Stability of βMnOOH and manganese oxide deposition from springwater
Beta MnOOH is precipitated preferentially (with respect to Mn 3 O 4 ) at temperatures near O°C when Mn 2+ is oxidized in aerated aqueous solutions. Upon aging in solutions open to the atmosphere a slurry of βMnOOH tends to disproportionate to form MnO 2 and Mn 2+ . In such aged solutions, Mn 2+ and H + activities can be constant, and both the oxidation reaction Mn 2+ +¼O 2 (aq) + 3/2H 2 O → βMnOOH (c) + 2H + and the disproportionate reaction 2βMnOOH (c) + 2H + → MnO 2 (c) + Mn 2+ + 2H 2 O can have positive reaction affinities. It is not possible for both reactions to be in thermodynamic equilibrium in the same system unless oxygen is almost completely absent. A value for Δ G f 0 of −129.8±0.6 kcal/mol was obtained for βMnOOH from experimental data by assuming that the reaction affinity for the oxidation reaction is equal to that for the disproportionation. A value for Δ G f 0 for βMnOOH of −129.8±0.5 kcal/mol was determined by measuring the redox potentials for the postulated half-reaction MnO 2 (c) + H + + e − → βMnOOH (c) at 0°, 5°, and 15°C and extrapolating to 25°C. Both these values are consistent with laboratory observations that βMnOOH is less stable than γMnOOH or Mn 3 O 4 at 25°C. Analytical data for manganese-depositing springwater samples are consistent with a nonequilibrium model involving disproportionation of either βMnOOH or Mn 3 O 4 .