Alcohol and acid formation during the anaerobic decomposition of propylene glycol under methanogenic conditions.

Intermediates formed during the anaerobic decomposition of propylene glycol under methanogenic conditions were studied using a serum bottle technique. The pathway is similar to the anaerobic decomposition of ethylene glycol as previously reported. For both compounds, the decomposition is believed to proceed via an initial disproportionation of the glycol to form equal molar amounts of the volatile fatty acid and normal alcohol of the same chain length. In the case of ethylene glycol, disproportionation results in the formation of acetate and ethanol, while disproportionation of propylene glycol produces propionate and n-propanol. Following disproportionation, the alcohols produced from glycol fermentation are oxidized to their corresponding volatile fatty acid with the reduction of protons to form hydrogen. Ethanol and propionate oxidation to acetate proceeds via a well-established syntrophic pathway that is favorable only under low hydrogen partial pressures. Subsequent degradation of acetate proceeds via acetoclastic methanogenesis with the production of carbon dioxide and methane. Despite the production of hydrogen in the initial steps of glycol degradation, both compounds are completely degradable under the methanogenic conditions tested in this study.