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

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.