Article pubs.acs.org/est
Carbon Isotope Fractionation in Reactions of 1,2-Dibromoethane with FeS and Hydrogen Sulfide Tomasz Kuder,*,† John T. Wilson,‡ Paul Philp,† and Y. Thomas He§ †
School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, SEC 710, Norman, Oklahoma 73019, United States National Risk Management Research Laboratory, Ground Water and Ecosystems Restoration Division, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, Oklahoma 74820, United States § R.S. Kerr Environmental Research Laboratory, National Research Council, 919 Kerr Research Drive, Ada, Oklahoma 74820, United States ‡
S Supporting Information *
ABSTRACT: EDB (1,2-dibromoethane) is frequently detected at sites impacted by leaded gasoline. In reducing environments, EDB is highly susceptible to abiotic degradation. A study was conducted to evaluate the potential of compound-specific isotope analysis (CSIA) in assessing abiotic degradation of EDB in sulfate-reducing environments. Water containing EDB was incubated in sealed vials with various combinations of Na2S (100 mM occur in sediments where biodegradable organic matter is abundant.55,56 As an example, if the concentration of FeS were 50 mM, the half-life of the reductive dehalogenation element of EDB degradation, defined by kprod‑ethe, would be 416 days at pH 6 and 278 days at pH 8.5. Even higher reaction rates can be extrapolated from the microcosm data shown in the Supporting Information. The reaction rates presented herein are directly applicable to EDB in groundwater at the temperature similar to that in the present study. At most sites, EDB resides at lower temperature, and the expected reaction rates would be lower. Kinetic data showing temperature effect for nucleophilic reaction with HS− are available elsewhere.6 Hydrogen sulfide at sulfate reducing sites often occurs at 0.03 to 0.3 mM,57 but concentrations as high as 19−1010 mM has been reported elsewhere.37 For the lower range of the observed S(II) concentrations, the combined yields from reactions with HS−, Sn, and/or FeS-bound nucleophiles can exceed those from reductive dehalogenation. While reduction converts EDB to nontoxic ethylene, the nucleophilic products may be of environmental concern. Based on pH-Eh stability fields of dissolved sulfur species, the dominant sulfur nucleophiles in groundwater at low and neutral pH are H2S and HS−.58 This would lead to the expectation that there will be limited potential for the substitution reactions. This study suggests that the role of substitution reactions is
two identified reaction pathways, the lack of variability among different treatments can be extrapolated to any combination of yields from reductive debromination and SN2 substitution (with HS− and/or polysulfides). Further insight into the reaction mechanisms can be gained through the estimation of kinetic isotope effects (KIE), according to eq 4.39 KIE is defined as k12C/k13C (see definition of ε in eq 3) at the reacting atom,39 as opposed to “bulk” ε measured by CSIA for whole molecules. KIE is obtained from ε (εbulk) by correcting for nonreactive atoms and/or for intramolecular competition. In eq 4, n = 2 is the number of carbon atoms in the EDB molecule, x = 2 is the number of reactive carbon atoms, and z is the number of reactive equivalent atoms (z = 1 or 2, depending on the reaction) KIE = 1/(1 + 10−3 × εbulk × z × n/x)
(4)
Determination of KIE requires a reaction scenario with values of x and z. In an experiment where the reaction pathways are problematic, calculation of KIEs permits testing the alternative scenarios, by identifying those where an empirical εbulk can be successfully converted into a meaningful KIE. The KIEs for alternative scenarios were calculated for the approximation of εbulk = −31‰. For stepwise reactions, z = 2 because the reactions involve only one of the carbon atoms (e.g., Figure 3, Reactions 1 and 9) and the value of KIE = 1.07. Nucleophilic SN2 reactions mediated by HS−, Sn and/or the hypothetical nucleophilic substitution by an FeS-bound S(II) species represent the stepwise scenario (Figure 3, Reaction 9). KIEs in miscellaneous SN2 reactions range from 1.05 to 1.09.39 In biological SN2 hydrolysis of the chlorinated analogue DCA, AKIE was ∼1.05.40,41 KIE = 1.07 fits well that range. The slightly lesser isotope effect in DCA is consistent with other studies where AKIEs of brominated analogues were larger by ∼0.01−0.02 than those of chlorinated analogues.42−45 For concerted pathways such as Reaction 3 in Figure 3, z = 1 because the reactions involve both carbon atoms at the same time and the value of KIE = 1.03. KIEs in reductive dechlorination of chlorinated ethenes and ethanes by reduced Fe(II) minerals and by zerovalent metals ranged between 1.02 and 1.04, including a KIE of 1.03 in a reaction of DCA with Zn(0).23,39,46,47 On the other hand, based on the isotope effects, the stepwise electron transfer pathway (Reactions 1 and 2, Figure 3) is unlikely, since its KIE would exceed the range observed in miscellaneous studies of reductive dehalogenation. This conclusion is consistent with the two-electron transfer mechanism of vicinal bromide reduction discussed elsewhere.11 The evidence against stepwise reactions other than SN2 is informative in limiting the options for explaining the low mass balance of the reaction products and the excess of DTA at low pH. Alternative reactions of the radical intermediate from a single electron transfer (Figure 3, Reaction 1) to form sulfide products are not likely to be significant. KIEs can be in part masked by nonfractionating steps preceding chemical bond cleavage.48 Apparent KIE (AKIE) applies in such situation. In abiotic systems, expression of KIEs might be restricted by physical barriers in reactant availability or by rate-limiting elements of the degradation sequence preceding bond cleavage (e.g., slow adsorption). In the present study, the observed isotope effects were near the theoretical maximum, and the overall rates were clearly controlled by the rates of EDB bond cleavage. 7500
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larger than might be anticipated, including low pH environments. In the presence of FeS in the aquifer sediments, reactions mediated by FeS-bound S(II) and/or Sn can exhibit significant rates in neutral and weakly acidic groundwater. In one available study of the fate of brominated hydrocarbons in contaminated groundwater, the degradation products from polysulfide-mediated reactions were identified.3 Monitoring for DTA and for cyclic alkyl sulfides may be informative at those sites where EDB or other halocarbons prone to nucleophilic reactions are present.
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ASSOCIATED CONTENT
S Supporting Information *
1) Analytical methods: analysis of organosulfur compounds and 2) results from incubations conducted under standard nitrogen−oxygen atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 405-325-3253. Fax: 405-325-3140. E-mail: tkuder@ou. edu. Corresponding author address: School of Geology and Geophysics, University of Oklahoma, 100 E Boyd, room 710, Norman, OK 73019, USA. Notes
This paper has not been subjected to the U.S. Environmental Protection Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The U.S. Environmental Protection Agency through its Office of Research and Development partially funded the research described here. Portions of this work were supported through IAG # RW57939929 “Identification of Mineral Substances that Enhance Natural Non-Biological Attenuation of Chlorinated Organic Contaminants in Ground Water” between U.S. EPA and the U.S. Air Force Center for Engineering and the Environment. We gratefully acknowledge the analytical support provided to the U.S. EPA by Shaw Environmental (contract #68-C-03-097).
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