Comparison of the reactivity of various manganese oxides with trivalent aqueous chromium: Microscopic and spectroscopic observations of dissolution, chromium sorption and chromium and manganese redox interactions

The complex interaction between CrIII_aq and manganite (γ-MnOOH) was systematically studied at room temperature over a pH range of 3 to 6, and within a concentration range of 10−4 to 10−2 M CrOH2+_aq. CrIII-hydroxy hydrate was shown to precipitate on the manganite surface while still undersaturated in bulk solution. Due to direct redox coupling with Mn reduction, Cr oxidation was most rapid in the lower pH range. Neither MnII nor CrVI were ever detected on manganite surfaces, even at the maximum rate of their generation. At the highest pH's of this study, CrIII_aq was effectively removed from solution to form CrIII-hydroxy hydrate on manganite surfaces and in the bulk solution, and manganite dissolution and CrVI _aq generation were minimized. This heterogeneity is a direct result of the heterogeneous semiconducting nature of natural manganite crystals, and is also an expression of the proximity effect, whereby redox processes on semiconducting surfaces are not limited to next nearest neighbor sites.; The interaction between CrIII_aq and seven different Mn-oxides (6 monomineralic, 1 synthetic) has been observed in pH 4.4 HNO ₃ and pH 4.4 ∼ 10−4 M Cr III_aq solutions. For each mineral-solution interaction, the aqueous chemical concentrations (e.g. [Mn]_aq, [Cr]_aq, [CrVI_aq]) were measured with time. Our surface sensitive FESEM and AFM observations tend to suggest that Cr-uptake is by isolated site binding, very small (<30 nm) surface clusters or monolayer scale films. Cr-uptake was followed by slow Cr-release on several of the solids (particularly the layered solids) after a substantial portion of the total aqueous Cr had been converted to CrVI_aq.; The oxidizing ability of the different Mn-oxides for CrIII _aq is evaluated with regards to the energy level of the redox couple (i.e. the redox potential) as compared with the Fermi energy level of the Mn-oxide. The Mn-oxides that exhibited the greatest and longest lasting Cr III-oxidizing power were the Mn-oxides containing Mn3+, and in particular those containing Mn3+ and Mn2+. It is believed that the combined presence of a reducible Mn ion (e.g. Mn 3+) and a highly soluble Mn2+ ion facilitates a sustained CrIII-oxidation reaction because fresh surface is exposed during the reaction. (Abstract shortened by UMI.)...