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Adsorption and Oxidation of Phenolic Compounds by Iron and Manganese Oxides¹

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.

¹ Contribution from the Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853.

² Professor of Soil Chemistry.

Received for publication February 23, 1987.

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