Oxidative detoxification of aqueous bark extracts. Part I: Autoxidation

The aqueous extracts of bark were studied as a model for wet debarking wastewater. These extracts are known to contain a high concentration of methanogenic toxic tannins. The objective of this study was to modify the native bark tannins (oligomers) with oxidative methods in order to decrease their methanogenic toxicity. The tannins were polymerized by autoxidation, forming colored high‐molecular‐weight tannins that were non‐toxic to methanogenic bacteria. The autoxidation of pine bark extracts provided complete detoxification. In the case of spruce bark, which was responsible for extracts of higher toxicity, the detoxification was either partial or complete depending on the specific sample of spruce bark from which the extract was prepared. The autoxidation of birch bark did not result in significant detoxification. Although the oligomeric tannins were effectively polymerized, birch bark extracts contain non‐tannin toxins which were not affected by the autoxidation and evidence that highly toxic intermediates were formed during the high pH autoxidation of birch bark extracts is presented. Autoxidative detoxification offers an interesting approach for pretreating coniferous bark wastewaters. The oxidation reactions of the pretreatment serve to detoxify the largely non‐BOD tannin fraction. Thus the methanogenic bacteria are not inhibited during the anaerobic wastewater treatment for removing BOD.