Effect of Terminal Electron Accepting Conditions on Chloroethene Oxidation

For ground-water contaminants, such as petroleum hydrocarbons, that serve as electron donors during biodegradation, the availability of competing terminal electron acceptors affects the efficiency of contaminant biodegradation. Oxygen-reduction is the most thermodynamically favored and, typically, the most efficient metabolic mechanism for biodegradation of reduced ground-water contaminants. Among the remaining terminal electron acceptors that are commonly observed in ground-water systems, the oxidation potential decreases in the order of NO₃ > Mn(IV) > Fe(III) > SO₄ > CO₂. Thus, in general, the potential for biodegradation of highly reduced ground-water contaminants is greatest under aerobic conditions and least under CO₂-reducing (methanogenic) conditions. Because dichloroethene (DCE) and vinyl chloride (VC) are relatively reduced compounds, a similar pattern of decreasing oxidation potential under increasingly reducing conditions would be expected.

The effect of redox conditions on DCE and VC oxidation was recently investigated in aquifer and stream-bed-sediment microcosms (Bradley and Chapelle 1998). In that study, mineralization of DCE and VC to CO₂ decreased under increasingly reducing conditions, but significant mineralization was observed for both sediments under anaerobic conditions. The rate and extent of VC mineralization decreased in the order of aerobic > Fe(III)-reducing > SO₄-reducing > methanogenic conditions. As one would expect given their difference in chlorine number, the rate of microbial VC oxidation was greater than microbial oxidation of DCE for each electron-accepting condition. For both sediments, the rate of microbial DCE mineralization under aerobic conditions was at least twice that observed under anaerobic conditions. It is interesting to note that the rate and magnitude of DCE oxidation did not differ significantly between Fe(III)-reducing, SO₄-reducing and methanogenic conditions. Based on this and other observations, it was concluded that microbial oxidation of DCE under these redox conditions involved an initial rate-limiting reduction to VC which was subsequently oxidized to CO₂. This in turn suggested that direct oxidation of DCE requires a more powerful oxidant than Fe(III)-oxides. A subsequent investigation demonstrated that aquifer microorganisms can anaerobically oxidize DCE to CO₂ under Mn(IV)-reducing conditions without an initial reduction to VC (Bradley et al. 1998). Because Mn(IV) oxides are common in alluvial and glacial aquifer sediments, microbial oxidation of DCE may be significant in some anaerobic ground-water systems.

References

• Bradley, P.M., and Chapelle, F.H., 1998,
  Microbial mineralization of VC and DCE under different terminal electron accepting conditions: Anaerobe, v. 4, p. 81-87.
• Bradley, P.M., Dinicola, R.S., and Landmeyer, J.E., 2000,
  Natural attenuation of 1,2-Dichloroethane by aquifer microorganisms under Mn(IV) reducing conditions, in Wichramanayake, G.B., Gavaskar, A.R., and Kelley, M.E., eds., Natural Attenuation Considerations and Case Studies: Columbus, Ohio, Battelle Press, p. 169.
• Bradley, P.M., Landmeyer, J.E., and Dinicola, R.S., 1998,
  Anaerobic oxidation of [1,2-14C]Dichloroethene under Mn(IV)-reducing conditions: Applied & Environmental Microbiology, v. 64, no. 4, p. 1560-1562.

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