Comparison of the influence of Mn(IV) oxide and tyrosinase on the formation of humic substances in the environment

Abstract

The catalytic effects of Mn(IV) oxide and tyrosinase on the oxidative polymerization of diphenols (hydroquinone, catechol, and resorcinol) were compared by measuring the degree of darkening and the formation of humic acids. The addition of Mn oxide or tyrosinase determined the darkening of the diphenol solutions over the pH range of 4–8. Mn oxide influenced the darkening of hydroquinone and resorcinol to a larger extent than did tyrosinase, while the reverse was true for catechol. In the tyrosinase system, the darkening of catechol was more efficient compared with hydroquinone and resorcinol. In both the Mn oxide and tyrosinase systems, the degree of the darkening of resorcinol was generally much lower than for the other phenols studied. The yields of humic acids were also significantly influenced by the kind of catalyst and diphenol used. In the Mn oxide system, the yield of humic acid was in the order: hydroquinone > catechol > resorcinol. In the tyrosinase system, catechol produced the highest yield of humic acid, followed by hydroquinone and resorcinol. These findings indicate that the relative catalytic effects of Mn(IV) oxides and tyrosinases in promoting the formation of diphenol-derived humic substances would vary with the type of diphenols in natural systems.