Handbook of Solvents - George Wypych - ChemTech - Ventech!

1584 Hanadi S. Rifai, Charles J. Newell, Todd H. Wiedemeier
2) chlorinated solvents amenable to halorespiration must be present; and 3) there must be an
adequate supply of fermentation substrates for production of dissolved hydrogen.”
Fermentation, the process that generates hydrogen, is a balanced oxidation-reduction
reaction, in which different portions of a single substrate are oxidized and reduced, yielding
energy. Fermentation yields substantially less energy per unit of substrate compared to oxidation
reactions which utilize an external electron acceptor; thus, fermentation generally
occurs when these external electron acceptors are not available. Bacterial fermentation
which can be important in anaerobic aquifers includes primary and secondary fermentation.
Primary fermentation refers to the fermentation of primary substrates such as sugars,
amino acids, and lipids yields acetate, formate, CO 2 and H 2, but also yields ethanol, lactate,
succinate, propionate, and butyrate. Secondary fermentation, on the other hand, refers to the
fermentation of primary fermentation products such as ethanol, lactate, succinate, propionate,
and butyrate yielding acetate, formate, H 2, and CO 2. Bacteria which carry out these reactions
are called obligate proton reducers because the reactions must produce hydrogen in
order to balance the oxidation of the carbon substrates. These secondary fermentation reactions
are energetically favorable only if hydrogen concentrations are very low (10 -2 to 10 -4
atm or 8,000 nM to 80 nM dissolved hydrogen, depending on the fermentation substrate).
Thus, these fermentation reactions occur only when the produced hydrogen is utilized by
other bacteria, such as methanogens which convert H 2 and CO 2 into CH 4 and H 2O.
In the absence of external electron acceptors, the hydrogen produced by fermentation
will be utilized by methanogens (methane producing bacteria). In this case, the ultimate end
products of anaerobic metabolism of carbon substrates will be CH 4 (the most reduced form
of carbon) and CO 2 (the most oxidized form of carbon). Methanogens will carry out the last
step in this metabolism, the conversion of H 2 and CO 2 into CH 4. However, in the presence of
external electron acceptors (halogenated organics, nitrate, sulfate, etc.), other products will
be formed. 1
There are a number of compounds besides the ones listed above that can be fermented
to produce hydrogen. Sewell and Gibson 54 noted that petroleum hydrocarbons support
reductive dechlorination. In this case, the reductive dechlorination is driven by the fermentation
of biodegradable compounds such as the BTEX compounds in fuels. Metabolism of
BTEX compounds to produce hydrogen likely requires the involvement of several strains of
bacteria. Although the BTEX compounds are common fermentation substrates at chlorinated
solvent sites, there are many other hydrocarbon substrates which are naturally fermented
at sites and result in the generation of hydrogen such as acetone, sugars, and fatty
acids from landfill leachate.
As hydrogen is produced by fermentative organisms, it is rapidly consumed by other
bacteria. The utilization of H 2 by non-fermentors is known as interspecies hydrogen transfer
and is required for fermentation reactions to proceed. Although H 2 is a waste product of fermentation,
it is a highly reduced molecule which makes it an excellent, high-energy electron
donor. Both organisms involved in interspecies hydrogen transfer benefit from the process.
The hydrogen-utilizing bacteria gain a high energy electron donor, and, for the fermentors,
the removal of hydrogen allows additional fermentation to remain energetically favorable.
A wide variety of bacteria can utilize hydrogen as an electron donor: denitrifiers, iron
reducers, sulfate reducers, methanogens, and halorespirators. Thus, the production of hydrogen
through fermentation does not, by itself, guarantee that hydrogen will be available
for halorespiration. For dechlorination to occur, halorespirators must successfully compete