Anaerobic biotransformation of methyl tert-butyl ether (MTBE) and related fuel oxygenates under different anoxic conditions

The fuel oxygenate methyl tert-butyl ether (MTBE) has been frequently detected in groundwater and surface water. Since contaminated sites are often subsurface, anaerobic degradation of MTBE will likely be significant for remediation. In this study, biodegradability of the fuel oxygenates MTBE, tert-amyl methyl ether (TAME), and ethyl tert-butyl ether (ETBE) was evaluated under different anoxic conditions. Anaerobic degradation of fuel oxygenates was observed in sediment microcosms from four different locations. After extended incubation and subculturing, stable microbial cultures that utilize MTBE and/or TAME under methanogenic and sulfate-reducing conditions were enriched. All cultures utilized both MTBE and TAME, regardless of their original substrate. No biotic loss of ETBE has been observed. Stoichiometric amounts of tert-butyl alcohol (TBA) and tert-amyl alcohol (TAA) accumulated, indicating that cleavage of the methyl group was the initial step in MTBE and TAME biotransformation. The inhibitory effect of alternative organic compounds on anaerobic degradation of MTBE varied depending on the type of compound and concentration. When methanogenesis or sulfate reduction was inhibited, the cultures continued to degrade MTBE, but a retardation of MTBE degradation was observed. The observation suggests that anaerobic MTBE degradation was not coupled directly to methanogenesis or sulfate reduction and the interactions among members of the MTBE-utilizing microbial community must be important for the overall degradation process. Carbon isotope fractionation during anaerobic biotransformation MTBE and TAME was examined. Significant enrichment of 13C in residual MTBE during anaerobic biotransformation was observed from different cultures under both sulfate-reducing and methanogenic conditions. Carbon enrichment factor (epsilon) values estimated for each enrichment are almost identical (-13.7‰ to -15.6‰), suggesting that environmental conditions may not be significantly influence carbon isotope fractionation during degradation. An (epsilon) value of -15.6 +/- 4.1‰ (R 2= 0.9690, n=55, 95% confidence interval) was obtained for anaerobic MTBE degradation from all data combined. An epsilon of -13.7 +/- 4.5‰ was obtained for anaerobic TAME degradation. The finding clearly demonstrates that carbon isotope fractionation has potential to be used as a tool to monitor in situ anaerobic MTBE and TAME biodegradation. As anaerobic degradation is likely to control the fate of MTBE and related fuel oxygenates, this study provides crucial information to understand the process as well as to appropriately manage MTBE-contaminated sites.