Nonequilibrium models for predicting forms of precipitated manganese oxides

Manganese oxides precipitated by bubbling air through 0.01 molar solutions of MnCl₂, Mn(NO₃)₂, MnSO₄, or Mn(ClO₄)₂ at a constantly maintained pH of 8.5 to 9.5 at temperatures of 25°C or higher consisted mainly of hausmannite, Mn₃O₄. At temperatures near 0°C, but with other conditions the same, the product is feitknechtite, βMnOOH, except that if the initial solution is MnSO₄ and the temperature is near 0°C the product is a mixture of manganite, γMnOOH and groutite, αMnOOH.All these oxides are metastable in aerated solution and alter by irreversible processes to more highly oxidized species during aging. A two-step nonequilibrium thermodynamic model predicts that the least stable species, βMnOOH, should be most readily converted to MnO₂. Some preparations of βMnOOH aged in their native solution at 5°C attained a manganese oxidation state of +3.3 or more after 7 months. Hausmannite aged at 25°C altered to γMnOOH. The latter is more stable than a or βMnOOH, and manganese oxidation states above 3.0 were not reached in hausmannite precipitates during 4 months of aging. Initial precipitation of MnCO₃ rather than a form of oxide is likely only where oxygen availability is very low.Composition of solutions and oxidation state and morphology of solids were determined during the aging process by chemical analyses, X-ray and electron diffraction and transmission electron micrographs.