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Competing Metal Ion Influences on Chromium(III) Oxidation by Birnessite
Author(s) -
Fendorf Scott E.,
Zasoski Robert J.,
Burau Richard G.
Publication year - 1993
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1993.03615995005700060019x
Subject(s) - birnessite , chemistry , oxidizing agent , sorption , chromium , metal , inorganic chemistry , redox , manganese , nuclear chemistry , adsorption , manganese oxide , organic chemistry
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‐MnO 2 . Aluminum, La, Co(II), Mn(II), and Cr(VI) were reacted with δ‐MnO 2 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‐MnO 2 . As pH increased from 3 to 5, the electrophoretic mobility (EM) of δ‐MnO 2 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‐MnO 2 (birnessite) is inhibited. High‐resolution transmission electron microscopy revealed that an Al‐hydroxide surface precipitate formed on δ‐MnO 2 , thus accounting for the sorption, EM trends, and Cr(III) oxidation inhibition.