Handbook of Solvents - George Wypych - ChemTech - Ventech!

1582 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
mary growth substrates (meaning the mediating organism obtains energy for growth).
When used as an electron donor, the chlorinated solvent is oxidized. Oxidation reactions
can be aerobic or anaerobic. Conversely, when used as an electron acceptor, the chlorinated
solvent is reduced via a reductive dechlorination process called halorespiration. It is important
to note that not all chlorinated solvents can be degraded via all of these reactions. In
fact, vinyl chloride is the only chlorinated solvent known to degrade via all of these pathways.
Chlorinated solvents can also be degraded via cometabolic pathways. During
cometabolism, microorganisms gain carbon and energy for growth from metabolism of a
primary substrate, and chlorinated solvents are degraded fortuitously by enzymes present in
the metabolic pathway. Cometabolism reactions can be either oxidation or reduction reactions
(under aerobic or anaerobic conditions), however based on data from numerous field
sites, it does not appear that cometabolic oxidation will be a significant process in plumes of
chlorinated solvents. Anaerobic reductive dechlorination can also occur via cometabolism.
The process of cometabolic reductive dechlorination, however, is “sufficiently slow and incomplete
that a successful natural attenuation strategy typically cannot completely rely
upon it”. 48
The types of biodegradation reactions that have been observed for different chlorinated
solvents are presented in Table 23.1.4. The remainder of this section will focus on
describing the various mechanisms shown in Table 23.1.4.
Table 23.1.4. Biological degradation processes for selected chlorinated solvents
Compound
Halorespiration
Direct aerobic
oxidation
Direct
anaerobic
oxidation
Aerobic
cometabolism
Anaerobic
cometabolism
PCE X X
TCE X X X
DCE X X X X X
Vinyl chloride X X X X X
1,1,1-TCA X X X
1,2-DCA X X X X
Chloroethane X X
Carbon tetrachloride X X
Chloroform X X X
Dichloromethane X X X
[From T.H. Wiedemeier, H. S. Rifai, C. J. Newell and J.T. Wilson, Natural Attenuation of Fuels and Chlorinated
Solvents in the Subsurface. Copyright © 1999 John Wiley & Sons, Inc. Reprinted by permission of John
Wiley & Sons, Inc.]
23.1.3.1 Halorespiration or reductive dechlorination using hydrogen
Reductive dechlorination is a reaction in which a chlorinated solvent acts as an electron acceptor
and a chlorine atom on the molecule is replaced with a hydrogen atom. This results in
the reduction of the chlorinated solvent. When this reaction is biological, and the organism
is utilizing the substrate for energy and growth, the reaction is termed halorespiration. Only