The effect of autoxidation on the methanogenic toxicity and anaerobic biodegradability of pyrogallol

In a previous study, catechin (a condensed tannim monomer) was polymerized by autoxidation treatments. The resulting oligomeric tannins were responsible for the methanogenic toxicity observed in the autoxidized catechin solutions (Field, J. A., Kortekaas, S. & Lettinga, G. (1989). The tannin theory of methanogenic toxicity. Biol. Wastes 29(4) 241-62). In this study the autoxidation of pyrogallol (a hydrolyzable tanning monomer) did not cause extensive polymerization. Initially, some polymerization occurred, producing toxic intermediates that were later destroyed by a destructive type of oxidation caused by prolonged autoxidation treatments. The first intermediate formed, purpurogallin (a dimer), caused a high level of toxicity to both the methanogenic activity and to the anaerobic degradation of pyrogallol. Since purpurogallin is a highly toxic autoxidation product that lacks tannic features, the changes in methanogenic toxicity induced by the autoxidation of pyrogallol cannot be estimated by changes in the oligomeric tannin concentration. Regardless of the reactions that take place during the autoxidation of tannin monomeric derivatives, the initial reactions can potentially lead to colored products of increased methanogenic toxicity. These are later detoxified by prolonged autoxidation, either by polymerization to high MW compounds (with condensed tannin model compounds), or by destruction of the initially toxic intermediates to low MW compounds (with hydrolyzable tannin model compounds).