 (C and D). The uncertainty bars for the measurement data are smaller than the data points. Solid and dotted lines have been plotted according to the Rayleigh equation (see text, eq 2) for δ124/116Sn and δ117/116Sn values, respectively. The kinetic isotopic fractionation factors, r, used in construction of the Rayleigh curves have been estimated from the best fit of the curves to the experimental isotopic data. This yielded r equal to 0.99866 for δ117/116Sn and 0.99859 for δ124/116Sn values at a pH of 5.6 (B), and r equal to 0.99975 for δ117/116Sn and 0.99858 for δ124/116Sn values at a pH of 7.8 (D). suggest that presumably the excess of methyl radicals in solution effectively increases the rate of methyltin formation. When a major part of methylcobalamin is decomposed, the photolytic degradation of methyltin dominates during further UV irradiation. Apart from formation of methyl radicals in solution the UV irradiation may result in increased oxidation of Sn(II) to Sn(IV). This, however, remains unresolved. As seen from Figure 2, the accelerated formation of methyltin under UV light is accompanied by enrichment of the lighter Sn isotopes in methyltin. The subsequent degradation of methyltin also favors preferential release of lighter Sn isotopes from the organic phase. A plausible explanation for the observed distribution of the Sn isotopes is a kinetic isotope effect, operating during both synthesis and decomposition of methyltin. An important observation is that at acidic and semineutral pH, the methyltin is selectively depleted in 117Sn and 119Sn relative to the other isotopes owing to mass-independent isotopic fractionation. In contrast, the isotopic composition of Sn in methyltin formed at a pH range of 7.9-8.7 is governed by mass-dependent fractionation only (Table S4). Buchachenko et al. (26, 27) demonstrated that photolysis of organotin compounds is a spin-selective radical reaction, accompanied by mass-independent isotopic fractionation. The magnetic isotope effect was shown to be the cause of mass-independent Sn isotopic fractionation during this process. This magnetic isotope effect separates isotopes by spin and magnetic momentum due to the difference in rates of spin conversion between triplet and singlet states of excited molecules for odd and even isotopes. As seen from Table S1, only 117Sn and 119Sn from isotopes of Sn measured in this 4402

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study have a nonzero nuclear spin and nuclear magnetic moments. Therefore, reaction rates of radical intermediates of magnetic 117Sn and 119Sn are much higher than those of nonmagnetic Sn isotopes. For molecules with magnetic nuclei, the radical reactions involving spin conversion may be reversible and result in regeneration of the starting molecules, selectively enriched in magnetic isotopes (26). Data obtained in our experiments are in line with the latter situation, showing that the starting molecules, i.e., the inorganic tin, are selectively enriched in 117Sn and 119Sn. The observation of the sole occurrence of a massdependent isotope effect at alkaline pH is presumably accounted for by the action of the OH- anion as the methyl radical scavenger. It is known that the OH- anion can inhibit the formation of free radicals in aqueous solution (31-33). The fact that smaller fractions of methyltin are formed at alkaline pH as compared to those at acidic to semineutral pH over the equal period of time is consistent with the conclusion that the methyl radical is efficiently scavenged and thus, not able to accelerate the rate of methyltin formation (Table S4). We believe that an influence of changed Sn speciation as compared to the acidified conditions on the reactivity of Sn toward radical pair formation is insignificant. Mass-Independent Fractionation during Photolytic Decomposition of Methyltin. To quantify the fractionation of Sn isotopes during the photolytic degradation of methyltin in solution, commercially available monomethyltin was used in the experiments. All data obtained in these experiments are shown in Table S5. The fraction of methyltin plotted versus time of UV irradiation shows that a slower rate of decomposition is observed in the solution with a pH of 7.8

as compared to that at pH of 5.6 (Figure 3). Previous studies suggested that photolytic decomposition of methyltin takes place via radical reaction which can include CH3• or the hydroxyl radical (OH•) (31). Inhibition of free radical formation by the OH- anion appears to be the cause of the slower rate of methyltin degradation observed at alkaline pH. It is worth noting that the commercial monomethyltin used in the experiments is isotopically lighter by -0.97 ( 0.02‰ in terms of δ124/116Sn than our in-house inorganic Sn standard solution. The photolytic demethylation is concomitant with Sn isotopic fractionation that results in preferential release of light isotopes. Figure 3 shows that the isotopic composition of methyltin in terms of δ117/116Sn and δ124/116Sn values plotted against fraction of methyltin remaining in solution is closely approximated by the classical Rayleigh distillation equation. The latter in δ‰ notation takes the form (34): (R - 1)ln f ) ln

10-3δs + 1 10-3δs,0 + 1

(2)

where R is the kinetic isotopic fractionation factor, considered as a constant during the progress of the reaction; in our study, f is the fraction of methyltin remaining in solution; δs0 and δs are isotopic delta values for methyltin at the beginning of the process and at a time of sampling, respectively. The Rayleigh curves shown in Figure 3 were constructed using eq 2 and the experimentally determined concentration data. The Rayleigh-type fractionation of Sn in methyltin suggests a kinetic isotope effect occurring during photolysis of methyltin. An important observation is that the variations in the isotopic composition of Sn during photolytic demethylation are mass-independent at a pH of 5.8 and mass-dependent at a pH of 7.8. Figure 4 shows that the deviation from massdependent scaling between Sn isotopes in the former case is due to selective enrichment of 117Sn and 119Sn relative to the other isotopes in methyltin. The magnetic isotope effect described earlier appears to be the explanation for this distribution of 117Sn and 119Sn. In line with the isotopic data from the experiments on the methylation of Sn(II) under UV irradiation, Sn isotopic fractionation during photolytic demethylation at alkaline pH is mass-dependent only. We suggest that effective inhibition of free radical reactions by the OHanion at alkaline pH can account for these observations. Implications of the Results for Natural Aquatic Systems and Future Studies. Data obtained in this study indicate that the isotopic composition of tin can be changed as a result of fractionation during chemical methylation and photolytic demethylation. The extent of variation in the isotopic composition of Sn in these processes is higher than that previously observed for geological and archeological samples (23, 24). It is known that methyltin can be readily adsorbed by the mineral phase and, due to its better lipophilicity than inorganic Sn compounds, it passes biological membranes, which results in accumulation in aquatic biota (35). The isotopic signal of methyltin can therefore be preserved for longer time by sediments or biota, including fish. Recent studies showed mass-independent fractionation of Hg isotopes in fish and sediments (36-38). It was also demonstrated that photochemical reduction of aqueous methylmercury species resulted in mass-independent Hg isotope fractionation (35). These isotope data provide valuable information about the biogeochemical cycling of Hg. We suggest that by analogy with these and other studies on fractionation of traditional and nontraditional isotopes, the isotopic signature of Sn can be used in identifying sources, pathways, and bioaccumulation of Sn in the environment. Mass-independent Sn isotopic fractionation observed during both the synthesis and the decomposition of methyltin

FIGURE 4. Sn isotopic composition of commercial methyltin during its photolytic decomposition, with (A) ∆117/116Sn plotted against δ124/116Sn; (B) ∆119/116Sn plotted against δ124/116Sn; (C) ∆119/116Sn versus ∆117/116Sn. It can be seen here that 117Sn and 119 Sn isotopes display mass-independent fractionation in the experiments at a pH of 5.6, whereas their fractionation is mass-dependent in the experiments at a pH of 7.8. The slope obtained for the plot of ∆119/116Sn versus ∆117/116Sn is 0.970 ( 0.015 at one standard deviation level. The uncertainty bars for the measurement data are smaller than the data points. under UV irradiation represents a unique isotope signature which reflects the specific mechanism of the reaction, i.e., a spin-selective radical reaction. Intense solar radiation can produce free radicals in natural surface waters, which may lead to mass-independent isotopic fractionation during methylation and/or demethylation of tin. If preserved in sediment or in aquatic biota, the mass-independent isotopic signature would indicate site-specific conditions of water chemistry at the time of its formation. VOL. 43, NO. 12, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The results of this study also show the importance of substances which can act as free radical scavengers in the aquatic systems for Sn isotopic fractionation during photolytic degradation of organotin. The action of the OH- anion as a scavenger of free radicals was suggested by us to be the cause of the absence of mass-independent Sn isotopic fractionation during photolysis of methyltin at alkaline pH. In contrast, the same process at slightly acidic pH is accompanied by mass-independent Sn isotopic fractionation. Due to the low concentration of dissolved species of Sn in natural waters, the use of an in situ preconcentration device will be necessary for sampling of these species for isotope ratio analysis. Evaluation of the performance of various types of in situ preconcentration techniques for studies on Sn isotope systematics in the environment will hence be required in the near future.

Acknowledgments This research was supported by postdoctoral fellowship grants for D.M. from the Swedish Research Council and the Natural Sciences and Engineering Research Council of Canada. F.V. acknowledges the Flemish Research Foundation (FWO-Vlaanderen) for financial support under the form of the research project G.0669.06. We are grateful to three anonymous reviewers for their comments on the manuscript.

Supporting Information Available Additional text, concentration and isotopic data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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