The effect of oxidative coloration on the methanogenic toxicity and anaerobic biodegradability of phenols

Phenolic compounds are often present in agricultural wastewater intended for anaerobic treatment. During industrial processing, storage or wastewater pretreatments, these compounds are susceptible to oxidative modifications leading to the formation of colored oxidized products. In this study, the role of the oxidative coloration on changing the methanogenic toxicity and anaerobic biodegradability of phenolic compounds was examined. Short exposures to air increased the methanogenic toxicity of monomeric phenols provided these compounds had neighboring hydroxyl groups, necessary for the coloration, and if their oxidized products lacked free carboxylic-acid groups. In contrast, a methanogenic toxic tannin, gallotannic acid (oligomeric), was detoxified by oxidative coloration. The extensive oxidation of a monomer, l-dopa, produced a dark-colored precipitate which was non-toxic. These results indicated that the initial products of the oxidation of monomers are toxic whereas highly polymerized products are non-toxic. The colorless phenolic compounds tested in this study were either partially or fully degraded (with the exception of catechol). The trihydroxyphenols were the most readily biodegradable compounds. The oxidized solutions of trihydroxyphenols were less biodegradable in proportion to their color.