Handbook of Solvents - George Wypych - ChemTech - Ventech!

1586 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
As with the ethenes, chlorinated ethanes will also undergo halorespiration.
Dechlorination of 1,1,1-TCA has been described by Vogel and McCarty 38 and Cox et al., 61
but understanding this pathway is complicated by the rapid hydrolysis reactions (e.g.,
half-life is 0.5-2.5 yrs) that can affect TCA. 30 Finally, halorespiration has been observed
with highly chlorinated benzenes such as hexachlorobenzene, pentachlorobenzene,
tetrachlorobenzene, and trichlorobenzene. 62-64 As discussed by Suflita and Townsend, 64
halorespiration of aromatic compounds has been observed in a variety of anaerobic habitats,
including aquifer materials, marine and freshwater sediments, sewage sludges, and soil
samples. However, isolation of specific microbes capable of these reactions has been difficult.
23.1.3.2 Oxidation of chlorinated solvents
In contrast to halorespiration, direct oxidation of some chlorinated solvents can occur biologically
in groundwater systems. In this case, the chlorinated compound serves as the electron
donor, and oxygen, sulfate, ferric iron or other compounds serve as the electron
acceptor.
23.1.3.2.1 Direct aerobic oxidation of chlorinated compounds
Under direct aerobic oxidation conditions, the facilitating microorganism uses oxygen as an
electron acceptor and obtains energy and organic carbon from the degradation of the chlorinated
solvent. In general, the more-chlorinated aliphatic chlorinated solvents (e.g., PCE,
TCE, and TCA) have not been shown to be susceptible to aerobic oxidation, while many of
the progeny products (e.g., vinyl chloride, 1,2-DCA, and perhaps the isomers of DCE) are
degraded via direct aerobic oxidation.
Hartmans et al. 65 and Hartmans and de Bont 66 show that vinyl chloride can be used as a
primary substrate under aerobic conditions, with vinyl chloride being directly mineralized
to carbon dioxide and water. Direct vinyl chloride oxidation has also been reported by Davis
and Carpenter, 67 McCarty and Semprini, 53 and Bradley and Chapelle. 68 Aerobic oxidation is
rapid relative to reductive dechlorination of dichloroethene and vinyl chloride. Although direct
DCE oxidation has not been verified, a recent study has suggested that DCE isomers
may be used as primary substrates. 68 Of the chlorinated ethanes, only 1,2-dichloroethane
has been shown to be aerobically oxidized. Stucki et al. 69 and Janssen et al. 70 show that
1,2-DCA can be used as a primary substrate under aerobic conditions. In this case, the bacteria
transform 1,2-DCA to chloroethanol, which is then mineralized to carbon dioxide.
McCarty and Semprini 53 describe investigations in which 1,2-dichloroethane (DCA) was
shown to serve as primary substrates under aerobic conditions.
Chlorobenzene and polychlorinated benzenes (up to and including tetrachlorobenzene)
have been shown to biodegrade under aerobic conditions. Several studies have shown
that bacteria are able to utilize chlorobenzene, 71 1,4-DCB, 71-73 1,3-DCB, 74 1,2-DCB, 75
1,2,4-TCB, 76,77 and 1,2,4,5-TeCB, 77 as primary growth substrates in aerobic systems.
Nishino et al. 78 note that aerobic bacteria able to grow on chlorobenzene have been detected
at a variety of chlorobenzene-contaminated sites but not at uncontaminated sites. Spain 79
suggests that this provides strong evidence that the bacteria are selected for their ability to
derive carbon and energy from chlorobenzene degradation in situ. The pathways for all of
these reactions are similar, bearing resemblance to benzene degradation pathways. 79,80