Abstract :
Through the use of thermodynamic relationships, the Gibbs free energy change of 12 reactions describing methane (CH4) formation, decomposition and oxidation was calculated for various sets of environmental conditions comparable to those of soil. The calculations showed that the possibility of CH4 formation out of formic acid and acetic acid was enhanced by a low CO2 fugacity. At similar CO2 fugacities, the CH4 stability area when produced out of acetic acid was greater than out of formic acid, indicating that much more conditions are in favour for formation of that gas out of acetic acid than out of formic acid. Low O2 fugacity enhances CH4 formation out of methanol. While, high H2 fugacity enhances CH4 formation out of CO2. Simple decomposition of the environmentally active CH4 to lesser active ones (C2H6, C3H8, C2H4 and C2H2) was enhanced by a low H2 fugacity. At similar H2 fugacity, the stability area of C2H6 and C2H2 was larger than that of C3H8 and C2H4. Hence, more easier formation of C3H8 and C2H4 out of CH4 can be expected. Methane is unstable and can easily be oxidized to CO2 in the presence of even very small amounts of O2 (10-27 Pa ), NO3 - or Fe3+ (10-9M). In contrast, some CH4 can be formed in the presence of SO4 2- at relatively higher activity (10-6M).
