Environmental Health - Toxic Substances
Microbial Oxidation of Vinyl Chloride Under Iron-Reducing Conditions
By 1995, efforts to utilize in situ bioremediation in chloroethene-contaminated aquifers had reached an impasse. By this time, microbial reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) was considered pervasive in anaerobic aquifers, but, in most systems, reductive dechlorination appeared to stop with dichloroethene (DCE) and VC. Likewise, the characteristic association of DCE and vinyl chloride (VC) accumulation with anaerobic conditions disqualified aerobic microbial metabolism as a significant mechanism for bioremediation of these compounds under in situ conditions. Without a demonstrable mechanism for in situ biodegradation of chloroethenes to non-toxic products, state and federal regulators were understandably reluctant to accept in situ biodegradation as a legitimate mechanism for remediation of chloroethenes in ground water.
The discovery in 1996 that microorganisms can oxidize VC to CO2 under anaerobic conditions paved the way for regulatory acceptance of in situ bioremediation as a defensible mechanism for remediation of chloroethene-contaminated aquifers. Oxidation of vinyl chloride can occur under anaerobic conditions, if a sufficiently strong oxidant is available to drive microbial degradation. Fe(III)-oxides are strong oxidants that are ubiquitous in ground water systems. In an experiment conducted with sediment from a Fe(III)-reducing aquifer, addition of Fe(III) to anaerobic microcosms resulted in VC mineralization rates comparable to those observed under aerobic conditions. Low but significant VC mineralization was also observed in anaerobic microcosms under ambient Fe(III) conditions. The results indicated that vinyl chloride can be mineralized under anaerobic, Fe(III)-reducing conditions, and that the bioavailability of Fe(III) is an important factor affecting the rates of mineralization. More importantly, microbial oxidation of VC under Fe(III)-reducing conditions provided a potential anaerobic alternative to the slow and inefficient reductive dechlorination of VC to ethene.