Reductive dehalogenation of chlorinated ethenes with elemental iron: the role of microorganisms.

Trichloroethene (TCE) transformation and the product distribution in an aqueous medium containing zero-valent iron (Fe(0)) was investigated in the presence of an anaerobic mixed culture to assess the potential role of microorganisms in permeable iron barriers. The presence of the culture increased the rate of TCE disappearance and changed the product distribution. Rapid formation and degradation of cis-dichloroethene (cis-DCE) was observed in reactors containing cells plus Fe(0) or H2 as a bulk reducing agent. High levels of vinyl chloride (VC) were formed and very similar profiles were obtained in the Fe(0) plus cell and H2 plus cell reactors, but not in Fe(0)-only reactors. The similar trends observed in Fe(0)-cell and H2-cell reactors suggest that most cis-DCE and VC in the Fe(0)-cell reactors were produced and transformed biologically rather than abiotically. Accumulation of methane in the Fe(0)-cell system indicates that hydrogen gas generated during anaerobic iron corrosion could support a methanogenic culture. Digital confocal images showed that the microorganisms were able to colonize the iron surface. The results suggest that potential development of dechlorinating populations in Fe(0) barriers may alter the TCE reduction pathway and produce VC, which would have significant impact on the performance of Fe(0) barriers.