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DIVISION S-9 - SOIL MINERALOGY

Oxidation of Chromium(III) to (VI) by Manganese Oxides

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843-2474
^b Dep. of Chemistry, P.O. Box 30012, Texas A&M Univ., College Station, TX 77842-3012

* Corresponding author (j-dixon{at}tamu.edu)

Manganese oxides in soils oxidize Cr from a stable form to a more toxic mobile form. The influence of Mn oxide diversity on Cr oxidation is the subject of this report. Oxidation of Cr(III) to Cr(VI) by coarse clay-size natural Mn oxides, birnessite, todorokite, and lithiophorite was studied at pH 4 and 7, and at 200 and 400 µM initial Cr(III) concentrations. The oxidation rate obeyed the first-order rate law at the 400 µM Cr(III) initial concentration; the 200 µM concentration was too low and reverse reactions may have interfered. Rate law is valid only far from equilibrium. The rate of oxidation was greater for todorokite and birnessite that contained the most quadrivalent Mn and least for lithiophorite that contained a greater proportion of trivalent Mn determined by x-ray photoelectron spectroscopy (XPS). The deviation of Mn oxidation state from the frequently assumed quadrivalent form in Mn oxides suggests the need for a thorough assessment of the ion species of Mn in Mn oxides that occur in soils. At 400 µM Cr(III), the rate of Cr oxidation by the pyrolusite sample was intermediate between birnessite and todorokite suggesting that the sample was not pure pyrolusite. Apparently, the synthesis process did not reach equilibrium with pyrolusite. The sample used as a chemical standard was apparently more soluble and more effective in oxidizing Cr(III) than would be expected for the natural mineral pyrolusite.

Abbreviations: BE, binding energy • FT-IR, Fourier transform infrared spectroscopy • IR, infrared • PZC, point of zero charge • USNM, United States National Museum • XPS, x-ray photelectron spectroscopy • XRD, x-ray diffraction

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