Table 8.1 presents several catabolic reactions which yield acetate and H2. First we shall have a look at Eq. (8.2), the degradation of propionate to acetate. It is interesting to note how far the concentration of hydrogen must be decreased to make the degradation of propionate possible.

This makes it necessary to study the equilibrium of Eq. (8.2). First, we write for the equilibrium constant:

S S3

SAc- SH

Process |
(kJ mol-1) | |

Fermentation |
C6H12O6 ^ CH3CH2COOH + CH3COOH + CO2 + h2 | |

by acidogenic |
C6H12O6 ^ CH3CH2CH2COOH + 2 CO2 + 2 H2 | |

bacteria | ||

p-Oxidation by |
CH3CH2CH2COO- + 2 H2O ^ 2 CH3COO- + H+ + 2 H2 |
48.1 |

acetogenic |
CH3CH2COO- + 3 H2O ^ CH3COO- + H+ + HCO- + 3 H2 |
76.1 |

bacteria |
CH3CH2OH + H2O ^ CH3COO- + H+ + 2 H2 |
9.6 |

(synthrophic |
CH3CHOH COO- + 2 H2O ^ CH3COO- + H+ + HCO3- + 2 H2 |
-4.2 |

reactions) | ||

CH4 formation |
HCO3- + H+ + 4 H2 ^ CH4 + 3 H2O |
-135.6 |

by methano- |
ch3coo- + H2O ^ CH4 + HCO- |
-31.0 |

genic bacteria | ||

Further |
2 CH3CHOH COO- + SO2- ^ 2 CH3COO- + 2 HCO- + 2 H+ + S2- |
-1120.5 |

respiration |
CH3COO- + SO2- ^ 2 HCO- + H+ + S2- | |

reactions |
4 H2+ SO2- ^ 2 H2O + S2- | |

2 NO- + 5 H2 + 2 H+ ^ N2 + 6 H2O |

After introducing partial pressures for:

Ch2_ RT

where R is the ideal gas constant and: k _ sH2O

The equilibrium constant is a function of the free enthalpies of reaction: (AG°- AG)^

where AG is the free enthalpy of reaction and AGo is the standard free enthalpy for T _ 0 K.

From Eqs. (8.9) and (8.10), it follows that: AG = AG° + RT(3 lnRT + lnk + lnSPr- - lnSAc- - lnSH

For given values of SPr-, SAc-, Shco-, pH and T, the reaction enthalpy can be calculated as a function of pH2 (McInerney and Bryant 1981; Pohland 1992; McInerney 1999). Curve 1 in Fig. 8.4 presents some results.

log pH2

2 butyrate 1 propionate |
3 ethanol \ \ 4 methane |

Fig. 8.4 Influence of hydrogen partial pressure pH2 on free enthalpy AG for anaerobic degradation of propionate (curve 1), butyrate (curve 2), ethanol (curve 3) and for CH4-formation from H2 (curve 4). Curve 1 is calculated from Eq. (8.12) using SPr- = 1 mmol L-1, SAc-= 1 mmol L-1, SHCO- = 50 mmol L-1, pH 7, T = 25°C (McInerney and Bryant 1981). -120 kJ mol Only at partial pressures pH2< 10 Pa or pH2<10-4 bar is AG negative at the given conditions and the equilibrium constant Kc increases, resulting in a higher rate of propionate conversion. Compared with that, for pH2 > 10 Pa or pH2 > 10-4 bar AG is positive, Kc decreases and the propionate concentration increases. But there is an important catabolic reaction utilized by some methanogenic bacteria which produces CH4 from H2 (see Eq. 8.5). This cooperation of acetogenic bacteria which produces H2 from butyrate and propionate (Eqs. 8.2 and 8.3), but which is not able to transfer hydrogen to other p-oxidation products and methanogenics using H2 is called p-oxidation (Stronach et al. 1986). If the thermodynamic equilibrium is calculated using the same method as described above, a straight line with a positive slope follows, resulting from the negative free enthalpy AGo = -135.6 kJ mol-1 (Fig. 8.4, curve 2). For pH2 = 2 ■ 10-6 bar, the difference of reaction enthalpies is nearly zero. Methane cannot be produced via the reaction in Eq. (8.5) and pH2 must increase. The reaction can only "move along the two sides of the triangle" formed by curves 1, 4 and the vertical line for AG = 0. But for other T, pH and concentrations of propionate, acetate and CO2, the triangle with the two paths of reaction is moved somewhat and a somewhat higher or lower pH2 must be reached. Following the same thermodynamic consideration, curve 2 can be obtained for butyrate and curve 3 for ethanol. For given conditions (not shown here) AG is already negative for pH2= 10-1 bar (ethanol) and pH2= 10-3 bar (butyrate). The fermentation of butyrate or ethanol, in each case by a pure culture, is inhibited by higher partial pressures of H2. However, for experiments with mixed cultures the lowest value of pH2=10-4 bar is crucial for the whole process. In nature this problem was solved billions of years ago by forming communities which included methanogenic bacteria able to use H2 and acetate. |

Was this article helpful?

## Post a comment