Adsorption and Oxidation of Phenolic Compounds by Iron and Manganese Oxides

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Abstract
The adsorption and oxidation of catechol and hydroquinone by Fe and Mn oxides has been investigated by Fourier transform infrared spectroscopic (FTIR) analysis of the adsorbed molecules and by the measurement of O2 consumption by aqueous suspensions of these oxides. Evidence for direct coordination of catechol and salicylate to surface Fe3+ on iron oxides was obtained by FTIR. The promotion of catechol and hydroquinone oxidation by Fe and Mn oxides was confirmed by measured rates of O2 consumption and by the appearance of Fe2+ and Mn2+ in the solutions. However, only trace levels of soluble Fe2+ were detected, suggesting that oxidation by Fe(III) oxides was catalytic in that electron transfer between the phenols and Fe3+ generated Fe2+, which was rapidly reoxidized by O2. Other adsorbates introduced into these oxide/phenol systems, such as acetate, phosphate, and Cu2+, diminished O2 consumption rates, but the effect was generally attributable to a Iowered pH that inhibited oxidation. A model of surface oxidation by Mn and Fe is presented in which coordination of the organic at the surface is a prerequisite to electron transfer. Oxidation of organics can proceed with or without the uptake of O2, depending largely on pH, which determines the rate of reoxidation of the reduced metal ions by O2. The results emphasize the difficulty in interpreting the effects that chemical buffers have on oxidation reactions at oxide surfaces.
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