BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 453

14.3 THERMODYNAMIC FUNDAMENTALS

The microbial conversion of organic substrates to methane-containing biogas is based
on a firm thermodynamic foundation. In a thermodynamically closed system, redox
reactions are driven in the direction of the thermodynamic equilibrium state. This can
most easily be explained by identifying the half reactions that describe organic carbon
oxidation to the most oxidized state of carbon (carbon dioxide) and comparing the
Gibbs free energy change per electron of the different reactions. Four example half
reactions are shown below:

6CO 2 +24H +1 +24e −1

Glucose : C 6 H 12 O 6 +6H 2 O !

G 01 =

Δ − 40 : 9 kJ per electron ð RX : 14 : 3 Þ

C 3 H 5 O − 3 +3H 2 O 6CO 2 +11H +1 +12e −1

Lactate : !

G 01 = Δ − 32 : 8 kJ per electron ð RX : 14 : 4 Þ

C 2 H 3 O − 2 +2H 2 O 2CO 2 +7H +1 +8e −1 Acetate : !

G 01 =

Δ − 28 : 1 kJ per electron ð RX : 14 : 5 Þ

CO 2 +8H +1 +8e −1

Methane : CH 4 +2H 2 O !

G 01 =

Δ − 23 : 6 kJ per electron ð RX : 14 : 6 Þ

Organic carbon is a strong electron donor, suggesting that upon oxidation of organic
compounds, Gibbs energy is generated as reflected in the negative G 01 values

shown. In the absence of an external electron acceptor, the oxidation of organic carbon
needs to be combined with the reduction of carbon dioxide to another organic com-
pound to close the electron balance. Evidently, the reduction of carbon dioxide to
organic carbon is thermodynamically unfavorable (endergonic). The overall redox
reaction is thermodynamically only favorable if the Gibbs energy content per electron
decreases upon conversion of one organic compound into another. Only thermody-
namically favorable (exergonic) redox reactions can sustain growth and maintenance
of a microbial ecosystem. Therefore, the four compounds shown in RX. 14.3

Δ

- 14.6

will be degraded through the sequence shown in Figure 14.7.
In more generalized terms, it can be stated that in a thermodynamically closed
microbial system (no input of energy source and electron acceptor), organic carbon
will be converted to the compound with the lowest energy content per electron. It
is therefore no surprise that when we compare the energy content per electron of all
organic molecules, the lowest value is obtained for gaseous methane (see Figure 14.8).
There is (of course!) one exception to this firm rule, and that is elemental carbon
(graphite), which has a Gibbs energy content per electron even lower than methane,
suggesting that (microbial) conversion of methane to graphite is exergonic, but to date,
no microorganism has been identified that is capable of catalyzing this reaction.
The thermodynamic basis of the anaerobic digestion process makes it particularly
suitable for bioenergy production from heterogeneous substrates. Independent of the
origin and composition of the substrate, the end product of the microbial degradation