COMPETING METAL-ION INFLUENCES ON CHROMIUM(III) OXIDATION BY BIRNESSITE

The oxidation of Cr(III) to Cr(VI) presents a significant environmenta l hazard due to the much greater mobility and toxicity of Cr(VI). Desp ite the importance of Cr(III) oxidation, many factors influencing this important chemical process in soils and waters remain unknown. In thi s study we investigated the effects of competing metal sorbates on Cr( III) oxidation by delta-MnO2. Aluminum, La, Co(II), Mn(II), and Cr(VI) were reacted with delta-MnO2 prior to the addition of Cr(III). Only A l affected the extent of Cr(III) oxidation at pH greater than or equal to 4, and at PH 5 the presence of Al, with 2.84 mu mol Cr(III) initia lly present, decreased oxidation from 2.80 to 0.28 mu mol Cr(VI). None of the competing ions influenced the degree of oxidation at pH 3. Nei ther of the oxidation reaction products, Mn(II) or Cr(VI), affected th e extent of oxidation. Electrokinetic investigations revealed that bot h Al and La induced a charge reversal on delta-MnO2. As pH increased f rom 3 to 5, the electrophorectic mobility (EM) of delta MnO2 sharply i ncreased at pH 4 in the presence of 2.90 x 10(-4) M Al. The sorption b ehavior and influences on EM of Al indicate that a surface alteration occurs whereby the oxidizing ability of delta-MnO2 (birnessite) is inh ibited. High-resolution transmission electron microscopy revealed that an Al-hydroxide surface precipitate formed on delta-MnO2, thus accoun ting for the sorption, EM trends, and Cr(III) oxidation inhibition.