Cr(III)-containing iron oxides are a common product of Cr(VI) reduction by Fe(II) species. Redox reactions between MnO2 and Cr(III)-containing iron oxides can affect the mobility of Cr(VI) in natural and engineered systems. We systematically evaluated the rates of Cr(III) oxidation by δ-MnO2 from Cr(III)-Fe(III) hydroxides. With a fixed manganese oxide concentration, the rate of Cr(VI) production exhibits a log-linear relationship to the predicted equilibrium dissolved Cr(III) concentration. We hypothesize that Cr(VI) is generated as a result of Cr(III) dissolution and transport to MnO2 surfaces where it can be oxidized. Cr(VI) can then be released to the aqueous phase with some of the Cr(VI) adsorbing to the Cr(III)-Fe(III) hydroxides. In addition to reactions with pre-existing MnO2, Mn(II) generated from MnO2 during Cr(III) oxidation could be oxidized back to MnO2 by dissolved oxygen. While abiotic Mn(II) oxidation by dissolved oxygen is slow in solution, it is catalyzed by Cr(III)-Fe(III) hydroxides surfaces and can be significant above pH 8. Both MnO2 formation and Cr(III) oxidation by MnO2 play an important role in Cr(VI) production rates in the Cr(III)-Fe(III) hydroxides/Mn(II)/oxygen system. Our results suggest that Cr(VI) can be naturally produced through oxidation of Cr(III) in Cr(III)-Fe(III) hydroxides coupled with the redox cycling of manganese.

Cr(VI) generation from Cr(III)-containing iron oxides associated with manganese redox cycling

255th National Meeting (2018)

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