Now the thermodynamic feasibility of aerobic biodegradation
of the dechlorinated byproducts increases
as the chlorination progresses, as indicated by the more positive Delta E
values.
So it is quite possible that these compounds
can be degraded by sequential anaerobic-aerobic processes.
For example, as they dechlorinate in an anaerobic zone, and then the byproducts
migrate into the aerobic zone.
Now the key points are that reductive dechlorination
requires appropriate electron donors to first induce anaerobic conditions
and stimulate dechlorination or, that is,
the removal of chlorine atoms, which will
decrease the toxicity with the noted exception of the formation of vinyl
chloride and enhance the solubility of these compounds.
Some strains can respire these chlorinated solvents
as electron acceptors.
And this results in faster and greater potential for complete detoxification.
These halorespiring organisms include those of the genus Dehalobacter
restrictus, Dehalococcoides ethenogenes, and Desulfomonile tiedje, among many.
It's interesting to point out that these bacteria are
pretty widespread among solvent-contaminated sites.
Although it is not always the case that they will be present.
And sometimes they're added in bioaugmentation schemes
to stimulate dechlorination.
Now in contrast to chlorinated solvents, hydrocarbons
are very reduced chemically.
So their oxidation is very favorable thermodynamically.
And that's why we use them as fuel in combustion engines.
Now hydroxylation, which refers to the addition of hydroxyl groups,
is often the first step.
This is mediated by oxygenase enzymes.
This transformation increases the solubility of the compound
and makes it more susceptible to subsequent metabolism.
Now this reaction requires molecular oxygen,
and often it is the diffusion and replenishment of molecular oxygen
what limits the rate of aerobic biodegradation.
One condition for the ring fission for aromatic compounds
is that they have to be di-hydroxylated.
That is, they need two hydroxyl groups inserted by these oxygenase enzymes
before the ring can be opened up.
Then once the ring is opened up, the byproducts
can be easily funneled into central metabolic pathways,
such as Krebs cycle, where complete mineralization to carbon dioxide
occurs.
Anaerobic degradation of BTEX compounds is
very important as a natural attenuation mechanism,
even if it proceeds at much slower rates due to the weaker electron acceptors
that are utilized in this process.
Benzene, which is the most toxic of the BTEX compounds,
is unfortunately relatively recalcitrant under anaerobic conditions,
where it degrades very slowly, if at all,
usually after the other alkylbenzenes are degraded.
And benzoyl-CoA is a common intermediate in these reactions.
It's reduced prior to ring fission by hydrolysis,
and the oxygen that evolves in that CO2 is usually originating from water.
A survey of plume dimensions that was conducted by GSI--
they considered hundreds of sites-- they showed
that BTEX plumes are relatively small, compared to chlorinated plumes.
And this reflects that BTEX compounds are relatively easy to degrade due
perhaps to their natural pyrolytic origin,
compared to these synthetic chlorinated solvents that
are relatively recalcitrant in nature.
So the key points are that chlorinated solvents like TCE and PCE
degrade relatively fast under anaerobic conditions, provided
that suitable electron donors, such as hydrogen and acetate, are present.
Some bacteria, known as "dehalorespirers,"
can obtain metabolic energy from this process
and grow, thus increasing the dechlorination rates
and dechlorinate TCE all the way to ethene,
resulting in complete detoxification.
And hydrocarbons degrade faster under aerobic conditions,
and their biodegradation rate are often limited by oxygen replenishment rates.
Anaerobic degradation of hydrocarbons, although slower,
is also an important mechanism for monitored natural attenuation.