Environmental Engineering Reference
In-Depth Information
5.2.4 Homoacetogenic bacteria
Hydrogen-consuming acetogens appear to be outcompeted by methanogens
for hydrogen. The net result, however, is the maintenance of low hydrogen
partial pressures and increased significance of acetate as an immediate
methane precursor (Zeikus, 1979).
5.2.5 Stoichiometry of acetogenic bacteria
In 1967, Bryant and co-workers showed that the M. Omelianski culture
contained two bacterial species (McCarty, 1981). One converted ethanol
(CH
3
CH
2
OH) to acetate (CH
3
COOH) and hydrogen (H
2
); the other
converted carbon dioxide (CO
2
) and hydrogen to methane (CH
4
). So, for
the complete oxidation of a simple compound such as ethanol to CO
2
and
CH
4
contributions from three separate species would be required, as
follows.
Δ
G (kJ/reaction)
H
2
O=CH
3
COO
þ
H
þ
þ
þ
Species 2
CH
3
CH
2
OH
2H
2
+5.95
Species 4.1 2H
2
þ
O.5CO
2
= O.5CH
4
þ
H
2
O
65.45
Species 4.2 CH
3
COO
þ
H
þ
=CH
4
þ
CO
2
28.35
Net
CH
3
CH
2
OH
= 1.5CH
4
þ
0.5CO
2
87.85
.
Species 2: OPR acetogenic bacteria. The standard free energy of the
reaction (
G) is positive. Thus, for this reaction to occur, the hydrogen
partial pressure has to be lowered. This is explained by Le Chatelier's
principle, which states that if some stress (in this case a reduction in the
concentration of the products) is brought to bear on a system in
equilibrium a reaction occurs which displaces the equilibrium in the
direction which tends to undo the effect (towards the product).
Δ
.
Species 4.1: Hydrogenotrophic methanogenic bacteria form a syntropic
association with the acetogenic bacteria. The standard free energy of the
reaction is relatively high in the negative direction, indicating the affinity
of the methanogen to hydrogen.
.
Species 4.2: Aceticlastic methanogenic bacteria. The standard free
energy of the reaction is less negative than for its hydrogenotrophic
relation, indicating the lower affinity of methanogens for acetate.
In the theoretical degradation of ethanol two thirds of the methane comes
from acetate and one third comes from hydrogen.
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