School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, SEC 710, Norman, Oklahoma 73019, United States.
Environ Sci Technol. 2012 Jul 17;46(14):7495-502. doi: 10.1021/es300850x. Epub 2012 Jul 6.
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 Na(2)S (<0.7 mM) and mackinawite (FeS) (180 mM). Degradation rates in vials containing FeS exceeded those in Na(2)S-only controls. In the presence of FeS, first-order constants ranged from 0.034 ± 0.002 d(-1) at pH 6 to 0.081 ± 0.005 d(-1) at pH 8.5. In the presence of FeS, products from reductive debromination (ethylene) and from S(N)2 substitution with S(II) nucleophiles were detected (1,2-dithioethane, DTA). Relatively high yields of DTA suggested that the S(N)2 reactions were not mediated by HS(-) only but likely also included reactions mediated by FeS surface. Significant carbon isotope effects were observed for nucleophilic substitution by HS(-) (ε = -31.6 ± 3.7‰) and for a combination of reductive and substitution pathways in the presence of FeS (-30.9 ± 0.7‰), indicating good site assessment potential of CSIA. The isotope effects (KIEs) observed in the presence of FeS corroborated the predominance of S(N)2 substitution by nucleophiles combined with two-electron transfer reductive debromination.
EDB（1,2-二溴乙烷）经常在受含铅汽油影响的地点被检测到。在还原环境中，EDB 极易受到非生物降解。本研究旨在评估化合物特异性同位素分析（CSIA）在评估硫酸盐还原环境中 EDB 的非生物降解中的潜力。将含有 EDB 的水在密封小瓶中与各种组合的 Na(2)S（<0.7 mM）和 mackinawite（FeS）（180 mM）孵育。含 FeS 的小瓶中的降解速率超过仅含 Na(2)S 的对照。在 FeS 的存在下，一级常数范围从 pH 6 时的 0.034 ± 0.002 d(-1)到 pH 8.5 时的 0.081 ± 0.005 d(-1)。在 FeS 的存在下，检测到还原脱溴（乙烯）和 S(II)亲核试剂的 S(N)2 取代产物（1,2-二硫代乙烷，DTA）。DTA 的相对高产量表明，S(N)2 反应不仅由 HS(-)介导，而且可能还包括由 FeS 表面介导的反应。观察到 HS(-)亲核取代的显著碳同位素效应（ε = -31.6 ± 3.7‰）和 FeS 存在下还原和取代途径的组合的同位素效应（-30.9 ± 0.7‰），表明 CSIA 具有良好的位点评估潜力。在 FeS 存在下观察到的同位素效应（KIE）证实了 S(N)2 取代与二电子转移还原脱溴的亲核试剂的优势。
