Oxidative transformation of aqueous phenolic mixtures by birnessite-mediated catalysis

The catalytic efficiency of birnessite in the removal of catechol, hydroxytyrosol, methylcatechol and m-tyrosol, four phenols commonly present in polluted wastewaters, was studied in mono-substrate solutions or in mixtures of two, three, and four substrates. In single phenolic solutions the transformation order of phenols was catechol > hydroxytyrosol > methylcatechol > m-tyrosol. With phenolic mixtures different responses were observed and the amount of each phenol transformed and the crossing effects among the various phenols depended on the type and number of phenols present in the mixture. In particular, general inhibitory effects were observed for hydroxytyrosol and m-tyrosol that were transformed less when present in combination with the other phenols. By contrast the effects by the presence of more than one phenol were basically annulled for catechol and methylcatechol at 24 h incubation in all the mixtures. A simultaneous, but often no stoichiometric, release of soluble Mn2+ in the reaction mixtures occurred. The multi-substrate systems were designed to mimic birnessite-mediated oxidative processes that could occur under field conditions. Therefore they could be of great interest to environmental and soil science. The use of birnessite as a potential tool for an effective detoxification and recovery of phenol-polluted systems could be also envisaged.