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

Abstract

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.

The controlling effect of pH on oxidation of Cr(III) by manganese oxide minerals

“…Cr(III) oxidation by γ -MnOOH was observed to be faster and more significant at pH 3.0-4.5, and when pH increased to 4.5-6.0 the reactions were minimized, due to blockage of reaction sites on the mineral surface by the produced multinuclear complexes or surface precipitation [12]. Cr(III) oxidation at pH 4 by several manganese oxides, todorokite, birnessite, lithiophorite, and pyrolusite, was much greater than that at pH 7 [14]. However, it was also reported that pH influenced oxidation of Cr(III) by manganese oxides in different ways in different pH range, and maximum oxidation appeared at a certain pH value or pH range [10,21].…”

Growth Response of Sorghum bicolor (L.) Moench. Cultivars to Trivalent Chromium Stress

“…Cr can be either beneficial or toxic to animals and humans, depending on its oxidation state and concentrations (Zayed et al 1998). Cr 3+ is less toxic compared to Cr 6+ and has toxicity 10 -100 times lower than Cr 6+ (Kim et al, 2002;Yu & Gu, 2007). Cr 3+ is considered to be a trace element essential for the proper functioning of living organisms.…”

Toxic elements in tobacco and in cigarette smoke: inflammation and sensitization

“…62 While it is generally presumed that most of the chromium in tobacco is in the chromium (III) oxidation state, 65 manganese oxides are known to oxidize chromium (III) to chromium (VI) in soil and solutions. 66 Manganese in one or more oxidation states is transported in smoke particulate, therefore it is possible that this oxidation could also occur in saliva or in smoke moist particulate droplets, and on moist surfaces in the lungs to some degree.…”

“…84 The (III), (IV), (V), (VI), and (VII) oxidation states are generally more toxic in uncomplexed forms. The capacity of manganese oxides to oxidize chromium (III) to chromium (VI) 66 adds the oxidationreduction dimension to potentiation of chromium toxicity. U.S. Environmental Protection Agency reports stated that compounds of manganese were suspected of inducing or exacerbating asthma.…”