Environmental Engineering Reference
In-Depth Information
it is a human-produced (anthropogenic) contaminant or an influx of natural
material, indigenous aerobic heterotrophic microorganisms metabolize the
organic material and consume the available oxygen in the process. When
this occurs, aerobic respiration slows and eventually stops. This allows for
the development of anaerobic metabolic communities utilizing other electron
acceptors. Under these conditions, the subsurface environment becomes elec-
tron acceptor limited and the rates of transformations of the introduced
organics are controlled by the availability of the various electron acceptors.
The introduction of the parent chloroethenes (PCE, TCE) does not supply
bio-oxidizable electron donors to subsurface environments, and thus will
not create anoxic conditions in the impacted groundwater. Chloroethenes,
however, are often codisposed with other organics or may mobilize native
or anthropogenic organics, which can serve as electron donors and can be
biologically utilized for the reduction of dissolved oxygen, other anaerobic
electron acceptors, or the chloroethenes. It is important to remember that the
reductive dechlorination process is one of several competing electron sinks,
and the amount of reducing equivalents that are utilized for dechlorination
of chloroethenes is determined by the energetics and relative concentrations
of all of the available electron acceptors. The parent chloroethenes (PCE,
TCE) are resistant to oxidative catabolism. This means that in the presence
of oxygen, the biodegradation potential of these compounds varies from not
detected (for PCE) to low (for vinyl chloride (VC)). TCE and less chlorinated
daughters can be degraded under cometabolic conditions. However, since
this process is not directly beneficial or may even be detrimental to the
microorganisms and requires the presence of an inducer, its relevance is
limited.
In the subsurface and other anaerobic environments, chloroethenes can
be transformed by microorganisms through a process known as reductive
dechlorination. This is a stepwise removal of chloride ions:
Reductive chlorination of chloroethenes:
R-Cl
x
+ H
2
R-Cl
x-1
+ H
+
+ Cl
-
(5.1)
This process transforms the chlorinated ethenes to nonchlorinated prod-
ucts (ethene, ethane), which do not pose a threat to human health or the
environment. Halorespiration is the term used to describe the reductive
dechlorination process when specific microorganisms obtain energy for
growth using halogenated compounds, such as PCE, as terminal electron
acceptors. Unfortunately, the more mobile and toxic daughter products, such
as dichloroethene (DCE) and vinyl chloride (VC), are intermediates in the
halorespiration pathway (Figure 5.1). If the process “stalls,” as it often seems
to in the subsurface, before reaching nonchlorinated end products, the reduc-
tive dechlorination process may increase the potential risks to humans and
the environment. Thus, the reductive dechlorination process can exacerbate
or attenuate the problems created by the release of chloroethenes to the
