Electron Transfer Processes Between Hydroquinone and Hausmannite (Mn₃O₄)

Abstract

Abstract--A kinetic study of the oxidation of hydroquinone by aqueous suspensions of hausmannite at pH 6 was conducted using an on-line analysis system. Electron transfer between hydroquinone and the oxide was monitored by ultraviolet and electron spin resonance spectroscopy to measure the loss of hydroquinone and the appearance of oxidation products. Although hydroquinone oxidized on the surface of the oxide and the oxide surface was altered after the reduction, hydroquinone and its oxidation products did not adsorb strongly on the surface. At a high concentration of hydroquinone, p-benzosemiquinone free radicals persisted in aqueous solution and were oxidized by dissolved 02. Calculations based on the thermodynamic stabilities of the oxide and the organic species involved show that the formation of p-benzosemiquinone radical by Mn reduction is feasible. The presence of the radicals indicates that the oxidation of hydroquinone by the oxide proceeded by a one-electron transfer process. At high organic/ oxide ratios, an increase in the amount of hausmannite dissolved with increasing hydroquinone concentration suggests that the reduction of the oxide by the organic was not limited to the surface layer of the oxide. At a high concentration of hydroquinone, polymers were detected in solution, suggesting that radical-mediated reactions played a role in the polymerization process. A reaction scheme is proposed to explain the effect of the Mn oxide to hydroquinone ratio on the consumption of O2 and the appearance of quinone, p-benzosemiquinone, and polymers in solution.

Oxidation of Dihydroxybenzenes in Aerated Aqueous Suspensions of Birnessite

“…Unlike 1,4-DHB oxidation by Mn oxide, which involved two separate electron transfers to form the semiquinone and then the quinone monomer (Kung and McBride, 1988), a more complex oxidation process was observed with 1,2-DHB. Figure 6 reveals that, as the 9-line ESR spectrum of o-benzosemiquinone diminished in intensity, it was replaced by a new spectrum which appeared to consist of four lines of nearly equal intensity.…”

“…Manganese oxides are particularly efficient in accepting electrons from phenolic compounds to form semiquinones and quinones (Ono et aL, 1977;Stone and Morgan, 1984;Kung and McBride, 1988). Reaction models generally assume the coordination of the phenolic group to a surface Mn atom, followed by the transfer of an electron.…”

Characterization and Properties of Metallic Iron Nanoparticles: Spectroscopy, Electrochemistry, and Kinetics

“…Two model contaminants were used as probe compounds to investigate the reactivity of nano-Fe 0 with solutes: benzoquinone (BQ) and carbon tetrachloride (CT). BQ was chosen because it is a well-established probe for the reactivity of iron oxides (55,56), and CT has been used in many detailed studies of the reactivity of iron and iron oxides (e.g., refs 57-64). BQ is likely to form comparatively strong inner-sphere complexes on iron and iron oxides (at pH > 7, as is relevant to this study; 56), whereas CT is believed to form weak and outer-sphere complexes with oxide-coated iron (65).…”

Electron Transfer Processes Between Hydroquinone and Iron Oxides

“…It is apparent that more detailed research is required to elucidate the mechanism of the oxide-organic reaction. Kung and McBride (1988) reported the kinetics of hydroquinone oxidation in an aqueous suspension of hausmannite (Mn304). Here, the oxidation reaction was apparently a surface process: the greater the surface area in suspension (with increased oxide loadings), the faster hydroquinone oxidized and the more Mn 2 § dissolved.…”

“…Here, the oxidation reaction was apparently a surface process: the greater the surface area in suspension (with increased oxide loadings), the faster hydroquinone oxidized and the more Mn 2 § dissolved. Kung and McBride (1988) continuously mon-itored the concentration ofp-benzosemiquinone anion radical, a suggested precursor of humic substances, by electron spin resonance of the solution phase throughout the reaction period. This radical, which is stable only at high pH, was found to persist in the Mn oxide suspensions at pH 6.…”