The effects of Fe(III) reduction on biodegradation of fuel oxygenates and chlorinated ethenes

摘要

Fuel oxygenates, methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA), and chlorinated ethenes (TCE, cis-DCE and VC) are two groups of contaminants prevalent in groundwater systems. Fe(III) reducing conditions are dominant in many sedimentary environments. However, the effects of Fe(III) reduction on biotransformation of MTBE/TBA and chlorinated ethenes are not well understood. This research investigated the biodegradation of these contaminants and related microbial ecology under Fe(III)-reducing conditions in both sediment microcosms and enrichment cultures.The primary limitation to understanding anaerobic MTBE biodegradation is the lack of liquid cultures with consistent activity. This study enriched three distinct MTBE-degrading, anaerobic cultures from contaminated aquifer material, and they use anthraquinone-2,6-disulfonate, sulfate and fumarate as the terminal electron acceptor, respectively. Phylogenetic analyses based on 16S rRNA gene suggested novel microorganisms involved in anaerobic MTBE biodegradation. These cultures are the first stable, sediment-free anaerobic MTBE-degrading cultures, which provide model systems for mechanistic studies of anaerobic MTBE biodegradation. Substantial [U-14C]-TBA mineralization occurred under Fe(III)-reducing conditions. The TBA biodegradation activity was correlated with the abundance of one dominant clone, which is closely associated with organisms belonging to the Alphaproteobacteria. The results provide the original evidence of the stimulative effect of Fe(III) reduction on anaerobic TBA mineralization, and give initial insight to the organisms that may catalyze the anaerobic TBA mineralization reactions.The experimental study on the effects of Fe(III) reduction on reductive dechlorination demonstrated that Fe(III) reduction did not inhibit complete dechlorination, which is in contrast to the common opinion that TCE dechlorination will be inhibited wherever there is bioavailable Fe(III). Fe(III) speciation has an impact on daughter product distribution and dechlorination kinetics. Quantitative PCR analysis revealed that Dehalococcoides and Geobacteraceae organisms were enriched concurrently in the dechlorinating Fe(III)-reducing sediments/cultures; Dehalococcoides abundance in the presence of Fe(III) was not significantly different from that in the cultures without Fe(III), meaning Fe(III) reducers would not outcompete and inhibit Dehalococcoides growth. Also, Fe(III) reduction may stimulate growth of G. lovleyi like organisms and contribute to TCE dechlorination to cis-DCE. Enrichment culture study demonstrated that Fe(III) reduction poised the hydrogen concentration at an appropriate steady-state that is within the ideal range for reductive dechlorination when acetate was amended as the sole electron donor. 10X excessive electron donor addition did not facilitate chlorinated ethene dechlorination, but led to considerably high methane production and enrichment of methanogens. The results suggest that adding surplus electron donor will not only cause substrate wastage and unnecessary cost in bioremediation, but also have adverse effects such as enhanced methane release.