Catalytic Effects of Manganese (IV), Iron(III), Aluminum, and Silicon Oxides on the Formation of Phenolic Polymers

The catalytic effects of three Mn(IV) oxides (birnessite, cryptomelane, and pyrolusite) and short‐range ordered Fe(III), Al, and Si oxides on the darkening of phenolic compound solutions (hydroquinone, resorcinol, and catechol) and the subsequent formation of humic acid (the precipitate formed by acidifying the darkening solution) were investigated. Manganese oxides are very powerful in causing the darkening of phenolic compounds. The rate and degree of the darkening vary with the kinds of Mn oxides, the chemistry of the phenolic compounds, and the pH values of the systems. In the Mn oxide systems, phenolic compounds are converted to humic acid with a relatively high degree of humification (Δlog K: 0.52–0.70; RF: 54–105) through oxidative polymerization. For example, at the initial pH of 6.0, the percent conversion of hydroquinone to humic acid ranged from 36 to 55%. The yields of humic acids formed in the Mn oxide systems are highly correlated with the degree of the darkening measured at 400 nm ( r 2 = 0.818) and 600 nm ( r 2 = 0.983). The catalytic effect of Fe oxide on the darkening and the formation of humic acid was relatively limited under the conditions studied. The catalytic effects were not observed in the Al and Si oxide systems. The results obtained in this study indicate that various Mn(IV) oxides common in the environment merit close attention in the abiotic formation of humic substances.