Handbook of Solvents - George Wypych - ChemTech - Ventech!

23.1 Natural attenuation of chlorinated solvents 1587
23.1.3.2.2 Aerobic cometabolism of chlorinated compounds
It has been reported that under aerobic conditions chlorinated ethenes, with the exception of
PCE, are susceptible to cometabolic oxidation. 30,53,81,82 Vogel 30 further elaborates that the
oxidation rate increases as the degree of chlorination decreases. Aerobic cometabolism of
ethenes may be characterized by a loss of contaminant mass, the presence of intermediate
degradation products (e.g., chlorinated oxides, aldehydes, ethanols, and epoxides), and the
presence of other products such as chloride, carbon dioxide, carbon monoxide, and a variety
of organic acids. 53,83 Cometabolism requires the presence of a suitable primary substrate
such as toluene, phenol, or methane. For cometabolism to be effective, the primary substrate
must be present at higher concentrations than the chlorinated compound, and the system
must be aerobic. Because the introduction of high concentrations of oxidizable organic matter
into an aquifer quickly drives the groundwater anaerobic, aerobic cometabolism typically
must be engineered.
23.1.3.2.3 Anaerobic oxidation of chlorinated compounds
Anaerobic oxidation occurs when
anaerobic bacteria use the chlorinated
solvent as an electron donor
by utilizing an available electron
acceptor such as ferric iron
(Fe(III)). Bradley and Chapelle 84
show that vinyl chloride can be
oxidized to carbon dioxide and
water via Fe(III) reduction. In microcosms
amended with
Fe(III)-EDTA, reduction of vinyl
chloride concentrations closely
matched the production of carbon
dioxide. Slight mineralization was
also noted in unamended microcosms.
The rate of this reaction
apparently depends on the
bioavailability of Fe(III). In a subsequent
paper, Bradley and
Chapelle 85 reported “significant”
anaerobic mineralization of both
DCE and VC in microcosms containing
creek bed sediments. The
sediments were taken from a
stream where groundwater containing
chlorinated ethenes con-
Figure 23.1.5. Reaction sequence and relative rates for tinually discharges. Anaerobic
halorespiration of chlorinated ethenes, with other reactions shown.
mineralization was observed in
[From T.H. Wiedemeier, H. S. Rifai, C. J. Newell and J.T. Wilson,
Natural Attenuation of Fuels and Chlorinated Solvents in the both methanogenic and Fe(III) re-
Subsurface, reaction rates description from reference 88. Copyright ducing conditions.
© 1999 John Wiley & Sons, Inc. Reprinted by permission of John
Wiley & Sons, Inc.]