The rate and extent of oxidation of dihydroxybenzenes (DHB) to quinones in acetate-buffered suspensions of synthetic birnessite were studied using Mn dissolution to monitor reaction progress. Concentration of free Mn2+ in the aqueous phase was continuously monitored by electron spin resonance, and ultraviolet-visible (UV-VIS) spectroscopy was utilized to quantify dihydroxybenzene and quinone concentrations. Although dissolution of the oxide and release of Mn2+ to solution generally accompanied phenol oxidation, a threshold oxidation level had to be exceeded before Mn2+ appeared in solution. Once this threshold was surpassed, the mole quantity of Mn2+ dissolved equaled the mole quantity of organic oxidized for 1,4-DHB, but exceeded the quantity of organic oxidized for 1,2-DHB. Thus, the latter phenol was more efficient in dissolving the oxide. Soluble phosphate suppressed Mn2+ release without influencing the degree of organic oxidation, suggesting that phosphate chemically interacted with reduced Mn to hinder its dissolution. UV spectra provided tentative evidence for the transitory existence of Mn3+-1,4-DHB complexes in the solution phase.

Infrared spectra of the birnessite after reaction with 1,4-DHB indicated some new features, which may have been a result of the reduction of surface Mn atoms to the 3+ oxidation state. These features were not present after reaction with 1,2-DHB, confirming that the latter phenol efficiently dissolved the oxide to release Mn2+. Although the initial Mn dissolution was very rapid and was attributed to a surface reaction, further slow Mn release accompanied by more complete oxidation of the phenols suggests a process limited by the rate of dissolution of the solid.

The oxidation of 1,2- and 1,4-dihydroxybenzenes (1,2-DHB and 1,4-DHB) by unbuffered aerated suspensions of synthetic bimessite was studied by continuously monitoring the H+, Mn2+, dissolved O2, and organic radical concentration of the aqueous phase during the reaction. The reaction rapidly generated a very high pH, attributed to oxide dissolution, and the alkaline conditions prevented Mn2+ release into solution over the entire reaction period. Semiquinone radical anions accumulated early in the reaction and then diminished. A secondary radical product appeared in solution during the reaction of the oxide with 1,2-DHB, and was tentatively identified as an hydroxylated semiquinone. The oxide/DHB ratio controlled the maximum concentration and persistence of these radicals in solution as well as the degree to which O2 was consumed as an electron donor. In general, low oxide/DHB ratios promoted O2 uptake by the system, consistent with the subordinate role of O2 as a competing electron acceptor in the presence of excess Mn oxide. Soluble phosphate suppressed O2 consumption, but the mechanism by which it interacted with the reaction system was not determined.