Carbon Isotope Fractionation during Anaerobic Degradation of Methyl tert -Butyl Ether under Sulfate-Reducing and Methanogenic Conditions

Abstract

Methyl tert-butyl ether (MTBE), an octane enhancer and a fuel oxygenate in reformulated gasoline, has received increasing public attention after it was detected as a major contaminant of water resources. Although several techniques have been developed to remediate MTBE-contaminated sites, the fate of MTBE is mainly dependent upon natural degradation processes. Compound-specific stable isotope analysis has been proposed as a tool to distinguish the loss of MTBE due to biodegradation from other physical processes. Although MTBE is highly recalcitrant, anaerobic degradation has been demonstrated under different anoxic conditions and may be an important process. To accurately assess in situ MTBE degradation through carbon isotope analysis, carbon isotope fractionation during MTBE degradation by different cultures under different electron-accepting conditions needs to be investigated. In this study, carbon isotope fractionation during MTBE degradation under sulfate-reducing and methanogenic conditions was studied in anaerobic cultures enriched from two different sediments. Significant enrichment of 13 C in residual MTBE during anaerobic biotransformation was observed under both sulfate-reducing and methanogenic conditions. The isotopic enrichment factors () estimated for each enrichment were almost identical (؊13.4‰ to ؊14.6‰; r 2 ‫؍‬ 0.89 to 0.99). A value of ؊14.4‰ ؎ 0.7‰ was obtained from regression analysis (r 2 ‫؍‬ 0.97, n ‫؍‬ 55, 95% confidence interval), when all data from our MTBE-transforming anaerobic cultures were combined. The similar magnitude of carbon isotope fractionation in all enrichments regardless of culture or electron-accepting condition suggests that the terminal electron-accepting process may not significantly affect carbon isotope fractionation during anaerobic MTBE degradation.Methyl tert-butyl ether (MTBE) has been used extensively as an octane enhancer and later as a fuel oxygenate in reformulated gasoline, as required by the Clean Air Act Amendments of 1990. The widespread use of MTBE to reduce air pollution has, however, resulted in major contamination of water resources (3,11,22,(28)(29)(30). There has been increasing interest in the development of effective technologies to remediate MTBE-contaminated sites, for instance, pump-and-treat techniques, biostimulation, and bioaugmentation (7,9,15,31,33). The full-scale implementation of these techniques is considerably costly and is currently economically applicable for only a few sites with highly sensitive receptors. Therefore, the fate of MTBE in the environment is mainly dependent upon natural remediation processes.Monitored natural attenuation is being increasingly implemented for a number of environmental pollutants. Natural processes, including dispersion, sorption, dilution, volatilization, and biodegradation, control plume migration and reduce MTBE concentration. Among these processes, biodegradation is the most effective in reducing the mass of contaminant in the environment in a sustainable way. One challenge, however, is to accur...

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