Coprecipitation mechanisms and products in manganese oxidation in the presence of cadmium

Journal Article · Sun Sep 01 00:00:00 EDT 1991 · Geochimica et Cosmochimica Acta; (United States)

DOI:https://doi.org/10.1016/0016-7037(91)90364-B· OSTI ID:7049769

Hem, J D; Lind, C J ^[1]

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1. Geological Survey, Menlo Park, CA (United States)

Manganese oxidation products were precipitated in an aerated open-aqueous system where a continuous influx of mixed Mn{sup 2+} and Cd{sup 2+} solution was supplied and pH was maintained with an automated pH-stat adding dilute NaOH. X-ray diffraction and electron diffraction identified the solids produced as mixtures of Cd{sub 2}Mn{sub 3}{sup 4+}O{sub 8}, Mn{sub 2}{sup 2+}Mn{sub 3}{sup 4+}O{sub 8}, MnO{sub 2} (ramsdellite), and CdCO{sub 3}. Mean oxidation numbers of the total precipitated Mn as great as 3.6 were reached during titrations. During subsequent aging in solution, oxidation numbers between 3.8 and 3.9 were reached in some precipitates in less than 40 days. Kinetic experiments were made to test effects of various Cd/Mn + Cd mole ratios and rates of addition of the feed solution, different temperatures from 5.0 to 35C, and pH from 8.0 to 9.0. Oxidation rates were slower when the Cd mole ratio was less than 0.40. The increased rate of Mn{sup 4+} oxide formation in the presence of Cd{sup 2+} can be ascribed to the formation of a labile adsorbed intermediate, CdMn{sub 2}O{sub 4Int}, an analog of hausmannite, formed on precipitate surfaces at the beginning of the oxidation process. The increased lability of this structure, resulting from coordination-chemical behavior of Cd{sup 2+} can be ascribed to the formation of a labile adsorbed intermediate, CdMn{sub 2}O{sub 4Int} an analog of hausmannite, formed on precipitate surfaces at the beginning of the oxidation process. The increased lability of this structure, resulting from coordination-chemical behavior of Cd{sup 2+} during the titration, causes a rapid second-stage rearrangement and facilitates disproportionation of the Mn{sup 3+} ions. The Mn{sup 2+} ions thus released provide a positive feedback mechanism that couples the two steps of the conversion of Mn{sup 2+} to Mn{sup 4+} more closely than is possible when other metal ions besides manganese are not present.

• OSTI ID: 7049769
• Journal Information: Geochimica et Cosmochimica Acta; (United States), Journal Name: Geochimica et Cosmochimica Acta; (United States) Vol. 55:9; ISSN GCACA; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
CADMIUM
CADMIUM COMPOUNDS
CADMIUM OXIDES
CHALCOGENIDES
CHARGED PARTICLES
CHEMICAL REACTIONS
ELEMENTS
IONIC POTENTIAL
IONS
KINETICS
MANGANESE
MANGANESE COMPOUNDS
MANGANESE IONS
MANGANESE OXIDES
METALS
OXIDATION
OXIDES
OXYGEN COMPOUNDS
PH VALUE
PRECIPITATION
REACTION KINETICS
SEPARATION PROCESSES
SORPTION
SORPTIVE PROPERTIES
SURFACE PROPERTIES
TEMPERATURE EFFECTS
TITRATION
TRANSITION ELEMENT COMPOUNDS
TRANSITION ELEMENTS
VALENCE