Competing Metal Ion Influences on Chromium(III) Oxidation by Birnessite
1993
Fendorf, Scott E. | Zasoski, Robert J. | Burau, Richard G.
The oxidation of Cr(III) to Cr(VI) presents a significant environmental hazard due to the much greater mobility and toxicity of Cr(VI). Despite the importance of Cr(III) oxidation, many factors influencing this important chemical process in soils and waters remain unknown. In this study we investigated the effects of competing metal sorbates on Cr(III) oxidation by δ-MnO₂. Aluminum, La, Co(II), Mn(II), and Cr(VI) were reacted with δ-MnO₂ prior to the addition of Cr(III). Only Al affected the extent of Cr(III) oxidation at pH ≥ 4, and at pH 5 the presence of Al, with 2.84 µmol Cr(III) initially present, decreased oxidation from 2.80 to 0.28 µmol Cr(VI). None of the competing ions influenced the degree of oxidation at pH 3. Neither of the oxidation reaction products, Mn(II) or Cr(VI), affected the extent of oxidation. Electrokinetic investigations revealed that both Al and La induced a charge reversal on δ-MnO₂. As pH increased from 3 to 5, the electrophoretic mobility (EM) of δ-MnO₂ sharply increased at pH 4 in the presence of 2.90 × 10⁻⁴ M Al. The sorption behavior and influences on EM of Al indicate that a surface alteration occurs whereby the oxidizing ability of δ-MnO₂ (birnessite) is inhibited. High-resolution transmission electron microscopy revealed that an Al-hydroxide surface precipitate formed on δ-MnO₂, thus accounting for the sorption, EM trends, and Cr(III) oxidation inhibition.
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