Stability of βMnOOH and manganese oxide deposition from springwater

Abstract

Beta MnOOH is precipitated preferentially (with respect to Mn304) at temperatures near 0øC when Mn 2+ is oxidized in aerated aqueous solutions. Upon aging in solutions open to the atmosphere a slurry of/3MnOOH tends to disproportionate to form MnO2 and Mn 2+. In such aged solutions, Mn 2+ and H + activities can be constant, and both the oxidation reaction Mn 2+ + x402 (aq) + «H20 •/3MnOOH (c) + 2H + and the disproportionation reaction 2/3MnOOH (c) + 2H + • MnO2 (c) + Mn 2+ + 2H20 can have positive reaction afiSnities. 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 AGf of -129.8 _+ 0.6 kcal/mol was obtained for/3MnOOH from experimental data by assuming that the reaction afiSnity for the oxidation reaction is equal to that for the disproportionation. A value for AGf for/3MnOOH of -129.8 -+ 0.5 kcal/mol was determined by measuring the redox potentials for the postulated half-reaction MnO2 (c) + H + + e-•/3MnOOH (c) at 0 ø, 5 ø, and 15øC and extrapolating to 25øC. Both these values are consistent with laboratory observations that/3MnOOH is less stable than 7MnOOH or Mn304 at 25øC. Analytical data for manganese-depositing springwater samples are consistent with a nonequilibrium model involving disproportionation of either/3MnOOH or Mn304.

Auto-Inhibition of Oxide Mineral Reductive Capacity Toward Co(II)EDTA

“…This is consistent with x-ray photoelectron spectroscopy which indicated the production of Mn(III) after birnessite was reacted with Co(II) (13). The production of Mn(III) is also noted in the oxidation of Mn(II) as either a transient intermediate phase, dominantly Mn 3 04 or β-ΜηΟΟΗ, or as a stable end product, γ-ΜηΟΟΗ (14)(15)(16)(17).…”

Oxidation of Phenol in Acidic Aqueous Suspensions of Manganese Oxides

“…Another fraction of the solution was used for determining dissolved Mn by atomic absorption spectroscopy (AAS). This procedure avoids any need for drying the solid (Hem et al, 1982). Dichromate titration was found superior to the commonly used permanganate titration, giving a much sharper end point, and reproducible results using commercial MnO2 as a standard.…”

“…Feitknechtite (B-MnOOH) was synthesized according to the method of Hem et al (1982) by raising the pH of 900 ml of 0.0098 M Mn(C104)2 to 9.00, and holding it at that pH by addition of carbonate-free 0.2 M NaOH until the reaction was complete. The temperature of the solution was held between 0 ~ and 2~ 3 Hem et al (1982).…”

“…(2) Oxide stability decreases in the same order as its ability to oxidize phenol at the slow rate: buserite > manganite > feitknechtite ( Figure 9). Feitknechtite and manganite are unstable at low pH (Bricker, 1965;Hem et al, 1982). They "leak" large quantities of Mn 2+ into solution generating high soluble manganese concentration.…”

The kinetics of iodide oxidation by the manganese oxide mineral birnessite

“…The solid was confirmed to be birnessite by XRD, and the surface area was determined to be 41 m 2 /g by 5 point N 2 -BET. The average oxidation state of Mn in the solid was determined to be 4.09 by oxalate titration (Hem et al, 1982). The resulting birnessite solid contained 52.4% (w/w) Mn and 9.2% (w/w) K.…”